Troubles with my 92 GMC 4.3 again


1 sec reaction King
May 24, 2001
92 GMC Sonoma 4.3 tbi

Ok guys, some may remember a few months back me doing several things to get it to run right, ended up being a fairly new fuel pump had gone bad. Here I am several months later and it acted up again. Changed out the fuel pump and filter. I cleaned out the throttle body and put a new gasket kit on the TBI. When trying to start it, it will turn over a few times then lock up like it's wanting to backfire, then start turning over again. After 15 seconds on the starter it will slowly start firing up and then run. I can run it up and down the street, but if it is put under too much load it will stumble like it's not getting enough fuel. The new symptom is the backfiring and stopping the cranking while trying to start. Any ideas? I put new plugs and wires on it thinking that I wasn't getting enough spark. It has a new coil, and ign module, no cat, clean egr, new temp sensor, and a few things I'm probably forgetting. Truck has just over 70,000 miles on it.

What did the wiring to the fuel pump look like? I know until I changed the pigtail to the top of the tank on our suburban ours acted up. Is the tank getting good voltage? Does it have a good ground?
Thanks for the input guys. I've been racking my brain with this one. Everything seems to be in good working order, so I said to myself, what has changed? I don't know if I am right, but this is the path I'm going to attempt today. The problem with this truck is that it was a salvage title in 1993 (it was less than a year old when totaled). Basically the bed was sheared off and a new one was WELDED on! So, when it was handed down to me with fuel issues, I cut a hole in the bed to access the tank, and I think therein lies the problem. I'm think I'm getting water in the tank.

So, with that said. What is the best course of action without dropping the tank? I will if I have to, but I have about 8 gallons of gas in it right now. If my assumption is correct, I have enough gas to water to run, but it runs like crap. I can try to syphon as much water off the bottom of the tank and add HEET type products. Is there better methods or products? Should I just try isopropyl alcohol, and how much?
Do you have a pressure port on the fuel rail? If so, remove the schrader valve and clamp a piece of hose on there into a gas can while the truck is idling. The pump should pump out fuel until it runs out and the truck stops. Then go from there.

How is Lauderdale? I haven't been back since 2004. I think we met at Tower Shops once?
Hi John,
I saw your reply and thought, "that name sounds familiar!" I'm pretty sure we did meet at towers. I'm not in Lauderdale anymore. I moved to Fort Wayne IN about 3 years ago. I miss it and will probably move back in the near future.

I don't think there is a pressure port. I'll double check. The reason I'm pretty sure there isn't is because the only way to test pressure is to unhook the fuel filter and put a gauge there.

