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Hesitation and backfire at cruising speed

Stone4140

NAXJA Forum User
Location
Caledonia,NY
Ive got an 88 Renix XJ that I’ve done quit a bit to as far as maintenance over the past few months. All of the priority steps Cruiser recommends. New wires, cap, rotor, plugs, coil, ICU, NKT O2 sensor among other stuff. I also have Nicks Renix Engine monitor which is great. However I can’t seem to isolate this hesitation problem. It’s right at one spot when I’m cruising in 3rd and 4th gear typically. If I hold it there without shifting or +\- the accelerator it can get pretty severe and start backfiring. Every time I think I fix it it seems to come back. My last attempt was “new” OEM Bosch 4 hole injectors. Those certainly help with the long start I had, but did not assist with this issue. My sensors all seem to be working and are in spec according to the Renix Monitor. My next steps are to try new TPS and MaP sensors. I also tested fuel pressure and that was in spec. Anything glaring I should be looking at that Ive missed?
Thanks
 
Hesitation can be caused by a lean or rich fuel supply. So I'd definitely look at the TPS,MAP, and O2 sensors. Though you've replaced the coil, the new coil could be breaking down under load and not supplying a high enough spark voltage. Or bad plugs, bad ground, bad plug wires or a weak battery/alternator could be limiting the spark plug voltage at cruise. Try running a #2 welding cable directly from the negative battery terminal to the engine block ground and see if that helps.

Best regards,

CJR
 
Pretty common to get a lean burn near peak torque and have your engine buck, almost feels like a torque converter issue. Usually an O2 sensor issue, either the O2 sensor or the O2 sensor wiring. But various things can cause a lean-burn situation. Try disconnecting the O2 sensor and see what happens.
Where the O2 sensor wiring runs up the front of the motor is a known trouble spot, especially if the water pump has been swapped out and the wiring inadvertently rerouted too close to the exhaust manifold. I've seen multiple times on various Renix XJ where the O2, Knock sensor and engine temp wiring was cooking on the exhaust. Really hard to see the trouble spot. I've had to disconnect the harness down below and pull that section up into the clear to see the problem.
You might also want to tighten your manifold bolts (except the end studs) and generally check for vacuum leaks.
 
Pretty common to get a lean burn near peak torque and have your engine buck, almost feels like a torque converter issue. Usually an O2 sensor issue, either the O2 sensor or the O2 sensor wiring. But various things can cause a lean-burn situation. Try disconnecting the O2 sensor and see what happens.
Where the O2 sensor wiring runs up the front of the motor is a known trouble spot, especially if the water pump has been swapped out and the wiring inadvertently rerouted too close to the exhaust manifold. I've seen multiple times on various Renix XJ where the O2, Knock sensor and engine temp wiring was cooking on the exhaust. Really hard to see the trouble spot. I've had to disconnect the harness down below and pull that section up into the clear to see the problem.
You might also want to tighten your manifold bolts (except the end studs) and generally check for vacuum leaks.

I disconnected the O2 sensor and it seemed to just exacerbate the issue. Same type of feeling, but worse. It happens in the exact same range on the MAP sensor, right around 18.5 on the Renix Monitor. I didn't see any issues with the wires as I was tracing them back, but in that same loom there is another "sensor" or something that appears broken. Any idea what is right above the oil pan closer to the engine than the O2 sensor? Same style in appearance as the O2 sensor but this had what appeared to be wrapped wires leading to it, with one wire apparently broken off and dangling. Thanks for the assistance.

Also I previously ran #2 gauge wire from the fire wall to the engine stud and cleaned the contact points from the negative terminal and engine block.
 
So i figured out what the broken sensor was and replaced it with a new Knock sensor. No help there. I checked the fuel pressure again yesterday and I had 35-37 with the key turned on,31 at idle and then when I pull the vacuum hose its starts jumping around erratically around 39, but it is not steady at all. Moves so fast it looks like a blur. Is this typical or could I have a bad regulater?

Thanks for the assistance.
 
Crank Position sensor
Crank Position sensor
Crank Position sensor
Crank Position sensor

Every single time I have ran into anything like this.
 
