96 1500 4.3 misfire

[danimal141]

I need help with my 96 k1500 4.3 5 speed. Im getting a misfire around 2500 rpm and this appears to have caused the cat to clog. I had it in the garage to replace the cat and they scanned the computer and found a code for random cylinder misfire. so far ive changed the cap, rotor, plugs, wires, fuel pump & fuel filter. I also borrowed the coil and ignition module from my wifes suburban to try. I also checked the tps with an ohm meter.I checked for vacuum leaks with a propane torch around the intake manifold.So far i cant track down this problem and i havent been driving this truck becouse im afraid to clog the new cat.

[manicmechanix]

What's the appllication have, CPI sequential injection? Dirty or bad injectors or maybe a problem in the fuel pressure like the the regulator can cause misfiring and even damage the cat. You would want to check fuel pressure, and with a voltmeter for proper injector resistance and run some good injector/fuel system cleaner like Techron through the tank. To really diagnosis the injector you would probably need to run a cylinder drop test and injector coil test, but that requires the equipment or taking it to a good shop.

Usually if a sensor's value like the MAP, TPS, CTS etc. was off it would set a code, but a fuel pressure or injector problem will just give a misfire code. It's funny you say it only runs bad at 2500rpms. That might indicate you're getting too much EGR or maybe a problem in the EVAP but I really doubt it. That'd probably set a code too. I also doubt your pick up coil/crank sensor would act up just at certain RPMs. You switched out all the other ignition parts to no avail. I think the fuel injection delivery is the most likely culprit.

[Jeremy-WI]

Did you happen to get a distributor cap with brass contacts? The ones with brass contacts seem to help with some misfire issues with the 4.3

A fuel pressure test may show a problem

[rhandwor]

I have found them with an after market Egr valve on them. These come with washers to regulate flow. A lot of people don't read the box and throw them away. They are designed to fit numerous vehicles and have a sheet telling what size washer to use. If your not looking for it its very hard to find.
Otherwise follow manic mechanics instructions.

[danimal141]

i finally got it running good again. it ended up being a bad ground on the ecm harness.thanks to all who replied, the ground wires were going to one of the bolts on the thermostat housing.while checking the egr valve i disturbed the harness and one of the 2 ground wires pulled out of the connector.i soldered on a new connector and truck runs great.

[manicmechanix]

Originally Posted by danimal141

i finally got it running good again. it ended up being a bad ground on the ecm harness.thanks to all who replied, the ground wires were going to one of the bolts on the thermostat housing.while checking the egr valve i disturbed the harness and one of the 2 ground wires pulled out of the connector.i soldered on a new connector and truck runs great.

Good to know. The dreaded hard to notice ground problem. It's always advisable to check connections and grounds when you have a drivability problem, but if someone were to say check your ground connections it sounds like a cop out answer lol.