Mercruiser 7.4 MPI Idle Speed Very High

[tpenfield]

I'm dodging rain drops this morning, but so far it seems that the timing is 8-10 degrees advanced.

I put the engine in base timing mode with the Diacom, then checked the timing. Looks to be advanced a bit . . . there is only a single timing mark. not graduated scale, so I would guess it is advanced by about 8-10 degrees.

Not sure how it came out of adjustment . . I still need to see if the distributor is loose. Just wondering if the ECM is malfunctioning, would it affect the base timing. :noidea:

 

[tpenfield]

I swapped the ECM's and the port engine ran fine with the ECM from the stbd engine.

Here are the stats of the port engine running on the ECM from the starboard engine. I have not run the starboard engine with the port ECM yet. Probably won't do that until tomorrow.

Anyway, The problem did not follow the ECM.

 

[Scotvl]

The only variation I see is that your motor with the high idle has the timing retarded 9.49* while the other one has 4.22* advanced timing, puzzling why they are different? AllDodge knows way more than I do about what might be causing the timing difference all I can think of is a vacuum leak causing a lean condition so the ECM might be adding more fuel. Does the manifold absolute pressure indicate the manifold vacuum? I would have thought it would be a negative value.

 

[tpenfield]

The only variation I see is that your motor with the high idle has the timing retarded 9.49* while the other one has 4.22* advanced timing, puzzling why they are different? AllDodge knows way more than I do about what might be causing the timing difference all I can think of is a vacuum leak causing a lean condition so the ECM might be adding more fuel. Does the manifold absolute pressure indicate the manifold vacuum? I would have thought it would be a negative value.

The only thing that I can make about the timing is that the base timing on the stbd engine appears to be advanced about 8-10 degrees more than spec. So probably 16-18 total advance, as the spec is 8 degrees BTDC. So the stbd ECM is trying to retard the timing as much as it can to slow the engine down.

Tomorrow, is supposed to be dryer weather, so I should be able to continue diagnostics on the starboard engine. Ruling out the ECM is a positive though, as those are pricey to replace and not all that easy to get fixed.

 

[tpenfield]

Hopefully today will show some progress towards a fix for the high idle speed. Based on what I discovered so far . . .

1) Base timing of stbd engine is too far advanced (yet it used to be dead nuts on 8 degrees BTDC and the distributor seems like it is not loose nor has rotated)

2) The problem did not follow the ECM when I swapped the ECM's between the Port and Starboard engines.

I am wondering if the Ignition Control (IC) Module on the distributor is the source of the problem :noidea:

I think I'll check the connections. I'm looking thru the shop manual (Manual #16) to see if there is a way to test the IC module.

 

[tpenfield]

Looks like I solved the problem . . .

Plenum gaskets. I had the plenum on my list of things to check today, and when I loosened it I could see that a couple of the gaskets had become broken/dislodged leaving an air gap. I had loosened the plenum earlier in my prep work to check a few things and the gaskets must have gotten disturbed in that process.

So, a new set of gaskets and the engine idles as it should.



I'm not a fan of the paper gaskets that are spec'd for the plenum/intake manifold joints. I think they would be better in a different material. . . . maybe a hi-temp rubber?

 

[Scotvl]

Wow I bet you're happy you don't have to look for a new boat, lol. Nice work finding the leak I thought it was going to be one of the vacuum hoses causing the ECM to try to compensate for it.

 

[nola mike]

I think Ted needs to stop looking at them fancy 'puter numbers. Keeps getting him on long diagnostic adventures...

 

[tpenfield]

Yup no matter how fancy the diagnostics, it usually comes down to something simple . . . like a sea shell stuck in the water inlet hose or a torn/crumpled plenum gasket.

Don't tell anybody, but I made a new set of gaskets out of a Bud Light 30 pack carton

 

[alldodge]

Glad you got it fixed, and sorry it took me so long to get back with, had some family issues that need attending

What I was wanting you to do is connect the vacuum gauge to the hose coming from the intake manifold and going to the fuel pressure regulator. With a vacuum leak you would have seen it on the gauge right away. I chose the fuel pressure regulator hose because it would be easy to get to. Guess I'll try to be more descriptive next time to help clear things up.

 

[tpenfield]

Glad you got it fixed, and sorry it took me so long to get back with, had some family issues that need attending

What I was wanting you to do is connect the vacuum gauge to the hose coming from the intake manifold and going to the fuel pressure regulator. With a vacuum leak you would have seen it on the gauge right away. I chose the fuel pressure regulator hose because it would be easy to get to. Guess I'll try to be more descriptive next time to help clear things up.

No worries AllDodge Yes, on the vacuum hose, I just was not sure if you were having me test the regulator or just grab some vacuum.

Anyway, it is all good . . . after reading the manual a bit I realized that the ignition system has 2 modes of operation, each of which it can advance the timing. 'Distributor Timing' mode and 'ECM' mode. It seems that the engine cranks in distributor mode and then the ECM mode kicks in once the engine is started. I was concerned that the ECM mode was not kicking in.

Once I found and corrected the gasket issue, all the bits and pieces came together. The engine idles better than ever. I could never get either engine to go down to the 600 RPM level, but now that is where they idle.

 

[alldodge]

The engine idles better than ever. I could never get either engine to go down to the 600 RPM level, but now that is where they idle.

Cool beans,

 

[tpenfield]

A Quick Update:

The engines have been running fine. I was a bit concerned about the starboard engine because the temp gauge has been hovering in the 175-180 range. From my past experiences, the gauges at the helm seem to run about 10-15 degrees F hotter than the ECT reading. . . Not sure why, but the 2 senders are at slightly different parts of the cooling flow.

Anyway, I wanted to check the ECT to verify that it was in fact running more in the 155-170 range as it should. When I hooked up the Diacom computer program, there was a MAP sensor code (33) on the MEFI. I think it may have been from when I was futzing with the plenum gaskets, cuz I replaced the sensor. Anyway, I cleared the code.

I took the boat out in the bay and ran it at about 30 mph for a couple of minutes, just to see if the coolant temperature rose more than it should. Here is the data from the ECM, as a video, which is sped up about 3X, The temp did not break 170 per the ECM, but my gauge at the helm was 180-183 ish.

ECT is the top graph . . . RPM's is the middle graph . . . Fuel Consumption is the lower graph.

The cooling system seems to recover fairly well. The Port engine is the one that gave me fits last year and that seems to run even cooler per the gauge, but who knows what the ECT sensor has to say about it. I will have to check that one at some point.

Looks like the fuel consumption is about 10 GPH (per engine) at a speed of 30 MPH . . . or 20 GPH total for both engines, which seem about right . .. 1.5 MPG :thumb:

 

[QBhoy]

You try taking the wee idle control muffler filter out ?

 

[QBhoy]

Oops. Just read the last page. Glad seems fine.

 

[alldodge]

Looking good

 

[tpenfield]

Looking good

Thanks, AD.

I just hope the MAP code does not come back, I can see myself pulling my hair out like some of the MPI/EFI threads that we have seen lately.