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Help me Gods and Men, Computerized motors beat me up really bad. I am black and blue!

Q

qhn

Regular Contributor
I am working on an '04 Mercury EFI motor. I've put in days of work and tons of money to get it right so I can sell it.
It starts easy, runs fine, but has a miss at one point throttling up and back down to neutral. If I gradually increased the speed, then it was fine. But if I slammed on the throttle then it sputtered/missed out.
Compressions: 165lbs on all cylinders
Spark: All cylinders are firing nicely
Vapor separator tank is cleaned out and pressure is at 130lbs
Injector heads clean out and all spray strong
Mechanical fuel pump works fine
Could it be the map, throttle control or idle sensors or all?
I have 2 motors and been swapping out and in parts for the past few days. The same problem occurs on both ECM, fuel distributor manifolds and injectors, both VST pump, both mechanic pump, both of everything electronically and mechanically...
I've worked on many EFI and carburated Yamaha 4stroke. But this Merc just doesn't seem to run into any of the common problems with 4strokes that I've known.
So, please help me if you have anything on mind for me or if you have come across this same problem.
I thank you very, very much.
 
It's a 40hp, Sir.
I only swap the rectifier and not the harness. Also, I am using the 8 pin plug control without tilt/trim. This one has tilt/trim. Could it be something to do with the harness?
Thank you very much. The harness will be tried out today.
 
Changed out the wiring harness, same problem. Will decarbonize the cylinders/valves and look into changing the TPS sensor next. But how can both TPS went out on the very same type of motors and even a third one which is an '02.
Will also run a leak down test to see if a valve job is needed. Also plan to put in on a boat and tow it down to a dealership for scanning the ECM...
Let me know if anyone has anything on mind.
Thank you.
 
It isn't the TPS because it doesn't miss w/a gradual increase in throttle. Just a WAG but check the trigger coil for a fine crack in the wiring. Move the throttle slowly and if revs normally. Jerk the throttle and the trigger cable moves quickly...it may be the culprit.
 
Check the CPS sensor as this engine does not have a trigger ,the ECM triggers the modules. Are you running this on a test gas tank or the boats built in tank? What your drscribing is sounds like a fuel restriction either to the motor or to the VST housing.
 
Thank you very, very much for all the advice.
I just decarbed the motor. No change. At the time it hesitates, it sounds like it's breathing hard for air at the throttle fly. I removed the plastic restrictor. No change.
The fuel pressure is actually 42psi as retested running the motor today and stays the same at rev up or idle.
Can the fuel distributor manifold be clogged?
I rechecked spark on each and every cylinder while it acts up, no bad coil or losing spark. All spark plugs burnt wet and dark brown.
I will check the pulse coil or so called trigger on others, CPS on EFI.
I test the motor using a 6gal plastic gas can, Mercury connector. It cannot be the can or hose as the injectors are spraying like 4 lions marking terriories...
After this process of elimination, I am still aiming at the TPS or condemn the ECMs. Both of them.
If the fuel distributor manifolds, then both of them are clogged.
I am stuck and another day of labor and hard work wasting down the drain. I have 9 days left to get this motor out to cover portion of my investment. Oh Lord....Deadline is reaching!
 
Sorry, if I confused viewers. When I said both of them, I meant that I have 2 and 1/2 EFI motors. Even parts on the other '04 and '02 does not help. They all are 40hp, EFI motors.
I can sell them cheap. But damn, I am an outboard motor mechanic doing mostly refurbishing, have fixed lots of Yamaha EFI, tons of carburated 4stroke and it seems these Merc EFIs are blocking my career path!
 
Checked out the Crank Position Sensors. 2 are in good shape, one has a cracked stud ring. Tried all 3. No change. They all worked the same way - gradually increase and decrease speed: No problem. Slam on it or back it down to neutral quickly: sputters, almost dies out, but revs up and idles down eventually each and every time. Tried to rev up faster: cut off. Put back to neutral, starts right up and idles down quickly and smoothly like a brand new motor.
Ran on and off and at idle: 6gal of gas already. Deep it in a large barrel of water, no change: starts right up just like on the earmuffs.
I will go try another block now and will let this Merc EFI haunt my dream for the rest of my outboard repair time.
I also want to add: Ohm reading on each injector right after shutting the motor off is 12 Ohm.
I will find out what's wrong next winter or some raining days that I have no works to do.
Take care, all.
 
Update, update!
Did a leak down test, found a leak.
Parted the blocked, found a crack internally.
What a hassle!
Thank you all for viewing and supportive advice.
 
did you ck the tp with an ohm meter should be smooth reading up and down no glitchs.also doe this have a rev limiter? ive had a few of these sounds like you ck most every thing if you run it at wide open throt. and its fine you would think all circuits are good fuel spark mabe tp? most tps when you ck voltage goes from 0 to mabe 4 volts but i have some that read backwards realy you need to ck book and make sure you have the right ser.number i dont no if this helps i have had a few that drove me nuts good luck
 
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