Tried a bunch of the fixes on here (thanks!)...and, when it's running, it's smoother than before....but at speed, it still hesitates...

Here's an update on what's been done, and if anyone has suggestions on what to work up next, it would be great to hear them! Full details on the symptoms below the list of things done to the Cherokee.

Thanks!

1995 Cherokee Sport.
4.0L, 4dr, Auto, 178k miles.

New:
Radiator, Water Pump, Thermostat, Belt.
Starter
Battery
Air Filter.
Crank Position Sensor.
Alternator replaced (2 y.o. unit)
Rear brake line.
Front Brake pads.
Swapped (chain grinding) 242 T-case for a 231.


New floor (sheet metal)

************************************

Distributor Cap, Wires, Rotor, Plugs.
U-Joints (rear shaft & front hubs).
Throttle Position Sensor.
Fuel Filter, Fuel pump (cheap OEM style).
Oxygen Sensor (NTK as recommended on here)....and as I cracked a pipe in the exhaust manifold trying to get it out...had to replace the manifold also.
New valve cover gasket (old rubber one had a break and oil was leaking around the whole bottom of the rear of the engine and trans).

It has a bad hesitation/stutter.
Happens at speed (50mph +), but has had the stutter once or twice while going to accelerate from a stop.
Happens when maintaining speed (not on accelerating or decelerating) .
Severe enough to shake the vehicle...feels more like a sudden stall (for a milisecond).
Happens when hot. I can predict about where the first one will happen after leaving the house....once the road I'm on frees up and you can move at 50mph (and the engine is hot).
Seems to get a bit worse when the vehicle is hot.
Intermittent (not at any regular interval...might get two at almost the same time, and then go 2 or 4 minutes before the next one).

I NOTICED it about where the red line is above...so the Distributor, u-joints, tps, o2 sensor, etc were all done AFTER the hesitation was noticed, and didn't improve it at all. The vehicle had not been running (no floor, grinding t-case, etc.) other than to start it and run it a few feet to move it...for almost 3 years total (I got it after it sat for 1 year and did the stuff above the red line before I was able to get it on the road).

It has the OBD I setup. The only codes it's throwing are 12 (battery disconnected in the last 50 cycles) and 21 (O2 sensor...but this is probably a stored code as I just replace the O2 sensor). The Check engine light doesn't come on during the hesitation (at all...even after running for a good distance).


I'm going to clear the ECU. I did clean out the throttle body when I changed the TPS, and some of the intake when I had it off. I just checked the vacuum lines (all are connected, no visible cracks, etc...didn't go under it to look at the one running to the front axle though).


Any thoughts?


Thanks!

 

Does it slowly bog down, like it's running out of fuel? Or does it stutter and jerk like the ignition is cutting out?

Open the hood at night, fire it up and mist the plug wires, coil and cap with water. See any sparking? Perhaps the coil tower is cracked and arcing to the mount.

If it's an automatic, have someone lightly power-brake it while you look for arcing. Putting a slight load on the engine will raise the combustion chamber pressures and likewise cause the coil to output more voltage, making any arcing problem more apparent.

 

The vehicle had not been running (no floor, grinding t-case, etc.) other than to start it and run it a few feet to move it...for almost 3 years total (I got it after it sat for 1 year and did the stuff above the red line before I was able to get it on the road).

Did you clean out that tank with 3+ year old gas in it when you changed your pump/filter etc? I'd guess it would need a thorough flushing after 3 years, probably lousy with water varnish & rust.

 

To answer the above two posts...

I mis-typed. The car sat for 2 years (plus the 1 I had it and was working on it...still 'sat' for about 3 years total with only occasional starts). The owner would occasionally start it. I got it, filled it with gas (it was nearly empty when I got it), and have filled it 2 more times (despite the hesitation, had to use it to help move...it was a short distance, but made lots of trips).

It is a stutter/jerk (definitely). I've put a full can of Seafoam through the tank with one of the tanks of gas.

I'll check on the arcing issue. Load (accelerating) doesn't seem to cause the problem...just maintaining speed.

Thanks, and any other suggestions will be welcome.

