Cherokee_of_Tennessee

Ok, I searched all the posts related to hesitation, backfire, etc. Nothing seems to match my situation, or if it does, nobody posted the final fix. A lot of the threads would suggest things to try, but the OP did not come back and say yes or no if that fixed it.(very annoying by the way)


So here is the story: 98 XJ Cherokee sport, 4.0 inline six, automatic transmission, 166,000 miles. Original engine, original transmission.


Symptoms: Rough idle at 600 to 1000 rpms. Intermittent hesitation and almost stall and backfire sometimes when cold, and after heat soak. Sometimes when I start it up cold, it will run rough and hesitate when I give it gas. If I let up on the gas, it backfires(loud thump under my feet) and then takes off if I gradually give it gas. If I just stomp the gas it will hesitate and almost stall out. Sometimes this will happen after I have been driving for about 10 min and stop at a redlight or stop sign. But, it will do it at least 80% of the time after I drive for a while, stop at a store, go shopping for 30 to 60 min, then come out and start back up. It will hesitate, back fire, then take off and its fine. About 5 days ago the check engine light came on and the code was "cylinder 1 miss-fire" according to advance auto's code checker. The code stayed on until this morning when the check engine light just went off. I don't know why it went off, I didn't do anything to make it go off.


What I have done: I replaced the fuel pump 3 years ago. Replaced IAC with original mopar IAC about 4 months ago in an attempt to smooth out the idle, and fix the "Idle air control system" engine code. It didn't make much difference in the idle, but it did get rid of the code. About 3 weeks ago the engine completely stalled and died. I knew the catalytic converter was going bad because it sounded like it had a cup full of bb's rattling around in it. Went to midas and they showed me that the exhaust temp upstream of the converter was about 250 and the exhaust temp downstream was 580+. They said the temp should not be more than 50 degrees or so different. So since the muffler was blown away, I had new exhaust installed with new cat. They did not replace the O2 sensors. I got a very noticeable increase in power, but still had the intermittent hesitation, rough idle, and back firing. This evening I replaced the spark plugs with NGK's and wires with BWD wires. I inspected all the plugs, but none of them looked really bad. I paid close attention to cylinder 1 spark plug, but it looked the same as all the rest. After replacing the plugs and wires, I did notice a slightly smoother idle. But, I test drove to the store. Went in for about 15-30 min, came out, and the friggin thing hesitated again.


My gut feeling: So I am thinking O2 sensors or throttle position sensor. The O2 sensors are not throwing any codes. Could they still be bad anyway? Is there a way to test the throttle position sensor to be sure it is faulty?


Many thanks in advance and I will definitely come back and post what fixed the problem so future posters may benefit from my experience.

bender32

I would replace the tps!

Cherokee_of_Tennessee

Do you know where I can get an original mopar TPS? I looked on wholesalemopar but they were discontinued.??

Cherokee_of_Tennessee

Tonight I got my multimeter out, and my digital multimeter also, and tested the voltages on my TPS. I pinned the wires right behind the connector to make sure I was getting good readings. At idle I get 0.83 volts. At WOT I get 3.72 volts. I have 5.2 volts on the supply side and really good ground path to the battery on the ground side. Does anybody know if these are good voltages, or if my TPS is out of tolerance and needs to be replaced? Or what voltages should I see?

Cherokee_of_Tennessee

Well, day before yesterday the TPS threw a code. TPS INPUT TOO HIGH. So I went to Oreilly's and got a BWD tps. Before everyone jumps me for not buying a mopar, I had to leave within 4 hours on a 300mile trip and I didn't have time to wait for amazon to ship me a mopar tps, and hope they sent me the correct one. The BWD has a lifetime warranty so if it craps out I will get my money back after I order and install a mopar. So I put it in and fired up the engine to see if it made a difference. My engine immediately sped up to 3000 rpms and stayed there. Also set another engine code. Code was "transmission control module failure". Thinking I had bought a dead on arrival tps, I was about to throw my hammer threw the windshield in frustration. But, as I went back to the engine to pull out the tps, I realized I had forgotton to reinstall the 3/8" vacuum lines back on the intake manifold. I had taken them loose to make it easier to install the tps. One goes over to the brake booster, and the other one goes to the top of the valve cover. So I plugged them back up and all was well... sort of. I get much better throttle response, but still have the hesitation issue. It only happens when the engine is dead cold, or has run long enough to get pretty warm. Then I shut it off, wait about 30 min, start up, and it will hesitate when I pull out. I reset the transmission fault by the way. I think it was just due to the fact that I had the vacuum lines disconnected when I started it up. So, anybody have any ideas what the hell this stupid hesitation could be caused by????

