'96 GMC Sierra almost stalls when gas pedal pressed

lrbarrios · February 15, 2016

I've been having some weird problems with my truck lately. Recently I started getting the P1870 code indicating a slipping transmission component. I had noticed some issues with the transmission, but mostly in reverse. Not the hard shifts between 1st and 2nd that most people experience. At any rate, I've purchased the Transgo 4L60E Shift Kit. It's on its way. While I'm waiting for the kit to arrive, I have started experiencing other strange issues. I'm getting many different codes, but not all at the same time. I've seen P1345 (Camshaft to Crankshaft Position Correlation), P0420 (Catalyst System Efficiency Below Threshold), P1351 (ICM Circuit High Input Condition), P0118 (Engine Coolant Temperature Circuit High Input), P0131 (O2 Sensor Circuit Low Voltage, Bank 1 Sensor 1). A little background... Back in the summer, I had to replace the timing chain because a new starter caused the old one (which was very loose) to skip a tooth or two. During the diagnostic process to determine that the timing chain was the reason it wouldn't start, the distributor was removed. When my son put it back in, he must've put it in a tooth off. Since then I've been driving around with a P1345. I recently had that fixed at a local shop (after I had tried several times to correct it myself -- I was happy to see that he was having the same issues that I did. $50 well spent). Just before I took the truck into the shop, the P1870 started showing up. I would clear it and it would come right back within a few minutes of driving. Fortunately I was able to get the truck to pass inspection anyway. Even though the timing was fixed (I saw it on the screen myself. It was dialed in to 0 degrees.) I haven't noticed any kind of performance (or gas mileage increase). During the whole time that the timing was off, I was getting about 5 mpg less and it would struggle to get up to speed. I would eventually get up to highway speed (65mph) and it would cruise along just fine. Starts from a stop seemed to be an issue. After the timing was fixed, I still have the same problem. Maybe the catalytic converter is a bit clogged (I had been driving for almost a year with a rich condition because the fuel injection spider was leaking -- I eventually fixed it, but shortly afterwards, I would occasionally get the P0420 code.) Another issue was the coolant temperature sensor. I had replaced it, but when I crimped on the new pigtail, I think I may have squeezed too hard and messed up one of the splice connections. Every once in a while, when I'm driving, the truck would suddenly act like it was about to stall and then it would return to normal. Then it would do it again. I cranked up my Torque app on my Android phone and saw that the coolant temperature was -40F! I wiggled the pigtail wire and everything would go back to normal until it decided to do it again.

The other night, while I was driving home from a late night job, the truck started doing something strange. I was driving down the highway at 65mph when, all of a sudden, there was a lack of power. The engine was running at 1000rpm but the speedometer kept going down. When I would press the gas pedal I would hear some popping sounds (kinda like a backfire). I was able to coast into the parking lot of the nearby mall. What I noticed was that IF I didn't press the gas pedal, the engine would idle at 1000rpm in drive and would actually move the truck through the parking lot at 20mph. Once I stopped it and put it in park, and then barely applied the gas pedal, the engine would threaten to stall. I was getting all of the codes listed above. In the Torque app, I was seeing that the O2 sensors were zeroed out. They weren't showing anything. Eventually, the O2 sensors would start reporting voltages again and when I pressed the gas pedal, the engine would usually rev up as expected. Sometimes it would struggle, but eventually it would do it. I turned the engine off (so I could do some online researching). After about half an hour, I tried to start it again, but it wouldn't start. It would turn over just fine, and I could tell that it was occasionally sparking, but it just wouldn't start. I couldn't hear if the fuel pump was pressurizing because of the nearby highway traffic noise. Eventually I got it to start by standing on the gas pedal. Once it started, it did the same thing as before -- it would threaten to stall when the gas pedal was pressed.

Just so you know... The O2 sensor just before the cat was replaced a couple years. I've replaced the coolant temperature sensor recently (I replaced the sensor, but not the pigtail). About a month ago, I replace the Ignition Control Module because the truck absolutely not start. It would turn over and barely spark. After quite a bit of troubleshooting with a multi-meter, I determined that the problem was the ICM. I replaced it and the truck started right up. Sooooo... Why is the truck all of a sudden throwing codes, at the same time, for components that I've replaced within the past couple of years? It doesn't make any sense. The mechanic has mentioned that he *thought* that *maybe* the passenger side O2 sensor might be going out because of what he saw on his scanner. But still, why would all the other codes be showing up? I've got an idea, I would just like somebody to confirm it for me. The truck is in the mall parking lot about 25 miles from me. Tomorrow I'm going to have it towed to a shop for them to diagnose and repair the situation. I need to get this thing running soon because it's my only mode of transportation. Any ideas? Thanks in advance for your input.

lrbarrios