[firechicken]

I bought this little car on a whim. It's quite fun but has an issue I'm trying to diagnose and rectify. I'm no expert with OBD2 stuff but do have a scanner and other tools. I know there's a few efi smart guys here that might be able to help.

This is a 1999 Jaguar XK8 with a 4l V8 and I believe it shares parts with Ford.

It mostly runs quite well but has a tip in stumble I can't isolate. Easing into the throttle is fine but a quick snap open gives what I'd call a lean lag like a faulty pump shot on a carb. If I snap to wot it bogs bad enough it won't recover and throws a lean code. I've smoked the engine and did find a small vacuum leak but nothing I felt would cause this.

Fuel pressure seems good. Coil packs and plugs are good. Fuel trims are not out of walk as far as I can tell but ST does go straight to 25 when I snap the throttle and create the bog. If I disconnect the maf it seems much better. I've cleaned the maf with no difference and it appears to be functioning properly on the scanner. O2 sensors appear correct as well. Throttle body seems fairly clean and working properly.. I'm pretty much stumped.

[Mr Twister]

Just a guess but is the IAC clean and functioning?

[TAKerry]

Have you found or posted on any Jag forums? These kind of cars are best when bought new and under a warranty. I once read that there is nothing more expensive than a cheap Jag (mercedes, bmw, etc. fill in the blank).

[mgarblik]

Couple no cost things you can look at. I am not familiar with the exact layout of this engine, but. German and some British cars are notorious for rubber parts failing. Now I am assuming there are accordion boots between the air box, MAF sensor and throttle bodies. Physically remove the boots, turn them over and inspect each one very carefully. It is common to have very small or possibly large splits in the pleats of the boots. They remain closed and look fine. But when you first accelerate, the engine torques and pulls on the boot (s), opening a split and allowing air to bypass the MAF sensor. It will act just like a bad accelerator pump on a carb car. Pull the boots apart and look in the pleats. If everything looks good, a defective MAF sensor would be my next item to deal with. The fact that the engine runs better with it disconnected, makes me think the sensor is skewed lean. If it is using a Ford sensor, that is fairly common on them. Good luck with it.

[firechicken]

I've been on the Jag forums yes. Some very helpful people there but it seems with OBD problems, most people just want to throw parts at it instead of actually diagnosing the issue. I know this can be difficult, especially through a forum.

Mr Twiser - I haven't looked at the IAC if there is one I'm not sure where it's located. Somewhere on the TB I assume. At least that gives me a direction for the moment thanks.

This car has 83k kms on it and every single thing works except this bogging issue. I know I took a bit of a chance but it was just a small investment.

Edit to add - I have checked the intake pipe and did find a small leak in the bellows. At first I thought, that was an easy fix but it made no difference to the bog. I have ordered a new maf just to see if I get lucky but it would be just that if it helps...luck. It does use a common maf so at least it wasn't too expensive.

[camerjeff]

I wish you luck, I have had good help with our old Mark 2 on Jag lovers forum. But I do not know how much help they would be with a more modern Jaguar. My wife really likes her old Jag, she just wishes it had A/C.
I hope you get it sorted out with the MAF sensor!

[mgarblik]

Thought of another diagnostic trick, possibly. If the engine has an external vacuum pressure regulator on the fuel rail. Try pulling the hose off and plugging it and go for a drive. That will give you about 8-10 extra PSI fuel pressure at high vacuum. If the tip-in goes away, the extra fuel pressure is helping. At that point a fuel pressure gauge needs to be installed to check pressure and pressure regulator function.

[KS circutguy]

Try a Ford forum,also we had a guy hear named Jagtech.Was a master Tech like me but specialized in Jags.
I think he changed careers awhile a go but may chime in.
I love my old E36 328I.
Good luck,

[Baron Von Zeppelin]

I like that body style Jag convertible.
And only 52k Miles !
But not in those colors .

MAF's can cause a world of unsuspecting symptoms sometimes.
Worked on a 98 TDI that had a terrible low power gremlin - only happened on initial start up and first acceleration.
After it hit 2200 rpms the first time it would run perfectly thereafter.
Until you turned it off and restarted it. Same drill.

After a barrage of tests and diagnostics getting nowhere , swapped out MAF from another TDI just to say I did ,
no more problems .
damn cars are crazy

Good Luck with your MAF

[geeteeohguy]

The Pole Barn Garage on youtube has a couple of helpful Jag videos. He's a Pontiac guy, to boot.

[Hikin Mike]

Originally Posted by geeteeohguy

The Pole Barn Garage on youtube has a couple of helpful Jag videos. He's a Pontiac guy, to boot.

When I read the thread, I immediately thought about him...and hearing how he said Jaaaag...lol!

