Mercury 280 bogs down sometimes when trimming, shifter sometimes shuts off the motor

[jwestern]

My 1990 STV has a 2-stroke Mercury 2.5L EFI SS (280 hp, powerhead is 2001, other components are older, from 2.4 L EFI days). It has been plagued with intermittent throttle power issues for years, with shops usually unable to identify the problems. I'll first describe the current issues. Then, in case it's relevant, I'll describe issues from the past few years. All of the issues exhibited inconsistent conditions for replication, which is a big reason why it's been so difficult to identify and fix.

Current issues:

When launching the boat this season, the battery was too weak to start the engine. I boosted it, drove a few km to my dock, and when shifting into reverse while parking, the engine shut off. The engine shutting off while going into either forward or reverse gear has occurred a few times so far this season (a few weeks). The battery was chargeable but wouldn't hold sufficient voltage under a load, as determined by a load tester. So I bought a new battery.

A few days later, while making a sharp turn and letting off the throttle (to avoid a hairy situation), the engine abruptly shut off. It started again and there were no further issues that day. The boat was fully occupied at the time (3 or 4 people total).

A couple of weeks later, I'm out for a drive by myself. After 30-40 minutes of driving, I experienced a momentary drop in power. With the height of my motor, to get past the galloping around 70 mph, I have to trim the motor out and accelerate rapidly. Once I'm around 80 mph, I'm past the gallop, so I trim back down a bit to get more grip on the water and level the boat out, as it feels safer. A split second after trimming down, the motor power bogged down. The power loss lasted a second or so, and was variable during that second, and prompted me to get off the throttle and baby it for a few minutes. 10 minutes later, while getting from idle onto plane, I again was trimming down and experienced a huge ~second long loss of power.

The next day, I went out again, and experienced a drop in power getting out of the hole (again touching the trim during throttle). What was new here was the issue happened immediately -- I didn't have to drive around for 30 minutes to experience it. Suspecting power draw as the issue, I began looking at my voltage gauge while driving and trimming. I'm not sure the gauge is accurate (many of my gauges are not accurate -- trim is slow to respond, water pressure reads 6 psi instead of 0). But the reading while driving around is about 13 V, and while trimming it drops to about 10.5 V. At idle it reads about 10.5 V. The engine does NOT bog down consistently while trimming, nor can I confidently say that it has ONLY happened while trimming (I don't recall the details of what I was doing in every instance).

Anyone know what's going on? Are there multiple core issues here (e.g. starter assist is dying, and alternator is dying) or is there a common cause? How can I or my local marina (not high-performance experts) diagnose?

Note I don't have enough domain knowledge to understand many replies I read on this forum (too much jargon or short-form terms), so longer explanations would be welcomed.

Issues from the past few years:

Also loss of power, usually at 5500 RPM, and not necessarily while trimming. This power loss is much more sharp, brief, intermittent, and repeating. But again, it was not very consistent. It got worse with time (usually no issue for the first ~20 minutes of driving, issues starting around the ~30 minute mark). The mechanic (a high-performance expert) wasn't able to identify the issue or replicate it during lake tests, but opined that it was just fuel issues from the boat sitting too long (he cleaned out some vacuum tubes and what not), and re-rigged the fuel pump to something more optimal (I can't describe the details). This servicing took place over the last two seasons (one visit per season). The cleaning out of tubes didn't seem to fix anything, but the re-rigging of the fuel pump seems to have stopped this.

The year before that, a more major check-up was performed by a different high-performance expert. He found the exhaust was from a 2.4L (probably the previous motor, which blew up), so he put in a 2.5 exhaust. Replaced reeds. The fuel pump was barely pumping 1 L of fuel in under 17 seconds, so he recommended replacing the fuel pump to a Bosch (I forget the model). I did so, which decreased the time to pump 1 L to ~10 seconds if I recall correctly.

----------------------------------------

So in summary, there have been intermittent, difficult-to-reliably-reproduce, power loss issues under throttle for several years. 3 visits to high-performance experts. This is year 4 of issues.

