tombstone

Hello all, I have a 1992 Chevy Cheyenne with a 4.3L with 150,000 miles on it that started having a rough start issue. It has a two-injector TBI, Auto trans, A/C, 2WD. The truck ran perfect for the past 10 years, then one day last year the following issue started – literally overnight.
Sorry for the long post - just want to give you all the symptoms...
Here are the symptoms. When the engine is completely cold, the truck will start immediately, then within a few seconds will start loping – or revving up and down, which I suspect is the IAC trying to adjust the idle – but something is fighting against it. If I just let it idle, it will lope (rev up and down) for about 3 or 4 minutes, then it will instantly smooth out and run acceptable. I have noticed that if the outside temperature is above 90 degrees, the engine smoothes out quicker – the colder the temperature, the longer it takes for the idle to smooth out. Also, I have noticed that if I put the truck in ‘Drive’ after about 30 seconds of idle, then coast a few hundred feet (and maybe depress the gas pedal a little) the truck will smooth out faster and idle fairly smooth – even when stopped and in gear. When the truck is cold, I have removed the air cleaner and noticed the injectors are dumping a lot of gas down the bore. The spray is in a nice cone pattern – it just seems like a lot of volume – but I don’t think I ever did this when the truck ran great, so I do not have anything to compare it to. The engine seems to never stall and die, just lopes and surges at idle. Also, the CEL does not come on at any point when running.

Again, after a few minutes, the engine smoothes out and it will idle OK, even thought the engine temperature is still cold (according to the temperature gauge). The truck does have poor acceleration and hesitation until the engine warms up – then it gets better when at operating temp, but has not ran strong since last year. Gas mileage has dropped also.

Now the issue when warm; Again, the truck runs and drives ‘acceptable’ when warm. If I shut off the engine once it’s warm, it will start right back up, but if I shut it off and wait longer than 20 minutes or so, the engine will start – but immediately die. When this happens, the only way I can get the truck to stay running is to feather the gas pedal for a minute or so. Even while feathering the gas, the engine bucks and runs very rough – like it’s running very rich. After feathering the gas to keep it running for about a minute, the engine will smooth out and idle on its own. One thing that did happen once that was different; I was driving at 75mph for about 20 miles with a nearly empty tank. When I stopped at a light on an off-ramp, the truck died and was hard to start. Once I got it started, it would drive fine until I came to a stop again, then it would die and hard-to-start. I drove it for about 5 more miles at highway speeds with no hick-up until I came to a complete stop. Once I filled up, that symptom never came back. It has a 34 gallon tank and when the gauge reads empty, it still has 8-10 gallons in it. I have recently ran it dry enough to fill the tank with 28 gallons with none of the stalling symptoms. Before last year, I have ran it so dry that I filled the 34 gal tank with 36 gallons---but never had stalling issues before the current issues arrived.

I have taken the truck to three different repair shops and left it with them for several days each. The only diagnosis they can give me is they think the ECM is having problems trying to run the engine in closed-loop until the O2 sensor heats up enough to start taking over, but they don’t know what is causing it. Since they could not pinpoint an issue, they only suggested to start replacing parts – which is no guarantee to find the problem, and can get expensive.

I have replaced (some temporarily) or checked, the following
Replaced;
Spark plugs and wires
Distributor cap & rotor
Fuel pressure regulator
Air filter
Fuel filter
Ignition coil
ECM Temperature sensor (2X)
O2 sensor
ECM (slaved in a spare from a similar truck for a few days then reinstalled the original)
Ignition modular (slaved in a spare for a few days then reinstalled the original)

I have checked the following;
Removed the distributer and inspected for wear and bad P/U coil
Properly re-set timing
EGR valve – the EGR does not hold vacuume, but I do not think the symptoms are consistent with a bad EGR or vacuum leak.
MAP sensor
TPS
IAC valve – removed and inspected for blockage or fouling
Probably have checked or changed other things that I forgot about
I have not checked the fuel pressure, or the fuel return pipe.

Any suggestions??? I’m at my whit’s end

Tom

oilcanhenry

That is exactly what you should be checking: the fuel pressure. Its not a wise idea to run an in-tank electrical fuel pump near empty. That pump depends on fuel to keep it cool and "lubricated", Buying the cheapest gas you can, is better then running an E-F-P on empty. As to the TBI injectors, it sounds like they are working A-Okay to me.