Back Fires, Runs Rough In The Rain, 89 4.0 Renix

[jiffyjet2]

The first time this happened was after going through a car wash. It suddenly lost power, although still running, and was backfiring through the carb, and on deceleration through the exhaust. I babied it along, keeping revs up and it got over it and I drove home, although still idleing rough and backfiring through exhaust. The second time was after sitting out in a long week of rain. It started out ok, then started "bucking" at low speeds then went into the back firing. I stopped look under the hood, which looked dry enough. I considered AAA and having it hauled home, but again, babied it along, it seemed to get over it and got home. Over night and it ran fine, but I took it to a Jeep dealer to check it out. They put a new EGR valve in it, a new O2 sensor, and rotor and cap. Yesterday, it rained again (I'm in Seattle). It started out with the bucking, then went into the same backfiring, snorting and generally running like crap. I got it home, put it in the garage and today it ran fine going back to the Jeep shop. The only connection I can figure out, is there is a leak of some sort that allows water into the cab on the passenger side. The carpet is soaked. It's dry on the Renix side, but is there something behind the glove box that is a critical part of the ignition system that could be getting wet? There's also the thing on the left fender edge that could be getting wet. Any ideas? I don't have a lot of faith that the mechanics at the dealer can find and fix an older Jeep that you can't plug into a computer for diagnosis.

Thanks for any advise.

Jess

'89 Comanche - 4.0 with manual transmission, 145,000 miles

[cruiser54]

No plug wires replaced?

 

How are the connections at the fuel pump ballast resistor on the driver's side inner fender?

 

Just for good measure, I would remove the left taillamp and refresh the ground behind it. That's for your fuel pump.

 

Actually, it would be a good idea to do the Ground Rfreshing and Connector Refreshing in the link in my signature.

[mvusse]

The 4.0 can't backfire through the carb because it doesn't have one. The throttle body perhaps?

 

The only times I've had these issues (2 different vehicles, neither was a Jeep) were a cracked distributor cap. Since yours has been replaced I would suspect the plug wires. Couldn't hurt to replace the plugs while you're at it. Use just regular good old fashioned copper core ones like the Champion truck ones. The 4.0 doesn't seem to like all the newfangled iridium, gold, platinum, or unobtanium ones.

[ftpiercecracker1]

Your problem sounds nearly identical to mine, "Backfiring and rough idle"

 

Physically put your hands on the dipstick tube and run your hand down its lenth, about half way down a support leg will be attatched to it. This leg is bolted to the block a few inches back via a stud in the block. On this stud there will be a nut and several wires. It is absolutely critical that these wires are super clean and have super good contact. They ground a multitude of crucial engine control systems.

[airspeed]

I agree, change the plug wires. You can take a spray bottle of water and with the engine running spray the plug wires and see what happens.

[jiffyjet2]

I'm quite aware that there is no "carb" on an MPI system, but it's a lot easier to type "carb" than "throttle body". Everyone seems to have understood what I was getting at.

In any case, the Jeep dealer was unable to duplicate the problem, drove it twice and called me to tell me that it was running fine. (It wasn't raining or wet out.) I asked him if they had a car wash at the dealer, he said yes, so I said run it through the car wash. He did and it finally went into the crap mode. The end result is they found the MAP sensor did strange things when it got damp. They replaced it and it really runs great now.

We'll see.

Thanks for all the advise.

Jess