I recently bought a 90 cherokee with the 4.0 as a bit of a project. I have been dealing with the idle issues from day one. I have read a ton of posts and here is where I am at. I have replace the TPS, IAC, O2 sensor, and MAP. I have also warmed it up and tightend the manifold bolts, redone most of the grounds, and added 2 extra grounds, one to intake and radiator clip. I removed the injectors and cleaned them. replace cap and rotor, wires and plugs. pulled the ECU, didnt see any capaciters that looked faulty (kinda over my head though) oh and pulled and cleaned the throttle body. Today I tested the fuel pressure, I got about 30psi and 37ish when I removed the vac hose to the fuel pressure regulator, however, then the engine idle dropped a bit and with the vac hose back on I was getting 37ish again, it would seem that I have a loss in vac pressure at idle, which makes sense cause the breaks suck at idle but there is a notcible differance when i increase the RPMs. I have sprayed brake cleaner and carb cleaner all over when running and cant find a leak... I am pretty sure I have already replaced parts I didnt need, help would be greatly appreciated!

 

Squeeze the hose to the booster shut and see how it affects the idle.

I know you've done some "ground" work but I would suggest completing Tips 1,3,4, and 5 in my Pink Link below.

 

this brings me back to my previous point on how 90's seem to idle weird. I really hope someone figures it out.

 

Mine doesn't. Idles just like my 88.

 

My 87 used to idle weird until I followed Cruiser54's tips. Also upgraded all grounds with 5-90's cables. Idles like champ now.

 

thanks for the response 54. I actually did try that with the hose already and it didn't make a difference. I will read all of this other post that you gave me, unfortunately I have already done most of the ground refreshing and also replaced all of the relay connectors. I think I may be at the point of tearing into the rest of the wiring harness. can you tell me if the timing on this year needs to be adjusted or if its done by the ECU?
My backup plan, has anyone converted theirs to a carbureted manifold? The number of sensors involved for this model is insane!

 

Not that simple everything works together sensor wise. Whats the idle high or low ?

 

I guess I would call it a rough idle, that will randomly go up. When I put it in gear(automatic) it will die or almost die.

 

Finish Cruisers tips. Hes the Renix God..And cable upgrades work wonders. Still running stock injectors ?

 

yeah they are stock. I found that the CPS was going bad and replace it. The knock sensor is also testing bad, I read some articles that said just to leave it unhooked? I am going to give that a try for now. Also just found that single ground wire that comes off the dipstick tube to about 10 other grounds, going to redo all that and ground it to the fire wall. hopefully all that will help....

 

Get all the grounds redone and the connectors refreshed. How did you determine the knock sensor is bad?

 

Multimeter set to ohms, nothing at all from the ground to one of the plugs?? I watch a video. After replacing the CPS it certainly starts a whole lot better. I did the cps and all the grounds on the portion of the wire harness that runs all those sensors and the ecu, I also adjusted my tps, and the knock sensor is still disconnected. It fires up good, but still drops to a low rough idle and sometimes dies, but I feel like the were some improvements. Now the brakes seem the vacuum pressure they need, its odd because the idle still is very low, also no when I put it in gear(auto) the idle is almost completely unaffected. I followed the throttle plate steps and backed off then in till if just moved, but when the idle was low I tried to adjust it to give it more... didnt do anything.

 

Its not a idle adjustment screw its all computer controlled . Search volvo injector upgrades it definitely a step in the right direction for old factory injector upgrades.

 

So an update. I took the advice of the last post and went out and got some 4 port injectors off a ranger. also did the fuel filter. I noticed a huge difference in throttle response, but it did not resolve the issue. Still idles like crap once it warms up, and the issue with the brake booster is not having the vacuum it needs is back. Also now when I take it out for a short drive the idle gets worse, most of the time it just slowly dies. I did find that the MAT(which I cant find a replacement for) is bad. Not sure where to go from here... Anyone know where a vacuum line that comes out of the firewall on the passenger side goes. There are two that come out, the black one is hooked to something, the gray one is just sitting there.

 

I wonder if your O2 sensor is bad.

Make sure your intake manifold bolts are snug.

Your EGR could be hanging open causing an undetectable vacuum leak.