JUST finished with the LED headlight install and backed the car out of the garage. Went in to clean up for maybe five minutes and came out to the car that I had left idling. Got in, put the car in drive, and stepped on the gas. At anything other than an idle the car immediately wants to die. The idle is also a bit rough. Ran perfectly up until now. Timing chain upgrades done. No codes or lights yet. Battery fully charged. It feels like there is almost no fuel getting to the engine. Classic - finish one job and Immediately something else happens. Dammit!!

 

OBD live data - check fuel pressure & fuel trims (LTFTs).

 

OK, ... I apologize in advance for this being such a basic suggestion, but....
Check to make sure your snorkle hasn't come undone from either the MAF sensor or the the throttle body, or that a huge leak has appeared in it.

 

Thanks, I'll check that. One other thing to note - after installing the headlights, I decided to adjust the hood fit, which has always been a little off. That turned into adjusting the latches as well, so there was a lot of opening and closing the hood, which of course puts a good deal of vibration/shock into that area. Maybe something came loose?

 

Limped the car to my independent on Saturday (good thing it's only a few level blocks from my office where I did the LED headlight swap) so he could look at it Monday. Naturally, Monday the car starts up and idles/runs like normal. Told him to go ahead and do an oil service (almost due) and also to see if any codes are stored. CEL is not on, but you never know.

Anybody else experience an intermittent fault where the car goes from running perfectly to (almost) not running at all and then back to running perfectly? I'm still leaning towards a near total lack of fuel going into the engine (only enough to barely idle) as any amount of throttle opening (more air) would cause it to stall.

 

Yep, Friday last week, car gave up and died on motorway 300 miles into a return journey, 30 miles from home.
When the breakdown truck turned up 40 mins later he tried car it started and he drove it onto his platform and drove it off at my home and it has run fine since!!
Cars especially Jags act strange at times?

 

My independent took the car out and measured running fuel pressure. Started out at 40 psi but after about 5 minutes driving, it began to fluctuate. It would go up as high as 50 psi and then drop to 30 psi, then back up again. I'd imagine the injectors want to see a constant pressure so this can't be right. They are talking about replacing the fuel pump or fuel pressure regulator.

The car desperately needed valve cover gaskets/coil boots/etc. and was due for plugs as well, so I'm having them go ahead and do that work, but I'd like to get some feedback on the fuel pressure symptoms before giving them the go ahead on anything else.

 

A couple of questions -

Does the fuel pressure regulator adjust fuel pressure based on vacuum signal to maintain a constant pressure drop across the injector?

If so, does this potentially account for the measured variations (down as low as 30 psi and spiking occasionally to 50 psi) in fuel rail pressure while the car is being driven?

Just don't want to have them replace the fuel pump if those are 'normal' pressure variations...

 

More or less, yes.

Post #2. Also check sensors for plausibility.

Could be an intermittent air leak, such as a hose that flexes.