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XJ rough idle at temp issue

DFW Rusty

DFW Rusty

Well-known member
Joined
May 21, 2020
Member Number
898
Messages
395
My good friend has a 97ish XJ, 4.0, auto, on tons. A couple months ago it developed this weird issue where it will idle like dog shit at temp. However, you can clear it by revving it to the moon for a few seconds, or kill it and start it again after letting it sit for about 5 minutes. It comes and goes. I dont know shit about cherokees, except that they seem to be plagued by engine issues once you make a high mileage one into a crawler.

Symptoms are as follows-
  • rough idle
  • gasping for air sound
  • it tries idling it's self up to stay running
  • almost sounds like it's about to run out of fuel
Things tried to fix the issue-
  • installed a cpu chip that supposedly bypasses sensors. (no improvement)
  • Replaced 02 sensors, tps, crank sensor,
  • new spark plug wires
  • checked for vacuum leaks. (found one on the brake booster)
  • new cold air intake
Things we though made an improvement but in the end did not-
  • when we found the brake boost vacuum leak we plugged it and it idled better but the very next time he drove it, it screwed up again
pic of jeep-

334768903_760647962163046_7245895968379223771_n.jpg
 
He is his fan on a switch, so no temp control on the fan.

Edit-
the vapor lock issue is something to look at. He did have to re-route the fuel lines during the 1 ton swap and maybe they are running too close to the exhaust... I just text him and told him to look at those tonight.
 
In my experience running like shit after warm up is almost always fuel, not ignition.

I'd go looking at the fuel system and the sensors that tell the engine how much air is going in.
 
Yeah, it's vapor lock because the injectors and/or fuel rail is getting too hot. I swapped in Junkyard Dodge Neon Injectors (5-hole, plastic body) 🆚 the stock Siemens (metal body, one hole) thinking the plastic wouldn't heat soak as bad and it helped a little. Maybe it was better atomozation due to the 5- hole injectors but I doubt it, fuel just gets squirted at the intake valve anyways. They make a heat shield kit but it didn't do shit for me, never tried the hood louver thing because it rains here, didn't seem like a great idea for a rig that sits outside. I chose to just deal with it.
 
DFW Rusty said:
He is his fan on a switch, so no temp control on the fan.

Edit-
the vapor lock issue is something to look at. He did have to re-route the fuel lines during the 1 ton swap and maybe they are running too close to the exhaust... I just text him and told him to look at those tonight.
Click to expand...
Did he have his fan actually on? Mine needed to be turned on before the factory wanted it to (215* ish) or I had to run the AC at all times to keep air moving across the top-side of the intake manifold. That, and hood louvres, and I haven't had this problem since. Those injectors, plus the low hood of an XJ, mean the fuel system gets rather toasty. My IAT regularly reads in the 170-180* range at lower speeds.
 
I appologize for never following up with the fix he found. He asked the same questions on a XJ group on facebook and got a shit ton of "try this or that". However one common thing he was being told was to un plug his bank 2 02 sensor. He did and now it runs like a champ. He can idle it for as long as needed. He has no explanation as to why it fixed it, other than he was told to try it and it worked. The plug is literally just zip tied up and out of the way.
 
Good deal.

On the lower temps discussion, add a 1" block between the hood hinges
 
DFW Rusty said:
I appologize for never following up with the fix he found. He asked the same questions on a XJ group on facebook and got a shit ton of "try this or that". However one common thing he was being told was to un plug his bank 2 02 sensor. He did and now it runs like a champ. He can idle it for as long as needed. He has no explanation as to why it fixed it, other than he was told to try it and it worked. The plug is literally just zip tied up and out of the way.
Click to expand...

I guess this means that “sensor bypass” chip is bullshit then?
 
Gravy said:
Good deal.

On the lower temps discussion, add a 1" block between the hood hinges
Click to expand...
Tradeoff there is less airflow through the radiator at speed since you have high pressure air from the windshield transition bleeding into the engine bay. There's a reason fresh air vents inside the cab feed from there.
 
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