blistovmhz
New member
Hey all. I picked up a 91 cherokee laredo a few months ago, and from day one, it would buck and surge when:
1. Engine is warm,
2. Engine is under load.
This happened throughout the RPM range, normally starting about 2000RPM.
The problem got worse as summer progressed, and I found a crack in the exhaust manifold, and some leaks in the intake. I noticed that when I unplugged the MAP sensor, the engine ran fine up till about 4300RPM, where it would start bucking again regardless of the tempurature. (this makes for very very poor mileage however).
I decided this may be caused by invalid O2 measurements in the downpipe because of the cracked manifold.
I just replaced the exhaust manifold with one from APN, and while I was at it, I cleaned the intake manifold and intake and exhaust ports on the block. (lots of crud in there).
Ran the engine for about 20 minutes driving like an ******* (its never gone 20 minutes without bucking before) and the problem is back. Its not as bad, its still driveable, but every time I put some torque behind the wheels, it starts again (which sucks when power gliding around icy corners and you lose power).
I checked for intake/vacuum leaks with some throttlebody cleaner, and found nothing this time.
The previous owner said he had a bunch of stuff replaced, trying to track down this gremlin, among other parts, the MAP sensor was supposedly replaced. (the shop may have lied and ripped him off)
Any idea's?
I've checked everything I can think to check.
PS. does anyone know if ALL 4.0L engines buck with the MAP disconnected?
1. Engine is warm,
2. Engine is under load.
This happened throughout the RPM range, normally starting about 2000RPM.
The problem got worse as summer progressed, and I found a crack in the exhaust manifold, and some leaks in the intake. I noticed that when I unplugged the MAP sensor, the engine ran fine up till about 4300RPM, where it would start bucking again regardless of the tempurature. (this makes for very very poor mileage however).
I decided this may be caused by invalid O2 measurements in the downpipe because of the cracked manifold.
I just replaced the exhaust manifold with one from APN, and while I was at it, I cleaned the intake manifold and intake and exhaust ports on the block. (lots of crud in there).
Ran the engine for about 20 minutes driving like an ******* (its never gone 20 minutes without bucking before) and the problem is back. Its not as bad, its still driveable, but every time I put some torque behind the wheels, it starts again (which sucks when power gliding around icy corners and you lose power).
I checked for intake/vacuum leaks with some throttlebody cleaner, and found nothing this time.
The previous owner said he had a bunch of stuff replaced, trying to track down this gremlin, among other parts, the MAP sensor was supposedly replaced. (the shop may have lied and ripped him off)
Any idea's?
I've checked everything I can think to check.
PS. does anyone know if ALL 4.0L engines buck with the MAP disconnected?
