SatiricalHen

This isn't a Cherokee I know, but this part of the forum gets more traffic than the Comanche part. If someone feels it needs moved please do so. This is a 1987 Jeep Comanche with 130,000ish miles. It has a 4.0 with an in-line 6 and manual transmission.
Things done to it and info that may be helpful:
-I've refreshed grounds
-deleted the c101
-open cooling system
-runs at the correct temperature
-no cat or muffler because they blew up from the backfiring
-replaced the fuel filter
-fuel pressures test about 30 at idle and high 30s wide open
-done cap, rotor, wires and plugs
-tried a new tps with no change
-cleaned iac and throttle body
-runs rough, stumbles and backfires under slight load. When trying to cruise or slight acceleration. If I let off it'll stop backfiring, if I hammer down it'll stop stumbling. Seems to be around 2000 rpms.
-driven it about 1300 miles

That's all I can think of, probably other things I'm forgetting. I plan to try indexing the distributor when a friend brings an old cap back with him that I can cut up.

Oswalla

How old is the o2 sensor, might look into testing it. Thinking along the lines of running rich to backfire out exhaust, lean usually backfire out carb/tbody. Another sensor is the coolant sensor on lower block, CTS. Basically any sensor that effects the computers decision to send extra fuel. Also the IAC could be bad even though you cleaned it, I didn't see where you said idle condition ?


I should add to check those intake/exhaust bolts, a leak there can give the o2 sensor fits.

SatiricalHen

Idle is good, was low before cleaning iac, but at 750 rpms. Pulled the plug on the coolant temp sensor with no change at all. Not sure how old the o2 sensor is since I just recently bought the jeep. Sealed a leak on the exhaust manifold with no change at all.

Oswalla

Wonder if an injector stuck open would fit your symptoms ? Never had one stuck open to know myself. Maybe not if your pressure holds up with engine off.

SatiricalHen

I'm not sure how long or if the pressure holds after the engine is off.

Oswalla

Probably kind of hard to see how the exhaust looks with most of it gone. But if its that rich to blow cat and muffler, it should be noticeable at tail pipe that lots of unburned fuel is getting out the back.


Story about my 1 and only blown muffler. Back in the day I put plug wires on wrong doing a cap/rotor, cranked for a while wouldnt start. My dad looks at it says I put wires on wrong firing order. Switched them around and cranked, BOOM, huge backfire. Shredded that muffler and gave my dad a good chuckle. He let me do that, you know teaching me the hard way.

freegdr

Check for vacuum leaks from map sensor to throttle body yet ?

cruiser54

Drive it while observing the fuel pressure gauge.

SatiricalHen

If that's the little tube on the side of the throttle body right? If so it's good.

SatiricalHen

I'm not quite sure how to do that since it's a rental and it screws onto the fuel rail.

freegdr

Was vacuum present when running ?

SatiricalHen

I'm assuming there is vacuum since I have functioning brakes, and when the vacuum ball is disconnected I can hear the vacuum

freegdr

At the line to map to be more specific.

cruiser54

Contrary to popular belief backfiring is generally caused by a lean mixture as in a fuel delivery issue.

Oswalla

I would have to agree, it is more common than a rich condition. Wonder how his temps are ? Lean can heat up the motor.

Satirical Hen, hows that exhaust look I mentioned earlier ? Rich condition will generally be excess black smoke when revved. Another rich sign is easy to check those plugs, if they look good and dry follow the lean condition. Get an extension hose for that pressure tester and run it up to windshield and drive it like Cruiser suggested.