need help with strange 7.4MPI problem
 

I have a 99 SeaRay with twin 7.4MPI inboards. The port engine just developed a strange problem that I need some help with.

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.

Water temp is normal, oil pressure is normal and battery voltage is about 13.8. It did this once before about two years ago (the rich running part) and the Mercruiser tech put a scan tool on it and saw no codes. It mysteriously went away until reoccurring again now.

Any idea on what could cause this and what I might look for?

Thanks!
Tom

The map sensor plays a big part in how much fuel the engine needs. If It has A short in it I could see it doing this to you'r engine and It might not in some cases do It long enough to have A code.

It's the damn EFI, have fun for the next month/year chasing it down.

Yep, it's a pretty sure bet all the engine manufacturers will be going
back to carbs in the next few years. Too much trouble with EFI. And no
one can fix them.

ZING !!!!!!!

How old is your battery or batteries?

Boat EFI IMO is more trouble than it is worth. Car EFI they seem to have figured out but not marine. A two minute problem for a standard setup becomes lots of fooling around with EFI. What is the benefit? Easy starting, are people rally that lazy? I never had any problem starting a carb setup. A few more hp? It is just not worth it yet. I have had both and know which way I will go next time I need a rebuild unless the technology improves markedly. Many fewer problems, much, much easier to repair. If a carb setup doesn't start, it is a two minute diagnosis, spark or fuel, if an EFI doesn't start it could be two dozen things.

The technology is cool and the maps are much tighter but who cares when you dead in the water, or under the hatch for days diagnosing, or sitting at a marina for even longer while they guess along replacing every sensor, charging you for each of course. Carbs setups are so much simpler for now and much cheaper to fix.

I will be out boating while you are zinging...

Seems to be speaking more of the repair shops you choose rather then the technology.

I highly doubt Merc now makes all there big HP motors EFI just to leave you strandedin the water one day.

Oh,,,, and try to get that carb motor through emissions ( you know its coming )

I would test the injectors if no codes are present ....One sticking and or not closing all the way will do what you're describing. With an injector tested you can build fuel pressure - then "pop" the injector for a set time - 500ms, 50ms, etc... and compare the pressure drop on each one. You may find during the pop test another injector fire as well meaning you have one shorted out. There is an "A" trigger and a "B" trigger for the injectors - obviously if the fuel pressure doesn't hold after cycling the key - you have one leaking pretty bad.

I agree boat EFI is a pain in the azz but with all the emissions laws it's here to stay. I think more of the EFI's bad rap is mechanics trying to diagnose and fix a customer's problem without the right tools or even a scan tool. I know in my town there is a place that wrenches on boats and he does alot of business and he doesn't even have a scan tool. For the cost of a spare prop you can buy a scan tool from Rinda and eliminate all the guess work on these EFI's. I bought one for my laptop and it tells you alot of info.

You guys certainly have a point on the emissions.

Get used to it, it is not going away. As emission laws get tighter, pretty much everything will be forced to go to it to comply. Plus, the greatly improved performance and fuel economy makes it a slam dunk. Regardless of any other issues, nothing would EVER make we want to go back to a carburetor and distributor on anything!

Tom

They are relatively new.

Tom

I have seen something as simple as a temp sensor doing what you describe,,,,, only dif being is is held a high idle on top of your simptoms,,,,not chocked it which leads me to belive the problem is after the controled circuit,,,,, also,,,,,, is there a ground cable between the two motors?

Another thought,,,,, do you have seperate tanks, or does both motors draw off of one tank?

They draw off of separate tanks but it does not act at all like a fuel problem so I don't see that as an issue. And yes, both engines have a common ground.

It acts like something is suddenly telling the EFI to dump in many times more fuel than is necessary or normal. If the engine happens to be at idle at the time it happens, the idle speed doesn't change much it just starts running very rough. The odd thing is that the problem goes away and stays away if I keep the engine at about 4K or above.

Check all your grounds including those at the bell housing. also if you have not changed your cap and rotor yet change it. these engines are very sensitive to cap and rotor wear or corrsion.

Just a thought, try checking you timing under load, with the scan tool and a timing light, if the timing isn't correct replace the distributor the whole thing. Mark @ Precision Marine mentioned this to me about 2 weeks ago I was doing some troubleshooting on a 575sci

once a good tuneup is verified,,,,, I would also change the fuel water seperators.

next Id lrun the boat with a scan tool hooked up,,,,, save you alot of time and potentially money of guessing and throwing parts at it.

There is always the old way of swapping parts between engines,,,,,, but thats kinda hackish IMO

The problem turned out to be a bad computer. Now all I have to do is try and find a 861720T2 EFI-3...

Thanks for the help!

Tom

Glad to hear you figured it out. We have been chasing an issue on a friend 7.4 for over a year. Can run fine all day on inland lakes connected by channrls, but get it on Lake Michigan where it can run at high RPM for longer periods of time and it appears to go into limp/safe mode. Turn the key off for a few seconds, restart and runs fine for a while.

We've checked codes - none stored, flowed injetors and changed out bad ones. Cant figure it out!!! Soemthing must be getting hot, shorting or grounding and making the computer go into limp/safe mode.

Or maybe its the computer itself.

Glad your issue is solved, because this sucks.

That only took a month to diagnose and fix. That's pretty good for EFI.

Actually, it sat for a number of weeks since I had other things to do in the interim. Once I got to it, it took only about half an hour. Based on the symptoms and reading the manual, I had pretty much determined that it had to be either fuel pressure or the computer. I scanned the codes (none were stored - I used a $39 Rinda checker I already had), checked the fuel pressure (which was perfect - I used my old A/C test set), and then swapped the computer in from the starboard engine. Success.

As I said earlier, even with this issue, nothing would ever make me go back to a carburetor and mechanical ignition advance.

Tom

with the factory workshop manual, a scanner and someone that can read , it was a 20 minute diagnosis and fix.

. .

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?

It turned out to be a bad engine computer (ECM) in my case. I found a used one for 280 bucks on Ebay and it solved the problem.

Tom

Here is the quote from the first message in this thread which describes the problem:

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

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.

That was true of the 16236999 version (which is what I had on my engines) but not the later 16237009 version (which is what I found on Ebay). The problem is, that ECM (EFI-3) is no longer available from Merc or anyone else. It went out of production with the 2003 model year engines (which moved to the EFI-4 and closed loop operation). The only option these days is to find a used one or use the 861720T2 upgrade kit which included an EFI-4 ECM and a wiring harness conversion kit. That's an expensive solution however.

There is no way to know definitively know what failed in the ECM though, as I said, it appears to be one of the injector drivers based on the symptoms. There is also no way to know what caused the failure though I suspect that it was a electronic component (mosfet, etc.) in the driver circuit and that the failure was not related to any specific cause and effect. The programming check code is unchanged and correct so it is highly unlikely that it was a programming error or overwrite. Remember that the ECM contains a lot of circuitry beyond just a computer, any of which can fail.