93 Zj terrible nasty idle...

C.Berry

C.Berry

Bad News
Joined
Aug 10, 2008
Location
Blacksburg, Va
so I met a nam vet at oriellys few weeks ago he's a older gentleman with a love for chevys,4x4s and good ole America. Well he's been throwing parts at a 93 Zj 4.0 that he bought to drive so he didn't have to drive his 76 short bed and put miles on the new engine, well he asked me if I could come take a look at the jeep and if I could fix it he would pay me... well I went over Thur for around 5 hours he has replaced fuel pressure regulator, tps,iac,rotor,plugs,wires dizzy cap! I took my fuel pressure gauge and found 18psi in the rail so we replaced fuel pump and now pressure is at a steady 33psi I know with my 96zj I have pressure in 40ish range the jeep has a terrible rough idle hard start and sounds like it's gonna stall unless gas is given to it!? Any ideas so I can help this fella out he thinks he's paying me but what he dont realize is he already has by serving in the 187th airborne for 20 years!!! since I couldn't serve myself I wanted to do what I could to help this guy out he has severe ptsd and told me he can't live with anyone since he got back from nam bc of nightmares an such, seems like a cool guy told me stories about ranger school him and his buddy went through I come from long line of marines so I can relate with him on ptsd bc I have family that's been through it! Any ideas what could be causing this crazy idle? He thinks it's a head gasket or cps I'm gonna do compression test next week on it but am I missing something? Went through timing all sis right never seen a 4.0 idle this rough tho...
 
Idles around 800rpmin park and 500-600ish in gear it seems like it's gonna die in drive but doesn't! jerks stumbles and just rough... cam shaft sensor possibly? Jeep has 207K on it!
 
Maybe a clogged fuel filter? Relatively cheap and easy repair. I didn't think to let pressure off my system and took a nice bath in gas as well
 
C.Berry said:
Idles around 800rpmin park and 500-600ish in gear it seems like it's gonna die in drive but doesn't! jerks stumbles and just rough... cam shaft sensor possibly? Jeep has 207K on it!
Click to expand...
Not a cam sensor. That PCM needs cam sensor signal to start, but not to keep running.
 
Crank sensor possibly. Or could be the coil or the cam sensor. Ours has been crappy forever and we found that it was only putting out 3-5000 volts to plugs. Put new coil in and it helped that. Make sure there's no crack in the plastic around the coil.
We Finally replaced the IAC that we had already replaced and it is a new Jeep now. Feel what the IAC is doing down in the throttle body and see if it seems to be regulating air or just doing the same thing all the time. If he didn't clean where the IAC fits in then it could be gunk in there. Scrub inside of throttle body with tooth brush and brake cleaner or carb cleaner.
Pull the vacuum line off of the brake booster and see what that does. The brake booster check valve can cause idling issues too. So not sure if this is any help. We chased ours (my sons) forever and it finally runs a lot better. Ours is a '91 XJ 4.0.
Edit: Just read Jody's post and didn't know about that year cam sensor, ours is built into the distributer.
 
93redzj said:
Maybe a clogged fuel filter? Relatively cheap and easy repair. I didn't think to let pressure off my system and took a nice bath in gas as well
Click to expand...
New filter with pump but he installed filter before hand could it be directional?
 
Ok I'm gonna pull iac and swap for new one to see what happens he already has the crank sensor so I guess it would t hurt to replace it? I'm trying to save him a few bucks bc he bought head gasket kit cps and few other things that he probably doesn't need!! What's avg compression on 4.0 I feel like a jeep newb all over again bc I ain't messed with a 4.0 in a hot minute...
 
I don't need recognition only reason y'all know he's a vet is bc I know there's some damn good people on here that still give a shit and a lot of ya know the 4.0 upside down and backwards... :usa:
 
If you've never replaced a crank sensor, either have some tiny hands available and/or a long 1/4'' or 3/8'' extension ready.
 
I've done a crank sensor unfortunently I have giant hands so bendable ratchet wrench will be my friend in this job! Fuel pressure reg is new unless this one is bad I didn't replace was replaced by him. I know the tbi 350 a coolant temp sensor can cause crazy stuff but is that true on 4.0? The coolant senor near firewall is broken but when I push it in and it shows engine temp no change in performance or idle... heading over Sunday if I can get a compression tester so I can rule a gasket leak out!
 
The fuel pump can be changed without changing the regulator.. it's cheaper to just change the pump, so ask if he changed the unit or just the pump. Also injectors may be leaking, pinch return/supply and that will show the leak source. CTS for ECM is located on thermostat housing. If you have to change the ckp, drop the trans cross member, makes it 3x easier for 4 extra bolts.

Reread original post... Check fuel pressure leak down.. bet you got a leaking injector or two.. plugs will tell you if that's the case too. Compression test will rule out head gasket.
 
Last edited:
Sorry works right on my heels, turns out to be a head gasket I'm not sure if he is wanting to replace it I told him I'd sale my Zj drivetrain for 300 and install it for him if he wanted for free and some cool stories. Lol So that's the plan I just have to fit work into equation and kids,woman,rental... I was in spring lake NC last week for the day and went to the army airborne military museum and his regiment was featured there the "Rakkasans", walked the museum with my ole ladies grandfather a retired army doctor, so a cool experience all the way around!! Any of y'all close by springs lake area?
 
Back
Top