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poor idle after 5 minutes idleing

A

aussie paul

NAXJA Forum User
Location
Mount Isa N/ West Queensland Australia
Any ideas guys? 94 xj sports, starts well runs and idles well when cold, idles well when hot for first 5 minutes then starts missing? and gets so bad engine won't rev up, and starts to get hot. Restarting seems to fix it.
Dealer says no fault codes in system.
New Crank Position Sensor 5000miles ago , new plugs at same time, replaced fuel filter, fuel pressure and leak down rate is fine.
New battery a month ago, new exhaust cat. 2 weeks ago.
Regards
 
not sure about the over heating part but that is classic 02 sensor when it starts to run funny after reaching operating temp.
02 sensor and map sensor are the only two things i can rember that are not used untill warm the rest is used even while cold
 
Thanks guys, I also thought of the O2 sensor and have disconnected it to see if it makes any difference (assuming it then reverts to programed idle) but has had no effect. Will look at MAP next but thought that just helped with allitude changes?
Aussie Paul
 
aussie paul said:
Any ideas guys? 94 xj sports, starts well runs and idles well when cold, idles well when hot for first 5 minutes then starts missing? and gets so bad engine won't rev up, and starts to get hot. Restarting seems to fix it.
Dealer says no fault codes in system.
New Crank Position Sensor 5000miles ago , new plugs at same time, replaced fuel filter, fuel pressure and leak down rate is fine.
New battery a month ago, new exhaust cat. 2 weeks ago.
Regards
Click to expand...
Is thr fuel pressure fine when the problem starts?
Often a plugged fuel filter will act like that.
Turning the engine off causes a little dirt to be backflushed from the filter letting you get more fuel for a short time.
 
Thanks again, but the fuel pressure is spot on at all time even when running rough and the leak down rate is well within the factory specs and fuel filter is brand new. I'll check the MAP and temp sensor readings against manual voltages today. It seem to be getting worse and now runs rough after a shorter idle time (3 minutes) also runs rough on hot restart after 30-40 min shut down.
 
Problem fixed! It turned out to be a blown HT lead from the coil to the distributor!
Why it only happened at hot idle I don’t know, must have changed resistance to the point where it misfired. Same as why it didn’t play up at other than idle when hot?
A classic case of looking too deeply for a simple solution. And it’s a huge relief as I’m driving to the coast next week, about 600 miles away!
Thanks for all your advice, and all the best from Australia
 
ht = High tension == high voltage - he means the high voltage wire from the coil to the center of the distributor cap.

I've had plug wires start to arc and glow when the car gets hot, but have not lost just a coil wire yet. Good to know!

-Ryan
 
Yes, sorry about the HT bit, Ryan is correct, I mean the high voltage lead from the coil to centre of distruibter. Local jeep dealer didn't have one and it was on order from Sydney (about 2000 miles away!) Anyway I've just returned from Townsville (1200 mile round trip) with a lead off a old 1973 VW beatle fitted..the jeep ran like a dream and the new lead set have arrived at the dealer, I thought it wise to replace them all after 12 years. Common universal leads from the auto discount houses won't fit my Jeep very well, and so I had to order an OEM set.
Regards, Aussie Paul
 
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