renix possesed question

My 89 jeep 4.0 aw4 has had some strange problems, all of which I thought were solved only to come back with a vengeance. I purchased it in December as gift to my wife but drive it much more than her because i had reliability issues and have her drive my 02 ram instead.
To make a long story a little shorter, the 4.0 (renix) was running rich and blowing mufflers off from backfire. I replaced the 02 sensor (precat) and it ran okay for about 2 days. I found the heat sensor (probably wrong term) that is in the manifold literally melted and replaced that. again, was alright for a couple days. i changed the fuel pump and same deal. finally, i checked the tps, which came in at 4.6 volts and adjusted to 82% for the other leg (end, whatever it's called) and it ran better. Actually, it ran really well off and on. I realize it's supposed to be 5 volts, I am hoping financially this is not the problem. Does anyone have any ideas? It's still backfiring intermittently. It will run really well and then putt or backfire. My next guess is the cps, though I am sick of changing parts. It is running rich even when it runs great. What sensor or part would cause this rich running? And if it is running rich is this why I am getting backfire? My third muffler is on now (my first one was put on at a muffler shop and replaced once free, I paid for the third one and put it on myself.)
I read someone Else's post that said they were good with the tps at 4.6 volts. Can anyone confirm this to be true or is this the root of all my problems?
just some technical info: it's a 89 xj laredo with only 96000 on the odometer, auto, air, cruise, 4.0 liter, new fuel pump, screen, fuel filter, precat 02 sensor, manifold temp sensor, thermostat, after cat exhaust, plugs, wires, rotor, cap, air filter, and of course oil change. The cruise control does not work.
The tell tale sign that it is about to buck or skip is the tachometer goes buggy, i.e., it shows zero rpms even thought it's running. When I see a steady tach it runs fine. The speedometer starts acting up also, which lets me know it's about to run bad.

Any clues would be appreciated.

I would guess CPS. I have experienced the sputtering and backfiring on an '88. That was my problem. The TPS Voltage is fine. You'll never have 5 actual volts present.

backfiring usually means MAP sensor, check for proper voltage via backprobing. I dont know the numbers off hand. This would account for it running rich too..

I would have to ask what have you tested? If you just swop/buy parts you may never find the problem.
All the stuff you bought you would have been better off going to the Dealer.
Do you at least have the Renix FSM?
PM me if you don't.

The tach goofing up is a sign that the CPS is going south. Try unplugging it and cleaning the connector first. Could also be a bad ground somewhere. On the cruise control, checkout the switch on the brake pedal as they tend to get gunked up and sticky. I think it's the one with the vacumn line on it.

my vote is either for the ground to the firewall or the map sensor ... when i had the backfire issue and trashed my cat it was the ground from the firewall to the engine.. i love that ground i tell everyone to check it on a renix

langer1 said:

I would have to ask what have you tested? If you just swop/buy parts you may never find the problem.
All the stuff you bought you would have been better off going to the Dealer.
Do you at least have the Renix FSM?
PM me if you don't.

Click to expand...

I had a shop test the fuel pump and replaced it due to low pressure. My local vocational school tested the manifold air temp sensor which was basically fryed. I tested the 02 which was probably way overdue (and also bad). I have a very expensive reliable multi meter so i'm pretty sure my stuff is accurate. I back probed the throttle position sensor and after adjusting the output voltage to 82% the thing runs much better, though it still smells a little rich. My mileage has almost doubled (which isn't saying much) to about 14 mpg all around compared to the 8 mpg I was getting. It likes to be warmed up or driven in warmer weather. I don't have the factory service manual, was trying to find one locally without much success. At any rate, all parts replaced were tested prior to replacement and found bad with the exception of the inline fuel filter which I regret replacing as I got a face full of grime, only to find the receipt from the previous owner who replaced it the month before I did! (this was before my garage found the low pressure and replaced the pump). My big concern was the iffy tach readings and now the speedometer?? Are they related?

thanks for your feed back
[email protected]

The thing is running fine now and have no problems other than the speedo working irradically in the 30 to 35 mph zone. It starts quicker and no hesitations of any type. I disconnected the battery after changing the tps to the proper voltage and let it set overnight. The more I drove it, the better it ran. Did it have to relearn the computer codes or something? I'm guessing after frying 3 mufflers that the cat converter is probably fried as well. If this is the case, anyone have an idea on what is the best and/or cheapest replacement cat? I am not sure of the new york state inspection rules, but i'm pretty sure they are more strict then in days past. One more unrelated question, the best viscosity/brand of oil to use. We are still talking about an 89 4.0 aw4 that is renix, not h.o.
thanks for anyone chiming in

davehoose said:

I had a shop test the fuel pump and replaced it due to low pressure. My local vocational school tested the manifold air temp sensor which was basically fryed. I tested the 02 which was probably way overdue (and also bad). I have a very expensive reliable multi meter so i'm pretty sure my stuff is accurate. I back probed the throttle position sensor and after adjusting the output voltage to 82% the thing runs much better, though it still smells a little rich. My mileage has almost doubled (which isn't saying much) to about 14 mpg all around compared to the 8 mpg I was getting. It likes to be warmed up or driven in warmer weather. I don't have the factory service manual, was trying to find one locally without much success. At any rate, all parts replaced were tested prior to replacement and found bad with the exception of the inline fuel filter which I regret replacing as I got a face full of grime, only to find the receipt from the previous owner who replaced it the month before I did! (this was before my garage found the low pressure and replaced the pump). My big concern was the iffy tach readings and now the speedometer?? Are they related?

thanks for your feed back
[email protected]

Click to expand...

Dave, hope you've got it fixed, but if the troubles return, I'd test the Ignition Control Module (ICM). The tach gets it's signal from it, and a bad ICM would certainly account for the backfiring, etc.

You can use your favorite brand of 10W30; I used Castrol GTX in my 89 with 3K oil changes. As I do with my 'new' 96.

Ground to the firewall! It took me forever to take off the engine side of this ground, but I sanded both ends off and bam! The thing runs like brand new!! I wonder if maybe i spent a few hundred dollars in vain (hopefully not). Thanks
wilcharl for your help! I have had no backfire, hard starts or stalling since I cleaned that ground strap.

wilcharl said:

my vote is either for the ground to the firewall or the map sensor ... when i had the backfire issue and trashed my cat it was the ground from the firewall to the engine.. i love that ground i tell everyone to check it on a renix

Click to expand...