4.0 hesitates at operating temp.

I'm looking at buying a 92 cherokee with a lot of miles on it.
It has a gremlin that three shops haven't been able to catch yet.
The engine runs fine at startup, drives perfect, lots of power.
When it warms up (this happens like a switch was turned on) the engine starts hesitating through the lower part of the RPM range, and has very little power at high RPM. Apparently;
- The spark plugs have been checked and gapped.
- The sparkplug wires have been replaced.
- The map sensor has been replaced.
- The ecu has been swapped swapped out then the original put back in.
- The cps has been replaced.

The owner says that when he got the Jeep back, after every visit to the shop, the truck ran fine for about a day. He traced it (he thinks) to the computer being reset.

Now, I'm not too concerned as to what the problem IS.
I'm more worried about what it IS NOT, and I'm hoping the forum members can rule out a problem with the block itself.

I'm thinking its likely the upstream o2 sensor, and/or cracked exaust manifold (cracked where it branches to the exaust pipe).
My logic is that the engine warms up the water temp sensor, which tells the computer to start enabling some emmisions control, which gets a reading (incorrectly) from the o2 sensor, and is in turn, running either very rich or lean. If not that, then possibly dirty fuel injectors, (or injectors getting too hot and vaporising fuel before it is injected?)

I'm going back tonight to take another look at it. I'll buy it if I can rule out the block (ie timing chain/cam which i can't see being a problem)

There are some other indicators, like does it happen at a specific RPM and/or throttle position. During acceleration or while cruising. Vacuum changes with throttle position, acceleration and even the RPM band.
O2 sensors often act up at or near peak torque, 2000-2500. while cruising. If the O2 runs the motor a little lean, it's shows up noticeably while cruising. And then can lean the motor out again at the high end of the RPM band. Both related to fairly high vacuum conditions.
I haven't heard a lot about excessive resistance in the CPS wiring in the 92, which is common in the Renix (pre 90 models). A poor CPS signal can cause all sorts of grief and replacing the CPS isn't always a cure.
A cam position sensor test is fairly easy to do and even if it isn't the trouble the 5 volt sensor voltage reading there can be an indicator of the health of many sensors. If there is a short in another sensor the 5 volt reading is often very low 2-2.5 volts or so.

I could be mistaken, but I recal the hesitation happening regardless of the load. It seemed to be very distinctly within an RPM range, somewhere around (wildly guessing here) 1500 - 2000? I haven't had it at cruising speed yet, but even at about 30km/h, the hesitation seemed to go away, and significantly more power was available. (although still only about half the power you'd expect from a 4.0L)
It feels like the timing advances too far (I'm pretty confident this isn't what happens, but i'm mentioning it in case it triggers some memory out of someone ) at a certain RPM and then fixes itself when you push through the hesitating range.
How much performance should I expect to lose because of a crack in the manifold?
Also, assuming it IS the o2 sensor, and air/fuel, could I just disconnect the water temp sending unit? Shouldn't that make the computer think the engine is still too cold to enable the emissions stuff?

blistovmhz said:

I could be mistaken, but I recal the hesitation happening regardless of the load. It seemed to be very distinctly within an RPM range, somewhere around (wildly guessing here) 1500 - 2000? I haven't had it at cruising speed yet, but even at about 30km/h, the hesitation seemed to go away, and significantly more power was available. (although still only about half the power you'd expect from a 4.0L)
It feels like the timing advances too far (I'm pretty confident this isn't what happens, but i'm mentioning it in case it triggers some memory out of someone ) at a certain RPM and then fixes itself when you push through the hesitating range.
How much performance should I expect to lose because of a crack in the manifold?
Also, assuming it IS the o2 sensor, and air/fuel, could I just disconnect the water temp sending unit? Shouldn't that make the computer think the engine is still too cold to enable the emissions stuff?

Click to expand...