Any help y'all can provide would be greatly appreciated! Thanks for reading.

[skialot2]

Sounds like something electrical. Bad grounds somewhere. Broken wire in the control harness. If you have another control harness with ignition you can plug in temporally just to test it. That way you can eliminate anything in the boat wiring.

"a more major check-up was performed by a different high-performance expert. He found the exhaust was from a 2.4L (probably the previous motor, which blew up), so he put in a 2.5 exhaust."

I could be wrong, but I don't think this is possible. A 2.4 Exhaust cover does not fit a 2.5. They have different bolt patterns. Unless you mean the adapter plate and tuner. But the powerhead has to come off for that.

[jwestern]

I could be wrong, but I don't think this is possible. A 2.4 Exhaust cover does not fit a 2.5. They have different bolt patterns. Unless you mean the adapter plate and tuner. But the powerhead has to come off for that.[/COLOR]

he said he has to "pull the powerhead" to change it, so it may have been what you said.

i dont have another control harness (i wouldnt even be able to identify it), but ill mention your suggestion to my local marina (not high-performance experts)

[LakeFever]

Gremlins like this can be challenging. Sounds like you need to go through it. I’m with skialot work through the wiring first. One wire at a time from end to end make sure all are in continuity and any poor splices need to be opened up and redone. Barrel connectors are not always great either so you can pull them apart and squeeze the female side back down to ensure a tight fit. If any of the barrel connectors are browned or darkened from heat, replace them. Add more grounds to your motor connecting any and all components with a ground strap. Make sure there is no corrosion on any connectors or eyes. Freshen any failing gauges and their wiring as well. You also should look at a fresh ignition switch too.

This is what I mean by going through it. Verify every single circuit and component is within spec. Use the manual to guide you through all this. It takes time but sounds like it needs to be done

[tlwjkw]

[QUOTE=jwestern;3430536]he said he has to "pull the powerhead" to change it, so it may have been what you said.[QUOTE]

he needs ta know what he lookin' for if he's gonna change it... there are (one piece) adapter plates out there with all different size discharge holes that will make a big difference in tha coolin' system.. tha holes in tha 2.4 plate are about tha right size for tha 280... you can use tha 2.4 plate as is or you can open up tha exhaust exit to same size as tha exit from tha block.. if you don't wanna do any grindin' on tha plate you can use tha metal infused power head gasket for tha 280.. tha exhaust will over lap a tad but you'll never notice any difference in performance.. that gasket will last for quite awhile before it goes south....

curious?... I'm to lazy to google... where is Bancroft? canada maybe?

[jwestern]

[QUOTE=jwestern;3430536]he said he has to "pull the powerhead" to change it, so it may have been what you said.[QUOTE]

he needs ta know what he lookin' for if he's gonna change it... there are (one piece) adapter plates out there with all different size discharge holes that will make a big difference in tha coolin' system.. tha holes in tha 2.4 plate are about tha right size for tha 280... you can use tha 2.4 plate as is or you can open up tha exhaust exit to same size as tha exit from tha block.. if you don't wanna do any grindin' on tha plate you can use tha metal infused power head gasket for tha 280.. tha exhaust will over lap a tad but you'll never notice any difference in performance.. that gasket will last for quite awhile before it goes south....

curious?... I'm to lazy to google... where is Bancroft? canada maybe?

To clarify, the 2.5 exhaust was put on the motor (a few years ago), so that's water under the bridge now.

Yes, Bancroft is in Ontario, Canada.


Gremlins like this can be challenging. Sounds like you need to go through it. I’m with skialot work through the wiring first. One wire at a time from end to end make sure all are in continuity and any poor splices need to be opened up and redone. Barrel connectors are not always great either so you can pull them apart and squeeze the female side back down to ensure a tight fit. If any of the barrel connectors are browned or darkened from heat, replace them. Add more grounds to your motor connecting any and all components with a ground strap. Make sure there is no corrosion on any connectors or eyes. Freshen any failing gauges and their wiring as well. You also should look at a fresh ignition switch too.

