Engine ECU swap for diagnostic purposes?

B

britandy

Member
Aug 19, 2019
39
Boat Info
340 Sundancer 2000
Engines
Mercrusier 7.4 MPI, inboard, V drives
Does anyone know if I can swap the engine ECU's to help me either find my intermittent port engine problem, or to at least rule out the ECU?

Boat is a 2000 340 Sundancer with twin 7.4 Mercruisers.

Thanks

Andy
 
You can, but here is the potential issue. Let’s say something is shorted or faulty and caused your ecu to go bad, now you put the second one in there and it also goes bad. Now you are out 2.
 
Thanks for the quick reply, and you do raise a very good point. I am thinking of purchasing diagnostic software. Old boat, old engines, I figure it will pay for itself.
 
Problem is intermittent. At times the port engine will struggle to get to 3700 rpm, the throttle lever will be all the way forward. When the problem occurs I can tell the power output is not what it should be from this engine. My speed is way down, maybe 16 - 17 knots. In addition according to my GPH gauge when this is occurring the fuel consumption on this engine is at least 30% more. This is also confirmed by the rapid drop in fuel level in the port fuel tank. There is no smell of gas so I have to assume it is all being burnt, just that the engine is not running efficiently hence the lack of power.

Problem first occurred two weeks ago. We were heading out for an overnight trip, at first throttle up problem was there. I ran like this for about 5 minutes. I throttled back down to idle for about another 5. Throttled back up, problem was gone. Boat ran perfectly all weekend.

A week ago we go out for a day trip. Problem occurred both out and back, I could not get it to "fix" itself.

This past weekend went out a couple of times. First trip no issues. Second trip happens again. This time I backed the engine down to around 2000 rpm, then immediately took it back up, problem went away!

Engine has new plugs, wires, cap and rotor and fuel filter. Engine oil is good with no signs of fuel in it (so at least it is not washing the cylinders down). I replaced the MAP sensor. I confirmed I have 5 volts at the sensor. Oil pressure and engine temp all good, at least according to the gauges. I did take fuel pressure readings at idle (35 psi) and 2000 rpm no load (32 psi) from both engines. Same readings from both engines. My theory is something is telling the ECU to add more fuel (at least when under load), which in turn results in an inefficient burn, so loss of power and unable to reach WOT.

Engines always start, run smoothly and stay in balance other than when the problem occurs. I notice the port engine is going to act up when I approach around 2000 rpm. The port throttle will start to creep ahead of the starboard with the differential between them getting worse as I try to reach cruising rpm/speed.

I do have a new fuel pressure regulator so intend to fit that next. I know I really need to get fuel pressure readings when under load.

This is my first boat, purchased it about 7 weeks ago, engines only have 340 hours. At 19 years old that is almost too little use.
 
The coolant temperature sensor is one other place to look. If you have the ability to monitor the coolant temp as seen by the ecm, you can confirm it rule it out as the cause of your issue.

At the very least, disconnect the 2 wire connector and check/clean it. On very rare occasions (2x in MANY years) I’ve seen one of the terminals in the sensor damaged by rust.
 
scoflaw, your thought on the injectors, I assume you think they are sticking open at times? Could this be a matter of running a good concentration of a fuel additive in the gas tank? I think I am fighting a previous lack of use on this boat. I have used it every weekend since purchase 7 weeks ago, more use than it has ever had. I can certainly see the injectors not operating properly after sitting idle for so long over the years.
 
At the very least, pull them and send them out. I just replaced mine on my 1999. 1 had failed electrically, and a couple of them broke getting them out. That plastic gets old and brittle
 
scoflaw, thanks for your comments, I think that is my next step, pull them and have them professionally check/cleaned. I just looked at the price of an injector, wow, they are not cheap, $360.00 or more each!!
 
There are better deals to be had. Boats Unlimited 817-658-7721 John. High quality $650 a set. Beware of the Chinese junk that look exactly like OEM.
 
In many cases, the injectors are rebuildable (what Scof likely meant by "send them out"). There are online places you can send them to - most big cities will have an injector shop somewhere nearby if you want to stay local. About $20 per injector to be professionally cleaned/rebuilt. This is a good idea, regardless of whether or not they are the issue.
 
I really appreciate everyone's comments. It does make sense to have the injectors cleaned/checked on both of the motors. By chance does anyone know of such a professional service in the Baltimore area. I have already found a couple of places on line where I will have to mail them out. I would rather do this locally. I will keep looking.
 
I don't know of any shops in your area, but just in case you are specifically looking for a "marine" injection shop... don't worry about that. There's nothing specific to your injectors where an automotive shop can't do them. Try googling for "fuel injection shop" and add your area to the search.
 
P1020369.JPG
P1020370.JPG


This guy is very thorough, as you can see, he's testing these things 9 ways to Sunday. These were the new ones he sold me
 
That is impressive. Might be worth me buying a set, as from your experience there's a chance one or more of mine will break, or possibly be beyond repair. I could then at least get one good set professionally cleaned after pulling the 16 of them from the two motors.
 
Well, I think I have found a local guy, just a few miles down the road from my house. Perfect. I am going to try and get the injectors off at least the port engine tomorrow. He has a 24 hour turn around. I could have them back on the boat for the weekend!! I will report back once a test run has been completed!
 
Good luck! At least this way you are eliminating a likely variable.
 
I went through a similar intermittent running problem concerning the ECM. The mechanic did tune ups including wires and it ran good for a brief time then coils then fuel injector rebuilds. The entire process took a season and a half before the ECM finally died. Mercruiser had discontinued the ECM by then and offered a retro-fit for a ridiculous amount of money. I ended up finding OBD Diagnostics 888-469-3274 guys name is Ken who tested and confirmed my ECM had failed. He carries ECM's so I bought one and haven't had a problem since.
 
Might also want to check the knock sensors. If it's getting to 3700, that might be it as a bad knock sensor (s) will usually limit the engine to around 3000 with the timing retarded all the way. I had a plug on one of mine come loose and it acted just as you described, but could only make about 3200.
 
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