2002 Tahoe 202 - Mercruiser 5.0 MPI fuel pump not priming

[ZLJUANN]

hello everyone, long time lurker, first time poster. I finally decided to create an account because I am stumped.

Preface: Been boating with friends and family for 10 years or so. bought myself a 2005 Ebbtide 2100 Funcruiser single console earlier this year with a Volvo Penta 5.0 mpi.

Problem: About a month ago, my brother bought a 2002 Tahoe 202 with the Mercruiser 5.0 mpi and Alpha One outdrive. We test drove in the local lake and everything ran great. Took it out for a week to Havasu, and on the second to last day, the engine cut off and refused to start. We got it out of the water and took it home.
Fast forward to last week. Finally got around to troubleshooting. Here is what I see:

-turn the key, no initial 2 beeps, no fuel pump priming. wait about 6 seconds, and the fuel pump kicks on with a constant alarm. You can hear both the alarm and the fuel pump cut in and out for a split second at times but both continue in a synchronized fashion. When the alarm sounds, the pump runs. When the alarm turns off, the pump turns off.
-engine refuses to turn over(run) until the alarm sounds and the pump runs. Once it does run, its rough and dies instantly unless throttle is given.
-Oil pressure is good at the dash gauge.
-swapped the 2 relays and the problem persists.
-My brother threw a new IAC and MAP sensor for the heck of it and no change
- drive lube is full and wiggled around the floater and no change - I can't physically see the floater with the fluid to the fill line.

Any ideas here? Any help would be much appreciated

Serial number: M346419
ECM 555

 

[Bondo]

Ayuh,...... Welcome Aboard,...... Check the key switch, to be sure it's powerin' the ignition circuit reliably,....

 

[alldodge]

Howdy
Thanks for the Serial number

Save some money and stop throwing parts at it, need to do some testing

I would start with removing both ends of the battery cables, from battery, starter post and engine block. Clean them up to nice shinny metal and reinstall.

While removing the cables look around for other things which are corroded

The only part which can be thrown at this motor is the distributor cap and rotor, they are known to go bad without warning, but right now this is not your issue

 

[ZLJUANN]

Howdy
Thanks for the Serial number

Save some money and stop throwing parts at it, need to do some testing

I would start with removing both ends of the battery cables, from battery, starter post and engine block. Clean them up to nice shinny metal and reinstall.

While removing the cables look around for other things which are corroded

The only part which can be thrown at this motor is the distributor cap and rotor, they are known to go bad without warning, but right now this is not your issue

thanks for the speedy responses. Will do that and update with results.

 

[ZLJUANN]

ok so we cleaned the battery cable leads at all locations and zero improvement. didn't see any other corrosion around really.

 

[alldodge]

Need a voltmeter and measure at the purple wire on the key. There should be 12V when key is turned to ON.

If there is 12V, then check one of the other purple wires connected to one of the gauges.

If 12V is there, check for 12V on pin D of the DLC connector

4 pin connector with a cap

If 12V is there, then find the 2 square relays on top the motor near the ECM, one is fuel pump and the other is main power. The MPR will have a Purple/.Dark Green stripe wire on pin 85 of the relay should be 0V on it and 12V on the Red/{urple wire pin 86

 

[ZLJUANN]

Need a voltmeter and measure at the purple wire on the key. There should be 12V when key is turned to ON.

If there is 12V, then check one of the other purple wires connected to one of the gauges.

If 12V is there, check for 12V on pin D of the DLC connector

4 pin connector with a cap

If 12V is there, then find the 2 square relays on top the motor near the ECM, one is fuel pump and the other is main power. The MPR will have a Purple/.Dark Green stripe wire on pin 85 of the relay should be 0V on it and 12V on the Red/{urple wire pin 86

Ok, went through this as well and have proper 12v where you indicated and 0V where it should. No changes.

 

[alldodge]

OK, if it goes to zero on pin 85 and the fuel pump is not running, then there is a bad connection or the relay is bad

Take the relay to the local auto parts shop and get one from them, they are the same

 

[ZLJUANN]

OK, if it goes to zero on pin 85 and the fuel pump is not running, then there is a bad connection or the relay is bad

Take the relay to the local auto parts shop and get one from them, they are the same

copy.

If I'm understanding this correctly, 85 is 0v ground, 86 is 12v ref from ignition, 30 is 12v from battery. when 86 gets 12v from key on, it closes the relay and should be sending 12v through 87. This is the same for both relays.

