need help.. out of ideas 92 gmc running like crap
 

I just finished the body work and new paint on my 1992 gmc this past week and got to enjoy it for a week before it starting to run rough.. mostly on idle.. This truck as always been reliable and ran great until a few days ago.
305 with throddle body, 145,000 miles

coincidence??
1. degreased engine and hosed down engine day before problems started
2. just filled up the gas before problem started, bad gas? how to check?

what I've done:
I thought maybe I got water in cap or something so I've done the following.
1. new cap, rotor, wires, plugs, ignition coil, and fuel pump.
2. there is gas spraying out of both throdle bodies.

I did smell gas off and on while driving it.
any ideas??? help me get her back on the road.
thanks, Mark
http://i147.photobucket.com/albums/r...k/GMC/gmc7.jpg

Re: need help.. out of ideas 92 gmc running like crap
 

The truck looks very nice.

It would be a good idea to check it to see if there are any trouble codes stored in the ECM.

My first guess would be that the degreaser damaged the TPS(Throttle Position Sensor).

If the ECM does not see and voltage on the TPS that shows it is at idle, the ECM will not move the IAC.

If you got degreaser in the Map sensor it could be damaged.

TPS (Throttle Position Sensor) testing:
Piercing the wires to do testing can cause future problems.
When I can’t use a scan tool, I unplug the sensor, and use jumper wires between the sensor and connector, when I do testing, which is easy when you have wire terminals like the factory uses. (Save old parts to take the terminals out of them)

With the key on and engine off:
With the wiring connected, test the voltage on the wire that connects to Terminal “C” of the TPS (possibly dark blue wire).
Engine off with key on for this test. It should be between .5 volt and 1.25 volts at idle and increase smoothly as you slowly open the throttle to at least 4 volts at wide open throttle. Some spots give a different allowable voltage reading at idle in GM manuals, but this is what I go by.

Spec .5 to 1.25 volts with throttle closed.

Many people just test it with the engine off using an ohm meter on the terminals of an unplugged sensor to test for a smooth change in resistance as the throttle is moved.





MAP sensor testing TBI

Manifold absolute pressure sensor

Do you have a hand vacuum pump for testing the MAP sensor?

A normal Map sensor reading on the wire that connects to terminal “B” (possibly Lt green) would be 1 to 2 volts at idle. With no vacuum to the MAP sensor the voltage should go to at least 4 volts.

NOTE: if it runs poorly it could have much lower vacuum at idle which would cause a higher voltage reading.

Terminal “A” (possibly Gray) is voltage supply to the Map sensor. (Should be close to 5 volts)

Terminal “C” (possibly purple) is the ground for the Map sensor. (Should have no voltage)

Re: need help.. out of ideas 92 gmc running like crap
 

Thanks for the reply. It ended up being a stuck egr valve. Put a vacuum pump on it at 10 lbs to check it. Sprayed a touch of contact cleaner on the shaft. Truck fired up and ran perfect again.
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Re: need help.. out of ideas 92 gmc running like crap
 

Re: need help.. out of ideas 92 gmc running like crap
 

Glad you got it figured out.

You may want to take the EGR off and clean the bottom end.

If it gives you more trouble, and you find it needs an EGR valve, I would suggest only using an OEM (Original equipment manufacturer) replacement EGR valve. These TBI truck are very picky about the EGR valve operation.

Re: need help.. out of ideas 92 gmc running like crap
 

thanks for the heads up. I went ahead and replaced the egr just to be safe. The truck has never ran better so the old one must have been on a slow decline. did pick up the oem unit.
Mark.

Re: need help.. out of ideas 92 gmc running like crap
 

yes.. contact cleaner is actually for electrical components. I didn't want to use a oil product that would atract dirt.
Mark.

Re: need help.. out of ideas 92 gmc running like crap
 

Glad the hear you got it fixed!

Thanks for letting us know what fixed it.