7.4 MPI intermittently hard to start/rough idle

[Mischief Managed]

My 7.4 MPI had a hot engine no-start issue early last Summer at a dock 10 miles from home. It would fire, then die. On a whim, I removed the IAC, cleaned it, exercised it and re-installed it. It started up perfectly and ran great for the rest of the day. I replaced the IAC with a brand new one the next day and it ran great for the remainder of the season.

Almost the same thing happened again two days ago, except it would start, run like crap for a few seconds, then die. I could make it run a little better, but not much, by giving it throttle. I took the IAC out and wiped it down, then put it back in. It started, ran roughly for a second or two, settled down, and ran perfectly for 50 miles of boating at various speeds from idle to WOT. I did not shut it down until it was on the trailer though.

There's plenty of power, it reaches 4600+ RPM at WOT with ease and has no issues accelerating. The plugs, cap and rotor are all new last year, and the fuel filter was new at the end of last season. I am running on the tank of gas the boat was stored with over the Winter. It was stored full and the gas was stabilized with the Blue marine Stabil at the correct mixture. I've used 49 out of the 74 gallons the tank holds. The temperature and voltage gauges read normally. There are no warming beeps other than the normal beep when the key is turned to "run" but the engine is not yet started.

Today, while testing on the muffs in my yard, it took about 8 tries to get the boat to start cold. It would fire, then immediately die. After it finally started, it ran perfectly and I could not reproduce the problem. I let it run until fully warmed up and it restarted fine. I let it heat soak and it started fine.

What are my next steps? Serial number is OL373137

 

[tpenfield]

Re: 7.4 MPI intermittently hard to start/rough idle

What are my next steps?

Yea, you might have to come to Cape Cod and go boating with me . . .

Something upstream causing the IAC to crud up ??

 

[Mischief Managed]

Re: 7.4 MPI intermittently hard to start/rough idle

Yea, you might have to come to Cape Cod and go boating with me . . .

Something upstream causing the IAC to crud up ??

The new one was not very dirty, just a light film. Ran crappy while clean today too...

 

[alldodge]

Re: 7.4 MPI intermittently hard to start/rough idle

Maybe its not the IAC, it just appears to be. What kind of fuel system do you have, maybe just the S/N would help me think

 

[Don S]

Re: 7.4 MPI intermittently hard to start/rough idle

You keep fixing the IAC, you should be fixing what is causing the IAC to fail so often.

 

[Mischief Managed]

Re: 7.4 MPI intermittently hard to start/rough idle

Serial number is OL373137, meant to put that in the original post...

I don't think there's anything wrong with the current IAC. I think it's something else. Don, what are you getting it? Is there a common cause of IAC failure that should be addressed?

 

[tpenfield]

Re: 7.4 MPI intermittently hard to start/rough idle

Maybe see if the engine is putting out any 'codes'. Got anyway to read the codes, or maybe will have to have a boat shop do it?

Is this thing going to kill your holiday weekend, or is it still 'operational'?

 

[Mischief Managed]

Re: 7.4 MPI intermittently hard to start/rough idle

Won't ruin the weekend at all. Getting help in case it fully fails on the water will be very easy, I can just call a neighbor for a tow. The worst case scenario is that we have to hang out with some of our best friends in a beautiful timber-frame lake house, swim from the dock, go sailing, go canoeing, and go out to eat by car instead of boat. I'm hoping it goes from intermittent to constant so I can figure out what is wrong.

 

[Mischief Managed]

Re: 7.4 MPI intermittently hard to start/rough idle

I've done some digging off iboats and am going to look into fuel delivery issues. I suspect that fuel delivery may have been the issue all along and it was coincidence that the IAC "fixed" it. Perhaps it just took many cycles of the ignition switch to build to the proper pressure on the fuel rail. I think I'll invest in a fuel pressure gauge today, it's one of those tools I should probably have.

 

[tpenfield]

Re: 7.4 MPI intermittently hard to start/rough idle

To me it sounds like it is running way rich at times, based on your description.

The intake manifold pressure sensor probably has the most to do with fuel mixture & delivery. So, maybe check/replace that sensor to see if it make a difference.

maybe someday we will be able to see all of this information on a screen up at the instrument panel, rather than playing guessing games or having to find the correct code readers and PC software, etc. :noidea:

 

[Mischief Managed]

Re: 7.4 MPI intermittently hard to start/rough idle

I don't recall any unburned fuel smell when the problem occurs.

It just started up perfectly fine a moment ago... Grrr, I hate intermittent problems.

