I have a 1995 XJ6. I have welded the cracked exhaust manifolds, replaced the fuel filter and installed correct champion plugs. The car runs ok during warm-up, may be misses a tiny but at idle, but does not stall.

The only codes I found were P0152, P0132 "O2 sensor circuit high voltage".

Things go wrong once the engine is hot enough for the cooling fans to start working. After I let the car sit idling for a while, either in traffic or in my driveway it starts stalling. If I am driving it usually happens when I stop at the light. In my driveway, I noticed the following bizarre behavior:

1. The fans start spinning a little bit, then they kick in for real.
2. The RPMs drop
3. The secondary air injection pump starts working? (WTF?)
4. the engine stalls.

Steps 3. and 4. happen at approximately the same time. It's a nice day here in Boston, 81 degree Fahrenheit outside. The cooling fans spinning implies that the engine is quite hot (and it is after sitting idling for a while). Why is the air pump starting to run? From what I remember about secondary air injection, it is only supposed to run at startup, and on relatively cold days (Below 45 or 35 Fahrenheit IIRC). One thing it's not supposed to do, is run on a hot day with the engine being hot as well.

I am wondering if the airpump is the cause of the stall or a side effect. Can the air pump enable an unmetered air leak?

I am not sure why I am seeing P0152, P0132 codes. I have cleared a lot of codes once before (the car was in bad shape). These two came back after I fixed the exhaust and drove the car for a couple of days.

 

I'd have a look at your coolant temperature sensor, perhaps it's faulty and the ECM is thinking the engine is cold (which would permit the air injection pump to run) and causing an incorrect mixture that's fine when the engine is warming up, but much too rich when the engine is at operating temperature. At least on my '92, there are two temperature senders, one for the temperature gauge in the cluster, and a separate one for the ECM. If the gauge sensor is correct but the ECM one faulty, you'd be seeing the proper temperature displayed on the gauge, bu the engine would think the car is always stone cold.

That would be my guess.

Nick

 

I'll swap the coolant temp sensor, it's cheap. My car does not stall when cold, only when warm. It could be the culprit.

Does anybody know the part number for the sensor in question? I am having a difficult time looking it up for some reason. Plenty of XJ40 sensors, but not X300 XJ6 4.0 for some reason.

 

Before replacing it, just unplug it when the engine is hot and check how the car behaves.

 

Juha,

The air injection pump will run for a short time (maybe 15 seconds), about two minutes after starting whether hot or cold. That is a normal self-test. Also of course, it runs at cold start up for a minute or so. I have two theories for your scenario:

1. Your O2 sensors are shot (not switching hi/lo) and they are preventing the proper fuel delivery, especially when the pump comes on to self-test.

2. A problem with power supply. If cooling fans are running, the air pump could then cause too much voltage drop if the car battery is weak or if there are any bad power connections. That could cause an interruption in fuel injection or more likely, coil spark.

A faulty air pump can leak air through it when off. But first I would check power connections, and get a scan tool to read the O2 sensor function.

Last year I had a stalling problem with the six. I was suspect of the air pump among other things. But ultimately the stall was completely cured with new spark plugs!

 

The codes (P0152, P0132) are for high voltage from oxygen sensors, so they may be indeed bad. I am unsure whether these codes deal with the sensors before or after the cats.

I also think that there is a restriction in the exhaust. When I start the car I get a lot more condensate (white-ish smoke) out of exhaust on one pipe than the other and the pressure of exhaust gasses seems to be stronger on that side. I am suspecting a plugged cat, I have an exhaust pressure gage on order. I'll check the exhaust for blockage, most likely I will need to address that. Then I'll swap the battery and oxygen sensors.

 

This could also mean a cracked head, hopefully not, also check your throttle body it my have smudge in it and when it drops to its lowest (warm idle state) It may be blocked more than usual, However at one point (not anymore) when hot enough outside my car would go into limp mode and or stall. To keep it from stalling I would double foot to keep the idle high. Whats your mileage at this moment?

 

Thanks for the tip, I'll clean the throttle body. My car has 108K on the odometer at the moment. Instead of double footing there is another trick:
1. Turn the ignition off
2. Turn the ignition on, but don't start the engine. This will "park" the IAC in high idle position.
3. Disconnect the wiring to the IAC (Stepper motor). Secure the wiring.
4. Start the engine and drive the car.
The engine will idle higher than normal, but it's easier to drive this way and it won't put as much stain on the transmission as double-footing.


I doubt that my car has a cracked head because once the engine warms up the white smoke goes away. Having owned many Rover V8 powered cars, I a lot more familiar with the symptoms of "head/head gasket" problems than I would like to be My Jag does not seem to pressurizing the cooling system with exhaust gases. If it's cracked head I'll just swap the engine, I have a spare. Ironically, swapping the engine will probably take less time than what I spent on troubleshooting the idle problems so far.

My plan is to check the exhaust pressure before and after the main cats, using the oxygen sensor ports. If the pressure is high before, but low after - it's the main cat. Otherwise it could be the secondary/auxiliary cats (the ones downstream of the second set of oxygen sensors) or muffler. I'll disconnect the muffler and re-check. If not muffler - disconnect the secondary cats and recheck.

Gotta fix the known problem before moving on and searching for unknown one

 

I don't completely agree with step 4, It would be sitting at the same idle as Double footing (about 1000) And if you're in gear the transmission will be engaged anyway, but yes it would be less physical driving work but I double foot anyway to rev match and brake haha.

If you have a exhaust with the other engine you could always swap the full system or go Cat-less for science!!!

 

Unfortunately I don't have a spare exhaust. I don't see a point in installing the cats unless the engine is running perfectly. The car had one cracked exhaust manifold, I would not be at all surprised if the cat (or may be even both cats) downstream from that manifold were bad. Exhaust leak could trick the oxygen sensor into thinking that the cylinder bank runs lean. The ECM would grossly overfuel trying to compensate for the non-existent lean condition, and that could kill the cats.

I'll diagnose , if the cats are bad I'll cut them and weld a straight pipe. Drilled out spark plug anti-foulers work wonders when it comes to convincing post-cat oxygen sensors that everything is good. Sometimes you have to use 2 or even 3 of them move the sensor away from the exhaust gas stream.

 

Wow...after reading all of this I am confused...my car is doing the same thing however after I drive it and it stalls several times...usually as outside temperatures drop...it will run normally the rest of the day....even after the car sits for hours....it will take a 5 hour rest to reactivate the problem...but once I get though the stalling it runs fantastic? I'm gonna try the o2 sensors first...