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Need Help - Jeep Surging/RPM Spiking

jeepster_259

NAXJA Forum User
98 xj classic, auto, 4.0.
So, I went out wheelin' today to take the jeep out and test run it since it had been about a month or so since I'd really driven it. Didn't beat on it too hard, but went through some mild water and got stuck in some mud up to top of the 33" tires a couple times.

Before going out today, I made the following repairs over the past month:
180* thermostat
new e-fan
new brake shoes, pads, calipers, rotors
new oil pressure sender
radiator flush

Also, upstream o2 sensor is bad so I had left it unplugged for the past 100 miles or so. when plugged in the jeep idles like crap and carbon comes out the tailpipe. When unplugged, it runs fine.

Anyway, with that background, I can't figure out what the issue is. Halfway through wheelin' today, the jeep starts to buck and surge in all rpm ranges and backfires in the manifold. At idle, it will drop down to 200 rpm, almost stall, then shoot up to 1500 rpm. With my foot on the gas steady it will spike and drop about 750-1000 rpm every couple of seconds. After shutting the jeep down for about 15 min. while we ate, the jeep was fine and made it out of the trail w/o an issue. While driving home, it started again. Stopped at parts store to pick up a few parts (shut down for about 10 min.). Jeep ran fine the rest of the way home.

Got home, was washing off jeep with it idling, started doing it again. Went to parts store, tried replacing TPS, IAC, and MAP sensor. Nothing. I thought it was the IAC at first b/c when I tapped on the old one while the jeep was running, the surging would stop for a minute or so. Put in a new one, still surging. Cleaned old one, put back in, still surging.

Any ideas??????
 
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Unplug your CPS clean it up real good and plug it back in. The connector is at the rear of the valve coer on the dr. side. Dirty/wet CPS can cause you some wierd problems also. I was gonna say you got your TPS wet and stuff till I noticed you replaced it. How much did you spend on sensors there parts swapping 200 bucks???
 
I spent $0. Bought the new sensors, plugged them in to see if it changed the problem. When they didn't I returned them! :worship: Made sure not to scratch them or anything.
 
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I spent $0. Bought the new sensors, plugged them in to see if it changed the problem. When they didn't I returned them! :worship: Made sure not to scratch them or anything.

Well that is some Cheap trouble shooting lol, I may have to try that. They always have young dumb kids working at O'reiley's partly the reason I don't shop there.
 
Be sure all connectors are clean/dry; Even a little bit of moisture intrusion into a sensor connector could create symptoms. If this problem began IMMEDIATELY after wheeling where any water got into the engine bay, gotta suspect moisture. Simple "cause and effect".

I imagine you have had a check engine light on prior to this problem because of having the oxygen sensor unplugged? May be a good idea to resolve that issue so that you have an engine with no check engine light and any future codes/check engine lights can help you isolate. Pulling codes may still help you though as even if the check engine light was on before, any new problems may still be setting codes that you don't even know about.

Good Luck!
 
my check engine light was on with the o2 sensor plugged in and unplugged. While this bucking/surging happens, i tried resetting the computer to clear any codes. 1/2 the time the CEL didn't even come on at all with this happening, 1/2 the time it did.

Don't have a scanner handy, so couldn't pull codes to see if there were any new ones.

I was just thinking in bed last night, since my 02 sensor is unplugged, maybe if the connector got plugged with mud/water, could it somehow send messed up signals to the computer causing it to do this? I'm going to get a new o2 sensor this afternoon and install it, see what happens.

Also, in thinking and reading other posts, it seems to only do it once the jeep is warmed up, which means it is in open loop, correct? So, if it only does it in open loop, what are the possible culprits...sensors, right? So, if I've eliminated the IAC, MAP, and TPS, and am about to eliminate the O2 sensor as culprits, what else could be the problem???

I really need some help on this one. My buddy and I had a road trip planned to go off-roading this weekend, but it looks like that's going to be postponed unless I can figure this out ASAP.
 
Be sure all connectors are clean/dry; Even a little bit of moisture intrusion into a sensor connector could create symptoms. If this problem began IMMEDIATELY after wheeling where any water got into the engine bay, gotta suspect moisture. Simple "cause and effect".

On the trail, it didn't really happen after any significant mud hole or anything, just kind of out of the blue. Then it went away. Then came back on the way home.

All my connectors are packed with dielectric grease to keep water out, so I can't see how any water would have gotten in them, especially since the water I went in was minimal. I checked the distributer, dry as a bone.
 
if you KNOW the upstream O2 is bad, you've got to change it before you will get any useful codes about the newest problem. Without the upstream connected, you'll never enter closed loop operation and will be running off rough estimate fuel mixtures.In open loop mode, if you get warm enough, the mixture will be too rich for the operating temps and can cloud the issue.
I'd change what I know was bad, reste/ clear codes and see from there what you're dealing with.
 
no I didn't try that b/c I know how hard that bastard is to get to. I'm hoping i can diagnose it as something else before that or be pretty darn sure it's that b4 I attempt it.


How hard it is to get too!?! The connector is sitting on top of the engine at the rear of the Valve cover. I am not talking about taking it off the bel housing let me see if I can find a pic... Pretty easy to get to and one of the easiest checks there is when your engine starts acting up!


Crankshaft_Position_Sensor_diagram.jpg
 
Ok, I thought you were talking about taking it off the bellhousing. Unplugging the connector is a piece of cake. And yes, i did check the connector, clean, no water. Nice and grease up like I left it!

And I will be replacing the upstream O2 sensor today. I've heard about there being problems with the jeeps and non-mopar o2 sensors. But I'm going to be getting a Bosch one as that is the cheapest and most readily available one I can find. Although it's still not cheap at $65!
 
Well, good news, kind of. Turns out it was just the oxygen sensor connector plugged up with mud. Must have been shorting out.

However, I went out and bought a new o2 sensor ($70 later) and plugged it in and the jeep runs great, nice and smooth, better power and shifting, CEL is out after resetting computer.
After a 20 min. drive, CEL comes on and it starts doing the stumbling at idle and running really rich, which is the whole reason I unplugged the old O2 sensor. I can smell gas while I'm driving and carbon comes out of the tailpipe pretty heavily.

So, now it appears that my old o2 sensor might not be bad. When i pull the codes it's saying o2 heater and misfire. If I unplug the O2 sensor, the codes stay, but the jeep runs fine. That's why I had left my old o2 sensor unplugged, but didn't think to cover the plug, so it filled with mud, causing the problems stated in the beginning of this post.

Any suggestions??? I'm about to go on a 4 hour road trip to go wheeling and would like to figure this out. I may start a new post on this, since it is a new issue.
 
Isn't the O2 heater sensor circuit fused on your vintage, possibly at the fuse/relay box under the hood? I'd check that out to be sure you don't have a blown fuse and also inspect your oxygen sensor circuit wiring closely.
 
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