thrstone

1991 Jeep Cherokee Laredo, 4.0 liter straight six, 300,000 miles, automatic.

Quick question, and I'm grateful to anyone who can help me.

When starting the Jeep after it's been sitting overnight, early in the drive it wants to stall - in fact, a couple of times it has stalled. The stalling or near-stalling is accompanied by a strong smell of gas. A good kick down on the accelerator and getting the revs up remedies the problem. After driving some miles, it settles down.

For various reasons, I know this car has been a bit neglected over the years (it came from a family member). The oil was changed a couple of times a year, but I know for a fact the plugs, wires, distributor cap and rotor are all 15 years / 100,000 miles old. I'll change those these weekend, but I was wondering what else I should be looking for if that doesn't solve the problem. I'm reasonably competent with basic maintenance on cars, but by no means skilled with working on engines. It'll be nice to change the plugs on a straight six engine. I did this job recently on my own car, a Buick Lacrosse 3800 V6, and two of them were an absolute pig to get to. Anyway, thanks for any help! Much appreciated!

agreen

Sounds like you might have leaky injector or 2. Pull the rail with the injectors attached and turn the key on. Nothing should leak. Turn the key off and watch for drips.

BlueRidgeMark

Possible. Might also be a bad O2 sensor. When they fail, the engine moves into "stupidly rich" mode.

thrstone

Am I right in thinking a bad oxygen sensor would also cause the tailpipe to have black deposits? I noticed today that the new tailpipe I had put on earlier this year (it can only have 3000 miles on it, tops) is now completely black inside, and black around the outside of the tip. It sounds like that could be a symptom of the engine running rich? I found a Youtube video of someone installing a new oxygen sensor in a 1980s XJ. It looks fiddly, but not too hard. From what I've read, a 1991 XJ should only have one sensor, near the exhaust header?

I had the new tailpipe installed because it needed it to pass Virginia inspection. I've been meaning to get to this Jeep properly, soon, but should have paid closer attention.

bluejeep2001

Those older jeeps, the 02 sensor has almost no authority on fuel flow...there is a vacuum line on your fuel regulator on the fuel rail...pull it off with the jeep running...if fuel comes out of the regulator then its bad. Also unplug the temp sensor on the thermostat housing...it will set a code but the coolant temp plays a big part of fuel regulation. I dont think its enough to stink like fuel , i suspect a blown regulator first but give it a try after you pull off the vacuum line

BlueRidgeMark

Bingo. Contrary to other advice here, the O2 sensor will do exactly that. Its only reason for existing is to tell the computer to change the fuel mixture.

I went from averaging 18 mpg to FIVE mpg when one of mine failed some years back. Lots of soot in the tailpipe.

thrstone

Thank you for the helpful information.

4.3L XJ

I would lean towards O2 sensors. Any check engine lights?

thrstone

No check engine lights. I drove it to the mechanic which was kind of scary (I should have had it towed but $80 is $80, did it early one morning on country back roads, it stalled twice, second time I thought I was done for and it would end up being towed, anyway, but it came back to life again by starting it while pumping the gas pedal).

Mechanic's impression agrees very much with yours - o2 sensors. I'll know for certain today or tomorrow.

BlueRidgeMark

Yeah, bad O2 sensors don't throw codes, and a bad CPS doesn't either. But both are essential. Go figure.

cruiser54

I bet the hose from the MAP sensor to intake manifold is messed up.