zman17,

even if the O2 sensors are switching, no error codes, no check engine light and the heating circuits checkrd out okay, would you still change the O2 sensor? With the O2 sensor only $40 i'll probably try it anyway.

The truck seems to 'wander' with the RPM's by 200 rpm or so when the truck is cold then seems to improve after 5-10minutes of driving now. If it was the TPS I would think it would be more of a constant problem and not intermittent.

 

Yes, I'd replace the precat o2. And are you or did you clean the IAC and port on TB?

 

Have u pulled the pre oxygen and start it . Good way to see if its clogged . Also check your compression you may have a cracked head

 

Okay here is what I have found and done thus far:

TPS - checked okay (tried new one just in case
MAP - replaced
Precat O2 sensor - pulled and ran without is (same problem), also the truck sputters when it is cold so the computer would be running open loop at this point anyway
crank shaft - sensor replaced
cam shaft sensor replaced
distributor ripped apart and all okay (spindle, cap. rotor, bushing . . .)

I found that the spark advance was running between 9 - 12 degrees advanced and would go down to 0 degrees, the engine would backfire and pick back up again. The spark advance would run as high as 18 degrees advanced.

I finally found something on the scanner that was CTK count on the output side of the computer that was erratic and all over th place. So i am thinking that it is either the computer or a loose/bad wire from one of the sensors.

Any thoughts guys?

Thanks

 

check the connectors at the pcm, they tend to get a bit corroded, might give you that erratic signal.

 

Okay - so computer is not the likely problem (by the way I really dislike the security module that required the VIN in the computer to match it . . . another story). The scanner stopped reading anything from the computer. I am now thinking there is a short somewhere in the wiring harness supplying the computer. I will post an update if I find anything. I hope this thread is helping a few people.

 

Yes, please let us know what you find. And post it in this thread please, don't start a new one. Thank You,

 

Another Update - Found out the scanner needs at least 7 Volts from the computer for communications. This was discovered when I plugged in an old computer from a 1998 5.9L to try it out as I thought the stock one was faulty. Since I did not have a scanner to flash the old computer to get the VIN number to match the security module in the truck (new about a $7500 piece of electronics) I put the original computer back in the truck - and guess what - no communication. I think the computer is powered from two points. I did a little digging and it would appear the computer pretty much requires 12 volts to operate or sporadic problems can occur . . . i.e. spark advance issues, sensor readings . . .

There is a factory splice in the wiring harness I am going to check out that apparently powers the computer. I will keep everyone posted.

 

Well, it was not the wiring splice and is most definately the computer. What a frustrating experience this is. So because their is a security module in the truck and the VIN in the module has to match the VIN programmed in the computer, i have discovered that only the dealer can reprogram a computer and have the VINs match (yes, even on a used computer).

Anyone have any solutions to this one - I'll have to find out from the dealer how much this is going to cost. At least I did all the troubleshooting so it won't cost an arm and a leg.

 

Call around to some of the independent shops. Someone may have the bitchin' cool tool that will let them address that for you. Even independents have to replace PCM's once in while, they should have the capability.