So managed to go for a run just now.
I noticed from the OBD2 live data, that BOTH 02 sensors were losing voltage to zero under medium throttle.
I know and can see that at wide open throttle, it briefly goes into open loop, though this doesnt fix the problem.
But I can watch it working fine, both sensors reading good voltage values and updating.
A bit more throttle, and they both drop to 0 voltage, while still in closed loop, and thats when the bucking starts.
Back off the throttle a bit, voltages return, and car stops backfiring.
Earth wire? Shorts?

 

The positive and earth (ground on this side) are for the heater in them. An unheated sensor has only one wire and it generates its own voltage

 

So checked vacuum today, it was good. Tank breather line to charcoal canister felt a bit blocked, blew it out. Also changed charcoal cannister for good measure. Downstream sensor was reading 1.0V and not moving. Replaced with a spare NTK sensor, and got it reading. NO change.
Checked all wiring to both sensors, both plugs. I couldnt see any faulty wires, no splits, cuts etc.
Put brand new conduit over both.
Undid the ground by the dipstick, cleaned, reattached.
No change.
Am I right that in a closed loop, sensors should always read voltage, and not drop to zero?
So annoying on first start up, it runs perfect in open loop, and perfect in closed loop.
But after 10 mins, it goes to crap. Then I can turn it on and off, and its fine again.

Whatever it is, its obviously past my ability to pin point.


I'm about over it and about to give up.

 

here are a few things to think about, owning 3 x '96, and following odd problems with them for quite some time

*bad distributor, the bush wears out..does it have any freeplay ?

*bad wire in the harness that goes behind the head, Jeep XJ have poor quality wiring imo, including bad wires to dizzy, crank sensor, and front O2 sensor. The rear O2 sensor has no affect on performance

*there is a purge solenoid, associated with the vapour system. Believe it or not, if this goes bad, it will give severe misfiring symptoms

From a fellow Aussie, yes Jeeps are way lighter than 2.5 ton Nissan or Toyota, gravity wins every time

 

left field idea from a mate.

What if there is crud inside the full tank, namely rust. Over the 10 mins, it is clogging the outside of the pump filter sock, to the point its getting starved of fuel. When you turn it off and restart it, the crud falls away, and the process starts again?

 

That purge solenoid interests me. It's operation is the kind of thing that could be on a timer 10 minutes in to run regardless of coolant temp. If it had a problem it might make the system so lean as to read 0 volts when it trips which would make the fuel trims go crazy, it would be super rich, enough to have raw fuel in the exhaust making all the pops.

 

My mate suggestion seemed to line up with symptoms.
So I got stuck into it, dropped the tank. Poured all the fuel out, it was filthy.
Acid washed it. Rinsed it out over and over and over.
Filled back up with fuel.
Took for test drive...

PROBLEM SOLVED.

If I had a fuel pressure gauge, I guess I would of narrowed it down earlier. But the only ones locally only went to 10psi ?!?!

I will be definately investing in one.

Thanks for all the replies blokes.

At least I learnt heaps about the jeep, its components. Tested about 87 spare parts, sensors, leads, rotors, etc. at least I know they are all good. Jeep has had a good tune up as well.

Something so simple. I dont know why I didnt think of it earlier. What a *****.

Thanks again all, and merry xmas from down under.

 

High pressure **** gas?

 

10psi? Do they only use carburetor engines down under?

 

That's standard values on no FI fuel pumps.

 

One of my '96 had that issue as well, I got it very cheap because it was a non-goer, and fixed it

even though the '96 has a unique fuel pump assembly, the actual pump itself is same as a 95 (I think, not the later models, but I could be wrong, came with a new sock anyway)

I have vehicles from 50s and 60s. It is essential to clean out the fuel tank periodically for 2 reasons.

1) Any large metal vessel precipitates condensation from air moisture, which settles on bottom of tank, and rusts it out eventually

2) the same problem afflicts fuel stations, so amounts of grit get pumped into your tank...every so often, you may get a real nasty batch of fuel