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[thedriver2005]

Hi all
I have a '98 4.0l Wrangler Sport and i have been having an ongoing problem.
While my wrangler is warming up (when it hits about 95 degrees) i lose all power, starts backfiring and hesitating on idle. If i can nurse the engine past that temp she seems to run fine.
When it happened in the past i had fitted a stainless exhaust manifold and a rr oxygen sensor and it seem to cure it. But i have just got back from cornwall (600 mile round trip at motorway speeds) and its worse than ever. I have disconnected all the sensors WD40'd them and made sure they are all connected securely, but no change.
Any help would be very much apprciated. Mark

[Dave69]

do you have normal driving when the engine is cold ? and the performance is dire when hot. the quick answer that jumps to mind is change the coil pack and see what happens. good performance when cold and poor when hot can lead to a coil pack breakdown (ref 4ltr engine with single coil pack)

[greggmo]

it is the oxygen upstream sensor or the throttle position sensor.

[Mike Pavelin]

Sounds like O2 sensor to me.

[thedriver2005]

Thanks to all who replied.
Just an update on the problem.
Obviously i am continuing to drive it until i can obtain parts.
The problem is now changing, its now running rough even from start up. She hunts at idle with a loud air induction noise and the engine revs up automatically when she is about to stall. When driving she is still hesitating and back firing and when the engine is hot she idles at 1500 rpm for a while then drops back to hunting.
Any further ideas before i buy parts?
Thanks, Mark

[Jonny Jeep]

Reading the initial post I though Upstream O2 sensor because that's what they do when the heater circuit in them goes bad.

With the newer symptoms I'm wondering if you've washed the engine bay as that can upset things including the throttle position sensor.

I'd swap the O2 sensor myself.

As it's a 98 the key trick won't work and you will need to find someone with a code reader to see if any codes are stored. The O2 sensor won't show up if it's only the heater circuit that's failed.

[haslamg]

Bonjour

Have you:-

1) tested for vacuum leaks
2) looked at cleaning the carbon out of the throttle body and IAC which is bolted to the back of it
3) considered testing the sensors against the resistance and voltage specifications in the Haynes manual (takes about 30 mins to do all of them)

GH

[wildwood]

If at  steady throttle you suddenly get a momentary cut out of literally a fraction of a second followed by a backfire and power again that gives the old drivetrain a good jolting, then that is the throttle position sensor, happenened to me it got very wet at Tong! Changed it and back to normal.

From what you wrote it seems that is one of your symptoms.

[greggmo]

it is the oxygen upstream sensor or the throttle position sensor.

[greggmo]

any joy sorting this problem then?

[thedriver2005]

Thank you all to who reponded!
Fitted new downstream oxygen sensor and TPS and she now runs like a dream, smooth idle, fast response and quick acceleration

[PtP]

Fitted new downstream oxygen sensor and TPS and she now runs like a dream, smooth idle, fast response and quick acceleration

If you don’t mind me asking, where did you get the parts from and how much were they? Only I occasionally get the odd misfire and a very, very slight hesitation whilst accelerating.

[Manda]

Mine's doing it again, I think it's because it's standing a lot of the time, I'd be interested to know too.

[Jonny Jeep]

Downstream O2 sensor (after the cat) won't have played any part in the better driveability. It's only function is to monitor the performance of the catalytic converter. It's the upstream O2 sensor just below the exhaust manifold that controls fuel mix.

[thedriver2005]

Hi all again

I got the parts from 'Westaway Jeep'
The TPS sensor was about 70 +vat
And the Oxygen sensor was about 45 +vat (old stock, glad to get rid of)
The oxygen sensor was the upstream one (before CAT) not the downstream one as i originally stated.
She now runs how she used to, smooth and fast  

Mark

[PtP]

Thanks for the info Mark.

[Bishops Finger]

Useful info