This is what I mean by going through it. Verify every single circuit and component is within spec. Use the manual to guide you through all this. It takes time but sounds like it needs to be done

Thanks for your replies. Under the cowl of the motor things look pristine. Inside the boat, not so much -- it's a mess. Definitely seems to be an accumulation of entropy from quick fixes over the years, and it's very difficult to get at those wires (it's an STV pro comp ski).

What should the voltage reading be at idle, at throttle, and with/without trimming? Can one test the alternator system easily?

Do I need to use the motor serial number to find the correct manual? I always thought these were frankenstein motors, rebuilds, custom, etc., for which manuals might not be accurate.

[LakeFever]

I’m not the most informed on 280’s. I have seen manuals for them come up for sale now and then so they are out there. I have a laser injected motor and the mercury manual has been excellent to have helping me get everything set to spec. I don’t know what the voltage at idle is supposed to be for the alternator set up maybe someone else familiar will chime in. I know all about tight quarters and rigging these speed boats we all have are brutal to access and service in the hull. I enlisted a young skinny fella to help me with mine for that very reason. It sounds to me like this is the place to begin by tidying up all your wiring with good fresh connections and heat shrinks. A quick tip that works good is to use barrel connectors but cut the insulation off. You can do this carefully with a razor knife slicing down the barrel repeatedly until it can be pried open and the metal crimp tube removed. Then using quality dual wall heat shrink tube you can crimp the metal down for a splice and heat shrink over the whole connection making a good strong clean water tight splice that is faster than solder and a lot easier to manage.

[jwestern]

Do you mean the owner's manual or the service manual? I see a wiring diagram in the owner's manual, but I'm not sure how complete it is.

[LakeFever]

You need the service manual it’s extremely comprehensive. The owners manual is basic. The manual for my motor is about a thousand pages to give you an idea. I have also seen them for sale on ebay. There are digital pdf downloadable copies out there but not that I have seen for 280’s or drags etc. as for identifying your motor if you took pics of the power head from a few angles someone on here should be able to tell you what model manual to look for and hopefully have more info that I do to share. Some of the most advanced members on here don’t get on that frequently so it may take a little time. A bummer at this time of year when the season is closing fast. You could also call some of the race shops around and they might be of some help. Either way if you have a birds nest of wire in the hull that seems a sound place to start to me

[25two.stroke]

Check voltage with a digital volt meter and multiple places in the boat and see if it is consistent. Also see if the analogue gauge is accurate. Many times those analogue volt meters are off. Sometimes significantly off (3 to 4 volts). The voltage at rest should be between 12 and 13. The voltage keyed up but engine not running should be no more than 0.2 v lower than with the key off UNLESS the fuel pump is running. If fuel pump is running the voltage may drop 0.5, 0.8 something like that. The voltage at the motor with it idling should be close to 13v. Then as the rpms vary it shouldn't ever go over 14.7v but it should remain above 12.5.

Do you have a fuel pressure gauge? I would suspect a fuel issue with exactly what you are describing. Either wiring to the fuel pump is bad, the fuel pump itself is bad, or the pickup in your fuel tank is sucking air. That is my best guess. The 280s had a pair of wires coming out of the harness that were for running the onboard fuel pump. 280 was the only motor that had this...not 260s or anything older. I have seen people run that straight to their bosch pump. I would never do that. I would only use those wires to activate a relay which would carry the load of the pump. Those wires come right out of the ecm and I do not believe there is enough current capacity to effectively power the fuel pump. Check into this in your scenario. See if your pump is powered from the motor, if it's powered off a separate switch on the dash, or if it's directly off the key? Either way if it is not on a relay it could be dragging the power down from whatever it is coming off of.

[jwestern]

Check voltage with a digital volt meter and multiple places in the boat and see if it is consistent. Also see if the analogue gauge is accurate. Many times those analogue volt meters are off. Sometimes significantly off (3 to 4 volts). The voltage at rest should be between 12 and 13. The voltage keyed up but engine not running should be no more than 0.2 v lower than with the key off UNLESS the fuel pump is running. If fuel pump is running the voltage may drop 0.5, 0.8 something like that. The voltage at the motor with it idling should be close to 13v. Then as the rpms vary it shouldn't ever go over 14.7v but it should remain above 12.5.

