'97 Cherokee straight 6 AW4

Was running pretty good.

Suddenly, when pressing the accelerator, it starts sort of hesitating/lurching almost wanting to die. Slightly rough idle at times.

Noticed the upstream O2 sensor wires were laying on exhaust manifold and a couple wires were burned down to bare metal (yikes).

Patched the two bare wires with new wire. Retest driving, same problem.

Rent fuel pressure test gauge and run it:
Key on, engine off, first time cold: 35psi
turn off and on a couple times, 45psi
notice that if I leave the gauge and key on, the pressure slowly drops... shouldn't it stay solid? (Bad check valve?)
engine on: 48psi - solid, no drop over time (not the stuttering while driving problem?)

Run codes:
P0305 - cyl 5 misfire
P0138 - bad voltage, o2 sensor 2
P0132 - bad voltage, o2 sensor 1

Speculating the o2 sensor codes were from the bad wires and/or me fixing them

Replaced all spark plugs (they were kinda nasty from oil from valve cover leak)

Test vehicle - same problem.

So - what should I do next?

 

Clear the codes, drive it, check codes again. If the O2 codes come back, you should probably investigate the wiring more or replace it if it's on the sensor side of the connector. The cylinder 5 misfire though, that's probably not due to the O2 sensor. Check the cap/rotor and see if it's corroded on the terminals and replace it if necessary. You should have changed it when you did the spark plugs, but you may have already done that so I won't judge. If the misfire persists, swap the #5 injector with another one. If it continues, check compression. Also, the wiring harness is known to rub right around the intake manifold between cylinders 5/6. So check that as well.

 

I did a quick look at the distributor cap and it looked OK, maybe a touch of corrosion on the contacts. I think I will replace that next.

Probably follow that by replacing the o2 sensor if still a problem.

If those don't fix it, I think it's back to a fuel pressure issue?

Is there any kind of fuel system cleaner you could recommend? Would sea foam possibly help?

Thanks.

 

BG 44K is the best I've used. Just checked at amazon and it's currently out of stock and they don't when or if it will be.
Only other product I've ever used to clean the fuel system is Marvel Mystery Oil. Works very good.

 

Have you checked the O2 sensor fuse?

 

No I haven't, but I will now. Which one is that?

 

Here you go:
http://www.lunghd.com/Tech_Articles/...tification.htm

 

A good way to test the upstream O2 sensor (which will cause issues like this) is to unplug it (the wiring connector that is) and drive around. It'll throw a code but if the sensor is bad, the engine should run clean despite the CEL.

Does it take a while to actually fire the first time you start it after it's been sitting for a while? That's usually the circumstances of a bad fuel pump check valve.

You might want to check the fuel injector on cylinder five too. Unplug it while it's running (probably like crap) and if it still runs like crap, you may have a bad injector.

Good luck and don't give up! Maybe get a multimeter, a test light and a factory service manual too...

 

Here's how dumb I am, I forgot about the fuse idea, and replaced the dist. cap and upstream o2 sensor, then remembered the fuse, checked it, sure enough, it was blown.

Replaced it.

Jeep runs good.

As a test I put the old dist. cap and o2 sensor back on.

Jeep still runs good ... it was the fuse. Probably blown by the bare wires that I had fixed at first.

Thanks everyone and BlueRidgeMark for the fuse thought -- not the first time you came through for us.

 

Yeah, melted wires and blown fuses have a way of hanging around together. Glad you nailed it.