Diagnose me...o2 sensor/fuse/misfire

So, just picked up a 98 XJ, 4.0 auto 4x with 128k miles.

Got it for a steal of a price with "electrical issues, but nothing wrong mechanically" so didn't even drive it. Just showed up with cash and bought it. Yes, it was a GREAT price.

Noticed on the way home it was doing some bucking on the highway. I was thinking it was a bad rear d-shaft u-joint catching, since I didn't notice any rpm lag when it would catch/buck.

Fast forward to later that night. driving a few blocks, the check gauge light pops on and the temp sensor pegs from 210 to the full red. Then, after 10 seconds, pops back to 210. After another minute it pegs again, then drops back down. Checked the coolant when I got to my destination, and it was sitting very low. Put 3/4 gallon into the radiator, and the rest into the bone-dry overflow.

Fast forward to the next morning. Check engine light pops on immediately, and its bucking worse than the day before. Now, the bucking does not occur at all when coasting. It does not occur when really getting on it. Only bucks when lightly on the throttle, ~2k rpm steady.

Get the CEL scanned, and comes up as a downstream o2 and a cyl5 misfire.

I do a little research, and find the blown o2 fuse is a common failure. I pop the box, and try to get the o2 fuse out by hand (no tools). I fail, due to it being -12 outside and not having tools on me at the time. But I had wiggled it a bunch before pushing it back down and giving up. It appears to be solid, not blown. Get back in the Jeep, turn it on, and the CEL is gone. Still bucks, but nowhere near as bad, very minimal.

So... is it jsut a connection at the fuse? Is it really a bad o2? Or, is it something else? I already plan on doing plugs, wires, and probably a cap/rotor, plus running a can of seafoam through it to clean the cumbustion chambers and fuel injectors. What else should I look for?

(sorry about the novel)

Check to make sure the wiring harness for the downstream O2 sensor is not hanging on your rear driveshaft.

Had this happen to me, blew the fuse for the O2 sensors, ignition coil, alt field coil, ECU, injectors... yep they're all on the same 12V feed! About 4" of insulation was missing off all 4 wires for the sensor and they were twisted together from riding on the shaft. Untwisted them, reinsulated with electrical tape, stuffed a new fuse in it, and drove it home.

kastein said:

Check to make sure the wiring harness for the downstream O2 sensor is not hanging on your rear driveshaft.

Had this happen to me, blew the fuse for the O2 sensors, ignition coil, alt field coil, ECU, injectors... yep they're all on the same 12V feed! About 4" of insulation was missing off all 4 wires for the sensor and they were twisted together from riding on the shaft. Untwisted them, reinsulated with electrical tape, stuffed a new fuse in it, and drove it home.

Click to expand...

And yours bucked like the rear axle was about to lock up? I am pretty sure that the misfire is the cause of it (yes, I will be swapping fluids in the near future too)... but want to make sure.

Yup... bucked like you wouldn't believe. I thought my tranny was dying on me again. Every time that wire shorts out, the ECU loses all power and so do the ignition coil and injectors, so the motor basically momentarily stalls out.

If it isn't that, I'm not sure what it could be, will have to think about it some more.

Awesome. Looks like I get to go through the harness looking for a short, beginning with the rear o2 sensor.

*As you mentioned, all fresh tuneup hardware. Champion coppers gapped to .035, new plug wires, distributor cap and rotor. Importance of KNOWING you have fresh tuneup hardware is critical so you aren't chasing your tail. Especially as you have no history here.

*A compression test on all cylinders is a good "snapshot in time" of the internal condition of the engine. 120-150, with no more than a 30 psi variation between cylinders.

*And isolate that cooling system leak! If it isn't obvious, a cooling system pressure test is helpful here.

birchlakeXJ said:

*As you mentioned, all fresh tuneup hardware. Champion coppers gapped to .035, new plug wires, distributor cap and rotor. Importance of KNOWING you have fresh tuneup hardware is critical so you aren't chasing your tail. Especially as you have no history here.

*A compression test on all cylinders is a good "snapshot in time" of the internal condition of the engine. 120-150, with no more than a 30 psi variation between cylinders.

*And isolate that cooling system leak! If it isn't obvious, a cooling system pressure test is helpful here.

Click to expand...

Plan to do the tuneup once it gets back above zero here. Perferably back into the ~30's. That was a given since before we even picked this up.

I think I still have an old compression tester in the bottom of a toolbox. I'll have to dig it out and check that. I am hoping running low on coolant didn't warp the head. It never actually overheated, but if the AL was without coolant in its passages, anything could happen.

I have been watching, and can't find any leaks. At the same time, the oil is black and the coolant still green. No froth in either, no evidence of mixing.


I just found it strange that simply jiggling the #15 o2 fuse was enough to clear the CEL, never removed it completely from the socket, didn't replace it. Just a shifting of the fuse was enough to clear the CEL.

Started poking around a bit... I think I found my culprit.

The temp sensor on the t-stat housing is broke where the plastic connection meets the metal. You can pull the prongs out at least an eighth of an inch.

If I got $2,000 off purchase price due to a $15 sensor that takes 5 min to swap, I will be ecstatic.

Wouldn't be the first time!

I know people who have gotten XJs for a few hundred bucks because they correctly guessed what was wrong with them and showed up with a spare alternator and CPS when they went to buy it. Installed them in the PO's driveway and drove them home. Electrical is beyond most home mechanics and even many shops ("throw parts at it till the customer gets fed up with the bill and scraps it, or it's fixed, whichever comes first") so if you know what you're doing you can save a ton.

Hope it works out for you, if this doesn't fix it, keep poking around!

Put on the new temp sensor.... runs worse than before, instant CEL. Still need to go get it scanned.

grrrr.....:soapbox:

kastein said:

Wouldn't be the first time!

I know people who have gotten XJs for a few hundred bucks because they correctly guessed what was wrong with them and showed up with a spare alternator and CPS when they went to buy it. Installed them in the PO's driveway and drove them home. Electrical is beyond most home mechanics and even many shops ("throw parts at it till the customer gets fed up with the bill and scraps it, or it's fixed, whichever comes first") so if you know what you're doing you can save a ton.

Hope it works out for you, if this doesn't fix it, keep poking around!

Click to expand...

I checked out a Cherokee at a dealer several years ago. It had a bad vibration and the harmonic balancer was wobbling badly. The dealer was going to let it go cheap because he said it had a bent crankshaft and needed a new engine. I almost bought it so I could replace the balancer and resell it for a profit.

You need to start with a visual inspection for obvious stuff like the CTS you just replaced. A scan is a good place to start since it is OBD-II.

Common, and likely suspects are TPS, O2 sensor, and wiring for both, vac leak at the MAP...and about 20 other possible issues with wiring, PCM, ignition parts...and so on. Most people have sense enough to try a tune up before dumping a vehicle.