Try unplugging the rear O2 sensor. This will likely cause some mileage loss, though I've heard conflicting results.
Sounds like the motor is running lean. Like you mentioned maybe an O2 sensor. Plugs will often tell you a story, I've found a five mile trip up the interstate (around 2000 RPM), then a short stop on the roadside to pull a couple of warm plugs, gives the best reading. Take a couple of extra plugs with just in case, been there done that, snapped off a plug during a color check miles from home.
With the OBD 1 type computers I've gotten good results on occasion just by disconnecting the battery for awhile (15-30 minutes) then reconnecting it. This can sometimes cure some troubles at least in the short term.
Can you get MIL codes from a flashing check engine light in your 92?
It depends on the manifold leak and it's location. I've seen very few that affect the O2 sensor reading much, but it does occasionally happen. It's more likely the hot exhaust gases are cooking some wiring or a sensor.

The origional owner said the same thing about the computer. Disconnecting the battery for 10 minutes or so apparently makes the truck run fine for a few days (though i haven't yet tried this).
I'm about to go buy the truck anyhow, this afternoon.
Can anyone give me any reasurance that this isn't a problem with the block (ie cam, lifters, something binding horribly)?
From what I've gathered thus far, this is most likely a problem with the emissions system, and should be fairly easily repairable, although perhaps a bit of a pain to track.

reasurance that this isn't a problem with the block (ie cam, lifters, something binding horribly)?


Does it knock when it does this? I think it is somthing easy, get him to give you a great price an go for it..

It does sound like its knocking. The problem really feels like timing.
Is it possible the stator is messing up somehow? I know nothing of electronic timing.

Also, reseting the computer did not help. The problem felt like it came back after hitting a bit of a bump in the road, then it goes away again when i hit another bump.

I'm going buy it tomorrow, but I'd really like to at least have a good idea of what I'm getting myself into. Hate to waste more money on another dead block.

Oh, I did reset the computer, and while it cleared the codes, it didn't solve anything. I did however, run the truck till the problem re-occured, and it gave me a code 54 (no signal from Stator i believe)
How much does the stator actually do though, once the truck is running?
And would its performance change at all once things warmed up?

(goddamn i paid the guy for the truck now and i'm starting to get paranoid about mechanical problems)

why, now you can fix it :roflmao:

yea, I also have an 83 eagle i just spent $4000 on, that i'm turning around and selling so i don't spend another $4000 to get it back on the road.
I'm really really hoping this is an electrical problem.

it is something simple

Where do I start though?
I know I'm throwing a code 54, but everyone seems to be up in arms about what the CAMSHAFT position sensor actually does.
Some say its only used during cranking, to determine when the #1 cyl is tdc.
Others say it determines injection timing during all engine operation.
The manual says nothing. Just to replace the component.

If im not mistaken the Cam sensor is used in conjuction with the Crankshaft sensor to tell the computer where in the engine is at so it knows when to spark and fire fuel injectors. I had a problem that sounds sorta farmiliar that turned out to be the upstream O2 sensor, it would run good until it warmed up then it would have absolutely no power. While the engine is warming up the computer runs in an open loop mode where it ignores the O2 sensor and runs fine. The only way I could drive it at all was to put the pedal to the floor because WOT is also an open loop mode for the computer. If it runs normal with the pedal to the floor that could be your problem. Also unpluging the front O2 sensor will cause the computer to always be in an open loop, and if the sensor is your problem it should run ok.

Which is the upstream o2 sensor? I found one about 9" before the bottom of the pipe, has a 4 wire connector (i assume this is the heated type).
Is there another one I'm missing?

Also, could a wire, or ground heating up, cause the CPS (crankshaft position sensor) to malfunction?

If it has a lot of miles on it, I would pick up a rebuilt distributor for about $70 and replace the entire distributor rather than trying to replace the cam sensor (stator, sync sensor, has many names) since the rest of the distributor is probably just about toast as well (bearings and seals might go next). The #54 code is saying that either the synch sensor in the distributor is bad, or the wiring between it and the ECU is bad. It is not a mechanical problem. You might want to get a wiring diagram and test the continuity between the synch sensor wire at the distributor and the ECU before replacing the distributor. You will need a long test wire and a volt ohm multimeter.

