Did you gap them or did they say pregapped and you went with that? Double check your gapping, never trust outta the box.

 

Ok, so I bought the plugs and gave it to my mechanic he changed it but I don't know if he gapped them, I ask him? How can the car run without the correct gapping? Wouldn't it be misfiring 99% of the time? that is just my question..

 

To short a gap, it will run sloppy, if at all. To much gap (unless excessive, it may not start), it will idle, but you will have no performance and acceleration will be very poor. Many American cars have a gap range, Jaguars are specific (I think its .44?) and like a cat, finicky because they have the air/fuel mixture so dialed in. Plugs are also so easy to check and important to be correct, which is why you will see many folks here say to check them.
How cold is it up there btw?

 

oh, alright I should check the plugs... What should be the correct gappin?!
weather is bad, yesterday was light snow and sleet, and 25 degrees F, and will be 17 tomorrow at 6am... How bout down there?

 

Good, you have the gap, its only about a 30 minute check.
Its a crisp 41 and clear (was colder last night with the wind), ..coldest its been down here in a few years.

 

I dont know different websites are telling me different things, like 0.39in and .43in. If somebody could tell me the correct gap.
wow its pretty cold there

 

Here's and older thread with good reading and gap....https://www.jaguarforums.com/forum/x...k-plugs-72757/

 

Appreciate it Man! I shall find out.

 

I doubt that your problem is badly gapped plugs. If those tensioners have not been changed it is quite likely you're out of time. With timing recently slipped.

 

Spark Plugs gapped to spec. Jim I doubt I skipped timing, I can still drive the car to whatever speed, I can do 70mph and above but just won't go above the 2,500 rpm. Gearbox is smooth. I called my mechanic, and he said it probably could be the MAF or he said it sounds like a failing TB. But he has to see the car first/

 

That's good regarding the plugs, that's a simple part that can cause issues that is commonly overlooked. So are you going to take it over to him to look at?

 

He said he is busy, and to come next week.

 

What could be another cause

 

Why not do the MAF test yourself?

 

A bit late, but TSB 303-29 revised spark plug gap for ALL V8 4.0 engines to .052, not sure if that's what you ended up gapping to.

EDIT: saw that was in the thread and only applies to AJ26 engines, so you'd need the shorter, later gap.
For a quick and dirty test of the TB, do you have a scanner that reads and/or logs live data? You could watch the throttle position sensor/pedal position sensor readings while cruising and so on, if it doesn't go above a certain amount that could point to the TB.

 

ok if I unplug the MAF sensor and if the car stops running, then its a good sensor?

 

No, mine wouldn't get out of its own way the other day, all sorts of hesitation. Took out the MAF, sprayed MAF cleaner on a Q-Tip, lightly swabbed the temp probe and the two filaments under the sensor top cap, then sprayed the filaments with the cleaner, re-installed, ran like a million bucks (ok, half a mil)

 

If I read the thread correctly then something stands out. In "P" the revs should be "limited" IIRC to something like 2500/3000 RPM, yet you said yours revs fine in park past 2500/3000 (am I mistaken in that?) Yet won't go above 2500 in "D" It almost sounds like it's doing the opposite of what it's supposed to do.

 

If you shift the transmission manually will the engine exceed 2500 RPM?

 

never tried it. if I do is it supposed to?