Car starts to run lean when hot

Boston GN

Well-Known Member
This issue just started creeping up recently. Car runs fine otherwise.
When fully warmed up and usually when sitting in traffic, the car will start to run lean according to my wideband and then. It will idle at 13.8-14.4 on the wideband and then suddenly hit 17.1 and actually have a slight smell of fuel. I'm wondering if something is getting heat soaked and starting to malfunction.
BLMs when this happened were at 129 and the narrow band O2 was reading 068 for a while until I blipped the throttle.
Fuel pump is a two year old Racetronix 340 with hotwire, new injectors. Charging system is all good too with anew battery and alternator.

Any ideas of where to look?
Are the CC's and 02's moving normally...or do they seem sluggish? If they're slower I'd suspect the 02 sensor is on it's way out.
02s were sluggish. When it hit 17.x on the wideband, the 02s were at a pretty steady 064-068 until I blipped the throttle, then they started moving better. I did not check the CCs, but will the next time out. I also kind of suspected the O2 as it hasn't been changed in quite some time
Ive seen cars at idle in traffic jumped out of closed loop due to the NB02 cooling off and do weird stuff like that. Changing to a heated o2 is the cure. Not sure what your setup is on ECM and such.
So why are you concerned about those wideband O2 numbers in a car with a 30 year old computer?

Does the car run bad, idle bad?

If you want to look somewhere, pull a couple plugs and see if they look normal or not.
Both the narrow and wide band sensors agree it's lean.

I would look for a vacuum or header leak. Also look for a post maf pre intake leak. Check the couplings and slinky pipe if applicable. Check all the vacuum lines. Something might have blown off.

Check for cracks in the header. #5 is notorious for cracking.

What chip are you running?
Like nasty said when a narrowband cools down during long idling they can read wrong. They need a certain amount of heat to work. Throttle blip seemed to confirm. Was it a long idle period?
The wideband confirms a lean condition or pre o2 exhaust leak and that sensor is heated....

Might cool off a o2 sensor at a stop light in the winter but I doubt there is any in the entire u.s. cooling off at a stop light when most of the country in being cooked..

Just sayin..
I have had them cool off and jump out of closed loop with aftermarket headers in the south in the middle of summer.
Most of the modern chips are open loop idle anyways.... That's more than likely why the ecm is not correcting for the lean condition.
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If it's suddenly going that lean at idle when hot, you might have one cylinder that's not firing, which could be a plug, or injector problem.