Mercruiser MPI Won’t Stay Running

[BradsBoat]

Greetings, I just did a complete gasket job on perfectly good running engine with 300 hours on it - head gaskets included. New Delco 41-993 plugs, cap, rotor, CPS, and Cool Fuel pump. The engine starts and runs smooth for a few seconds, then stalls. By feathering the throttle, it’ll try to stay running, but it struggles and pops. Rechecked the distributer position which is pointing directly within the lines indicated in the picture below while at TDC, rechecked the firing order, and rechecked the valve rocker arms adjustments. The fuel pressure is 38-43psi. Vacuum is stable and in range. The Mercruiser/Rinda scan tool shows no codes. Tested the following using the Mercruiser/Rinda scan tool:

Ignition Coil
Injectors
IAC

All seem to be good since they all made noise as the scan tool cycled through. Also pulled the coil wire and watched it throw a strong spark. Put the old cap and rotor back on, and it still has the same issue. Added 15 gallons of fresh marine fuel to the approximately 12 gallons that were in the tank from last summer. Still, no change.

What am I missing?

NOTE: It’s a 2004 and the serial number is OM69XXXX

 

[julians]

I know you say you've tested the iac, but a failed iac can cause the issues your seeing.

I had a 5.0 mpi, an dthe iac would fail every 3 years consistently, and it wouldnt throw an error when it failed, the engine would just fail to idle. You could rig it to work with a faulty iac by adjusting the throttle so it was slightly open as a get you home fix, so maybe worth trying this?

 

[spannerman]

You should be able to see the IAC duty cycle on the Rinda tool. Watch it on the live data until the engine dies out, it should work to pull the revs back up but if the reading is stable as the engine dies its a safe bet its defective. Also when you first turn the ignition on it should self test and cycle from full open to closed and back. If everything is normal then check the other sensors especially the MAP sensor, popping is usually a sign of a weak mixture on a Mercruiser.

 

[david_bagshaw]

run it long enough to warm up, or is that only with the carb versions?

 

[BradsBoat]

Thank you all for the great responses. I ordered an IAC and will post the outcome!

 

[Antony Collis]

Also you don't mention that you bled the fuel line properly. After that rebuild it will have air in the line.

 

[spannerman]

MPI engines bleed thenselves, the pump primes the system everytime you turn the ignition on.

 

[QBhoy]

MPI engines bleed thenselves, the pump primes the system everytime you turn the ignition on.

Hi spannerman. I haven’t found this to be the case at all. If the fuel system like filter had been opened and taken in air...I’ve never seen them bleed themselves at all. Always had to bleed at the rail using the test fitting.

 

[QBhoy]

Hi
Firstly I’d say that despite adding fresh fuel, if the old fuel remaining (especially in this quantity stated) is badly contaminated, you will continue to have issues with the fuel until you completely drain the tank, on most occasion to be honest.
I would also say that despite replacing the Diz cap, just double check that cap again. They are notorious for being easily cracked when tightening the torx fasteners back down and even notorious for failing right out the box. These don’t get the name “crap cap” for nothing on these MPi. They can gather moisture over winter by their very design too. I’ve been through 5 of them in 5 years now. One was right out the box and failed.

With regards to the setting up of the Diz gear itself, although the manual advises that you simply set it up between the lines, then the ECU takes over from there. I’ve rarely seen this to be the case. The local merc dealer confirmed that I wasn’t being silly when even he told me that sometimes they need a little fiddle by only the tiniest mm one way or the other to get spot on and stop the engine giving the odd pop or bang (if the chance of water in fuel is totally ruled out).
To summarise;
Get totally fresh fuel in it or run it off a remote tank to be sure your fuel is good.
Check that Diz cap for any damage when assembled
Then if all the above is ok for sure, get a 13mm and loosen the Diz assembly a little and adjust one way or the other by literally tiny margins until she stops the popping.
Best of luck.

 

[dpb]

I had a similar issue with my efi, eventually proved to be the connection terminals to the IAC rather than the IAC itself. Cleaning and spraying with AC50 have kept it going for the last two years.

 

[spannerman]

Hi spannerman. I haven’t found this to be the case at all. If the fuel system like filter had been opened and taken in air...I’ve never seen them bleed themselves at all. Always had to bleed at the rail using the test fitting.

