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1998 Jeep Wrangler with rough cold start

CAR TALK, April 6, 2010



To: Tom & Ray



I have a 1998 Jeep Wrangler that runs rough and backfires when first started–for about a mile or so, or about several minutes of driving, and then it is smooth and has good power and acceleration. I bought it new, and have maintained it diligently.



To try to correct the problem, I have changed:

Spark Plugs, Ignition Wires, Cap, Rotor, Air & Fuel Filters,(checked timing),Fuel Pump–



Also CHANGED SENSORS: Coolant Temp. Sensor, MAP Sensor & TPS Sensor–Used GM Upper Engine Cleaner, GM88861802.



Two Jeep dealers have used diagnostic machines and found NO codes and no problem. The roughness is somewhat intermittent, and since the problem is present for a brief period, I ask them to start it and begin driving immediately, but I suspect they take too much time before moving.



I really love this vehicle, and this problem is driving me nuts–plus, I worry that it’s a sign that the engine will quit completely at some inconvenient time.



I am not hallucinating; my wife and kids have reported the same behavior.



Is it possible that the Engine Computer that controls fuel mixture and timing advance in start-up conditions is dead or defective? Could that function be separate (or different) from controlling fuel and ignition during normal running? HELP!

I have the same exact problem, what did you find out?

I have an 05 wrangler (4.0) that had the identical problem. Don’t know if your 98 has a crankshaft position sensor, but changing the CPS fixed the problem in my 05. Actually, since it was new it seemed to ping under only rather light load, and changing the CPS cured that, too. May have been a faulty part from the factory. It did not throw a code.

THE OXYGEN SENSOR!!
THE OTHER SENSORS I CHANGED WERE RELATIVELY INEXPENSIVE, SO I CHANGED THEM IN A DESPERATE ATTEMPT TO SOLVE THE PROBLEM OUT OF BLIND LUCK.
I FINALLY GOT A DEALER TECHNICIAN TO HOOK UP THE DIAGNOSTIC DEVICE BEFORE HE STARTED THE JEEP IN THE MORNING (I LEFT IT THERE OVERNIGHT) AND HE FOUND THE OXYGEN SENSOR READING WAY OUT OF RANGE ON START-UP.
THEN, AFTER A FEW MINUTES, THE READINGS WERE IN RANGE.
SO, IT SEEMS THAT THE JEEP’S COMPUTER BELIEVED THE START-UP READINGS WERE TELLING IT THE MIXTURE WAS TOO RICH AND IT LEANED WAY DOWN. WHEN THE READINGS GOT IN-RANGE THE MIXTURE WAS BROUGHT RIGHT WHERE IT SHOULD BE. THE O2 SENSOR WAS APPARENTLY ON IT’S LAST LEGS AND DYING GRADUALLY AND INTERMITTENTLY. IT’S NOT CLEAR WHY THE COMPUTER CODES DIDN’T SHOW AN OXYGEN SENSOR PROBLEM.
SO, FOR $300+, THE JEEP IS RUNNING LIKE NEW!

I DON’T KNOW ABOUT A CPS PROBLEM, OR IF THE '98 WRANGLER EVEN HAS ONE-- PROBABLY NOT, IT’S THE OLD IN-LINE 6, WHICH IS WHY I’LL KEEP IT FOREVER.(I ALSO HAVE NOW AN’09 UNLIMITED AND A '10 RUBICON, BOTH WITH V6s, THEY’VE BEEN OK)
BUT, THE PROBLEM WITH MY '98 WAS THE OXYGEN SENSOR!!
THE OTHER SENSORS I CHANGED WERE RELATIVELY INEXPENSIVE, SO I CHANGED THEM IN A DESPERATE ATTEMPT TO SOLVE THE PROBLEM OUT OF BLIND LUCK.
I FINALLY GOT A DEALER TECHNICIAN TO HOOK UP THE DIAGNOSTIC DEVICE BEFORE HE STARTED THE JEEP IN THE MORNING (I LEFT IT THERE OVERNIGHT) AND HE FOUND THE OXYGEN SENSOR READING WAY OUT OF RANGE ON START-UP.
THEN, AFTER A FEW MINUTES, THE READINGS WERE IN RANGE.
SO, IT SEEMS THAT THE JEEP’S COMPUTER BELIEVED THE START-UP READINGS WERE TELLING IT THE MIXTURE WAS TOO RICH AND IT LEANED WAY DOWN. WHEN THE READINGS GOT IN-RANGE THE MIXTURE WAS BROUGHT RIGHT WHERE IT SHOULD BE. THE O2 SENSOR WAS APPARENTLY ON IT’S LAST LEGS AND DYING GRADUALLY AND INTERMITTENTLY. IT’S NOT CLEAR WHY THE COMPUTER CODES DIDN’T SHOW AN OXYGEN SENSOR PROBLEM.
SO, FOR $300+, THE JEEP IS RUNNING LIKE NEW!

I am going thru the exact same symptoms and it is driving me crazy too. Today I have just cleaned the Throttle body and the various sensors. I have also welded up all cracks on the exhaust manifold which Jeep claimed to be the reason for the rough drive during cold starts. I test drove it afterwards and it was a little rough in idle but it did not do the misfire ( running out of gas symptom ). Tomorrow when it is cold I will try again, but my research points me towards the o2 sensor. ( Air / fuel mix ) It only makes sense as the symptoms matches exactly that. If you let too much air in then this can happen. Please write what you have learned so far!

Since I changed the oxygen sensor in April of 2010, the Jeep is starting and running like a champ. As I answered Windcaptain and 03impreza, the failure mode of the O2 sensor was intermittent and for a brief few minutes on start-up. I have never used one of these diagnostic machines that read a car’s computer codes, but it seems strange that, on the several times I had the Jeep diagnosed after an episode of very rough start-up for about a mile, no codes were found… not until the Technician hooked up the diagnosis equipment BEFORE starting the Jeep and got the temporary out-of-range readings.
The O2 Sensor IS expensive, but that’s the price of clean air!