need help with strange 7.4MPI problem
05-10-2009, 03:21 PM
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05-10-2009, 03:28 PM
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As I said earlier, even with this issue, nothing would ever make me go back to a carburetor and mechanical ignition advance.
Tom
Last edited by tomtc; 05-10-2009 at 04:12 PM.
05-10-2009, 04:06 PM
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08-02-2009, 08:43 PM
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I know this is an old post but what did you find? I'm dealing with a similar issue like this as well on a 98 7.4mpi. After 45 min of run time I lose power bad, the fuel pump is louder than sh!t. If I let it sit for a bit I can run for awhile before it happens again. I have replaced the injectors so far(had 3 that tested bad) and now i'm going to order a new pump and regulators.The injectors helped getting on plane but the 45 min problem is still there. I am really confused on this one. Whats up with the fuel pump whining so loud?
08-03-2009, 09:15 AM
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At all RPMs below 4,000, it occasionally starts running VERY rich and power and rpm drops way off. It will do this sometime for just a few seconds and other times for minutes. I have a Floscan and when the problem occurs, the fuel flow will jump way up. For example, at idle it will jump from about 1.2 gph to 5 gph or so. At 2,200 rpm (about the fastest it will run when the problem occurs), it jumps to something like 19gph. When the problem occurs, you can smell the strong odor of the unburned fuel. It does this both when the engine is basically cold and when it is fully warmed up.
If above about 4,000 rpm it runs just fine and the problem does not reoccur until I slow back down and the rpm drops.
This weekend, the problem took on a different nature. Now, the opposite is happening. The fuel flow is very low (lean) for the engine speed and load, and the engine is running VERY rough. For example, it is only showing about 5.5gph at 2,500 rpm, about the fastest it will now run under any circumstances. The problem is now also no longer intermittent and is occurring constantly.
The ECM on the engine is a computer and like any electronic device, it can develop a problem. In this case it appears it was the driver for one of the two banks of injectors. Since everything was fine as far as the computer was concerned (it has no way to know that one set of injectors was not firing properly - this was before the later versions which can do an injector test), no codes were set. It's rare for an ECM to fail but it can happen.
Tom
Last edited by tomtc; 08-03-2009 at 07:07 PM.
08-03-2009, 10:40 AM
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Merc had a problem with a bunch of bad ECM's on the 7.4L MPI early in it's run (1998 & 1999 models). I bought a used 1999 Crownline in Sept. of 2002. In April of 2003, I was having hard start problems. My Merc tech sent the ECM into Merc and even with it being well out of warranty Merc shipped back a new ECM free of charge.
08-03-2009, 10:52 AM
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Merc had a problem with a bunch of bad ECM's on the 7.4L MPI early in it's run (1998 & 1999 models). I bought a used 1999 Crownline in Sept. of 2002. In April of 2003, I was having hard start problems. My Merc tech sent the ECM into Merc and even with it being well out of warranty Merc shipped back a new ECM free of charge.
08-03-2009, 06:58 PM
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tomtc - I did not mean the boat. I am talking specifically
what is wrong with the ECU/ECM? Programming error?
Something caused your engine to perform poorly. I am
thinking of the next guy who may have the same situation.
If he knows what chain of events brought your engine's
ECU down it would save him time. My Cobalt is a '99 and
came with a 7.4L MPI. It ran perfectly until the day I sold
it and dropped in the 502. I am sure someone knows the
real story of the "bad ECUs". Did your low voltage issue
cause bad info to get stored in the ECU never to be
overwritten? As a programmer I am often told to chase down
a "programming error" or "fix a bad program". I laugh to
myself since it is always something else. Now I get that
most mechanics or owners don't really care what the real
problem with the boat was just that it runs well and only
cost a few $100's. I am just looking for info from someone
who knows the Merc MEFI ECU's and the real cause of
your engine's problem.
what is wrong with the ECU/ECM? Programming error?
Something caused your engine to perform poorly. I am
thinking of the next guy who may have the same situation.
If he knows what chain of events brought your engine's
ECU down it would save him time. My Cobalt is a '99 and
came with a 7.4L MPI. It ran perfectly until the day I sold
it and dropped in the 502. I am sure someone knows the
real story of the "bad ECUs". Did your low voltage issue
cause bad info to get stored in the ECU never to be
overwritten? As a programmer I am often told to chase down
a "programming error" or "fix a bad program". I laugh to
myself since it is always something else. Now I get that
most mechanics or owners don't really care what the real
problem with the boat was just that it runs well and only
cost a few $100's. I am just looking for info from someone
who knows the Merc MEFI ECU's and the real cause of
your engine's problem.
Last edited by tomtc; 08-03-2009 at 07:09 PM.
