Questions and Answers
Powerless Pontiac
Q. Hello, I found your site through a search engine and it seems like I found what I was looking for. Okay, here we go. I have two problems with one car. I just got it. It's a 1991 Pontiac Firebird convertible with the 3.1 liter MPFI V-6 VIN "T" motor with the digital EGR.
Had drivers' side airbag but was removed to swap in "sportier" steering wheel. It also has the GM VATS system. (Resistor in the key). I am reasonably handy with cars and do (or at least try) anything that doesn't require a lift myself. The car has two problems.
First the A/C compressor doesn't kick on. System was R12 but was converted to R134a before I got the car. System does hold a charge and the two pressure switches are good. The compressor has two connectors going to it, a rear one; two wires, does get some kind of power, and the front connector (clutch) those don't have any power. I don't know what else to look at. The guy I got it from "says" it worked last summer.
Now my bigger concern, it refuses to run right. Idle properly and is powerless. It has 140,000 miles, and the motor does not tick, and sounds pretty good. It just runs very bad. This is what I have done to solve this problem, I changed plugs, wires, fuel filter, ignition coil, remanufactured distributor with new module and pickup coil, gutted the cat, muffler is good, new coolant temp sensor, new TPS, can't find any vacuum leaks, holds about 16" of vacuum at reasonably steady rate, cleaned the EGR, throttle body, ran fuel injector cleaner and filled it with 93 octane gas.
Now here's what it does, at a cold start it idles high but as the engine warms it does not come down, if I shut it off and turn it back on it idles low until the gas pedal is depressed, once the rpms pick up they don't come down automatically, (throttle moves freely) it is also powerless. It will lightly backfire / misfire if under quick acceleration, whether its in drive or park. Under a load (actually driving it) the motor shakes slightly (trembles) and revs up very slowly. In a no load (neutral) it revs up reasonably quick but with a flat spot at about 1100 rpm. Feels like the timing is off. I tried for a very long time to time it but it does not make a difference.
The Chilton says to set a base timing of 10 degrees with the ECM grounded. The A & B terminals. I can't even get the engine to run with the ECM grounded. It does crank and tries to start but doesn't, (back fires, mis-fires more than usual) fuel pressure is good at 46 psi with a 3 psi drop while its on. Could it be anything with the charcoal canister? It is cracked and the crack was taped over. It has a vent, line with "timed vacuum" and a electronic purge valve that gets very hot.
I don't really know what more info you would need but just email me with any thing else to help me diagnose it. It getting very frustrating and I can't figure it out. Please help. Hope I didn't bore you to death with all this reading but I really don't know what else to do. Oh yeah, I get no check engine light, and there are no stored codes, and yes the light does work.
Dimitr
A. 1. A/C is usually controlled by the ECM on these. There should be a relay under the hood somewhere that controls the clutch provided that the ECM is getting a A/C on command. It then turns on comprssor through the relay.
2. All readings sound pretty good to me. These have been known to "slip" the intake gaskets and suck air from inside the engine causing rough running etc. If you pull off the oil fill cap does it idle rougher? Try spraying carbutetor cleaner around and check for vaccum leaks.
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