I am probably going to order a new CPS from the Jeep dealership near where I work and continue to troubleshoot between now and then. In an earlier post I mentioned that when I pulled the vacuum tube during a fuel pressure test the gauge needle became very erratic. It went to 39 but moved up and down so fast that it became a blur. Is that something I should consider an issue to resolve? I'm now leaning more towards the CPS because of the comments here and every sensor on my Renix Monitor shows it is performing properly, but there is no indicator in that monitor for the CPS. I also replaced the TPS recently to no avail.

Thanks,
 
I called my local dealership and they said MOPAR no longer carries an OEM part. Are there still ways of getting an original part or is it all after market?
 
Do you have a tach? If it's the CPS, it will waver during this failure.
Sure it's not the torque converter locking and unlocking rapidly?
done Tip 10?


I have a tach that is original to the the vehicle. It is completely steady when the issue happens. I have thought about the torque converter but I get a soft backfire during the hesitation, so that leads me to believe it is a spark or fuel issue. I have not done step 10. I'll get at that this weekend.
Thanks,
 
I took it to my mechanic to see if he could find anything I was missing. He seemed to believe it is a power delivery issue. He unplugged the AC and things improved greatly but not completely. He said that the larger tires (31x10.5x15) could cause the hesitation because they are taking too much power . He also said the distributor is slightly out of "curve". I'd definitely still like to use my AC. Could this be the actual issue? Also I'm not sure what he meant about the curve. Could he mean the indexing? He said to replace the distributor and bring it back if it wasn't better. Iv'e been reading up on how to check the indexing and will complete that process soon.
 
There are other causes of what you are describing. Bucking at around 2400 RPM is often a fuel issue. On a *warm* motor, it will run normally lean at cruise speed, there is a fine line between efficient and fuel-starved. O2 sensor on a warm motor mostly controls fuel along with the MAP sensor. If your plugs are white after a run up the interstate it is often a sign of lean-burn, if they are sooty it is usually a sign of too rich.
Too rich can be a MAP issue, tiny leak in the vacuum line for the MAP is the more common cause. That line can rub on a wire for years and eventually develop a hairline crack. It also gets very brittle with age and should be handled with caution.
The O2 sensor wiring can be a trouble point. Maybe OHM test your O2 sensor ground. If you really want to check it out disconnect the ECU connector and ohm test the wire from the O2 sensor to the ECU.
Distributor indexing basically adjusts the envelope for the timing, minimum timing and maximum timing. If the adjustment is too far off the ECU loses minimum or maximum timing. The ECU is going to adjust the timing, really no adjustment except the envelope. Timing is regulated by multiple sensors, mostly by TPS.
In an ideal situation retarding the timing will help with the lean burn under load. The ECU controls the timing, no real way to retard timing. The knock sensor is supposed to retard timing when it detects detonation.
It threw me when you said disconnecting the O2 sensor made it worse. In my experience disconnecting the O2 sensor should default the ECU to basic setting, most often a little rich. You might try a drive on the interstate on a cold motor and see what happens. When my Renix was bucking it would happen on the same stretch of interstate and started about 4-5 miles from home, about the time the motor warmed up. One time it was the O2 sensor the other time it was the wiring.

Just a WAG but are you sure the number one plug is at the number one position on the distributor cap?

If I had to venture a guess I'd guess there is something in the O2 sensor wiring. And or multiple little issues that add up to a large problem.

Hook your vacuum gauge up to the intake manifold and see what the needle does, more the motion than the reading. Double-check for vacuum leaks, a lean burn can be a vacuum issue, most likely loose intake bolts. Note; don't try to tighten the end studs on the intake exhaust manifold they are often frozen and weak from rust etc. and very rarely the source of a leak. Intake leak is usually nearer the center of the manifold.

I'm out of ideas.
 
You might also do a plug reading to see how carboned up the engine is. Glowing carbon at highway speeds inside the cylinder will cause pre-detonation.
 
Just wanted to conclude with my resolution in the event someone down the road has a similar issue. I ended up dropping in a new distributor and it 100% solved my issue. Runs just like it should and I waited a week or so to post to make sure. Thanks for all the assistance, it was driving me nuts!
 
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