Hesitation Missing and Aggressive Auto Shifting

Hey Guys-

I have an issue with my 94 XJ that is driving me crazy.

I read many posts on this topic before asking these questions.

Here's what's up:

I bought this 94 4.0 Auto XJ last year with 90K on it, but it needed some love here in the NE rust belt (ha. 90K and newly replaced floors). When I got it, it did not ping or hesitate. Over time it started to ping going up hills- the pinging would go away if I either put Seafoam in the tank or ran 89+ Gas.

OK, so I figured I'd get around to misting through a vac hose with either water or Seafoam assuming it was carbon. It might be relevant that it was running hot due to a leak between the block and water pump (but never overheated)- maybe that contributed to carbon? Anyway-

I replaced the water pump, hoses, the rattling PS pump, idler pulley, everything happy, not pinging at this point.

Pinging over time starts getting worse, but I'm not pushing it and doing other repairs to get to work and pass PA inspection- totally new exhaust end to end, O2 sensor, brakes, shocks, steel brake lines, fuel filter. Still not hesitating, and shifting right.

At first I used 3/8 rubber line for the filter and just clamped it down- so after reading when the hesitation started, I thought I might be sucking air so I replaced the rubber fuel line with 5/16 and correct clamps. Still hesitated.

The symptoms are for one as if on a flat road going 55 you are rapidly switching off the ignition and back on again- it can do it once, or repeatedly, but changing the throttle seems to make it go away for a little (getting on it or backing out of it)...and then it starts again in the same 45 min trip to work.

At the same time, the Auto trans started to shift as if I put a shift kit in it.
1st winds up more than it used to, and it (too firmly) shifts into 2nd. The shifting and hesitation started simultaneously, so it seems like they might be related.

OK- NEW:
Cap, Rotor, Wires, Air Filter, fuel pressure regulator, 90 deg regulator rubber vac end, inspected all other vac lines and rubber ends (but not replaced), new "higher end" Standard Ignition TPS. (this after trying an Advance Auto TPS which sucked bad)

Still has hesitation and Firm shifting.

I did not replace the plugs- I'm still planning on misting to remove carbon and don't wanna foul new plugs, plus it seems unlikely to me that plugs are the root cause here.

I did not touch:
The Idle "plunger" on the throttle body
The MAP sensor
Air Temp sensor
Crankshaft Sensor
Fuel Pump

It starts and idles fine.

The steel fuel return line is ugly as is the large vapor hose going to the under hood canister.

From what I've read in the forums, it seems I could chase this until I've replaced everything. Bleeding on a 20+ year old XJ every spare moment is starting to lose it's shine. :gee:

Any helpful suggestions?

Thanks

BUMP
Too much detail?

A bit.

You mention crappy Chinese Throttle Position Sensors and shifting issues. You mention 55 mph and feels as if the engine is lightly bucking.

It has been very well proven over many years and dozens of forum threads that: 1) All engine sensors not made by Jeep are either suspected garbage, or proven garbage, and 2) The Jeep 4.0L does not like Bosch O2 sensors.

Start with replacing any non-Jeep OBD sensors and/or any suspect sensors with genuine Jeep Parts. Next, if the O2 sensors are not NTK brand, what Jeep installed when your was built, buy NTK direct fit O2 sensors. Inspect for a cracked exhaust manifold. Inspect the transmission fluid level and condition. Confirm that the Cherokee AW-4 automatic transmission is filled to the correct level with clean Dexron-III/Mercon fluid and not anything else. The Jeep 4.0L runs best on plain old Champion or plain old NGK spark plugs, anything else is a waste of money.


The TPS has direct input to the Transmission Control Unit computer, and non-Jeep TPS are usually garbage.

Bucking/hesitation is almost always either a leaky exhaust manifold, or a Bosch O2 sensor. Nearly all XJ Cherokee have a cracked exhaust manifold which causes a lean condition.

Good Luck.

.

Thanks-

all good observations.

The detail was about trying to head off all of those questions, and give an exact picture of what was done... and maybe help someone else.

The top two issues you identified:

I replaced the 02 sensor with a Bosch "wire-it-yourself" sensor.
I'm certain that I got the wires right since that's my industry, AND I checked it with a meter, AND it ran fine...for awhile.

I mentioned I replaced the entire exhaust.
"entire" meant Header back with stock parts except for the Dynomax muffler and tail pipe which fits like stock. No exhaust manifold replacement.

