giarc

here's the problem: truck stumbles, then dies, or backfires. plugs, wires, cap, coil,no change. map sensor, no change. cleaned throttle body, O2 connections, no change..checked voltage at crank sensor, 5 volts,,,checked it at cam sensor, 5 volts. now, after every one of those "fixes" it was ok, for maybe 24 hours, but then started doing it again. have looked at a few posts on here, but am very confused here. any help is appreciated, thanks in advance!

jnicewan

Backfiring out of the tail pipe, or through the throttle body and any codes present?

If through the throttle body you might suspect the fuel system for a lean condition. You might want to test fuel pressure and see if it is in spec. Also check for vacuum leaks, vac lines and plenum gaskets. At the very least you might want to replace the fuel filter and maybe try a bottle of a higher end injector cleaner in the fuel tank.

giarc

here is more info...she starts right up,first time, every time. once it gets goin, say maybe 3-5 minutes, it will "buck" the rpms drop, sometimes it comes right back, on down the road no problems. other times, it basically die out, and backfire through the exhaust.
plugs, wires, coil, new muffler(the old one blew up)checked and cleaned most all connectors i could get at. cleaned out the throttle body. stp injector cleaner, inside a tank of high octane. was ok for, maybe a day or so. then right back. dash light came on, pulled code for crank sensor. done. also code for map sensor. done. again slight reprieve, then right back.
so, my thoughts are to just go ahead and do the cam sensor, and maybe reset the pcm? this gremlin is gettin bothersome...

by the way, i like this site, have gotten great info for my past jeeps, always found a fix through searching, but this one has got me stumped!

celicoupe

Take your crank position sensor out and clean it. Or replace it.

jnicewan

So do you have no codes now, after replacing those sensors? Does it smoke or anything during the problem? did you check the vacuum line for the MAP, and check that the MAP is getting good vacuum when the engine is running?

I know it may seem primative but sometimes sensors can fail "within-range" and not set a DTC. You might try to unplug various sensors one at a time and see if your results change (i.e. MAP, IAT, ECT, O2). Keep in mind you may make it run different and may still run a little funny but you are just trying to maybe see if you can eliminate the concern you have with the backfiring/stumble. There are a few sensors that simply cannot be unplugged but that is kind of obvious, and you will need to unplug battery or reset PCM aftward too.

Thinking about it now, it seems like mine did something simular a long time ago and it was a TP sensor, granted it did it all the time and instantly after the engine started.
With yours happening 3-5mins into run time tells me it may either be heat related, or a fault is happening in "closed-loop", O2 sensor starts reading in closed loop and the PCM starts compensating fuel trims in closed loop too. Sensors like O2, IAT, ECT have a little more bearing to a closed loop condition. This could still be something simple like a distributor cap/rotor too?

98xjclassic350

Having the same problem! Did ya figure anything out?

SlimBillyJimBob

saw a post on another forum that these people had this problem and the ECU was shorting out to the firewall loosen two mounting screws on bottom out 1/4 turn and it stopped ? CNET car tech forums is where I found that info , that was before I found this forum of course ......

CrazyMacaw

The "screw" fix for the PCM is semi bs. The screws hold one of the two PC boards inside the PCM in place against the cover and can not short out anything inside. The only reason it may give a temporary fix that could last a day or a year is because in loosening them you allow the PC board to flex a bit and re-seat a typically faulty solder connection. The other way it has the potential to fix the issue is if it's actually the connectors to the PCM that are at fault, in removing and re plugging them to get to the screws you could be re-seating a faulty/corroded pin like the coil driver output.

A simple test is to wiggle the wires/connectors at the PCM while the engine is running and see if it causes a stumble or stall. Also try pushing on the face of the PCM with your fingers or tapping it on the side with a block of wood or screwdriver handle looking for a stumble or stall.

MadderThanMax

I noticed that you've change the cap - have you checked the cap for hairline cracks at all?? As the cap heats up the problem would appear. Had this on another vehicle many years ago. The back firing suggests its a timing issue or a fault on the HT side of the ignition (High Tension) hopefully this may give you some clues where to look...

sportrider

my guess is the crank position sensor, my ZJ did the same thing. no codes, sometimes it would fire off, sometimes not. it would pop & miss like crazy but never throw a DTC. replaced the crank sensor and it's been fine since.

giarc

found the solution, got myself a very large zip tie, and cinched it up around the plugs on the pcm. no more troubles, runs like a champ....go figure....

dave1123

Do it right! Pull the plugs and clean the pins! Why wait for future problems?

fisherman797

Just some info. If you pull the old crank sensor out make sure to air gap it. The new ones come with a piece of felt on the end that shears off when engine is started up.