1998 Mercury Pro Max 300 HP bogging on acceleration after injectors flowed

[superboop]

1998 Pro Max 300 Mercury HP
Boat was running good maybe a little rich and a little smokey on start up but I ran 75.4 mph on GPS with my starflite. This was my first 3.0l pro max and one thing I noticed is it reved really slow off an idle. Compared to a 2.5l it had absolutely the slowest spin up in neutral if you tap the hot foot. If in gear it had pretty good takeoff
with a full load. With a 26p trophy plus I plugged all vent holes and used an exhaust ring. Then I had the injectors flowed over the winter. They were rich by anywhere from 6% to 10%. Compression test was even at 125 across the board. I replaced all plugs with champions, replaced the fuel filter and put back together.

After putting it together it is now super sluggish out of the hole and it
will bog so bad that when I have 3 people in the boat it struggled to get enough rpm to get on plane. Once it gets above 3k rpm then it takes off with full force and
blows the prop out. It’s like
something is stuck and then all of sudden it takes off and has all of its power and rpm. Fuel pressure is 37 psi. Do I have a bad tps or map sensor and it’s now
in some kind of limp mode? Any ideas on what I should test. I don’t have any guardian lights. I don’t have a ddt to scan for codes.

[David - WI]

Possibly the wrong spark plugs? (And not Champions!)... if that's all you changed?

[LakeFever]

Raise engine height or remove the prop ring see how it goes. Keep an eye on water pressure if you raise it

[skialot2]

Make sure you put all the linkage back together correctly. Do a complete link and sync.

[superboop]

Yes it has the correct champion spark plugs. I did a complete link and sync. While I was doing the L&S I used leads connected to the TPS connector and idle was 1 volt and WOT was only 3.6 volts. It is supposed to be .95 volts and 4.0-4.2 volts. I adjusted and never could get it above 3.6 volts. So I ordered a new one and it did the same thing. No matter what I do, when I set it to .95 volts. Then go wide open it only goes up to 3.6 volts. When I let off it returns to 1 volt not .95. Is there something with the butterfly’s or throttle linkage that would cause this? I didn’t adjust either when taking the plenum off so I don’t know what would have changed?

[superboop]

I disconnected everything from a throttle cable and linkage standpoint and if I rotate the the throttle cam that is attached to the butterfly from lock to lock I can only get to 3.6. In order to get to 4v I have to adjust the TPS to an idle setting of 1.28. It’s like the butterfly doesn’t have enough travel. What am I doing wrong? I have a service manual and am following to a T.

[XstreamVking]

Aside from the TPS have you checked the fuel press before the fuel rails and on the return. Sounds to me that it may be running very rich. How did the old plugs look? Black and wet or black and sooty?

[superboop]

Aside from the TPS have you checked the fuel press before the fuel rails and on the return. Sounds to me that it may be running very rich. How did the old plugs look? Black and wet or black and sooty?

So my prop is very black and sooty from the exhaust so it is running very rich. However, I saw on the Allison forum that the TPS doesn't have authority over 3500 rpm so you should adjust it to idle and hole shot correctly and not worry about WOT voltage. Is that correct? If so then I assume what happened is when I installed the plenum I installed the TPS slightly more rich at idle. My plan is to simply lean it out at idle and not worry if its 3.5v or lower at WOT? Is that ok or am I risking a lean condition at WOT?

[superboop]

So I adjusted my TPS and it is now very clean and crisp off an idle if I am reving it up not in gear. My TPS, hot foot pedal, and the link and sync all had to be adjusted. I only have 3.6v at WOT so I am hoping that is ok.

I took my plugs out and they are very very rich. Like clean metal rich with a little bit of black oil on the tip. The longer I idle the longer it takes to clear itself out. But it is doing the same thing that if I hammer the throttle it will bog until its like a switch flips and it just blows the prop out. Again I am running the same prop as I had been and it never did this before I took my plenum off and had my injectors flowed. All of the hoses are routed correctly on the MAP sensor, VST etc.

[Chaz]

Sooo ... on top of the VST tank you will see a little chrome dome lookin guy held down with two or three screws. That is your fuel pressure regulator. Take it off, look down the hole, you will see a screen in there ( i throw it away) take it out and look thru it, more than likely it's clogged. Clean it and (throw it away) put it back in. Your problem should clear itself up.

[superboop]

That was it. I had 37 psi which is 1 psi higher than the service manual indicates. Took the screen out and it had a little bit around the edges. I cleaned it off, now my key on pressure was down to 36 psi. Took the boat out and voila!! I've got all my power. With new plugs and flowed injectors she felt as strong as ever!

[billybob]

One thing you may need to do is unhook the temp. sensor wire. (BROWN w/ BLACK tracer). To get correct TPI reading on that motor. Try It. I don,t think 1psi of fuel press. is going to do all that.

[Chaz]

It's true that a bad inlet air temp sensor or cyl head temp sensor can make the computer think it needs to stay in "choke" mode. Easy enough to test. These numbers are from a late model which uses the same sensor for both.

99% of the time though, it's the little screen guy ...

Left column is temp in F. Right column is Ohms ... (early model may differ, but I doubt it)

[superboop]

I did try unhooking the temp sensor wire and it didn't change my TPS readings. No matter what I do I can't get up to +4v if I set the idle to .95. I do believe my alarm started going off when I disconnected it. Just like when I unhook my TPS or put my TPS completely out of range like under .5v and above 4.5v. Under those conditions my alarm goes off if I have the key switch on.

Keep in mind I don't know what my fuel pressure was at WOT or any other condition. I was just testing my key on fuel pressure. Honestly I changed nothing else but take that screen out, clean it, check for vacuum on my MAP sensor and boom it was like a light switch I had all my power back right off of an idle. So I do believe that was the issue. Brucato told me that right off that bat but when I tested my fuel pressure I figured I was fine. So note to everyone else don't go by fuel pressure just pull it out and clean it.

[superboop]

My motor has always ran pretty cool on the gauge. I replaced my thermostat and my gauge is working but is just a analog needle gauge so I don't know exact temps it is running at. Everything I read is that the pro max runs pretty cool especially when water temps start dropping. I will check with an actual meter with my service manual.