obd1 reads 4cyl instead of 6cyl [also code 41]

[vagabond80]

Howdy

I've been fixing up this recently purchased (abused) '90 2.9 5spd 4x4
//Context: There was a pretty significantly blown headgasket around #6, replaced both at my friend's shop, runs well now except for occasionally wanting to stall when throttle is applied from a stop on a cold engine. I've yet to test the TPS or IAC valve on that, but I'm working towards it, and want to get a new radiator in first because the original is leaking near the top and so I'm assuming there could possibly be a coolant/sensor issue associated with that, and want to have the system 'right' before I dig in. Oh and my professional mechanic friend noted a leaking FPR and replaced that for me.

There has been a CEL that comes on(even prior to head gaskets) after driving 5-10 miles. I did the jumper test and got a 41 in memory. Replaced the 02 sensor with the Delphi OE type, not universal. Code 41 still comes on after driving. No other codes.
Broke down and paid the $30 for the Innova ford obd1 reader to make things easier. That initially showed nothing, even in memory, KOEO.
Today I drove it to work, got the CEL to come on, and without turning the engine off (CEL still on) ran the KOER test with the reader. Code 41, check. *BUT* the reader starts off by giving the code for the number of cylinders it reads. My 2.9 read out a clear "4 cyl". I suppose my searches were still too vague because I get pages of entirely unrelated topics. So I'll ask here directly, anyone heard of this or have experience/insight?

I read another user recently trying to track down a 41, he had bad injectors he believed were causing him to run rich, he replaced them and that seemed to fix the running rich but he still pulled a 41[which 'should' mean lean]

also pulled this from another site; haven't checked this yet, but doesn't address my two AWOL cylinders:
The common reason for the code 41 and a rich condition in most cases is a missing or open ground to the PCM (computer) on pin 49 at the PCM.
With KOEO and the O2 sensor disconnected, probe the O2 sensor Green/Purple signal wire on the harness side of the connector. The voltage reading on the Green/Purple wire should be less than 0.10 volts. If voltage is greater than 0.10 volts, check the Orange ground wire bolted to the rear of the passenger cylinder head, just below the valve cover. If it is loose, clean as necessary, tighten the bolt and retest. If the connection is good, check the resistance of the Orange wire between pin 49 at the PCM and the cylinder head. It should be less than 1ohm.

Truck still seems to run fine, but it makes me anxious. Thanks for any thoughts

 

[vagabond80]

Just ran it again KOER, registered 6 cyl this time, no code 41 even in memory[CEL doesnt come on for a few miles], but got a code 21 (cooling temp sensor out of range or ECT out of range. so maybe the engine just cooled off too much in an hour. its still in the low 40s here)

do these do some sort of limp mode with the CEL on? could that be why I showed 4cyl?
I guess either way I'll be checking the PCM tonight

 

[RonD]

Yes, pull the computer and have a look inside

The blue capacitors, 2 or 3 of them, do fail and leak, so look for signs of corrosion under them also move them with finger to see if 1 pin is broken
Under $5 to replace these

Also general corrosion on the circuit board will cause odd issues

 

[vagabond80]

Yes, pull the computer and have a look inside

The blue capacitors, 2 or 3 of them, do fail and leak, so look for signs of corrosion under them also move them with finger to see if 1 pin is broken
Under $5 to replace these

Also general corrosion on the circuit board will cause odd issues

caps were good and the circuit board was shiny and clean. I hit it with some deoxIT for good measure before putting it back together.
time to dig in to the wiring.