1996 low idle/stumbling at start - high idle after driving

So I went to the dunes for Halloween weekend, and destroyed alot of stuff on the xj - not due to bad dune manuevering, but lots of speed/flooring and a radiator that cracked. Front driveshaft put a hole in the exhuast right before the 1st O2 sensor (getting that welded up this weekend) from a real hard bottoming out, radiator is need of replacement (hopefully happening this weekend), bent lca mount (replaced already), and it seems to idle low or even die (gotta hold the pedal down for a minute and then it seems to hold idle fine at around 650rpm) but after driving for a while, it will idle higher at around 1000rpm in Park and Nuetral.

so im trying to figure out what has happened to cause this; i swapped in my spare IAC and TPS and no luck. i checked all the vacuum lines that are accessible easily under the hood and didnt see anything...

as for the radiator and exhuast problems, the reason i've put repairing these things off is that my mobility is limited at the moment due to me dislocating my knee cap and being on crutches since that weekend as it happened on the last day of the trip where everything got destroyed - jeep/knee.... wonderful.

so besides swapping those sensors, im not sure what to check next. Has anyone had similar symptoms? i know its dumping fuel due to the exhuast leak right before the O2 sensor, but i dont think that would effect the idle a whole lot on startup.

Well, i went around and tightened all of the bolts on the exhuast/intake manifold since i figured they could have become loose considering the Front DS ate the hole in the exhuast right after the gasket for the exhuast/exhaust header. Amazingly, some of them were barely even hand tight!!

Apparently this had been causing a slight vacuum leak, because I can say that the idle seems more stable after having done that. It still runs a little rough though around idle. it seems at idle it will sometimes run around 1000rpm still (once warm?) and then drop to 700 with the A/C turned on. Vacuum problem still, or simply the engine raising the idle because of the O2 Sensor info due to the exhaust leak? I'll be repairing that at the end of the week hopefully so i guess ill find out then; but if theres something else that it has to be that needs love/replacement i'd like to get on getting those parts to work on this weekend as well if anybody knows b/c they've had this problem before.

Yeah, um, if you have a hole in your exhaust right before your O2 sensor, that's gotta mess SOMETHING up....

EDIT: Maybe get some bumpstops so you dont bottom out like that again

yeah. my guess is that the PCM runs the idle higher when the O2 sensor is mad.... i just wasnt sure why the RPM would drop to normal with the A/C on :/

also, i did order some 2" longer bumpstops. I trimmed my fenders way up out of the way of the tires, i figured the stock bumpstops would keep the undercarriage and exhuast from ever being hit by the Dana 30 as nothing else was changed, but I guess you can compress the stock ones a bit haha.

Hmm, started it from cold (coldest its been run since problems started probably since its 2AM and its almost chilly), and it barely held idle.... like 300rpm at first for a while.... the Iac is brand new, a tps would send the idle higher instead of lower... the O2 sensor doesnt matter untill the ECU/engine goes into closed loop at higher temperature, and i think a vacuum leak would send the idle higher rather then lower; or could a vacuum leak work both ways??

im stumped right now... but ill keep on keeping on with the fixing stuff up, and hopefully somebody who knows more then me will stumble upon this thread and point me towards the solution.

My 96 recently was having a stumbling/low idle problem as well. My check engine light was on due to the o2 sensor being bad. I replaced the sensor and sense have had no issue with it. Could be related or could be completely separate problems but i have no clue as im no expert.

hmm, ok. thanks for the that, I was hoping it could possibly be caused, and therefore possibly fixed by the o2 sensor.

Hmmm... I just noticed this morning when I stopped in traffic and shifted into park... As i let off of the brake, the idle would actually change - going up...

so basically everything on this XJ is pissed off at me I guess; but what hell; the brakes too??

if i tap the brakes real hard/fast (ABS via my foot basically) in park the idle keeps going up in small increments as well. what the hell is this XJ's problem?? I havent done anything to the brakes really recently, I did do the front pads two and a half weeks ago just prior to my trip but didnt notice anything then, and it was just pads. I did notice going down a hill the other night with the truck loaded up a bit the brakes seemed a bit weak, but it was a temporary sensation it seemed and they havent really acted up since... Could this have something to do with my idle problem???

The brake booster works off of engine vacuum, and I noticed on my 89 when I pump the brakes at an idle, the RPM's will go up.

You could try taking the vac line off the booster, and sticking a wooden dowel in it or something to try and plug the line, and then start your motor up and see if it's any different. Don't try driving it though as you will have really really hard brakes.

Well, i had the exhaust sealed back up last night, CEL didnt come back on, and it seems to run/start alot better. Almost all the issues seems to have gone away except if I start it up when its at operating temperature right now though, it is normal idle in gear, but High Idle in Park and Neutral! Something around 1150 give or take 50 rpms... And no, i dont have a high idle switch or anything...

Well, CEL came back on... so i cleaned out the TB real well and put it back on the vehicle, cleared codes... Code was back on as soon as it started back up; so it couldnt be vacuum right? sensor/electrical problem is the only thing that could bring back the code (P0505) back instantly right? IAC/TPS have been changed out, new spark plugs, wires are good, no missing... checked dist. cap... Could the O2 Sensor that was previously (before exhuast leak) now be fouled even though the exhuast is sealed back up? Or could the Map sensor be triggering the High Idle and P0505? I've doubled over everything besides those two sensors (vacuum/electrical) multiple times and im getting pretty frustrated now. I just dont want to end up with a spare o2 and spare map sensor in addition to the spare IACV/TPS sensors I now have.

well, it was a brake booster issue. it was intermittent it seemed. I would always have my foot on the brake when i started the car out of habit and then let off of it - causing the leak at startup and then sealing it before I would find it - b/c it would only leak with a lot of pressure on the pedal.

just thought I'd post that up in case anyone were to look on here with the same problems. (which i doubt since the number of problems were numerous) but anyhow.... thats my followup.

CommandoXJ96 said:

well, it was a brake booster issue. it was intermittent it seemed. I would always have my foot on the brake when i started the car out of habit and then let off of it - causing the leak at startup and then sealing it before I would find it - b/c it would only leak with a lot of pressure on the pedal.

just thought I'd post that up in case anyone were to look on here with the same problems. (which i doubt since the number of problems were numerous) but anyhow.... thats my followup.

Click to expand...

Yeah leaking brake boosters suck bad. I had one on my '90 for the longest time and everytime I got on the brake my idle would jump like 200rpms. It took me a long time to figure out what made the transmission shift hard into gear.