Recording Wrong RPM on ECM

[fallflight]

Hey guys,

Did some more datalogging and had overlooked a major issue.

This application is my boat.

350 CID
TBI with 16168625 ECM

I had the boat running down the river at WOT. Really moves out fast. I know that the Tach on the dash is correct. Engine is racing. It was running at approx 4900 RPM on my dash, yet my ECM logged only ~3600 RPM. How can this be? bad distributor?

Also at 3100 RPM on the dash, ECM is recording 2400RPM.

This would make sense as to why all my data is lean. If the ECM thinks it is only running 3600 when it is actually running 4900, would cause a problem.

Thoughts?

-Fallflight

[EagleMark]

What distributor is in the engine?

[Six_Shooter]

This seems to be an issue with the P6 (IIRC that's the designation) PCMs, where they control both trans and engine. The engine indicated RPM seems to be lower than the trans data stream engine RPM.

It's something I need to look into a little more on the '7060, as well.

[fallflight]

The only p/n that I see on it is 1103479 which is 1 number short of the GM nomenclature. Does this help?

-fallflight

[EagleMark]

What distributor is in the engine?

This seems to be an issue with the P6 (IIRC that's the designation) PCMs, where they control both trans and engine. The engine indicated RPM seems to be lower than the trans data stream engine RPM.

It's something I need to look into a little more on the '7060, as well.

When I did the kids IH truck with the 16197427 we just used the extra white wire off coil like always and it's spot on to TP data. His 8625 is same type PCM, so not sure what the differnce is there or what's happening?

The only p/n that I see on it is 1103479 which is 1 number short of the GM nomenclature. Does this help?

-fallflight

No, is it a small cap EFI distributor? Just wondering if you had a tach filter with the howell conversion you got going on? Also could look at tach and see if it's set to 4, 6 or 8 cylinder, like said above I just used the extra white wire normally off coil and it worked fine... have we looked at your bin file on that chip yet?

[fallflight]

It is the small cap distributor. I dont lknow that it has a tach filter from Howell. I was just over to see them as I live close to them. they didnt mention anything about it, and we discussed this RPM issue. How would I know if there is a filter?

Also, if it is displaying the 3600 RPM, is the ECM/PCM thinking the engine is running at this RPM or ? This would explain the lean conditions

[RobertISaar]

hmm.... almost seems like the V6 and V8 RPM conversions are getting mixed up? my P66 unit displays the RPM perfectly.

perhaps swap out the reference period RPM for the normal RPM X 25 one to confirm?