94XJS 4.0 Hard Starting After NewFilters

I have looked through a lot of posts but found nothing to point me toward a solution.
Hoping to get some direction…I’ve had the car, my first Jag, a 94 2+2 5 speed about 4 years now working through some minor issues and loving the car.
Changed the fuel filter and the air filter last week and now the car cranks and cranks and after about the 5th round of cranking finally starts and runs fine from there. It started fine on the first attempt before the new filters.
Any thoughts on where to start looking would be appreciated.

John Bradshaw

Check the fuel filter connections for leaks. I could be the residualvfuel pressure is bleeding down between starts because of a leak, and it takes several cycles of the key to build pressure back up.

If that isn’t the problem, check the vicinity of the air filter for any electrical or hose connections that may have been knocked or left off when you changed the filter.

Finally, can you hear the fuel pump run each time you turn the key to run (not start), off, then on again?

Try turning the ignition key to the ‘run’ position and then off a few times (<>5) and then see if it starts on the next attempt.
If it does then this points to the fuel pressure bleeding off after shut down.
Not sure why it would suddenly start to do this after a filter change.

Also, Jag has a sneaky little (well, maybe no so little in diameter, but THIN) o-ring fitted between the air filter housing “exit” and the intake horn for the M.A.F. assembly. I have r/red my air filter on Superblue (and Superblack, before her) many times, but never spotted it. :slightly_frowning_face: Presumably then they either somehow rotted and fell off/out, or a PO of both of them discarded it and never replaced it (e.g. when changing out the filters). I guess the purpose of the ring is to make sure there is an airtight seal (or conversely vacuum seal) between the two units when assembled. I only happened to find out about this o-ring recently when I was looking at the “exploded parts” drawing of the air intake assembly on the Jag classic parts website. I don’t know if it makes any difference, as my XJSes have always idled smoothly and, as mentioned, I have never had the o-ring before in place on either of them to compare running. :confused: If missing on yours, perhaps so. :thinking: I do know whenever I tried to order new ones, our usual suspects all have them as “N/A” anymore. Grrrrrrr … :angry:

Anything upstream from the MAF sensor is unmetered air. Wouldn’t matter how air is getting through to the MAF.

Actually, I think u’re right there, VK, now that I think about it … :thinking: but so then what is the purpose of the rubber o-ring? :confused:

Thanks for the thoughts guys. All valid suggestions and I’ll follow up later today or tomorrow. The fuel pressure leaking off makes sense to me. I’ll report back.
Appreciate your help.


I tried switching on the ignition without cranking the engine 5 times and on the 5th tried to start. After a few cranks it started and ran. I think we’re onto something about a leak.
I’ll pull the filter I put in and make sure it’s tight. If that doesn’t sort it then I’ll follow up looking at the fuel tank and pump. Hoping it doesn’t come to that.

Thanks for the help.


Put a pressure meter on the fuel rail and do a leak down test. Report back.

There’s nowhere to attach a pressure meter on the 6 cylinder engines…he’ll have to add one.

There’s no Schrader valve on the 4.0 rail ? :confused:

No + twenty characters

How dirty was the old air filter? if quite dirty,it might have lessened the avaiable air and thuslly the car enjoed the rich mixture to start. A choke as in olden days, so to speak.

The new clean filter flows more air the mixture is eaner.

Now, that means, if i am correct, that t had masked another fault. teh means od cold start enrichment. that is as far as i can go. not familiar ith the engne.

Then how are we supposed to determine the fuel pressure at the rail? :confused: Dumb move on Jag’s part, IMHO …

Bryan N has an answer to that question, it involves making a ‘Tee’ that gets inserted into the fuel rail

An update……I listened for the fuel pump as advised. It runs for a second when the ignition is turned on. I turned it on and off 3 times and on the 4 th time I cranked the engine and it started right up. Fuel is getting to the fuel rail, it’s losing pressure while the power is turned off.
Thanks for all your comments.


Could it be the check valve in the fuel pump assembly? I started having difficult starts and pulled mine and found the check valve in the plastic housing FUBAR. Just added a aftermarket one in the fuel line and solved the problem.

The easiest place to attach pressure gauge is in your boot, regardless of the engine (XJSs with inline filter behind spare wheel). Right after your fuel filter. It’s not ideal to measure pressure in your lines at different revs, however works perfectly, you can even see the injector’s working (glycerine-filled gauge). That’s waaay more comfy when compared to other cars.

The facelifts have their fuel filter at the drivers wheel……

There’s nothing in the boot to attach a fuel pressure gauge to.