7.4 MPI won't start when hot

[dronthelake]

Scratching my head and looking for suggestions.

1998 Mercruiser 7.4 MPI
784 hours
Serviced religiously
89 octane (no ethanol), my marina
Twin 122 gallon tanks, half full

Last March we went to the cove for the night. The winds picked up in the middle of the night so I decided to moved the boat about 50 yards. The starboard engine would not run more than a couple second then it would stumble and die. The next morning it fire up and has ran perfectly until last weekend. I did change the fuel filters last March and found no water.

Last Saturday we took the family for a cruise, swim, and dinner. At 8:00 PM it was time to take the guests back to the dock. The starboard engine would not run for more than 1 second. It had been three months since the previous episode. We cruised back to the dock on the port engine. Inboards don’t back up easily on one engine. Like rowing with one oar. We stayed in the slip that night and the next morning it fired up and ran perfectly. I checked the filter and found no water. We cruised down to the cove for a swim and 3 hours late it would not start. Again back to the slip on one engine.

I stopped by the boat this evening and it fired up as if there was never a problem. I didn’t have time to make a run.

I don’t believe it’s the fuel as no one in the marina has reported a problem Also the port engine and generator have not had any issues.

I’ve read various posts on the web and found nothing other than speculations. This problem reminds me of the old carb vapor lock issues but I personally haven’t experienced VL with an injected engine.

I have the service manual and it lists all kinds of tests. I am hoping someone has experienced this problem and can save me hours of testing.

Any suggestions?

Thanks,
David

[mmwjr]

David, you have not said but I am assuming the engine cranks but is not starting or starts and dies in short time.

Runs good sometimes and not all other, being MPI I would have the engine checked for error codes from the computer.

Have you check for spark and fuel when it's not running? This will narrow it to ignition vs fuel delivery.

[dronthelake]

Yes it cranks fine, will fire up for 1 second then die as if it is starved for fuel. The last to trips down the lake it would not start have it warmed up and set for awhile. Starts and runs great when cold.Last night it started instantly as always and ran perfect and smooth. I didn't have time to take a cruise.

[Kevlar7r]

Does that have electric choke? Could be that's gone wonky and is choking the engine when it doesn't need to.

[mmwjr]

MPI engine so no choke. Verify fuel pump relay is not dropping out. Else maybe IAC.

[dronthelake]

Fuel injected.

[Phillbo]

I agree, replace the IAC.

[dronthelake]

I'll get back to the boat later this week and will check the IAC. My curiosity makes me ask. What would cause the IAC to only act up after the engine is at operating temp and has sat for a while? After the engine cools down it runs perfect.

[mmwjr]

Good question. Many sensors probably best to try having codes read out of the computer.

[capn_billl]

Under the engine there is a plastic box where the fuel line goes through a heat exchanger.

It looks like a kinda cheesy setup. If there isn't sufficient water flow, you will be boiling the fuel instead of cooling it.

My mechanic took one look at it, and insisted I replace it, (1996), with a newer system Mercruiser came out with later, and has as an upgrade.

He stated there were several problems with the old system.

I would check water flow through it, then verify there are no holes between water & fuel. Check for leaks, cracks, sucking air, or dripping fuel. See what manufacture date it has. Check Mercruiser catalog for part number changes.

You can easily check for spark by cranking engine when it is too warm to start. (there are hundreds of $1 spark testers that are basically a neon bulb). If you have spark & the starter is cranking, the only other things it can be is fuel, or bad computer.

I would assume you've checked the oil, and coolant, and the engine is running normally otherwise.

There is a temp sensor, but I would check basics first, unless you have a diagnostic computer handy. (Mercruiser takes a special one)

[mmwjr]

I think you are referring to mercs cool fuel, I have heard the older ones had paint inside that would flake and clog the fuel line.

[capn_billl]

Yes that is correct.

[dronthelake]

Good idea Bill. I am still waiting for the mechanic to get by and run a scan just to see if there are any codes.

Saturday evening we cruided down to our normal cove. Engine ran perfectly. After sitting 90 minutes it started and ran fine. Another 90 minutes passed and again it started an ran good. On Sunday we cruised back to the marina with no issues.

[dronthelake]

Hope everyone had a great and safe weekend. Other than a brief verbal confutation with a guy who tried to blame me for him anchoring to close to my boat we had a great weekend on the water. The boat ran perfectly all weekend.