vac pump problems?

[6gmc5]

I have a 94 1500 with the six five turbo and I have never heard anything run like this one!!!

When starting the engine will run as smooth as could be for about 3 to 4 seconds. then she will begin to spiratically rev up and down the rpms. touching the acc. pedal only makes it higher.BUT... When taking the serpentine belt off the engine will only miss a little but does not rev at all.
i sure do hope that someone has heard of this situation which leaves me in awe!

thanks! -6gmc5

[sixnickel]

Without hearing it myself, with what you describe, shooting from the hip I would say it is a fan clutch spazzing out. It's the fan RPM changing not the engine RPM. With the ENGINE OFF I repeat ENGINE OFF !!! keys in your pocket!!! Grab the fan and turn it and see how it feels it should turn smooth and easy if it turns real hard or locks up like it's binding up it could be your culprit. Rember I'm shooting from the hip on this one!!!!

[6gmc5]

no it is definately not the fan. It also rolls thick black smoke. i have seen the rpm guage and seen the engine jumping up and down myself. if you need more info I can try and get as much as I can.
thanks for replyin! -6gmc5

[1grand6]

DID YOU CHECK THE BEARING ON THE BELT TENSIONER?
My 93 also had those issues mentioned and it ended up being the engine temp sensor shorting out. Replaced it (dealer part) and smoke on start up went away, as well as jumpy idle. Is the smoke at idle?

[6gmc5]

the previous owner did say that the temp guage light came on while he drove it last... thats when the jumpy idle could have started. i will check into it. but it seems like it would be something more. and it does smoke a little at idle. thanks! -6gmc5

[6gmc5]

okay today we installed a new temp sensor... and it did seem to limit the reving and it definately stoped the thick smoke but it still has a slight jumpy idle and now has white smoke. could it be the throttle position sensor? I will try to get my hands on a scan tool sometime this weekend and run the codes hopefully that will tell us more. thanks! -6gmc5!

[racechaser8]

If its a 94 you dont need a scan tool Email walt on how to get the codes. Racechaser8

[Craig]

Quoted from SSDiesel FAQ

• Q: How do I read my DTC (Diagnostic Trouble Codes) on my 94-95 truck??
•

A: Locate the "Data Link" connector below the steering wheel on the rim of the dash. With the key in the "OFF" position, jumper the top two right terminal pins (termnals A&B) with a paperclip or short piece of wire. Turn the key to the "RUN" position, but DO NOT START. The check engine light will begin flashing a sequence of codes. If the diagnostics are working and there are no codes in memory, a code 12 will flash 3 times. A code 12 is a flash once, followed by a quick flash two times. Basically a 1-2 flash is a code 12. If no codes, only the 12 will flash. If codes present, the sequence will start with the 12, then flash each code 3 times before moving onto the next in numerical order. When 12 flashes again, you are done. Again, count the flashes and write them down, they will flash 3 times each code. To clear the codes, depress the brake pedel and accelerator pedel simultaniously for 15 seconds while still in diagnostic mode. NOW shut off the key and remove the jumper. The codes are now cleared. NEVER-EVER remove the jumper while still in the diagnostic mode, turn key to off first. NEVER-EVER start the truck with the jumper in place. ECM damage may result.

Email Walt the code numbers that came up along with symptoms and Walt will help you diagnose your truck.
•

[6gmc5]

okay i used the jumper wire and all it would come up with was 12 so i am guessing that whatever is causing this is not tripping a fault code. now where can i turn? what could be some other things to try. like i wrote before the temp sensor did help some by limiting the reving to only a few rpms and it only has white smoke now instead of thick black smoke. but it still revs too much for me to even think about driving it!:( i sure hope somebody can help me!! thanks!!!! -6gmc5

[6gmc5]

Good News! I finnaly got the code off the truck. it only came up with #19. which was the crankshaft sensor. when we pulled it out the wires broke and the end was covered in metal which i figure was causing the sensor to give a faulty reading. after replacing the sensor she fired right up and runs like a champ! Thanks Guys for all your help with this! This is a really useful site! I will post pics of my baby soon! Thanks again!!!! -6gmc5!:D