I have a 2000 XJR that has a drivability issue. I had a small engine compartment fire in the front passenger (US) engine area that caused damage to some wiring and was hot enough to melt plastic covers along the firewall and along the driver side fender. Wiring was repaired/replaced. Since that time the car has run fine when using light to moderate throttle or steady speed. When under load (or accelerating past 3500 rpm) the engine coughs and stumbles - sometimes backfiring. This does NOT result in codes or CEL being thrown.
I have replaced the fuel filter, both fuel pumps, and all spark plugs. Before I continue throwing parts at the problem I am seeking advice from the forum of where the issue could lie. Thank you in advance!

 

hmm, so if it was a misfire, it should throw a code. I know you replaced a lot of the fuel components, but wondering if you've actually measured fuel pressure. Perhaps you have some sort of collapsed or melted fuel hose that passes enough fuel for low speed but can't handle volume demands at higher rpm/load.

Unfortunately I think you will need some diagnostic tools to really diagnose. specifically, looking at ignition timing and a/f ratio when this misbehavior occurs would be very telling.

Was the fire anywhere near the maf? That seems to be a common trouble spot for poor running conditions.

 

The fire was near the MAF, but I replaced the MAF also. I've got another FPR on-hand that I can install, too. Could injectors (clogged?) play a part?

 

I take it the intake tube was also replaced with a new or used one? If used, check the underside of the bellows. Do you have an aftermarket MAF? I haven’t had much luck with those. I’d replace it with a used oem one instead of an aftermarket one.

Even though you installed new fuel pumps, I’d jump the secondary FP relay to make sure that it activates.

 

Intake tube was replaced, MAF was replaced with OEM - I'll try jumping the secondary fuel pump to be sure.

 

Sounds like the secondary fuel pump is not activating

 

This may sound unlikely; however, I remember helping a friend with a similar problem. It turned out to be the battery/charging system.
We replaced the battery and the situation improved, and later found a slight short that was causing a voltage drop.
In your case it may be a long shot, but it's easy enough to low amp.charge the battery overnight, then "fire for effect"
If you still have problems, my next step would be to check/replace a few coils, or O2 sensors.
Having no codes is problematic in any case of misfire.

 

spark plugs

 

although the op stated that the spark plugs have been replaced, a close look at the gap may be worth it. If you are blowing out the spark, it may indicate too big a gap or some ignition coils that didn't take too kindly to the heat.

 

Here is some interesting food for thought: I live in Tulsa, OK, and we had some mild, low humidity weather for a few days - and the car ran great with no hesitation. Warmer temps and humidity returned = hesitation returned.

Thoughts?

 

That is some interesting news. If I were you, I’d pull all 8 coils out and see if there’s any condensation (or oil) on the stems of the coils themselves. Condensation will cause the misfires. If it’s wet inside the spark plug tubes, it needs to be cleaned and dried. Dielectric grease on the spark plugs should eliminate the misfiring.

 

If not already done, check the bulk head ground mount and the connector pins for the MAF. What is the condition of the parts load breather?

I like the spark plug gap suggestion, never trust the box statement "pre-gapped".

Did you have oil in the plug holes when you changed them?...as Addicted alluded to.