HudsonN

Found a jeep on Craigslist.

99 XJ/4.0/AW4

200k miles.

It has a really bad misfire. Pulled 9 engine codes. 2 or 3 related to 02 sensors and the rest are multiple cylinder misfire, cylinder 5, cylinder 4, etc misfire. I've never seen a 4.0 idle as rough as this one.

Do you guys follow my logic?

- Check for crossed plug wires then replace; Plugs, wires, cap, rotor, and the coil.

Going to do a compression test as well before I purchase it.

Roler

Thought you already bought it. So you let the owner do the tune-up or are you planning to do one, once you bought it?

Don't forget the O2 sensor codes....(what were the codes, btw?)

A compression test before purchase is always a good idea.

(and no, not sure if I'm following your logic, but it's what I would do )

Firestorm500

Cracked head and/or blown head gasket can do that.

CCKen

While you're changing the rotor, check for side-to-side play in the distributor shaft. A little rotational play is normal, but side-to-side is unacceptable.

There wouldn't be a camshaft sensor code in that mix would there?

Generally any O2S code diagnosis should start by checking the O2S heaters fuse in the PDC. It's a 15A mini fuse in the top row (looking towards the firewall), second from your left.

HudsonN

You guys were right. Head is most likely cracked. Still bought it though. Not exactly sure what I want to do with it but the jeep is surprisingly clean.



Compression (Dry Test) Results;

#1 - 175 PSI
#2 - 150 PSI
#3 - 0 PSI
#4 - 0 PSI
#5 - 178 PSI
#6 - 185 PSI

Oil Pressure;

The oil pressure on this jeep holds right around 40 PSI at a warm idle. No milkshake in the oil at all.

Current codes the jeep is throwing;

P0300 - Random Misfire Deteched
P0132 - 02 Circuit High Voltage (Bank 1, Sensor 1)
P0135 - 02 Heater Circuit (Bank 1, Sensor 1)
P0303 - Cylinder 3 Misfire Detected
P0304 - Cylinder 4 Misfire Detected
P0442 - Evaporative Emission System Leak Detected (Small Leak)
P0455 - Evaporative Emission System Leak Detected (Gross Leak/No Flow.)

Firestorm500

Put a brand new head on it and enjoy. Nice looking Jeep.