88 C-1500 Miss-Fire Back-firing on Hot Re-start

chevrolet
1500
misfire

#1

My '88 Chev pick-up normally runs fine, but occasionally, esp after a five minute stop, it acts like the timing is way off, as when I give it any gas, it instantly bogs way down and starts back-firing, climbs hills only in kick-down mode,but sounds almost perfect at cruise throttle position. Exhaust smells (rich?), idle is a bit rough, mileage is down, it can drive like this for an hour hoping there are no hills to climb, until I park it and let it cool for 30 minutes. The hand-held scanner says everything is OK, but does say temp is close to 250 as it sat and got hotter in those 5 minutes. Guessing ECM is adjusting timing for a false positive sensor reading such as temp, O2, knock, etc. No trouble codes, and closed loop. Oxygen sensor does it’s normal high-low thing with the normal rich-lean reading. Wonder if a high resistance ground after heating on the O2 sensor? Does this once or twice a week, summer or winter, humid or dry. Rest of the time it’s tip-top. Did find one of the AC plugs cracked and signs of spark jumping a gap inside the porcelain.New cap, rotor, wires. Inside of cap was not clean. Typically truck sits for three days, then drive.


#2

If your scanner shoes live engine data, you should be able to see sensor readings? Use a gage and check fuel pressure? I thought the gage might also show a pressure bleed-off,but, if you have TBI, there may be an internal pressure bleed-off, normal. You can check for correct pressure.


#3

Yes, scanner data readings all seem similar when it’s running fine and when not, so that’s why I wondered if a sensor could be could be off and then the ECM adjusting something such as timing to bring that sensor to the numbers it would like to see. In 1988 there were only a dozen sensors, so I thought maybe something like a miss would make the O2 sensor tell the ECM we’re too rich, and it leans me down to where there’s no power. Then she cools down and that plug that was missing, is OK again.I’m just guessing here. I’ve got the full set of GM manuals, and tried testing and swapping most of the sensors with my buddy’s truck and can’t seem to reproduce or solve this intermittent problem. Kinda wish it would fail permanently so I could find it. I removed the AC plugs last night and installed Bosch. Seems like idle is a bit smoother. I didn’t have a fuel press. gauge, but did swap out the TBI bleed-off with his to see if it helps, I’ll let you know. Thanks Kill.


#4

I think I figured it out. It was a problem in the distributor cap. The distributor seems a bit loose. Whether it’s a bearing or seal that’s supposed to keep any crankcase vapors from coming up into the cap is letting water vapor into the it. That must be why it was worst when I’d shut it down for a bit, then re-start it. I’m guessing vapors come up even more as the engine gets hotter after shutting off, then they condense inside the cap, especially on cooler days, then though it starts back up fine because the plugs are firing OK, when I try to accelerate and put a load on the engine, it’s harder for them to fire under the increased compression, and the spark would rather go to ground through that moisture in the cap, thus the back-firing of the unburned fuel. Then I’d park and let it cool down for an hour and it would run fine after that moisture evaporated out of the cap sitting on that warm engine. I also noticed a bit of white crap that would build on the electrodes that I could easily scrape off which also seemed to help.

If anyone can concur with my assessment, or has any tips for solving this problem, I’d appreciate it.