My 1990 Cherokee 4.0 started stumbling and hesititating about a week ago. I checked and saw that two of the hard plastic vaccum lines were cracked. I presumed that it was vaccum leak and replaced the front/back vacuum harness that comes from the valve cover. It's still stumbling under a load, backfiring when revved in park and dying after revved in park. I went through and pinched off every vaccum line one at a time and could not isolate a leak. Its stumbling under load and if in park and revved it will stumble and die. Occasionaly it seems ok. I have replaced many parts within the last year including all sensors, computer, injectors, distrubutor, coil and others. I am so frustrated with it and ready to send it to pick n' pull. Any suggestions would be appreciated.

 

Have you tried to see if it's putting out any codes?

 

It's a Renix, Robert.

OP- do a fuel pressure test as your pump may be onn it's way out. Gotta elim inate that as a possibility first.

 

Checked fuel pressure. The pump filter and regulator are all about 7 months old . I tried pinching various vacuum lines but seemed to have no affect. I don't have a vacuum line diagram so not sure I hit them all. I have a ton of new parts on it but getting frustrated with it. It is stumbling and backfiring in neutral also now. Could it be an egr valve malfunction? Thanks

 

Here's a diagram. Perhaps the eGR is hanging open?

 

I thought it might be a clogged cat, so I unbolted it and it still does the same thing, just loud. I manual acutuated EGR vavle and it would die so I assume it is not stuck open. Not sure what else to check, the vaccum system seems like it shoudl be intact. Is there not vaccum resevior on a 1990?

 

What were the fuel pressure results?

 

if you have access to a vacuum pressure gauge, take your brake booster vacuum line off at the manifold. at see if its holding good pressure. I cannot say what its supposed to run with these engines exactly but I know with SBC it should be around 15-25 at idle.

you need to find out what normal operating vacuum is for sure, check it and that will least least you know if you have a vacuum leak.

if it is leaking(wont hold manifold vacuum) take a can of brake clean, and start spraying it while idling around the manifold, throttle body(or carb) and see if it idles up.

you can sometimes pin point a leak easily this way. sometimes not so much depending on where the leak actually is.

also, could be a valve spring, out of adjustment valve lash, or worse a collapsed lifter.