Strange power issue?

So I have a fresh 4.0 in my 1987 cherokee. It runs very good when it is cold all the way up to about 150 degrees, then once the engine gets hotter than that it feels completely gutless. When it is cold if I stomp on it you can really tell but when its warm to hot you would hardley know I just punched the gas pedal. Also the hotter it gets the higher it idles, cold it is at 750 RPM but warm it might sit at 1,200 RPM. I know it is temperature related because if I turn my auxilery fan on to cool it down, it starts gaining power again. Any ideas?

jeepboy381 said:

So I have a fresh 4.0 in my 1987 cherokee. It runs very good when it is cold all the way up to about 150 degrees, then once the engine gets hotter than that it feels completely gutless. When it is cold if I stomp on it you can really tell but when its warm to hot you would hardley know I just punched the gas pedal. Also the hotter it gets the higher it idles, cold it is at 750 RPM but warm it might sit at 1,200 RPM. I know it is temperature related because if I turn my auxilery fan on to cool it down, it starts gaining power again. Any ideas?

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If the problem is consistently coolant temperature related, a good first suspect would be the coolant temperature sensor. There's also a manifold air temp sensor.

Here, from a chilton Fuel injection manual, are ohmmeter readings for a healthy CTS (fahrenheit degrees, ohms with a digital ohmmeter) the CTS is located on the left side of the block, below the exhaust manifold. Do not confuse it with the sensor for the gauge:

212degrees = 185 ohms; 160 = 450 ohms; 100 = 1600 ohms; 70 = 3400 ohms; 40 = 7500 ohms; 20 = 13,500 ohms; 0 = 25000 ohms.

REsistance values for the MAT sensor are the same, it appears. This sensor is, oddly enough, located on the intake manifold.

I found it, I am not an idiot it is just blocked by the engine mount. I tested it and it passed the resistance test fine, any ideas on what else it could be?? I also checked the MAT and it passed the test.

I'm kind of stumped about what could be so specifically heat-related. Ordinarily wiring faults that occur when things heat up would not be so responsive to putting the fan on. Various changes occur in the engine control system dependent on whether the engine has warmed up, so perhaps the sensors that are working properly to tell the engine it's warm are triggering something else. It could be a pretty tough hunt. According to my book, which is not as informative on all points as I'd like, it appears that the only major component which is left out of the loop during warmup and added during hot idle and cruise is the oxygen sensor, which might warrant further investigation. but the O2 sensor is also supposedly cut out of the loop under wide open throttle mode. I'm afraid your guess is probably as good as mine.

After doing some wheeling my 91 would run fine until it warmed up then it would start to miss and back fire real bad except at idle and full throttle.
After chasing rabbits and pulling hair for about a week tried changing the tps and fuel pump with known good ones with no luck.
I changed the O2 Sensor now all is fine.
Worth a look,

73's KG4BET

Okay I figured it out on accident, it was a extremely bad catalitic converter. No clue how many miles were on it but I changed it out with a High Flow Magnaflow Cat and what a difference. The engine runs cooler, accelerate's twice as hard when it is cold and hot, gets much better mileage, and sounds extremely cool. I have 2-1/2 flowmaster exhaust and that Cat Converter really made it sound awesome. While I was at it I also made the adjustable MAP Sensor voltage thing, that gave me alot more horse power and I also have the ability to turn the voltage down to get much better mileage. I am going to buy a Air/Fuel Guage that hooks to the Oxygen Sensor so I will know how rich/lean it is running. Thanks for all the info guys!

JeepBoy318 that is the same problem i had. except it was bad power loss no matter what the temperature was. I mean my friends saturn beat me it was that slow. so since i was stumped i took it down to my local shop and they said. its the cat. when wheeling sometimes it hits so hard that it breaks inside and creates a backpressure backinto the motor. extremely bad. so i had them make me custom exhaust. 2.5 from the downpipe to cat, with magnaflow high flow cat, to 2.5 out to muffler the again 2.5 to tail pipe. it now sounds awesome, i ofter get the question what size of motor is in that thing. they all laugh when i say straight 6!

The innards of my cat fell out into the old muffler. New muffler, but same HIGH flow cat! :shhh: It even passed the sniffer test with flying colors, with a DEAD EGR valve!