[firechicken]

Originally Posted by mgarblik

Thought of another diagnostic trick, possibly. If the engine has an external vacuum pressure regulator on the fuel rail. Try pulling the hose off and plugging it and go for a drive. That will give you about 8-10 extra PSI fuel pressure at high vacuum. If the tip-in goes away, the extra fuel pressure is helping. At that point a fuel pressure gauge needs to be installed to check pressure and pressure regulator function.

I have checked the fuel pressure. Bought a kit just for this as my first thought was a fuel problem of course. Put a new fuel filter on as well. The pressure at the rail is in spec, regulator working correctly. I did try unplugging the vac to the reg as well but no change.

The only thing I can do to make any change is unplugging the maf. This puts the engine into a restricted mode and won't rev over 3k but makes the bog basically disappear.

I'm sure hopefull a new maf sensor is the solution but the scan tool shows otherwise.
Pretty frustrating. A nice Qjet would probably fix it right up!

[mgarblik]

You probably know this already, but restricted mode puts the engine essentially in open loop. so fixed low ignition timing, rich fuel delivery and no EGR if it has that system. BTW, if the engine has EGR and the MAF sensor does not clean it up, the next place I would go is to that system. I would disable the EGR valve and drive the car to see if it cleans up. If it is an engine with EGR and an electronic transducer to modify the vacuum, it may be allowing too much EGR on light acceleration that could make it bog. These type issues can be a bitch to work out sometimes. Sounds like your hot on the trail.

[firechicken]

I didn't know all that thanks, again, I'm a novice with this OBD fuel injection stuff and really appreciate any thoughts, even basic stuff.

To be clear, the engine runs fine under light loads, part throttle and any conditions where the throttle is eased open and even all the way to wot. It's only a quick snap open that causes the bog, like a lean stumble. It happens with the trans in park when free revving from idle or while driving and stomping on the go pedal.

I'm uncertain on the egr and will look into that next. The IAC seems to be working correctly. The idle is stable and remains good when the fans and/or A/C kick on and off, putting in drive etc.

I thought the injectors might be plugged but it will rev out to 6800 redline at wot as long as I ease into it. Weird...

[firechicken]

Originally Posted by mgarblik

You probably know this already, but restricted mode puts the engine essentially in open loop. so fixed low ignition timing, rich fuel delivery and no EGR if it has that system. BTW, if the engine has EGR and the MAF sensor does not clean it up, the next place I would go is to that system. I would disable the EGR valve and drive the car to see if it cleans up. If it is an engine with EGR and an electronic transducer to modify the vacuum, it may be allowing too much EGR on light acceleration that could make it bog. These type issues can be a bitch to work out sometimes. Sounds like your hot on the trail.

Turns out the egr was deleted by '99. I checked the car anyway but there isn't one. Another dead end but thanks for the thoughts.

[Sirrotica]

Originally Posted by Hikin Mike

When I read the thread, I immediately thought about him...and hearing how he said Jaaaag...lol!

Put a Pontiac Stratostreak in it, solve all your problems, and have a Q Jet too.

Dalton (Pole Barn Garage) bought his first one with a SBC in it. He's a glutton for punishment because he also bought another with a V12 in it, it didn't run when he purchsed it, but he managed to get it limping along enough to drive down his driveway.

I do like the body styling on the XJ8, but having worked on cars, and equipment with Lucas electrical systems, I would eliminate every circuit that was British in origin.

The reason the Brits drink warm beer, is because they have Lucas refrigerators....LMAO.....

[firechicken]

I'm not certain but I believe most if not all the electrics in this car are Ford stuff. Probably why everything still works. I don't know the whole story of the Ford takeover but it must have been well underway by 1999. I know the engine is similar to a Lincoln engine of that era but the trans is a ZF with a Jag independent rear.

[Baron Von Zeppelin]

Originally Posted by firechicken

.... The only thing I can do to make any change is unplugging the maf. This puts the engine into a restricted mode and won't rev over 3k but makes the bog basically disappear.

I'm sure hopeful a new maf sensor is the solution but the scan tool shows otherwise.

Sounds like your Jaggy is trying to cooperate with you.
As unlikely as it can seem using general logic , I think you may get lucky if you ordered a decent quality part.
Cheapo maf usually just causes more problems.

I would stop messing with it until the new sensor comes.
Have already verified enough systems are proper.
You could slip a wrench and break an expensive Jagwire doo-dad .

The TDi I described earlier wasn't tripping any scan codes either.
No one on 2 different VW Diesel forums guessed at the MAF (including me)
That is how highly unlikely it was.

Disconnecting the MAF on a TDi puts them in snail mode (and trips CEL) , and snail mode was the original issue trying to conquer.
So no cooperation from that car in diagnosing it .

[KS circutguy]

Don't buy a cheap maf.
Here is where I get most of my BMW parts,they do jags as well.
Life time warranty on all parts.
https://www.fcpeuro.com/Jaguar-parts...=&keywords=maf
Great company.

[Half-Inch Stud]

Be sure to have fresh fuel too. Over winter fuel would be weak and induce stumble.