I have one of these for sucking fuel out of tanks. Oil it after use as the gas dries out the seals. I just siphon it into a five gallon jug or three depending in how much is in the tank. Much easier to drop when empty.
Water in the gas shouldn't cause locking up while cranking, I don't think so anyway. I say this because it would be metered just as though it were gasoline. That being said it will sound strange obviously the water will cause no start or poor starting and running.
My test for you is this. Jack up the truck in a way that the heavier water will run to a corner of the fuel tank. Carefully observe how it starts and runs like this. Water is heavier and will barely mix with gas.
As far as locking up try pulling the plugs and listen to it crank. Should be nice and smooth. Could be a bad starter I've heard them get a flat spot.
No disintegrations, I think you may be right, I don't think its the water causing the odd starting issue. The truck finally threw a code. The code was a 45 o2 rich. Correct me if I am wrong, but if it is reading rich, it will pull fuel and cause it to run like a bad fuel pump. I wouldn't think this would cause the odd starting issue would it? I would think the o2 sensor is out of the equation while starting. I've pulled the injector wires off and it turns over just fine. It is when the injectors are firing it is like it is 180* out of timing and firing at the wrong time, which stops the engine from rotating. After 10 seconds of cranking it will finally and slowly start up. If the engine is cold, it fires on the first crank with no hesitation. I hate throwing $50 parts at this thing and getting nowhere. Although the 02 sensor could be the original one.
Just after you crank it and it tries to kickback, stop and pull the plugs. See if you have any fuel soaked plugs. That might give you a clue.
A few of them were wet when I changed them out yesterday. That's what is confusing me and let me to think of bad spark. Dumping fuel, bad spark, finally sparking with a lot of fuel, but if the 02 sensor is showing rich it should be pulling fuel. I wouldn't think that is an issue at starting since it should be in open loop. I'm an amazon prime member and just found the exact same bosch 02 sensor that is on Autozone for $50 for $14 plus $.38 shipping on Amazon. Guess I'll order one. Its one of the few things I haven't replaced on this truck.
I think I got it. Just went and scraped all the skin off of my arm to unplug the 02 sensor. Who decided to put the plug up behind the trans!?!? Engine was cold and fired right up, which it normally does when cold. It instantly started flashing the CE light. I didn't think it would do that until out of open loop from my research. I took it for a drive and it ran fine. It's probably running rich, so I didn't beat on it, but did put it under load at low speeds and at highway speeds. No sputter, backfire or lack of power was present. Drove for about 10 minutes to get it warmed up, pulled in the driveway, shut it off, let it set for a few seconds and it fired right back up on the first crank. I let it set for longer and it fired right up again. I'm not sure if this is the mysterious obd 1.5 or something, because it goes into closed loop instantly if the truck has been warmed up before. I don't know if the computer was pulling fuel when i was trying to start it, and it was already warm. That seemed the case, very little fuel injected while cranking, super lean mixture causing it to backfire. I've been fighting this for a week, and it wasn't until today that it threw a code. 99.99% sure its not a mechanical issue, but a bad 02 causing all the problems. Oh well, it has a new fuel pump and filter, tbi is clean with new gaskets, new plugs and wires. It should get some good gas mileage once the new 02 sensor gets here. Hopefully I can get the 02 out without too much of a hassle.

Thanks for all the input guys!
Someone mentioned the spider under the intake. I am thinking they are right.
Yeah, I've heard nightmares about those, and it's an expensive part. I think those are '94 or '95 and up for the v6 and '96 for the v8. My '92 is just a simple spring FPR and 2 injectors shooting into a hole for each bank. Basically a carb with 2 injectors instead of floats and bowls. Much more basic and easy to work on. I can have my TBI off apart and back on in 15 minutes.
FIXED! The 02 sensor issue wasn't it. I had changed everything on the fuel side and most everything on the spark side, and was still having troubles. Come to find out the new ICM was bad, but that only partially fixed it. I went to a local garage, spoke to the owner and he rattled off several things with my response as "already replaced that". He was then fairly convinced that it was the distributor. Ran to autozone and bought a new one with cap, rotor, ICM included for $110. I marked where the rotor was pointed and the orientation of the distributor, took the old one out, and put in the new one. There was the same amount of play on the old one as the new one. The old one showed no wear on the gear and looked mechanically fine other than being old. Tightened everything down, plugged it all in, and now it runs perfectly.
Ok, rather than start a new thread and try to explain everything that has been done to the truck, this thread has it all, but I have a new problem. Was merging on the highway, semi beside me so I went to accelerate, the truck downshifted and backfired. Let of the gas and it ran fine. A few days later I'm cruising along about 60, the ac clutch kicks on and it backfired. Had been driving for 10 minutes with the ac on. Turned the ac off and it ran fine. Got home, turned the truck off and an hour later it wouldn't start. Injectors were not spraying. The next morning I went to check it out, and it was running fine. Later that day it wouldn't start and hasn't started for a few days. Injectors aren't spraying. I took the TBI apart to see if there was a blockage, and its fine. It will fire with gas poured into the bowls. It has a new ICM and I swapped out a known good ICM with now change. I bought a new ECM, swapped out the chip, no change. Some sensor or something is not letting the injectors fire. I've been on several boards trying to figure this out with no luck. Any help appreciated.