Do you have a fuel pressure gauge? I would suspect a fuel issue with exactly what you are describing. Either wiring to the fuel pump is bad, the fuel pump itself is bad, or the pickup in your fuel tank is sucking air. That is my best guess. The 280s had a pair of wires coming out of the harness that were for running the onboard fuel pump. 280 was the only motor that had this...not 260s or anything older. I have seen people run that straight to their bosch pump. I would never do that. I would only use those wires to activate a relay which would carry the load of the pump. Those wires come right out of the ecm and I do not believe there is enough current capacity to effectively power the fuel pump. Check into this in your scenario. See if your pump is powered from the motor, if it's powered off a separate switch on the dash, or if it's directly off the key? Either way if it is not on a relay it could be dragging the power down from whatever it is coming off of.

Thanks. I'll check voltages as you describe. I don't have a fuel pressure gauge -- how would it behave if it were a fuel issue vs not a fuel issue?

[billybob]

The first thing you need to do is get a Boat in A box. Simply an accessory key switch. Hook it to the motor let it sit in back seat. Use it to start and run motor, then use control box to shift & throttle. Run it hard & see if problem happens. If it does problem is in motor, electrically. If not prob.does not happen it is in boat. harness or such. That is always first step in trouble shooting that problem.

[jwestern]

535414

Ohh by "harness" you guys mean the cabling from the shifter unit to the motor, not this pictured monstrosity hugging the motor.

[LakeFever]

Yes to above. The harness plugs into the motor near the switch boxes. It runs up to the control and has some various leads for the key switch, trim, gauges, tether etc etc. I see plenty where the splices here are crap and not using marine grade wire and have who knows what accessory wired in and then cut off. Plenty of who knows what possible in there. That’s where I would start looking for problematic wiring

boat in a box is an awesome tool but a bit pricey for a one boat owner to use a handful of times. Cool tool though you can fire up any motor anywhere anytime basically

[jwestern]

Update: my local marina were able to identify a wiring issue with the fuel pump. The fuel pump is indeed on a relay (thanks 25two.stroke for asking about that), which they tell me allows the fuel pump to prime for a few seconds when the key turns one click, then turns off. But the wiring to the motor side was too short, so it was being tugged on when the motor trimmed down. Also the bullet connectors for these wires at the motor weren't tight -- I'm guessing that explains the dependence of the power loss issue on run duration (temperature and/or moisture level).

They gave some more slack to the wires and tightened down the connections. On my first run out, there was a momentary bogging down immediately after getting onto plane. Not sure what caused that, but it was an unusual situation where I was turning into a wake and flooring it to get out of the hole. My marina speculates that it was just a cold motor (I was only idling for a minute or two prior to this). Or maybe the fuel sloshed to one side of the tank so the pump sucked air for a moment. Following that, I ran for 30 minutes and didn't notice a power loss. Was able to trim down to neutral at 80 mph and increase speed to 97 mph.

So I'm hoping the issue is fixed. But I bought a borescope which I'm excited to use. I'll use it to inspect the wiring in the cramped areas like behind the fuel tank.

Any other tips/advice from y'all is welcome, and thanks for teaching me.

Separate question: is there a way to record all the available running data from the engine? It would be helpful for diagnosis purposes to always have a digital recording of data, rather than trying to rely on my faulty memory. Maybe a special gauge set that plugs into a separate computer?

[Dutch]

the reason for the relay is because back when people ran 260's powering the fuel pump from the key harness the draw from the pump exceeded the little wire most people pulled the power from. the 280's have power coming from the ecu so that when you turn the key on it primes the fuel system and the ecu will cut the power to the pump if it senses the 280 isn't running. I have seen 280 fuel pumps run with and without relays, depends on who you talk to and how much you're twisting it up. A fuel pressure gauge wouldn't hurt, and get a ddt if you want to plug into the motor for some data but it won't save it. You mentioned everything is from a 2.4 except the power head. I would then check your fuel pump that you have the newer style, not the one that the barb out of the pump is only 5/16".

