91 cherokee bucking.

[blistovmhz]

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?

[mud4feet]

You mentioned invalid O2 measurements.......have you checked the O2 sensors? When mine went bad in my Wrangler, it made for a very poor running vehicle - both a powerless and gas guzzling 4Banger.
I think my originals were rated for 50K or so, but I ran them a lot longer than that.

Just a thought......hope it helps.

[blistovmhz]

O2 sensor is brand new, and reading deadly accurate.
I've checked it hot and cold, (self heating type) at high and low rpms. I've tested artificially richening the fuel/air mixture to be sure the sensor reported properly.
The TPS (if i recall correctly) read between .2 and .8 V low at WOT, but I've heard from several other Jeep owners that my actual specs matched theirs.
IAC shouldn't be an issue either way, but it came dead within spec as well, as did the Cam pos and Crank pos sensors.
The only one I can't verify the full range is the MAP, because there's really no definitive low/high reading, just a fluctuation depending on vacuum.
If the MAP is dead, that'd be great. :)
Can anyone tell me what happens at WOT at +4200RPM, with the MAP electrical disconnected, in their Jeep?

	Throttle position sensor A throttle position sensor (TPS) is a sensor used to monitor the position of the throttle in an internal combustion engine. The sensor is usually located on the butterfly spindle so that it can directly monitor the position of the butterfly throttle valve. The sensor is usually a potentiometer and therefore provides a variable resistance dependent upon the position of the butterfly valve (and hence throttle position). The sensor signal is used by the Engine Control Unit (ECU) as an input to its control system. The ignition timing and fuel injection timing (and potentially other parameters) are altered depending upon the position of the throttle, and also depending on the rate of change of that position. For example, in fuel injected engines, in order to avoid stalling, extra fuel may be injected if the throttle is opened rapidly (mimicking the accelerator pump of carburetor systems).

[blistovmhz]

UPDATE

I replaced the MAP sensor, but the problem still persists.
Arg arg arg.