 

[alldodge]

Re: 7.4 MPI intermittently hard to start/rough idle

I'm thinking part of the problem could be its vapor locking. First time it happened it was hot, tried cranking several times to only spit/spudder and die. You have one of the earily cool fuel systems which work ok but still have issues, hence why they no longer use this type. It could also be combining with fuel pressure delivery problems. The manifold pressure sensor and/or fuel pressure regulator.

 

[Mischief Managed]

Re: 7.4 MPI intermittently hard to start/rough idle

I made a code reader with a paper clip and a 12 volt LED as seen here: http://forums.iboats.com/mercruiser...timing-tool-make-1-00-a-49800.html#post448079.

The ECM does not go into diagnostic mode with A and B shorted together. I just get a steady battery voltage (+13.7) between F and E on the DLC. Does this mean I have a faulty ECM?

Bear in mind, the engine starts perfectly today.

 

[alldodge]

Re: 7.4 MPI intermittently hard to start/rough idle

I made a code reader with a paper clip and a 12 volt LED as seen here: http://forums.iboats.com/mercruiser...timing-tool-make-1-00-a-49800.html#post448079.

The ECM does not go into diagnostic mode with A and B shorted together. I just get a steady battery voltage (+13.7) between F and E on the DLC. Does this mean I have a faulty ECM?

Bear in mind, the engine starts perfectly today.

It tells me no codes have been recorded by the ECM, everything is ok'ey dok'ey today

 

[tpenfield]

Re: 7.4 MPI intermittently hard to start/rough idle

I made a code reader with a paper clip and a 12 volt LED as seen here: http://forums.iboats.com/mercruiser...timing-tool-make-1-00-a-49800.html#post448079.

The ECM does not go into diagnostic mode with A and B shorted together. I just get a steady battery voltage (+13.7) between F and E on the DLC. Does this mean I have a faulty ECM?

Bear in mind, the engine starts perfectly today.

Wow . . . I'm thinking of spending the $700 +/- for the computer program, since I got 'twins' to maintain. . . I guess the LED and the paper clip are at the other end of the spectrum.

Interesting that the ECM is not giving you any signal on the LED . . .

I wonder if this will help :noidea: Pirate4x4.Com - The largest off roading and 4x4 website in the world.

Hopefully the thing will behave over the long weekend so you can go boating. I'm leaving work about mid-afternoon to go wait in traffic to get to 'the Cape'.

 

[alldodge]

Re: 7.4 MPI intermittently hard to start/rough idle

Sorry, I have to retract a bit, I jumped the gun when I saw 13.7 volts
The battery should read 12, 12.5 with the engine off. The LED code reader when connected and engine off, key in run position should blink the number 12, and repeat that 3 times if all is OK. A stright 13.7 volts doesn't add up if it was in diagnostic mode and the engine running fine.

 

[Don S]

Re: 7.4 MPI intermittently hard to start/rough idle

There is more to the system than just setting codes. You could have a faulty ECT that tells the ECM the engine is colder than it really is. It would richen the fuel mixture, but there would be no code set. It doesn't know real cold from warm, just resistance readings from the ECT.

To many people get trapped in the idea that if there are no codes, it's either perfect or the ECM is bad. Both are wrong.

 

[Mischief Managed]

Re: 7.4 MPI intermittently hard to start/rough idle

There is more to the system than just setting codes. You could have a faulty ECT that tells the ECM the engine is colder than it really is. It would richen the fuel mixture, but there would be no code set. It doesn't know real cold from warm, just resistance readings from the ECT.

To many people get trapped in the idea that if there are no codes, it's either perfect or the ECM is bad. Both are wrong.

When pins A and B are shorted, shouldn't the ECM send a 12 code to output on pins E and F, if the ignition is on but the engine is not running, even if there are no codes? That's what my manual says it's supposed to do. I get nothing but a steady battery+ voltage on pin F.

 

[Don S]

Re: 7.4 MPI intermittently hard to start/rough idle

Pull the paper clip out of A and B, put the LED in E and F. The LED should be on when the key is on. Now put the paper clip in A and B and it should flash the codes.
I think it only goes through the codes 3 times, after that the light may be steady. I don't remember for sure.

 

[Mischief Managed]

Re: 7.4 MPI intermittently hard to start/rough idle

Pull the paper clip out of A and B, put the LED in E and F. The LED should be on when the key is on. Now put the paper clip in A and B and it should flash the codes.
I think it only goes through the codes 3 times, after that the light may be steady. I don't remember for sure.

I did that, remains steady the entire time the key on, paper clip inserted, or not.

I removed both plugs from the ECM and inspected all 4 connectors, they look brand new and neither plug was loose. There's no sign of any external damage or corrosion on any of the wiring harness. The engine has very little corrosion since the boat is trailer kept, always covered, and rarely sees salt water.

I did not test continuity from A + B of the DLC to the ECM connector yet; I gotta look up the data first.