Turbo X_J

Sounds more like a wiring issue than a component. Have you cleaned all the under hood ground connections?

Cherokee_of_Tennessee

I have not cleaned the grounds, however, today I was driving and it really started going to hell in a hand basket. At about 75mph, I noticed it would get sluggish if I gave it more gas. Once I stopped, if i give it full pedal, it sputters, hesitates, backfires like crazy in the manifold, and dies. I also smell gas in the engine when it does this. As long as I let up off the gas, it just purrs. Is there an airflow sensor that could be giving me issues that anyone knows of?

Turbo Supra

The high voltage detection (P0123) can also be the clockspring. You can remove the steering wheel bezel and unplug the two plugs and do an ECU reset. If this resolves your issue, you are in the same boat as me. Bad clockspring.

Turbo Supra

I can't say this is the definite solution to your problems, but eliminating the possibility takes all of 10 minutes.

https://www.cherokeeforum.com/f2/tps...2/#post2985806

Is the code back? When was the last tune up? Maybe an ECU reset is all she needs.

Cherokee_of_Tennessee

I actually repaired the clockspring about a year ago due to cruise control failure. Has worked fine ever since.

Cherokee_of_Tennessee

So, yesterday on the way home from work, the jeep just flat out quit. Stalled out while waiting at a red light. Some helpful citizens pushed me out of the intersection. They offered me a ride and I had them take me to orielly's where I bought both the upstream and downstream O2 sensors. Before I put them in, I fired the jeep back up, and it started just fine. I shut it off and replaced the sensors anyway. Started back up, got about 1.5 miles down the road, and it died again. I called a tow truck. After waiting 30min on the tow truck, I started it up, and drove another 2 miles, then died again. This time I stayed put and waited on the tow. Towed me to midas, since they were the last one to work on my jeep. They called me today and said the ECU is bad. Said it is fine until the engine heats up, and then it starts freaking out and throwing several codes. I hope this fixes it because the only one they can find is from the dealer.


So, If anyone is starting to have a hesitation and back firing issue, get your ecu checked out immediately before you get stranded like I did. I will post back in about a week to let everyone know if this actually fixed the issue when the ECU comes in and the repair is complete.

SomeGuy138

I could be wrong but I bet its not the ECU,Them things rarely go bad but I guess you will find out.

Had a friend have this happen and it was the Crank Sensor.

Cherokee_of_Tennessee

I told the mechanic to specifically check the crank sensor, he said that was his first suspicion, but it checked ok. Then he moved on to the ecu. Either way, if it is not the ECU, then I get my money back and they fix it for free.

Cherokee_of_Tennessee

FIXED!!

I have been driving on a new ECU now for about a week and all is well. This is a remanufactured ECU installed and flashed by the dealer. Still runs a little rough at idle, but she has idled a little rough ever since i have owned her. Maybe my eninge mounts are worn out and I am just feeling vibrations??? But she runs like a scalded dog now with no hesitation, no backfire, and no extraneous engine codes popping up. I am pleased except for the $700 repair bill. But hey, its under warranty for a year and if I did it myself and screwed it up, I would not have anybody to yell at.

Cherokee_of_Tennessee

Well, Ran great for a week. Then yesterday I started the jeep up from cold, and it died as soon as I put it in gear. Cranked it up again, pulled to a stop sign, died again. Drove to the auto shop that changed my ECU, back firing and running rough the whole way. Car shop mechanic was stumped. Crank sensor checked fine, but he went ahead and changed it anyway. Then he test drove it and got stranded when it died on him too with a new crank sensor. He finally got it re-started, drove it back to the shop and we could not make it die again. The only thing we could think of was bad gas. I have been getting ethanol free gas from a Gas Plus in North Augusta, South Carolina. The mechanic said that gas station is notorious for bad gas. So they put some gumout and octane boost in the tank and I drove it for 4 hours and ran out the "bad gas". Put new gas in from a Shell station. So far so good. They said the ECU was definitely bad because when they tried to re-flash it, it would not take the new flash program. So I didn't pay for the crank sensor since it did not fix the problem. We will see what happens from here. Starting to get a little frustrated.....

By the way, this is everything either I or the car shop has done so far to fix this issue:
Replaced ECU
Replaced TPS
Replaced champion plugs with new NGK's gapped to .035
Replaced plug wires with standard wires
Replaced IAC valve
Replaced fuel injector on cylinder 5
Replaced both O2 sensors
Replaced catalytic converter and muffler

Checked distributor cap and rotor
checked ignition coil
Checked for vacuum leaks
checked and cleaned all grounds
Checked head gasket
Checked and cleaned throttle body
Checked air filter for clogging


Can anybody suggest something I might have missed?