[jwestern]

Just discovered the nut on the positive post on the fuel pump is spinning. Thread must be stripped.

[jwestern]

the reason for the relay is because back when people ran 260's powering the fuel pump from the key harness the draw from the pump exceeded the little wire most people pulled the power from. the 280's have power coming from the ecu so that when you turn the key on it primes the fuel system and the ecu will cut the power to the pump if it senses the 280 isn't running. I have seen 280 fuel pumps run with and without relays, depends on who you talk to and how much you're twisting it up. A fuel pressure gauge wouldn't hurt, and get a ddt if you want to plug into the motor for some data but it won't save it. You mentioned everything is from a 2.4 except the power head. I would then check your fuel pump that you have the newer style, not the one that the barb out of the pump is only 5/16".

Are you saying that if the fuel pump primes and then shuts off, that does not necessarily mean the fuel pump is on a relay? I.e. the ECU is shutting it off because the motor isn't running, and it's not because of a relay? If so, then perhaps I do NOT have the fuel pump on a relay.

I checked the fuel pump hoses are all 3/8".

[jwestern]

Dutch 25two.stroke

I discussed what you said about relays with a friend. Now I understand better what you're talking about. My pump is not on a relay -- the wires go straight from the ECU to the pump.

Today on a perfectly calm lake I was able to notice small intermittent power loss when completely flooring the throttle at high speed. The amplitude of the power loss is much less than before, maybe 10-15%, so it's probably a separate issue from before. Perhaps the lack of relay is the culprit now.

What are the specifications I need to do the relay? I guess I need to know the power output of the ECU, what gauge wires to use... and understand the wiring diagram, and where to mount the relay.

[tlwjkw]

this or similar.. power tha relay with any "purple".. easiest in tha boat...tape up tha pump feed from tha ecu....

Mr. Gasket 40205G Mr. Gasket Fuel Pump Relays | Summit Racing (https://www.summitracing.com/parts/mrg-40205g)

[25two.stroke]

I don't think your 10-15% power loss is the lack of a relay; but it could be. Always better in my opinion to put it on a relay. Not good for the ECU to have to put out all that power all the time. I've seen a cheap relay kit like that in the link used and it works...I would look for a sealed relay like what merc uses for the power trim. Mercury sells an "accessory power kit" for use on 14 pin harnesses where you wanna wake up the whole dash by the key. This kit has a waterproof relay and the proper colored wires coming out of the holder. If you don't care about the wire colors being correct then cut up a power trim relay harness.

[jwestern]

Thank you both. I ordered that Mr. Gasket kit before reading your post 25two.stroke. Can I seal the relay using silicone?

I poked around the boat and motor today, and uploaded some photos of the wiring in this google drive folder: https://drive.google.com/drive/folders/1dUe9wAwFvOjqolMin4NUguZSnP5oZAIR?usp=drive_link

I can identify two relays. What do they power?

There's also this plug with one white and one blue wire coming out, which was unplugged. What does that do?

I also had a closer look at the fuel tank. It's got a plastic stand pipe, and the connection at the top is a sharp 90 degree bend, rather than a more gradual 90 degree turn like Diamond Marine recommends. Perhaps that flow restriction is causing the 10-15% power loss at full throttle? There is not enough clearance for a gentle 90 degree bend due to the master switch. But the opposite side of the tank has a plugged port and lots of room for such a bend. So I think I will switch the standpipe over to the other side and get a gentle bend in there. This will also shorten the hose length from the tank to the pump, since the pump is mounted on that side.

[25two.stroke]

Just mount the relay where it won't ever get wet.
Relays on the motor are the main power relays. When the motor is keyed-up they turn on and supply power to the CDMs, the ECM and that pigtail for the fuel pump. These items all share power off the Red/Blue wire in the harness, which is why I like putting the fuel pump on its own power source in the boat. If your fuel pump were to short or draw too much its taking power straight from the ECM and the CDMs...not good. If its on its own relay there is never enough power draw to take away from the engine management.
2 wire plug is for diagnostic; leave it.