AFARR

 

My 90 started all sorts of trouble. The whole works. It would hesitate, miss, pop back out the intake. (the confetti blowing out the grille is how I found the mouse nest in the air cleaner). No start a few times. Run like a (frightened) ape if you floored it, but stumble on even throttle. Or run fine for days. Found that most of the usual suspects all seemed to be about $50 each, and some are a ***** to change. I didn’t want to just throw money I didn’t have at it, so I just did normal tune up stuff. Odd symptom (you may or may not have). Could start and run all day , no problem. Turn it off for 2 minutes then “the tantrums”. The sitting business really had me. Oil dripping around the CPS sensor, engine heat migrating to the coil or TPS or something? Something involving the manifolds and a vacuum leak? Catalytic converter. Had to be heat migrating while it was off. It took over a year to figure out that the HEAT SHIELD over the manifolds was expanding into the injector wires at the front. I JUST MOVED THE WIRES. Problem solved.

 

If you could find a shop that has an actual live oscilloscope, you'd have the problem nailed for a half hour labor charge. Too bad most mechanics nowadays don't even know what that is or how to use one.

They went out of favor when distributorless ignition systems started becoming popular. Then when old guys like me drifted away from the biz, we were replaced by new guys who never used a scope and knew jack about how ignition systems worked. Screw the diagnostics, it must be that $300 ignition module, huh? Replace it and see what happens... nope, that wasn't it... how about the coil pack? Nope... that wasn't it either.

I worked at a Buick dealer a long time ago when one of these new guys was throwing parts at an intermittent miss on a 1990 Lesabre (new at the time). He'd replaced just about everything he could think of and the car still had a stutter.

I went in the back room and wheeled out the old SunTune oscilloscope. Two minutes after hooking it up, I saw what appeared to be a sparkplug shorting out.

Pulled the plug and didn't see anything wrong with it... until I turned it upside down and the broken ceramic insulator slid down over the electrode, covering it up. Bingo! He didn't think to check the spark plugs because the car was brand new.

The moral to the story is, more often than not, it's the simple and cheap things that cause the biggest problems.

 

I'm with you there Harper. 2 minutes and 6 inches of tape after nearly a year and a half. Did buy plugs, cap, rotor, wires and filters, but that's all. Afarr's DOES store codes, but that doesn't seem to be helping. My intermittent short was front and center, but still hard to spot.

 

I'll look for a shop with one of the older oscilloscopes. Not sure who might have one locally. I may throw a new coil in before that (since I've replaced most of the ignition stuff anyway.

I hate to throw parts at a problem...but since the Cherokee sat for so long, I figured it was worth it to at least change out a bunch of the usual maintenance items (have no Maintenance history on it...and the plugs I pulled out gapped at .070).

Someone (on a different board) recommended a PCM/ECU fix (swapping out some capacitors) to fix a lazy PCM...in looking it up, it seems that there are some issues with them as they age in jeeps. Doesn't look 'too' hard (famous last words...).

Thanks again..

AFARR

 

I'd like to get my hands on an old SunTune oscilloscope and two-gas analyzer. As far as finding a shop with one of the old CRT scopes, look for a run down building with an old grey haired mechanic. He's likely to have one in a back room.

 

Did you ever resolve this hesitation issue with your jeep?? I have a 90 cherokee that does the same thing at 65, and only with the pedal is held steady. I have spent several hours testing and replacing parts, and have gotten no where.

 

I had the same problem. I changed the CPS then in two days it started again. The new OEM CPS was bad. Changed it and all has been fine with the hesitation.

 

Don't tell me that, that thing was a pain to put in!! HA. I got mine from a parts store and it did not change a thing. Do you think I have a bad one?? I have spent 4 months trying to figure this problem I have about had it with my little Cherokee.

 

Not entirely. I did the coil and checked the vac lines (didn't have the right size to replace them yet), and pulled the fuel pump and replaced the small bit of hose above the pump. It runs MUCH better and only has the hesitation about 10% of what it used to.

Just logged on to check on what proper fuel pressure should be...looks like the cheap replacement I put in isn't working quite right (or my gauge is wrong).

AFARR

 

Did you check the throttle position sensor?

 

I changed my fuel pump, and my hose looked good, but after reading alot of threads, i guess they have problems on the inside. What was your fuel pressure reading at?? I can't remember what mine was at, I did it about a month ago.

Hopefully together we can solve this problem. I have spent months reading and testing. And you are the only one I have seen with this problem like mine.