I have a 1990 GMC "Jimmy" that I use as my daily driver. It is a 4.3 TBI V6. For a while now, it has had a poor idle. It shakes quite a bit at idle, so bad the wheel shakes and the antenna whips around. I am just about ready to give up on it as I can't figure out why it is happening.

I have replaced:
injectors (2)
ECM
Map sensor
blocked off the EGR
plugs
wires
cap
rotor
ignition module
set timing
coil
IAC
O2 sensor

A friend of mine connected a Snap on scanner to the truck, and it wouldn't go into closed loop. The O2 sensor read in the 400's, and did not bounce at all. it moved very slightly (485, a while later 487, etc.)

I installed a new O2 sensor, and it doesn't seem to help. It will go into closed loop, but the O2 sensor is VERY slow. The sensor will read in the high 400's again, and then it will move sometimes to the high 300's and has even jumped to the 500's where it sort of stayed. I know the reading are supposed to move around, but it only moves a few mV at a time (like 465, 468 20 secs later, etc). It also came out of closed loop at idle, after it had been closed for like 10 minutes. I reved the throttle, and it went to closed again.

Any ideas where I can start looking to get this thing running better?

Thanks in advance,

Tony

 

Whats the engine temperature? If the thermostat is stuck open it maybe running too cold even though it maybe right around 175 degrees thats just on the threshold of going into closed loop. Check that out and see it that helps you. Most GM EFI cars and trucks has a 190 or 195 degree stat in them and they run around 185 to 190 degrees. There is a minimum temperature for the system to go into closed loop. Another problem maybe that the o2 sensor has been relocated and can't maintaing its operating temperature.

 

The coolant temperature was in the high 180's. I really don't know what thermostat it has, as I have never replaced it. Also, the sensor is in the stock location, so I would hope it would work properly there.

I for got to mention the the integrator was at 121 and the block learn was at 95. It appears the ECM is adjusting the fuel quite a bit.

Tony

 

When you set your timing did you disconnect the set timing connector? Check the fuel pressure? Any air leaks around the TB or intake? Check the firing order for crossed wires?

 

It took me a while, but I did find connecter for the electronic spark control (it is inside the cabin, under the carpet on the passenger side :crazy: ). So, I did unplug it when I set the timing.

I have looked at all of the vacuum line and they appeared good. I also went around the throttle body, intake, etc. with my propane torch, but it did not affect the idle.

The wire are for sure in the correct order. The truck runs pretty good everywhere except for idle.

As far as fuel pressure, I suspect that may be the problem. It runs pretty rich. The injector I replaced was fron Auto Zone, and I wonder if it is the proper flow rate. I still want to supect the computer as well, but would expect some other anomalies as well if that were the problem.

Tony

 

Is your exhuast really hot? if so you maybe burnig fuel in the exhaust ratther than the motor.