Strange as in 24 yrs on Mercruiser I have never once had to bleed them on the rail, had the odd one that wouldn’t pull the fuel from the tank so had to fill the fuel filter. I always close the the cock if there is one when changing filters to prevent run back.

 

[Antony Collis]

Strange as in 24 yrs on Mercruiser I have never once had to bleed them on the rail, had the odd one that wouldn’t pull the fuel from the tank so had to fill the fuel filter. I always close the the cock if there is one when changing filters to prevent run back.

The issue here is not to get the engine running but to get it running better quicker. I'm sure given the time and running it lumpy for long enough it may pass all the air via the injectors. However, this can be sped up by allowing the excess air to bleed out of the schrader valve at the end of the fuel rail. This is an easy investigation without any tools or replacing anything. If the engine still runs lumpy, move onto something else....

 

[Croftie]

Am I missing somethimg on this engine? Having worked on thosands of EFI auto engines I have never had to bleed a system. The system does need it, fuel is pumped to the rail and held at pressire by the pressure reg. The pump supplies >5bar so the excess bleeds back via the regulator back to the tank maintaining 2-3 bar pressure in the rail. The schrader valve is there as a conveniant connector to attatch a test pressure guage not for bleeding air.

 

[Antony Collis]

I think you may be missing something. The fuel circuit goes round in a circle at the engine and is kept from boiling by the cool fuel device.. Nothing goes back to the tank so air continually is pumped round and round until it is eventually drawn into the fuel rail. At this point it cannot get out unless it passes through an injector or is drawn off at the end via the schrader valve. No one is saying this is the procedure set out by Mercruiser but it does help if the engine is running lumpy after a rebuild that has opened the fuel system..

 

[Croftie]

I think you may be missing something. The fuel circuit goes round in a circle at the engine and is kept from boiling by the cool fuel device.. Nothing goes back to the tank so air continually is pumped round and round until it is eventually drawn into the fuel rail. At this point it cannot get out unless it passes through an injector or is drawn off at the end via the schrader valve. No one is saying this is the procedure set out by Mercruiser but it does help if the engine is running lumpy after a rebuild that has opened the fuel system..

Thanks for that. obviously totally different to what is used in the auto world,

 

[spannerman]

Am I missing somethimg on this engine? Having worked on thosands of EFI auto engines I have never had to bleed a system. The system does need it, fuel is pumped to the rail and held at pressire by the pressure reg. The pump supplies >5bar so the excess bleeds back via the regulator back to the tank maintaining 2-3 bar pressure in the rail. The schrader valve is there as a conveniant connector to attatch a test pressure guage not for bleeding air.

+1, unless the system is completely empty they will start on whats in the rail and in the hoses and pumps. If they are empty, key on and off half a dozen times primes them up just fine. Mercruiser don’t return to tank though it returns to the fuel filter and goes round again.

 

[Croftie]

Mercruiser don’t return to tank though it returns to the fuel filter and goes round again.

Interesting, so I assume the HP pump is after the fuel filter?

 

[QBhoy]

Strange as in 24 yrs on Mercruiser I have never once had to bleed them on the rail, had the odd one that wouldn’t pull the fuel from the tank so had to fill the fuel filter. I always close the the cock if there is one when changing filters to prevent run back.

100% I’ve to bleed a mpi fuel rail to get it to start properly, on any MPI post 2003 GM engine I’ve seen.
Flat battery would be other option

 

[QBhoy]

Am I missing somethimg on this engine? Having worked on thosands of EFI auto engines I have never had to bleed a system. The system does need it, fuel is pumped to the rail and held at pressire by the pressure reg. The pump supplies >5bar so the excess bleeds back via the regulator back to the tank maintaining 2-3 bar pressure in the rail. The schrader valve is there as a conveniant connector to attatch a test pressure guage not for bleeding air.

Yeah.

 

[QBhoy]

+1, unless the system is completely empty they will start on whats in the rail and in the hoses and pumps. If they are empty, key on and off half a dozen times primes them up just fine. Mercruiser don’t return to tank though it returns to the fuel filter and goes round again.

That’s not in the manual at all...with the key on and off running pump. I’ll try it next time but I’m sure I’ve done it at least half a dozen key on/off previously. Same airing through pump sound.