# 1990 rough running chevy truck



## chevythang71 (Apr 5, 2012)

Hello, I own a 1990 chevy 1500 2wd truck it has a tbi 350 that me and my dad rebuilt several years ago sold the truck to brother in law and just bought it back It was running a little rough at first then I had the truck painted and now it has a extremely rough idle. Will cut off in reverse and some at stop signs ( once engine is hot). also rpms go up and down when you hold a constant speed say at 45mph. Also has a small miss it seems at a idle and is idled high to my ear truck does not have a tach. so here is what I have done

1. Complete tune up all plugs/wires/rot/button/filters

2. Have changed all vacuum lines

3. EGR sensor

4. Rebuilt the entire TBI all new gaskets and o rings. 

Up until I changed to TBI there was no codes no it throwing a code 22 which is the TPS so thought i would post here before I spend any more money any input is greatly app. 
thanks 
drew:banghead:


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## octaneman (May 13, 2009)

Hi chevythang71


Remove the O2 sensor then have it tested with a cheapie propane torch and DVOM if its out of spec replace it, the next step is to clear out the IAC with decorbonizer including the port hole. Erase codes and re-start the engine if idle doesn't stabilize test IAC motor. 


post back your findings.


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## chevythang71 (Apr 5, 2012)

Ok O2 sensor is new about 2 months before I got it. Also where is the IAC on these trucks?


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## octaneman (May 13, 2009)

The IAC is screwed on the throttle body.


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## petercj (Oct 21, 2004)

Have you tried another distributor?


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## chevythang71 (Apr 5, 2012)

Ok should to IAC move freely. I have worked on alot of trucks but first one that is a TBI seems that its almost loose of sorts. How does this function inside? Distributor is not the problem already ruled that out. It acts as though its a vaccum leak or a bad air/fuel mix.


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## octaneman (May 13, 2009)

To test the IAC properly use a DVOM to read its internal resistance and compare it with your spec sheet. Another test is with a 9v battery the plunger on the IAC motor should move out when you apply positive voltage and retract when you reverse polarity. The IAC plunger should move freely in either direction, if any of these tests fail replace the IAC. 

If the IAC passes move on to the O2 sensor. Inspect it for carbon deposits the O2 not only tells the computer to switch from lean/rich, but it also tells the computer how much time in milliseconds to turn on the injectors.


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## chevythang71 (Apr 5, 2012)

ok sorry it took so long to get back I am in school and work full time so been messing with the truck. 
replaced the IAC it was bad. Didnt change how the truck is running though
Replaced to TPS since the code 22 it is showing is that and now truck still runs the same and still have code 22?

The truck misses at a idle and surges at low speeds if you floor it it seems to run ok and code light goes off soon as you let off the gas code is back and truck starts to stall and act like its going to die. Really getting on my nerves also it is IDLEed very high. has no rpm gauge but it is high as heck you can tell by ear. help please thanks


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## octaneman (May 13, 2009)

Code 22 is a low voltage to the TPS take your DVOM and test the ground wire for open to ground.


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