Reluctantly seeking help! 1992 XJS N.American spec 4.0 manual. 88,000 km / 56,000 miles.

Today: from a cold start it idles at 1000 rpm but touch the throttle and it pops a little backfire and will not increase revs. The car has been mostly sitting since June 2019, driven only once and run only occasionally. Seemingly working perfectly when parked.

Aug. 2019: the one time it was used, suddenly the engine wouldn't rev above about 2000 rpm with applied throttle. No other symptoms. I only drove it because I had an appointment to have a windshield installed. Difficult drive.

2020: engine would start and be able to rev to about 1200 when cold, increasing to 2200 when warm, but no higher.

And now it won't rev above idle at all.

Done:
-swapped TPS from another car, no difference
-swapped fuel pressure regulator from another car, no difference
-new fuel filter and the pump does run for about two seconds when you turn on the ignition
-ignition coil resistances are in spec
-cap and rotor look like new
-spark plugs are all covered with black soot, as are the tops of the pistons (I have only used Shell premium since 2011). Two plugs have light ash on the threads. There's only fresh 91 in the tank now.
-I think I did this correctly: MAF continuity test yields 400 ohms and power and ground to pins 36 and 9 yields 0.002v. These appear to be out of spec per the green Jaguar service manual (page 20 of the 4.0 supplement) but I'm not confident...any comments?
Was going to remove and look at the IACV but I don't have the right tool....deep 7 mm socket? Anyway any suggestions as to where I should look next will be greatly appreciated. Or comments on that MAF. Thank you.

 

Someone will correct me if I'm wrong.

The IACV shouldn't cause the engine to die when gas is applied. A failure would cause the car to die when at idle. There is a manual way to reset the IACV, you should do a search. It requires turning the car (not the engine) on/off three times with some plugging and unplugging of the IACV. You will at least hear the thing operate.

I'll flip through my manual for the MAF later today...but it could certainly be the cause. What happens when you start the car, then unplug the MAF?

4 plugs with black soot, and 2 with white ash is an indicator....of what I don't know. Two plugs appear to be working normally, whereas the other four are being flooded. Could easily be the cap/rotor and/or coils, but you say they're fine. Could also be injectors, but four?

I have an AJ16. but do you have a Crank Position Sensor? DBC11501. Common failure.

 

I just started it (50deg. F here) and it idled smoothly at 750. Wouldn't rev on increased throttle.
Unplugged the MAF and it sped up to 800 rpm and then hunted around that level indefinitely.

Sorry, all 6 plugs are fouled black, and two have additional ash.
There's a CPS, I haven't tested, tach shows 200 rpm when cranking...
I had originally suspected over-fuelling when the first symptom appeared in Aug. 2019. Idling was ok but rev'ing quickly caused the cats to glow red. However the injectors all sound the same to me (then and now) when I use a stethoscope (16" dowel).

Thank you, Vee!

 

When was the last time the oxygen sensors were replaced?

I have to believe it's the MAF because the oxygen sensors wouldn't show a cause of any problems until the engine warmed up. The MAF is read right away. The TPS is also being read, as is the coolant temp sensor. I discount the TPS because you've already switched that. I don't believe the CTS can cause this much of a problem, but if you unplug the coolant sensor and jump the two female connectors on the harness, that would certainly clear that up. Unplug it, jump it and then start the engine?

It does sound like the oxygen sensors though...

 

Hi Mkii250

Have a look at Fuse Number 7 Passenger Side Knee Bolster Fusebox and also the Relays above it (try swapping them like for like)

Could also be blocked Cats if you had yours glowing Red

 

O2 sensors are probably original...I thought they couldn't be at play here as Vee said, too early for them...maybe I can test them.
Ok thanks guys, I'm on it, will report back.

 

blocked cats….of course….that could certainly be an obvious problem! Sometimes it could just be the most obvious thing. Why not?

it’s not a common occurrence on the 6 cylinder engines, but it should be considered!

 

Air intake leak?

 

It would have to be monstrously significant to cause this kind of problem….the kind that a faulty MAF could certainly provide.

 

Logical first place to start given the symptoms; however, have you checked the TPS signal at the ECU? You could have a good TPS, but a wiring fault or high resistance from the TPS to the ECU. Just a thought.

 

MAF has three pins.
pin 1 = 12V - Ignition Dependent Supply Voltage
pin 2 = 0-5V - Signal - low current
pin 3 = 0V - ground via ECM

I can list the nominal output voltage tied to airflow (kg/s)

0.813V : 0.002
0.982V : 0.003
1.117V : 0.004
1.555V : 0.009
1.927V : 0.016
2.344V : 0.028
2.751V : 0.045
3.103V : 0.065
3.451V : 0.090
3.867V : 0.125
4.305V : 0.170
4.700V : 0.220

 

I couldn’t find any references to the pins and results you referenced. Based on codes, which you don’t seem to have, there are certain MAF pins and ECM pins that should give you results.

I’ll see if I can scan them in.

 

Correct, no codes, no check engine light (except when I ran engine without the MAF, then disconnected the battery after to let it reset). Exhaust is coming out of both tailpipes but I will do all of the above-recommended checks today. Yesterday's cold wind kept me away. I did read somewhere that the backfire just coming off idle is an indication of excessively lean mix, so I'd still like to check fuel pressure...

Thank you everyone!!



edit...just realized it doesn't matter that exhaust is coming out of both tail pipes since the system is one pipe further forward.

 

Not sure what I'm looking for in the fuse boxes...the under-dash ones don't seem very applicable.

Right side



Left side



And of course my MAF has 6 pins. The ones I mentioned above are shown, 36 and 9.

 

If I had to guess, I’d say those were pins at the ECU. The ECU has two 36-pin connectors, a black and a red.

To follow this test procedures, it appears you have to have a machine that can custom generate voltages on demand. I don’t have anything that can do that.

 

…this is, unless you do Test 2

 

By the way ignore my Post #11, I thought the AJ6 and AJ16 used the same MAF. I only have 3 pins on my MAF.

 

I retested the MAF and now say with confidence that it is within spec. I read 0.32v for test 1 and up to 3.7 by blowing through it for test 2.

Hoping to lay my hands on a fuel pressure tester then will think about investigating the cats.

 

Was the CTS ever checked? A quick jump at the two pin harness should cross that off the list.

 

I tried to do the CTS test but the engine just turns over, not firing.
I have intermittent spark (determined with inductive timing light).
Does the ignition amplifier go out, and do so gradually?