Why does a 1990 jeep cherokee, 4.0, surge and hesitate?

Question

We have replaced the ecm, knock sensor, plugs, plug wires, cap rotor, coil, ignition module, fuel filter, and fuel pump. It will surge and hesitate bad after warming up for about 5 minutes.

so far none of the answers have helped. We are and ASE certified shop with 4 master technicians and they are all stumped. We have recently found that the fuel pump regulator does not seem to change the fuel pressure when you unplug it, and it should increase it by about 10 pounds of pressure. The engine has a VERY slight knock at initial start up, so we unplugged the knock sensor, not the oxygen sensor, and nothing has changed. Almost everything on the vehicle has either been tested to within specs or replaced. I was hoping this was a common problem, but nothing yet. Thanks for your help.

Answer 1

bad car,,, when it acts up, jump out real quick with a big stick, wack it across the back end four or five times and tell it how bad it is

Answer 2

This must be something common with Jeeps...I have a 1995 Jeep Grand Cherokee and mine does the same thing sometimes. My brother who works on cars says that it is because the engine is not getting enough gas (which I noticed if I am going at a steady pace on the highway it does not hestitate but it does if I am driving in town where the traffic is stop and go). So try putting your foot down on the gas a little more. I've noticed that helps with mine. I've had mine for about 6 years now, I bought it used and it has hesitated since I got it and it never seemed to be a major issue as far as driving the vehicle.
You also have to think about the fact that the vehicle is 16 years old.....

Answer 3

I assume by "knock sensor" you mean the oxygen sensor...if not, I'd look into that. Does your Check Engine light come on when the hesitation, etc. begins? If so, I'd go someplace and have them read the trouble codes for you. You could have a different sensor that's bad. I'd also check the EGR valve if your Cherokee has one...I think the 4.0 does. On a 1990 vehicle it could be ready to fail and can cause the problems you mention.

Hope this info helped a little.

Answer 4

You may have water in your fuel filter. Try adding some gasline antifreeze, see if it runs a lot better, and if it does, consider changing your fuel filters near the gas tank on the lines.

Answer 5

have your sensors checked especially the throttle position sensor

Answer 6

carburetor adjustment, may need a re-build

Answer 7

carborator need to be cleaned