1998 Wrangler TJ 2.5 5-speed: engine hunting / surging issues

[Mick's Jeep]

Dear fellow wrangler enthusiasts,

I've purchased my first wrangler a couple of weeks ago; 1998 TJ, 2.5 L, 5 speed manual gear and it's pretty much stock, apart from some exterior parts. Whilst test-driving, there weren't any apparent issues with the engine. A few days after I bought it though, the engine started behaving weirdly. About 1-2 minutes after starting the engine (cold start) it starts surging/hunting. The RPM goes very irregular and sometimes drops to as low as 400 before it shoots up to >1000 again. This happens both when the engine is idling or when driving in gear. It doesn't happen always, but about 95% of the time. When it happens when driving, I'm usually still in 1st or 2nd gear and doing not more than 30 mph. The RPM's drops and the engine starts surging as if it's not getting enough fuel. Depressing the throttle does sometimes help it get back to normal RPM's, but definitely not always. I've also noticed that when the car is parked with the engine idling after starting, an increased amount of condense (water) is coming from the exhaust as soon as the surging starts.

When I stop the engine and start it again the issue is almost always resolved, and I can drive 100 miles without any problems. When starting the engine when it's still warm (e.g. after a short stop at the gas station), it doesn't have any issues either.

Since I've bought the car, I've had it serviced by my local garage. The usual oils, liquids, and filters have been replaced and a general check on the vehicle was done. The mechanic said there weren't any fault codes active. An internal engine cleaning additive was also added to the tank. He recommended driving it around for a while and seeing if it would help at all. That was 2 weeks ago and so far it hasn't helped at all.

My best guess would be that the engine problem originates with a faulty/dirty TPS, MAP or O2 sensors, but I'm not sure where to start in all honesty.

Does anyone have experience with similar problems or can give me some advice on where to start looking?

Mick

 

[macleanflood]

I'd look for vacuum leaks...and...

Thread 'How to clean your IAC (Idle Air Controller) valve'

Jan 2, 2016

If you're experiencing idle issues on your Jeep Wrangler TJ the most likely culprit (and the first thing you should check) is your IAC (Idle Air Controller). Often times the IAC gets very gunked up causing the Jeep to idle improperly. A simple cleaning is usually all it takes to get it running as good as new.

If you end up needing to replace your IAC valve (they do fail on occasion), you can find the part numbers (and the links to purchase them on Amazon) below:

2005-2006 IAC Valve (Part # 4861552AC)
1997-2004 IAC Valve (Part # 4874373AB)

There is no need at all...

• Chris
• iac idle idle air controller idle valve
• Replies: 191
• Forum: TJ How-to Guides

• 

-Mac

 

[Chris]

Yep, I'm going to second looking for vacuum leaks. A smoke test will be your friend!

 

[JeepDrivesBest]

Beyond vacuum leaks/IAC, check the coolant temp sensor and its connector/grounds—if the PCM sees a false cold reading it over-enriches and hunts, then behaves after a restart.

 

[Mick's Jeep]

Thanks to all of you for your advice. I'm gonna have a look this weekend as I'm outta town the coming few days. I'll keep you updated!

Cheers!

 

[Mick's Jeep]

Hello again,

Took me a while but I've had a look at the engine issue again. Here's what I've done so far:

- Visual inspection of vacuum system; no leaks or cracks found.
- Smoke test (DIY); haven't observed any leaks with the smoke.
- Cleaned upstream O2 sensor.
- Cleaned IAC valve; was a bit dirty.
- Checked coolant temperature sensor; I get good resistance values that change with the actual temperature of the coolant.

The situation is as follows after all above mentioned checks:
- Problem with engine hunting still persists, although not as frequent as before. It is now hit or miss really with the engine starting to act weird about half of the time after starting it. It should be mentioned though that, whereas before it always happened once, but was rectified by killing and starting the engine again, it now sometimes happened twice in quick succession after driving off, i.e. it happens, I restart the engine, start driving again and within a short while it starts 'doing it' again.
- I've noticed that when I drive off immediately after starting the engine instead of letting it warm up for a little, the engine is less likely to start having issues.
- Still no 'check engine' light or any other indication of a problem.
- Once problem has gone away, can drive for a long time without issues.

Something in me says it's the upstream O2 sensor. I've tried to measure the sensor and sensor heater but struggle to get a reliably reading as my probes can't exactly fit in the sensor connecter.

Does anyone have any thoughts on the matter? Any pointers on where to look next?

Enjoy the rest of your weekend,

Mick

 

[macleanflood]

Watch ODB live data... monitor the O2 voltages. They should range below.9v.

-Mac

 

[Mick's Jeep]

Hi all,

Had to wait a couple of days for the ODB scanner I ordered to arrive (didn't have one yet). I've hooked up the scanner, monitored the live data and found the following:

- No codes (as expected).
- O2 sensor voltage with the engine stopped is 1.0 V. When the engine is started the voltage on both sensors remains 1.0 V for a little bit. The voltage on the B1S2 sensor starts dropping off after a little while. The B1S1 sensor voltage however first jumps up to approx. 1.2 volts before slowly dropping down. At this very moment the engine also starts running rough. As the voltage drops the engine also starts running better again.

Would it be safe to assume that the problem lies with the O2 sensor as the voltage spikes out of normal range?

I'm planning to install a new sensor, but they're quite expensive, so I would like to get your opinion before I order.

Cheers,

Mick

 

[macleanflood]

Yup any O2 that doesn't range below .9v is considered bad.

We just had someone spray a can of brake cleaner on an O2 and revive it. If you want a hail Mary...you could try it.

-Mac

 

[Mick's Jeep]

So... swapped out the upstream O2 sensor for a new one (Walker, took the number from one of the treads on here). Unfortunately, this didn't solve the problem. Still have the upstream sensor voltage jump up before it goes down to normal range. The new sensor voltage does seem to fluctuate a lot quicker than the old one, which is a positive I suppose.

Did a test drive with the scanner connected to read live data. Whereas before the sensor voltage would start changing from the (default reading?) of 1,000 V within 3 minutes of starting the engine, it now took about 15.

All this leads me to believe it's a wiring/grounding issue? Checked all the grounds I could find with my multi-tester. None of them have over 6 ohms between the battery negative and the ground which seems good to me. Haven't figured out where the O2 sensors are actually grounded. Gonna try and trace the sensor cabling somewhere coming week in the hope I can find the cause there...

 

[Mick's Jeep]

Hello all,

Pleased to inform that the problem has been resolved. After swapping out the upstream O2 sensor checked and cleaned all grounds (on engine, from battery negative, on firewall etc) to ensure propper grounding. Checked and cleaned the connector on the PCM that holds the O2 sensor wiring.

Since then I've started and driven with the car 6 or 7 times on seperate occations and haven't had any problems! There's still a lot of work to be done on the Jeep, but at least the engine is reliable now.

Thanks Mac and everyone else for helping me out!

Mick