94 mercruiser 350 low idle after thermostat change

[Blizzmet06]

Got a ‘94 Malibu ski boat with 5.7 350 EFI last fall. Had sat for 10 years but seller got it running and would idle and go up to 2500 rpm no problem. When I got it I Cleaned out the vst tank and got the rpm up to wot no problem. Never got up to temperature and found that the thermostat was installed improperly and was getting water all the time. Straightened that out and now the idle has dropped from 750ish to 630ish and will stall out. Also had it on the lake for a run and would get to about 3500 rpms and that’s it. Seemed to run good at higher rpms just not idle. Temperature was always 150-175ish depending on how fast I was going. I’m not sure if I burned out of all the old fuel or if the tank has a screen or filter. I replaced the fuel/water filter already no change. Start of the year I filled the tank with fresh gas after winter. Maybe less then 5 gallons were in it previously.

Thoughts on on where I should start?
Could the VST be clogged again?
is there a screen or sock in the tank?

 

[alldodge]

Need a serial number to verify, you may have a MEFI 1 ECM and TBI.
Could have 1 to 2 issues, fuel pressure could be down, and/or IAC might be an issue.
https://www.mercruiserparts.com/350-mag-efi-ski-tbi-gm-350-v-8-1994-1-1

I would start with measuring fuel pressure. To measure pressure you'll need a fitting that attaches to the TB and Merc has one, but may also be able to find one at an auto store (maybe rent a tool)



Would be 30 psi

 

[Blizzmet06]

Serial 3-57K111GS

Could this be vapor lock? Looks to be really common in with this model and ethanol.

Thinking the previous owner could have intentionally installed the thermostat incorrectly to keep engine running cool.

 

[alldodge]

Your serial number is one number, one letter and six more numbers (exp 0F215561)

The symptoms does not support VL. VL happens after the motor is all warmed up and then shut off for about an hour. It will either then start and die or in most cases not start at all. The time period is needed to heat soak the fuel and cause it to expand into gas vapors instead of a liquid.

Running the motor cooler can help with VL, but can not get rid of them by it self

 

[Blizzmet06]

Serial 0F230168

Boat died coming back to the ramp when I shifted into neutral and wouldn’t restart.

Just cranked

 

[alldodge]

The SN indicates this parts list
https://www.mercruiserparts.com/350-mag-efi-ski-tbi-gm-350-v-8-1994-1-1

If its the IAC, then you should be able to move the throttle forward (neutral, throttle only) just a little and keep it running. Soon as you go back to idle it dies.

 

[Blizzmet06]

Yeah can keep it going that way.

Would that hat explain why it wouldn’t restart though? I even tried restarting in throttle only with a little gas.

The boat boat also has the Malibu computer which throws a TPS error.

Previous owner did say he replaced it but with one meant for his truck. I have the original he took out. Could this be my issue. Should I put original back in before messing with iac?

Would IAC or TPS not allow me to get to WOT when I was on the lake?

 

[alldodge]

Yeah can keep it going that way.

Would that hat explain why it wouldn’t restart though? I even tried restarting in throttle only with a little gas.

I'm sure you understand but this is both sides of the question about the IAC

You can keep it going in throttle only, but not when it goes to idle, that's a IAC issue

You cannot restart in throttle only, even with gas

Would IAC or TPS not allow me to get to WOT when I was on the lake?

Both can give you trouble, but TPS would be my first thought after fuel pressure has been checked

 

[Blizzmet06]

Got gone from work and started tonight for the first time since it died at the ramp. Started right up but rpms were all over the place. No longer low were jumping between 1100 and 1400 and seemed like there was a pounder then normal noise coming from the throttle plate area.

 

[Blizzmet06]

With idle all over the place where should I start.

Adjusting the IAC or replacing the throttle position sensor?

 

[Blizzmet06]

Turns out previous owner didn’t replace the TPS it was the distributor module that he replaced with one from his truck.

 

[alldodge]

You need to do some testing first.
Measure fuel pressure 30 psi
Check for codes using a led and paper clip. With key ON and not running count flashes.

The paper clip is also used to set motor in base timing mode for adjusting timing

 

[fishngolf]

Is there a way to test the IAC?

 

[alldodge]

Voltages can be measured and also use a scan tool

Is there a way to test the IAC?

 

[fishngolf]

Took the boat to the local marine mechanic. Busy this time of the year and he's trying to figure out how to test the fuel pressure with the VST. Looking for the adapter that you mentioned so we can test the fuel pressure. What size threads? Where can I buy?

 

[alldodge]

Need part number 91806901 or similar. Its a GM TBI test fitting

 

[fishngolf]

Mechanic ran the scanner on the ECM and is thinking that's the problem. He tested the signal to the IAC and only got two of the four signals that he should have gotten. Should I have him replace the IAC before replacing the ECM? ECM seems to be hard to find and very expensive. ECM Model number 16159919. Is there any way to reset it?

 

[alldodge]

Would need more info to determine what the two out of four signals are.
As for the ECM, if you really think its the problem, send it off for testing, to a place like OBD diagnostics. The part number is a MEFI-1 and does not provided much info as to the problem.

 

[fishngolf]

Sent the ECM out to OBD diagnostics and it checked out fine. He was able to pull the engine hours 724 and logged codes which he cleared. M43 ESC M42 EST M33MAP M21 TPS M14 Coolant. Said it was good on harness with injectors and all sensors and relays and IAc was good. He did mention a lot of dirt in J1 and J2.

Not sure where to go now.

Could the idle problem be related to changing the thermostat and now that the engine is actually heating up it’s getting less fuel from computer till it relearns through a few cycles now that it’s been reset?

 

[alldodge]

He did mention a lot of dirt in J1 and J2.

Don't see how there would be a lot of dirt in the connectors unless the boat went under water or was left open to the elements. What your dealing with may just be a bad harness because it was left outside and the elements got to it. Open the harness covering up and see if there is other crud inside

Maybe time to get the injectors cleaned and flow tested by a place that does marine injectors (not cars), they operate at higher rpms and load rates