Even with "stock" formed pipes, I had some rattles and might have stressed the manifold by adjusting the rest of the exhaust.

I'm getting a real O2 sensor, and inspecting the exhaust now-

Thanks

Based on what I have read, my money is on a small hard to see vacuum leak on the MAP sensor Vacuum line causing the hesitaion.

Next up is a CPS going bad. Maybe both!

You need to check the fuel pressure at high demand, like WOT. Pressure at idle does not require as much flow volume as a heavy load at speed during acceleration.

Transmissions fluid may need a few transfusions, but I doubt that is the only problem. Add a pint of Trans-X added to the fluid, put about 20 miles on it, then drain what you can and refill with DexIII. You made need to do this 2-3 times if you drain at the drain pan plug, as that only drains about 1/4 of the Trans fluid. It will loosen and dissolve and clean any varnish making the valves stick that control shifting.

Test and clean the IAT Intake Air Temp sensor. It is no doubt fouled and is thus not seeing rapid temp changes under heavy throttle as fast as it should.

If you had not already changed the TPS, it would also be at the top of the list.

Get a voltmeter and back probe the O2 sensor, engine running to see if it is running lean, thus indicating a fuel pressure or injector(s) issues. Is it switching to closed loop. Google "Ecomike Jeep O2 sensor test Naxja.org" for testing details.

Make sure the old plugs have a proper plug gap and are not totally fouled at the gap. Test one plug with the gap increased to .060 (4.0 Spec is .035) to make sure the HV coil and ICM are giving you a solid spark.

Let us know how that all works out.

I have 3 XJs, I can easily replace the MAP Vac line and see what happens.

Correct on the "maybe both"- hence my comment on what I take away from hours of reading the forums is that it could be anything- but there are things I have not cleaned or adjusted or replaced.

Checking the fuel- I had 31 lbs with the old factory regulator, but read enough not to trust the static reading, and considered flaky intermittent performance as a possibility. New high quality regulator, but I have ran the tank very low since replacing the filter. Seems unlikely I clogged a 4 mo old filter, but not impossible...

Trans-
I have not replaced the filter, I understand running some additive to clear varnish, and flushing- not against that at all, but for everything to go downhill at once seems like too much of a coincidence for an Xj that was running "fine" until several things changed at once.

That said, the fluid level is topped off, and doesn't smell or appear burned.
Flushing/Filter is on the list... and will get bumped up if I fix the hesitation, but the shifting remains harsh.

I have not tested or cleaned the IAT yet.

TPS is changed to a quality (but not Mopar) sensor- that change alone feels better but still leaves the hesitation, as did the original Mopar, and the incredibly bad Advance Auto part which negatively affected the throttle response AND had the hesitation.

So things I can easily eliminate:

Run new vac to MAP
Clean IAT
Check exhaust for crack
Replace Bosch 02
Check/gap plugs larger gap- forces coil to produce more energy to jump

Thanks- this is a fresh look at the issues for me.

Quick update on progress:

Because the Previous Owner told me that "it was just tuned up" I never checked the plugs assuming they were good, and I had some sensor/vac issue.

I cleaned the air sensor which was somewhat cruddy, pulled and held a vac on the MAP sensor line, and took out what were the worst plugs I've ever seen come out of a running engine.

Wasted center electrodes, almost non-existent ground electrodes, all covered in bone white debris with a gap exceeding .060 easily, and one with actual broken porcelain tip (that I hope came out of the exhaust).

After that, the motor still ran strangely, so I reset the ECU by grounding the Pos cable, and it runs better and better as it learns the new tune up.

No pinging, no hesitation.

Still, I have to account for the extreme lean condition of the plugs, so I'm replacing the Bosch O2 sensor for insurance.

The trans shifts better, but still feels like there is a shift kit in it, so I'm going to flush it as suggested, and replace the filter.

Thanks to everyone who commented.

I would start the proper new plugs, run it for about 20 minutes at idle or drive it, then pull the plugs and see what the new plugs look like on the lean question. I would not assume it still has a lean issue from the old plug autopsy.

Good point, the motor has been in "many states of tune" getting it where it is.
I have however noticed some very light pinging, so something isn't completely happy yet.
Also stalling if I don't let it warm up and/or goose it in the mornings.

If you're reading this mess, I should add that now that I'm remembering, the trans did start to shift more firmly when I began putting it in "3" instead of "D" to limp it up hills with the bad pinging. (i.e. it shifts too firmly now even when in "D")