blistovmhz
NAXJA Forum User
- Location
- Vancouver, BC
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?
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?