99 7.4L MPI will not rev up, runs smooth.

[capz]

It just looks like the ecm is not controlling timing. Or it is but then reverts to timing advance at the distributor. But if the cause of that is an electrical fault in the Reference low or bypass lines you would get a code 42. The fact that you're getting no codes points to a bad sensor. You'll only get a trouble code if the ecm detects a shorted sensor, not a sensor giving bad readings. A bad MAP sensor will affect fuel and timing. You should have an intake air temperature (IAT) sensor.
Regarding the knock sensor. Make sure you don't have the knock sensor module mounted on the distributor. See here on page 7

http://www.*******.com/merc/Techbk/95/95HGB4.PDF

 

[capz]

guess I can't link there. I'll try and find another picture

 

[Adamo]

IT IS FIXED....... After checking all the sensors and replacing the cool fuel system as well as all the injectors we finally decided to check the wiring harness and the ECM. Unfortunatly we were not able to find a good used or new MEFI 3 wiring harness to try with my existing MEFI 3 ECM with so we ended up trying a MEFI 2 harness and MEFI two ECM and that it was it. The boat is running great. Finally...What I need to do now is source a good used or new harness for my 99 7.4L MPI engine and try it with my MEFI 3 harness. If that still leaves a problem I will need to also track down a MEFI 3 ECM. The good news is I now have a running boat. My mechanic wants me to put on the MEFI 4 kit which is a new harness and ECM as a kit, as Mercruiser no longer makes a MEFI 3 available as DELPHI the manufacturer no longer makes them. Anyone have experience or thoughts on going to MEFI 4.

Thanks again to all who helped me out here. Adamo.

 

[Lazy Daze]

Don't you just hate electrical problems?! They're such a pain - one little wire somewhere...

Glad you got it figured out!

 

[Adamo]

Me too and thanks again for the help.

 

[capz]

Nice! :thumbsup:

 

[fc3]

Glad you found it. There's nothing wrong with upgrading to MEFI-4, but doing so is not inexpensive. It can buy you some new capabilities, but you need to have someone upgrade the programming in order to use them. For example, MEFI-4 can read data from wide-band O2 sensors. Therefore you can get precise A/F ratio control, which can be good for economy and performance, but you need someone to program those look-up tables. Not something for the average person.

Best regards,
Frank

 

[Adamo]

Wanted to put in a final comment to close this thread but let future readers know the final solution was not the ECM. I found a used harness on EBAY for the MEFI 3 and voila! Engine ran great. No obvious visual problems looking at the harness but I suspect a broken wire or faulty connector would fail once the RPM came up and the voltages increased. Hugely hard to troubleshoot. Older engines using EFI opens up the engine to major corrosion headaches. Moist environment and electronics, not such a good combo. Thanks again to all who helped me with this. Boat is now sold and new owners loving her so I am pleased. I am now in diesel land with a 02 41DA and totally impressed with diesel power. Cheers again to all.

 

[efoxner]

Great info Adamo! I am on this forum for my 1987 Sundancer 300 (for sale if anyone is interested!) and found your thread due to the same issue I am having with my 1999 Maxum 3700 SCR with one of my two Mercruiser 7.4 MPI's. I cannot wait to get back down this weekend to check out the harnesses on my misbehaving motor.