91 4.0 bucks and surges on acceleration

Hey all.
This is an old issue and has been discussed in a few different threads. Figured with new information, and significantly more knowledge, I'd start a new thread that I might not throw in a bunch of useless/incorrect information.

I have a 91 Cherokee Lareado with a 4.0L I6 and automatic trans.
Runs fine when cold, or for the first few minutes. Runs fine forever when it is below -40C.
Once it warms up to operating tempature, during acceleration, the engine bucks and surges. The longer I drive, the more it does this. When it first starts, it seems to buck every time I hit a bump. Eventually (around 5 minutes in normal weather) it starts bucking any time I accelerate.
This seems to be very ambient tempature sensitive, as the problem never shows up when below -40C, even when coolant temp is normal.
The problem pretty much goes away when I unplug the MAP sensor electrical connector.
Unplugging any other sensor makes no noticable difference.
This leads me to believe that the truck never makes it to open loop operation (unplugging the O2 sensor should throw it into closed loop, but doing this has no effect on performance).
I get between 1 and 3 Km/litre (works out to I believe, 1/4 - 1-10 normal fuel economy)
The engine has lots of power before it starts bucking, and still lots when I unplug the MAP sensor.
Engine starts fine, takes about 3-4 cranks to fire (~1 second when warm, about 2-3 when below -40C)

Replaced O2 sensor and TPS.
Tested all sensors except for the Stator.
All sensors read correctly as far as the Haines manual states, but many of the tests look like they're designed only to test if the sensor works at all, not if it is actually giving correct values.
TPS was reading 800mV closed and 3.8 @ WOT, but I've now replaced it.


Exhaust manifold was leaking, so I replaced it with a proper header, and replaced the intake/exhaust gasket. Tested for leaks by lightly spraying intake/block mating area with carb cleaner. Could not find leaks.

Intake vaccum at idle is nominal, and drops momentarily when I hit the gas, but comes back up to normal levels within about 1/4 second.
IAC responds quickly, hot or cold, and continues responding when MAP is unplugged.

I don't think the truck ever goes into open loop operation. I'm not sure how the ECM responds to a no signal from the MAP. I'm assuming the ECM just provides a predefined value for MAP data based on RPM (someone correct me if I'm wrong).

Anyone have any ideas?

Aww comon... No response?

you have your loops backward...(no big deal..)

Engine Start up mode is OPEN LOOP
Engine Warm up mode is OPEN LOOP
Engine idle at operating temp is CLOSED LOOP
Cruise mode at operating temp is CLOSED LOOP
Aceleration mode (abrupt) is OPEN LOOP
Deceleration mode at operating temp is OPEN LOOP
WOT mode wide open throttle is OPEN LOOP

Anyhow...couple of thoughts here...maybe an intermittent engine harness problem or vibrational connection problem (motormounts?) if bucking occurs when you hit bumps.. Can you reproduce the stumble in idle if you wiggle connections, jiggle the engine harness, tug on ground connections...especially back by the MAP sensor....

second thought is your description of the problem says problem kind of goes away when you unplug the MAP sensor...try a new MAP sensor once you are sure all of the wiring and vac line for the sensor is solid .

Cannot reproduce the problem with my head under the hood. Only during accelleration. I've tried wiggling every wire I could find, no change.
The MAP is new, I've tested it in another Jeep to be sure the new one wasn't faulty as well.
Motor mounts are in good shape, engine is very securely attached to the frame
I don't know what else to do other than start replacing all my sensors, and then replace all the wires if that fails.
No one knows how the ECM responds to a no signal from MAP, so I can't think of a way to trace the problem back to any particular wire or sensor.

Does this year use a CTS?

whats a CTS?
Stator?

Maybe plugged catalylic converter.

Doesn't explain why unplugging the MAP fixes it.
I'm goona check the fuel pressure again once i get access to a shop.
It really feels like a timing issue.
Especially when its only when the engine is actually under load.
If i'm just reving the engine, it works fine.

blistovmhz said:

... Runs fine forever when it is below -40C.

Click to expand...

Doing better than me. Mine really hates to start below about -20*C.

Are the vacumn lines to the MAP good and hooked up correctly?

Vacuum line seems good.
I can't force a leak.

Hallo. Same experiences. Next can be bad when hot:
Connectors Injectors,Ignition cables,Ignition Coil.
All will hang and change and move when heated up.
Good luck in finding.:wave1:

I originally thought it was the ignition coil, until I discovered that unplugging the MAP electrical connected fixed it, which keeps leading me back to sensors, or sensor wires.
The only mechanical issue I can think of is valve timing. Maybe when my engine advances too far, the valve timing doesn't match the rest of the engine (bad valve clearance).
I'm not sure if that makes any sense though

blistovmhz said:

I originally thought it was the ignition coil, until I discovered that unplugging the MAP electrical connected fixed it, which keeps leading me back to sensors, or sensor wires.
The only mechanical issue I can think of is valve timing. Maybe when my engine advances too far, the valve timing doesn't match the rest of the engine (bad valve clearance).
I'm not sure if that makes any sense though

Click to expand...

Hallo I think, that the valve timing issue is reverse thinking.
Do all the wiringchecking again. Dont ask me why.:idea: good luck

Hrm, well seeing as this is directly controllable by unplugging the MAP, do you happen to know what is directly affected by the MAP sensor data? IE, the ECM responds to data from MAP, and does what?
I know that the ECM adjusts the ignition timing, and possibly the air/fuel mixture, but what about injector timing?

blistovmhz said:

Hrm, well seeing as this is directly controllable by unplugging the MAP, do you happen to know what is directly affected by the MAP sensor data? IE, the ECM responds to data from MAP, and does what?
I know that the ECM adjusts the ignition timing, and possibly the air/fuel mixture, but what about injector timing?

Click to expand...

Hallo, I think that the MAP,MAT ,CTS ,TPS and O2 sensors are related with the mixture, but the CPS and the O2 sensor have to do with the timing. Not only the for the Ignition ,but also for the Injectors.
There should be also a sync pulse switch within the distributor for synchronizing the injector opening? I dont known exacly.:wave1: