1999 Merc 200hp 2.5L EFI Issues

[Riptide]

Issue - Before running tests and replacing parts the motor would run perfectly with a small little glitch.
Glitch - The motor will periodically and sporadically drop 400 rpm anywhere between 3500rpm and WOT.
I would shut the motor down, restart and the problem would seem to clear up for a short time and then reappear.

Tested the following per service manual with the following results.
Compression: #1) 130, #3) 135, #5) 132, #2)129, #4) 128, #6) 125.
Fuel Pump & Fuel pressure regulator - rebuilt.
Stator replaced w/ new.
Trigger Assembly - good.
Switch Boxes - Have not tested yet. How often do these this really go bad? Will Check though.
Rectifiers - replaced w/ new.
Coils - good.
Plug Wires - good.
Spark Plugs - replaced w/ new.
Fuel Lines - new.
Fuel Filters - new.
Oil injection system removed. Pre-mix.
Water impeller replaced.
Water pressure - 18#'s.
Engine temp - 140-160 high side.
Thermostat - good.
Water Hoses - new.
All grounds disconnected, cleaned and reinstalled.
All wiring gone through - no breaks, bad wires, etc.

On my last water test outing, motor ran flawlessly for about 3 hours and then the rpm drop started up again.
On the way back to the docks, 3 times at 5000rpm the motor completely shut down to the point of idle state. Never completely shutting down.
When it did this the idle was around 650 rpm and still purred like a kitten.

This issue is driving me insane.
What am I missing folks???

All comments, ideas, directions of attack deeply appreciated.

Thanks

[Sonik]

Have you checked out the TPS yet???? Looks like you covered most of your bases but didn't see that mentioned in there anywhere. They typically develop a "flat" spot in the midrange of the potentiometer as that's where they spend most of their time. Give that a whirl.

[Riptide]

Have you checked out the TPS yet???? Looks like you covered most of your bases but didn't see that mentioned in there anywhere. They typically develop a "flat" spot in the midrange of the potentiometer as that's where they spend most of their time. Give that a whirl.

According to service manual, TPS only affects low speed and mid range operation,
not WOT.

So how would the TPS affect WOT shut down and mid range to WOT rpm droppage?

[Sonik]

I am no self proclaimed expert by any stretch, but given the extent to which you've gone thus far, I'd say that your culprit list is getting pretty short, unless one of your new components was taken out again by the original problem. I'd still check full range of motion on the TPS with a voltmeter and ensure it is within the manual specs (harness as well if not already done). A DVA might be in order to start testing components under load and/or heat soak conditions. Are you 100% certain that you also have a healthy ECU?

[transomstand]

So if the TPS went out and got drunk and fell down and hit it's head and while hungover decided to tell the ECU that it was at idle (because it felt like it was gonna puke if it went too fast) while the throttle was actually wide open .....it would have no effect on rpm?

[Riptide]

A DVA might be in order to start testing components under load and/or heat soak conditions.

Are you 100% certain that you also have a healthy ECU?

A little hard since the RPM issue has a mind of it's own coming and going whenever it wants to.
Additionally, there's a poling platform in the way of removing cowling while on the water and under way.

99.99999% sure, but any thing is possible at this point. Has gotten so aggravating I had to step away from it and let my head clear.

[Sonik]

The 400 RPM drop kinda sounds like the motor dropping a cylinder to me honestly. Could be an ignition component failing under load and/or heat, or an intermittent bad driver in the ECU, OR the TPS.

[rock]

Due to the fact you say it still purrs like a kitten while having limited rpm I would suggest a fuel delivery issue. Depending on the model you may have a cone filter plugging up. There is a hose on the bottom of the vst. Some of them have a cone shaped filter in the hose. This is not a filter you can see externally but instead is on the inside of the hose. You may have something in there occasionally impeding flow. Good luck.

Rock

[Bullet89]

Get yourself a CDI test manual and a dva. There is a lot of info in there that is not in the Merc manual. Find a friend that has a known good ecu and swap them before you go any farther.

[Riptide]

Gave the testing procedures the day off but did run across a very help trouble shooting guide apparently published from CDI Electronics corp.
Bullet89, is this the manual you were referring to?

Very informative info.

http://outboardparts.com/troubleshooting/printguide.htm

[bobthebuilder]

I had same issue here on a customer's boat.. Found it to be the ECU.. If you know some one who has a 1996 to 1999 200 EFI merc 200 then borrow the ecu for a trial... The one I had here dropped sometimes but not always but always dropped at around 3500 to 4000 rpm and sometimes would just drop to maybe 1500 and sometimes would just knock off.. it would restart most of the time. I swapped his ecu with one I had here and it ran perfectly.

[Riptide]

Unfortunately I don't know anybody with a ECU I can borrow.
Will test the switch boxes tomorrow to eliminate that possibility.
If they check out OK then all I can do is pull the ECU and send it somewhere to be tested for internal failure.

Any suggestions of whom and where to send unit out for thorough testing?

Thanks

[rock]

I think Tony Brucato checks them

[bobthebuilder]

I forgot to mention that ground wires can cause the symptoms of a bad ECU.. Check each and every ground.... The grounds on that engine is very important..

[Riptide]

1 step ahead of ya.
That was the very first thing I did.
Removed, inspected, cleaned and applied dielectric compound to ALL grounds, re-installed.

But you are absolutely correct, very important and quite often over looked.