XJ 4.0L 98 Sport Engine Lag O2 Problem??

[bartman]

Hi there,
You are all going to love this fault and I would love if someone could give me some idea WTF is wrong.

Loom got ripped out of "downstream O2 sensor", replaced sensor and still get the following problem.

This is after Jeep has sat still and off for awhile (15+minutes). As I accelerate up at normal speed the engine will bog down and lose rpm for about 5-10 secs. The car will then right itself and will continue to run smoothly until the next shutdown.

Might seem to be using a little more fuel, but not real sure. But other than that the car is behaving normally.

Had Jeep placed on the computer at Jeep mechanic and the problem is the downstream oxygen sensor circuit is not behaving correctly. It is constantly staying at approximately 4.5 volts, whereas it should be a little bit lower that what is being measured at the upstream sensor. Which was measuring approx 0.9 volts at 1500 rpm I think.

Would really appreciate any thoughts.

Bartman

[Dave'n'Wen]

This is after Jeep has sat still and off for awhile (15+minutes). As I accelerate up at normal speed the engine will bog down and lose rpm for about 5-10 secs. The car will then right itself and will continue to run smoothly until the next shutdown.

I get this problem all the time in hot weather. From what I can work out, the fuel rail sucks in a lot of heat from the exhaust manifold. This causes a fuel vapour and the engine, for about 5-10 seconds, where the lag happens.

DC did a recall on a heat shield but not sure if this sorted the problem as they said it was was due to leaves that could build up on top of the inlet manifold, and cause a fire.

[Steve]

I think Dave is correct on the heat thing. Ask GoJeep or check his website because if I remember correctly it's the No 3 cylinder injector that gets too hot. I think he installed or made his own heat sheild that corrected the problem. My TJ even does the same thing on the odd occasion.

Edit: Here ya go:http://www.go.jeep-xj.info/HowtoIntakeShield.htm

[bartman]

Thanks for the replies fellas, but this can be a cold thing, I could take off first start of the morning (Canberra so less than 12 degrees at moment in morning) and it will do it. It is only occurs when the car has been off and motionless for 15 minutes or more.

It all started when that mongrel loom got ripped off. JUst that I have spent $450 fault finding the mongrel thing, and all that is left is wiring that I can think off. ANd I am sure that will take them a few mechanic hours to solve.

Really appreciate your replies though

Might leave this on the floor and see if anyone else has had this most annoying problem.

[bartman]

new symptom now: I can feel a little flutter in the accelerator pedal, when the problem occurs

Thanks again

[bartman]

Oh god,
Rumour has it, it could be a computer or PCM as it is sometimes called. Has anyone ever seen a price for them, Used or New? And should I be afraid?



Bartman

PS. Is it possible to get little backfire like popping in the Intake Manifold, as I think that accompanies the accelerator flutter that I felt the other day.

Oh my poor Jeep.

[Dave'n'Wen]

If its the computer, try reseting it. Disconnect from the battery for about 5 minutes and then re-connect. May work. A popping in the inlet mainfold could mean a blockage, check the air intake filter, check the manifold for a leak. May need to put a spanner to the nuts to check to see there tight. If as you say there is a flutter in the pedal, may be the manifold is moving enough to feel in the pedal.

Worth a look.