The Cam/Synch sensor in the distributor helps the ECU/PCM more precisely control fuel injection timing, not the duration of injection, but when to first open the injector. If it is not working, the ECU/PCM must guess when to open the injector, so it can adversly affect power. The O2 sensor helps (as do other sensors) the ECU/PCM decide how long to keep the injector open. The CPS helps the ECU time the spark, but the ECU also uses data from other sensors (air and coolant temperature and the MAP sensor) to slightly modify the spark timing too.

If replacing the distributor (and getting it installed correctly, more on that later) does not solve the problem, I would test the air (IAT) sensor and coolant (CTS) sensor with an ohm meter at cold and hot engine temperatures to see if they are with in spec. Also test the TPS (ohm meter). A bad CTS, IAT, or TPS can also cause your problems, but will not cause the engine error code #54!

If you replace the distributor, be patient as it sometimes takes several attempts to get the distributor installed exactly on the right timing tooth. If it is off one tooth, it will start most of the time, but will not run well, cold or hot. Most of us have had to try several times to get it set right. There are right ups here on how to get it installed right.

I think the 92 only has one O2 sensor. As I recall 96 was the year a second O2 sensor was added. So far it sounds like the CPS is OK.

Bad grounds can cause problems too, but you need to fix the recurring #54 error first and see what that does! It might be the only problem.

I don't know about 1992 but newer cherokees had another O2 sensor right after the Catylitic converter. It might just be the OBDII, if i had to guess i would say yours only one. Sorry i guess i missed the last post.

how adversely would a non-functional o2 sensor affect my engine when its hot?
I checked out everything else, and all the other sensors check fine, and i remembered i MAY have disconnected the Camshaft Pos Sensor while it was running last time so that 54 may have been bogus.
Would a dead o2 sensor really render my engine useless?

Just disconnect the O2 sensor. The computer will run the engine on open loop (good power, smooth operation, but it guzzles gas!). If disconnecting the O2 sensor solves the problem, it needs replacing. Dead O2 sensors are usually locked out by the computer and do not hurt engine power, but kill gas mileage. Partly working but partly defective O2 sensors can fool the computer and cause poor operation.

If that does not solve the problem, go on to the next most probable item(s), like the IAT, CTS and TPS.

blistovmhz said:

how adversely would a non-functional o2 sensor affect my engine when its hot?
I checked out everything else, and all the other sensors check fine, and i remembered i MAY have disconnected the Camshaft Pos Sensor while it was running last time so that 54 may have been bogus.
Would a dead o2 sensor really render my engine useless?

Click to expand...

*sigh*
I was actually just trying that.
I checked continuity on the white wires to the o2 sensor, and they came back in spec, so I started the engine, warmed it up and periodically checked the o2 sensor feedback. Once the sensor was at operational temp, it started reporting the .1 - 1.0 volts like its supposed to.... but thats not all.
It started climbing higher. it would start climbing in voltage till it hit about 4V, then it would drop to 0V for a while, then it'd start fluctuating between 0.1 and 1.0 again.
I disconnected it entirely and let it run, no problems so far, but the exaust looks a bit smokey. Could be too much fuel, or possibly a bit of oil (oil looks brand new, and the level hasn't dropped noticably yet.)
I'm hoping this IS just a buggy o2 sensor, as it looks like its never been replaced. Could be a short somewhere, but that wouldn't explain the climbing voltage.
Could be my meter as well I suppose. I'm goona keep playing with it until tuesday when the insurance co is open and I can get this thing on the road for some real tests. Going to pick up a spare o2 sensor as well, and possibly do some cleanup on the wiring this weekend.

The problem though now, is that the previous owner said that sometimes the truck would run fine for a week at a time, so I suppose I won't know anything for a few weeks