Hello Mechanical Guru's!

First some info on the truck;
1990 Chevy 1500, 350 Engine, Fuel Injected, Standard Transmission.

Last year I was having a problem with the truck either idling too high, or not at all (dies if foot was removed from gas, when supposed to be idling - not driving down the highway) and while driving down the high way the engine would cut out, as if not receing any fuel, but when foot was removed from the accelerator, it would come back to life. So in turn we borrowed a friends code reader and received 3 codes:

21
Throttle Positioning Sensor (TPS) signal voltage too high.

22
Throttle Positioning Sensor (TPS) signal voltage too low.
Fuel cutoff relay circuit - open or shorted to ground.

34
Mass airflow sensor - signal voltage or frequency is low during engine crusie.
Manifold absolute pressure sensor - signal voltage is low during ignition on.

So I replaced the TPS and the idiling problem went away, we can idle the truck with out holding the accelerator down, and it does not idle high. However there is still something wrong - haven't noticed the truck 'choking' while driving, but have now noticed that when the clutch is pushed in when coming to a stop, and while still rolling the truck seems to cut out, but by releasing the clutch the engine will fire back up as if bomb starting. Also noticing a sputter and loss of power at times when leaving a complete stop -- seems to happen more when the engine is not warmed up.

Having felt all that I bought my own code reader and I am receiving 3 codes:

22
Throttle Positioning Sensor (TPS) signal voltage too low.
Fuel cutoff relay circuit - open or shorted to ground.

34
Mass airflow sensor - signal voltage or frequency is low during engine cruise.
Manifold absolute pressure sensor - signal voltage is low during ignition on.

45
Rich exhaust indicated (Left side on dual Oxygen models).

So after reading all that background, my questions are:
1. What is my next step?
2. Where do I find the Fuel cutoff relay circuit
(assuming this is the issue - TPS is new, and code 21 is gone)
3. How do I determine whether it is a Mass airflow sensor, or Manifold absolute pressure sensor ?
(Again assuming the problem I am experiencing can be described as "signal voltage or frequency is low during engine cruise" I'm guessing mass airflow sensor)
4. How do you fix rich exhaust? Does that have anything to do with the exhaust manifold? (part of the muffler is missing... )


Thanks in advance guys/gals, and thanks for reading such a long winded question - but I think its always good to have some background saves you guys from having to ask questions about questions :-)

SB

I also have a 1990 Chevy P/U but with a auto trany. I bought a cheap scanner and it said low voltage on TPS, so I replaced the TPS. That did not fix it. Did you ever figure your trouble out?

ECMs are giant power transistors. Since so many sensors are reporting low signal voltages, the ECM may be failing. Take a multimeter and check the reference voltage to the TPS, when the ignition switch is turned to ON. Red wire to the TPS connector and the black wire to ground. It should record around 5 volts. If it's low (likely around .35 volts), the ECM is bad.

It's also possible the Ignition Control Module (ICM) is failing. This is a problematic component on vehicles of this generation. I recommend changing ICMs every 120,000 miles or 10 years, whichever comes first.