 

[alldodge]

copy.

If I'm understanding this correctly, 85 is 0v ground, 86 is 12v ref from ignition, 30 is 12v from battery. when 86 gets 12v from key on, it closes the relay and should be sending 12v through 87. This is the same for both relays.

Yes, there should be 12V on 85 only because the ECM has not applied a ground to that side of the coil. Once it does, it goes to 0

 

[QBhoy]

Check the lanyard kill switch isn’t temperamental

 

[ZLJUANN]

Yes, there should be 12V on 85 only because the ECM has not applied a ground to that side of the coil. Once it does, it goes to 0

Bear with me here, trying to be very thorough:

MPR - on key on, 86 gets 12v, 85 goes from 12v to 0, and 87 goes to 12v - working as intended it seems

FPR - on key on, 86 gets 0 volts, 85 gets 0 volts. After a few seconds, when the pump starts running, 86 gets 0v, 85 gets fluctuating voltage, and 87 gets fluctuating voltage. - This doesnt sound right at all. If anything, 86 should be getting fluctuating voltage right?

 

[alldodge]

If anything, 86 should be getting fluctuating voltage right?

The MPR sends power to 2 other fuse, one of which is for the FPR. When MPR connects there should be 12V on pin 85 and 30 (not 86) on FPR along with connection to the IAC. So there is a bad connection or MPR coluod be the issue

 

[ZLJUANN]

The MPR sends power to 2 other fuse, one of which is for the FPR. When MPR connects there should be 12V on pin 85 and 30 (not 86) on FPR along with connection to the IAC. So there is a bad connection or MPR coluod be the issue

copy. So the fluctuation was from the multimeter accidentally getting set to the wrong setting.

Traced everything again and yes both relays work as intended based on the circuit diagram you gave.

So it seems like its likely its the connection to the cool fuel pump. The pump is below and behind the engine on this boat. It seems like its going to be near impossible to get to it without removing a lot of stuff, or pulling the engine. That is going to suck.

 

[alldodge]

The pump is in a bad spot for sure. Its behind the port motor mount.
It has a plastic cover which latches on top/bottom in the middle. There are 2 nuts which hold the bracket attached to the motor mount. If you remove the cover, might be able to remove 2 smaller nuts which hold a brass bracket and the pump

 

[ZLJUANN]

The pump is in a bad spot for sure. Its behind the port motor mount.
It has a plastic cover which latches on top/bottom in the middle. There are 2 nuts which hold the bracket attached to the motor mount. If you remove the cover, might be able to remove 2 smaller nuts which hold a brass bracket and the pump

I can touch the box but no way I can get it out without pulling the exhaust manifold or the whole engine.

i was not convinced it was the fuel pump so I did some testing myself.

I bought a Rinda Diacom kit. I connected it to my ebbtide with the Volvo penta just fine. Tried it on the this Tahoe with the mercruiser and it would not connect to the ECM. So back to electrical.

you can physically feel and hear the MPR getting energized almost exactly 6 seconds after you open the key every single time. I tested the purple/green cable at the relay pin 85 and ECM connector A pin 22 and sure enough it’s at 12v for about 6 seconds then drops to zero and intermittently the ECM stops sending ground and you will see the voltage jump to 12v and hear the MPR click on and off.

BAD ECM? Voltage is steady at the batteries, leads and anywhere else I can test 12v constant.

Would you know what pins power and ground the ECM? Or a diagram? I could not find one.

 

[alldodge]

Power is A23 in post 13
Purple from key B18
Ground A16 and 24

Check the kill switch

 

[alldodge]

The fuel pump is turning ON but you not getting spark, so that sends me to distributor and crank position sensor

Can you see the tach come up to 300 rpm or so?

If pump is running then you should see fuel pressure build at the schrader valve on the rail

 

[ZLJUANN]

Power is A23 in post 13
Purple from key B18
Ground A16 and 24

Check the kill switch

A23 gets 12v when the MPR finally kicks on.
B18 gets instant 12v when key on
A16 and 24 are good ground

It fires on but stumbles and dies. Fuel pressure was 43 when fuel pump finally kicks on.
tach jumps 300~ rpm when cranking

kill switch works and tested well. It’s an off/run switch on this boat, not a lanyard

 

[alldodge]

Does the pressure stay up or come back up after it starts?