Engine ECU swap for diagnostic purposes?

[britandy]

I got the injectors back after having them cleaned/checked by a local shop. They gave me the before and after readings for the fuel flow, before they were all a little different and actually on the lean side, after cleaning all are now identical (sorry I do not have the results printout with me as I type this). I think three of them were originally 18% down on flow. So overall well worth the effort to remove and have them checked out. I will at some point do the same to the starboard engine injectors. As far as did it fix my problem, I am really not sure...yet. I took the boat for a run last Saturday and she ran well, both throttles in the same positions, according to my GPH gauge both using the same amount of fuel, did get up on plane and was running 20 - 21 knots. However, I am not convinced this has fixed the issue, after all it was intermittent. I really need a few more runs before the results are in! With only a couple of weekends left this season I might not truly figure this out until next year.

One other strange phenomena that did occur on my run, was with the engines at around 1100 RPM and running 5 - 6 knots out of my creek, the RPM gauge on the port engine would at times suddenly swing up to 2000 RPM, then back down. It kept bouncing like this on and off, until I started to throttle up. The engine was not actually changing from 1100 to 2000 RPM, there was no change to engine noise and boat was not suddenly speeding up, or pushing to starboard. Not sure if this could be related to a general ECU issue that is manifesting itself into various problems. Anyone know if the ECU signal to the gauge comes directly from the ECU?

I think over the winter I will send the ECU to the OBD Diagnostics guy to have it checked out.

Thanks again for everyone's comments.

 

[hottoddie]

Mmmmmm! I could be a bad tachometer. Maybe pull the tach and cycle the 4/6/8 switch on the rear of the tach back and forth. Be sure to put it back on the 8 cyl position. Or you could disconnect the tach and see if the problem goes away.

 

[britandy]

So, to close this one out, engine running problem is fixed. Over the winter I did several things in the engine bay, new manifolds, elbows and risers (not that I thought they had anything to do with the port engine issue, they just needed replacing) and replaced the fuel pressure regulator on the port engine mainly because access is so much easier with all of the exhaust system out of the way. I did send the port engine ECU to OBD Diagnostics and they returned it saying it passed on their test rig as good.
Boat went back in the water early April but due to Covid 19 in MD recreational boating was not allowed. In early May that restriction was lifted. After a couple of test runs found that the port engine issue was still there and had not magically disappeared over the winter. I finally decided to switch ECU's and when I did, the problem moved to the starboard engine. (I assume due to the intermittent nature of the problem OBD Diagnostics' test rig at that time found nothing wrong) Finally, a resolution, albeit a VERY expensive one. I purchased the MEFI4 kit (MEFI3 ECU's no longer available) and fitted it yesterday. Took boat for a good run, with plenty of throttle up and down cycles. Boat is running as it should.
At this point I deem this issue closed.