Hello;
I'm trying to troubleshoot an issue that I have been recently experiencing.
My XJ8 4.0 NA runs great. No issues at all.

EXCEPT:

When doing a HARD acceleration, between 4,000 and 5,000 the engine bogs down and hesitates a little bit.
It does not happen on normal acceleration, only when I punch it and drive it hard.

No other issues at any time other than this hard acceleration issue.

It happens in all gears. It does NOT happen on the "shift" cycle, rather during the acceleration before any shifts occur.

I was thinking maybe the fuel filter/2nd fuel pump maybe?

This engine is modified with a cold air intake, and a cat-back free flow magnaflow exhaust system.

Like I said, the car drives great. No issues. Decent gas mileage.

BUT, when I "punch" it, I get a hesitation.

Any educated guesses where to start?

 

MAF sensor is always good to check & clean. I had bogged down acceleration when mine was gummed up.

 

+1 MAF sensor. Most likely is a really dirty throttle plate and bore. Next would be an air leak in the modified air induction system. Maybe too much play in the accelerator cable. Assuming you made the mod, did the instruction tell you to use the original MAF?

Have codes? What LTFTs?

When I first bought my last XJ8 in 2013 (I wish I still had it), it bogged down just as you noted. Cleaning the throttle plate and bore and taking the slack out of the cable fixed most of the problem. New air filter gave some punch, as did new plugs and SeaFoam treatment. But these were hardly noticeable compared to cleaning the throttle and taking the slack out of the cable.

Lots of threads on this forum.

 

+2 for MAF. Mine did the same, hesitated on acceleration. Cleaning the MAF helped once or twice, but a replacement MAF fixed it properly

 

If you have a NA engine there is only one fuel pump, but it could be supplying insufficient fuel. Certainly the filter and MAFS should be considered first. But it could even be the injectors. Take it one step at a time, and don't throw parts at it.

 

I've also seen an engine mount so bad that the engine would physically lift up and the air tube at the throttle body would get crooked and cause an air leak. Just food for thought.

 

The only reason the previous owner of my XJ8 was selling it was because he said it was "noticeablely slower than it should be, it's got a problem", however on the test drive I could feel the slack in the accelerator cable through the pedal. Once I'd knocked him down on price based on the slowness, I drove round the corner and fixed the problem by simply adjusting the cable. It's not even a 5 minute job to do, try that first as it won't cost you anything.

 

I had a similar problem caused by the fuel pump. Could also be the fuel pressure regulator. I diagnosed it with a fuel pressure gauge with a long enough hose to watch the gauge while driving. You could see the pressure drop under load. It should actually increase, as the FPR is connected to manifold vacuum. More load, less vacuum, higher fuel pressure.

 

A new fuel filter is never a bad idea for how little they cost.

bob