Jeep Wrangler lurching violently

jeep
wrangler

#1

I have a 97 Wrangler 4 liter that lurches violently. It runs fine until the engine is warmed up. After the engine warms up it it begins lurching. When I put the clutch in, the lurching stops. When I let the clutch out, it is okay for 1-2 seconds (i can get power and advance the vehicle), then I put the clutch in and repeat that process. When I give it a little gas with the clutch in, the engine runs smooth - so it doesn’t appear to be a gas problem. When I give it gas with the clutch out it lurches violently. After pulling it into my garage I let it idle to about 5 minutes. It idles rough and is about to stall, when it apparently shoots a little more gas preventing it from stalling.
The check engine light has come on. I took it to my local mechanic who cannot find the problem. The “computer code” indicates a faulty oxygen sensor. He also checked the fuel pressure which is okay.
It has 120K miles and otherwise has been a great vehicle. It has been well maintained.


#2

Did you replace the 02 sensor? The cat or muffler could be bad and restricting exhaust flow. Look for a more skilled mechanic or even take it to a Jeep dealer. More diagnostics are needed before someone starts throwing random parts at it hoping something will work.


#3

The O2 sensor was not replaced. My mechanic indicated that the O2 sensor would not cause those problems. He did suggest taking it to a Jeep dealer. The Jeep dealer is too far to drive it (in it’s current condition). I’d have to have it towed. I was hoping someone had witnessed these symptoms and new what to do. Do you recommend any particular diagnostics that I could try to narrow down the faulty part(s)?


#4

A false input from the upstream oxygen sensor can cause the engine to run leak. Engine power will drop off then recover resulting in a surge. This may be observed after a few minutes of operation after the computer switches from open loop to closed loop operation.

The oxgen sensor input and fuel trim can be monitored with a scan tool, if a scan tool in not available unplug the upsteam oxygen sensor and drive the vehicle. Without the O2 sensor input the computor will operate with default values, the engine should run normal.

The upstream oxygen sensor is very likely the problem.


#5

@msunix

I agree with @Nevada_545

If in doubt about a sensor, unplug it. If the engine runs better with a sensor unplugged, that was most likely your problem

I clearly remember a situation where an engine wouldn’t start, because of a faulty mass airflow sensor. Once unplugged, the engine started and ran pretty well, in default mode, of course.

BTW . . . can you please provide us with the exact fault code . . . P0130, etc. . . . not the interpretation, just the actual code


#6

I disconnected the upstream O2 sensor and took it for a test drive. Initial indication is that that fixed it. I do notice a small hole in the insulation on one of the 2 lead wires of the O2 sensor. I suppose it was shorting out against the manifold. I’ll get a new O2 sensor tomorrow and give it a more extensive test and then report back.
Thank you very much for your help.
BTW, I don’t have the exact fault code - my mechanic (who btw is really a transmission specialist - but I use him cause he’s very honest) had told me it was the O2 sensor. I don’t have the appropriate test computer, but if I can get it tested again I’ll report back.


#7

I replaced the upstream O2 sensor and the lurching has ceased. While I was at the auto parts store, they checked the fault code, which was P0132. So, all seems to be good at this point.
I didn’t know I could unplug the sensors to help narrow down the problem. Thanks for that tip, that will be useful in the future also. Thank you again for your help.


#8

@msunix

Congratulations on a successful repair!