[skialot2]

Use a waterproof marine relay. Thay are cheap.

Amazon.com: True MODS 1 Pack 5-Pin 12V Bosch Style Waterproof Relay Kit [Harness Socket] [12 AWG Hot Wires] [SPDT] [30/40 Amp] 12 Volt Automotive Marine Relays for Boats Auto Fan Cars : Automotive (https://www.amazon.com/ONLINE-LED-STORE-Waterproof-Harness/dp/B01N66W2XF/ref=sr_1_4?crid=1YKKA838LXNE9&dib=eyJ2IjoiMSJ9.cRrv_UjVuzPGPl7TkJVVpwszvJ0_FaCzCILGUEBd1RLl3pVxeBUI_ZdUWxlO3xImGoq1DeRx5KnDXaJbJm9 uJhVrF5D44hNE74wI0P-NIcCayQ069UyZKgc_HMcpYiT_7i7jFY2HByISK5OMB0exOhZEkFgX5AXnDFgdbYMsA5xWCl4aCLkz8J1igpoBOvkUA9m3rqDSeNP zNKQQQlsRgC0weFcibhRhd_0Es72NddU.5gRIIXbYEP1aGYTTU6M1dMW2r-mSiT3FSFszIP6RjN0&dib_tag=se&keywords=marine%2Brelay&qid=1726937318&sprefix=marine%2Brelay%2Caps%2C91&sr=8-4&th=1)

[jwestern]

I put the fuel pump on a relay. It did not solve the bogging at full throttle.

While doing so I noticed a few problems with the fuel plumbing, in addition to the sharp 90 degree turn at the top of the stand pipe.

1) The fuel line from the fuel/water separator and the pump inlet is 5/16". It looks to be squeezed over the barbs, like it doesn't fit them.

2) The fitting at the fuel pump outlet is a 90 degree tube (not a sharp bend), and quite narrow in diameter. Not sure how crucial the outlet side of the plumbing is, compared to the inlet side which I understand is rather important. The 90 degree tube is presumably because there's no room to run straight out (there's something else in the way, a starter or trim motor or something, I can't identify it).

Is it plausible that this plumbing is causing the bogging at full throttle?

Also, I might be crazy, but there seems to be an imperfect correlation between mechanical motion of the boat and the bogging. In my last test, the bog seemed to oscillate in strength in phase with the boat's porpoising at 75 mph. Not sure what to make of it -- e.g. I don't think the porpoising was crazy enough to slosh the fuel so far forward in the tank that the stand pipe would suck air. The bogging seems to get worse with time; one run with some friends we topped out at 88 mph due to bogging, an hour later on another run we topped out at 77 mph due to bogging.

Next I plan to empty the fuel tank and pull it forward so I can take out and inspect the standpipe, and get rid of the sharp 90 degree elbow by switching the standpipe to the other fuel tank port. I'll also replace that small segment of 5/16" hose with 3/8" hose. But I'm curious to hear all your thoughts.

[jwestern]

Update:

While inspecting the standpipe, I found a small piece of debris caught by the mesh filter at the top of the stand pipe. I'm guessing this piece of debris was causing the bog at full throttle, as it was reducing the cross sectional area of fuel flow by probably 1/3rd.

I also cleaned out the fuel tank and redid the plumbing on the pump inlet side, so that all the fittings are 1/2" and the hose is 1/2" and the run is a couple of feet shorter. I did a lake test and didn't notice any bogging at full throttle.

I did however notice that the fuel pump I thought was in there (Bosch 044) had been swapped with a Bosch 957. The shop that did that didn't tell me. I'm guessing it was a mistake they made while moving things around back there, but the guy denies it happened and doesn't seem interested in correcting it. Oh well, I bought a Bosch 200 (apparently the successor to the 044) and will install it for next season.