Last week my Jaguar started to idle funny, stalled and a gust of gas was coming out of the drain at the bottom of my intake manufold. Tried different things, checked vaccum hoses, and rerplace couple of paqrts but noting seemed to improve.
finally, I checked the fuel pressure and reralized that it was pretty low.
I then decided to change the fuel pressure regulator (took me 10 minutes) and the problem disappeared. Car is running great again.
It may be a good idea to keep one in stock in case yours breaks.
JP

 

Great problem/fix summary!

Do you or anyone know how the regulator actually works. I believe there is a vac hose to it but just curious if it is a mechanical valve of some sort that either is set by the fuel flow itself or by the vac??

Thanks

 

Yes, mechanical. Vacuum acts on a diaphragm which opens/close to allow more/less fuel to flow into the return side of the fuel system. More fuel flowing into the return side = lower fuel pressure.

Low vacuum gives higher fuel pressure and vice versa.

Cheers
DD

 

Hi Randy,
The regulator controls the pressure and flow of fuel to the injectors by varying the amount of fuel returned to the tank. Fuel from the pump comes in to a chamber on one end, through a spring loaded valve and out the other end. Fuel to the injectors comes from a connection to the inlet chamber. On the return/outlet side of the regulator is the vacuum line. The pump in any system is capable of supplying more fuel, at a higher pressure, than is required by the injectors. The regulator is needed to prevent damage to the system and flooding the engine at low load.

The operation of the regulator is controlled by a heavy spring on the outlet side holding the valve closed against pump pressure and engine vacuum trying to pull the valve open (against the spring pressure). With the engine idling, high vacuum/low load on the engine, the combination of inlet fuel pressure in front of the valve, and the high vacuum behind it, fuel/pressure to the injectors is low because most fuel is flowing straight through the valve and back to tank. As load is increased on the engine, the vacuum reduces and the heavy spring begins closing the valve which increases the pressure/flow to the injectors from the inlet side. At WOT there is virtually zero vacuum so the valve spring is applying it's full force to close the valve and create the maximum pressure/flow to the injectors. The spring is calibrated for the vehicle/system and will have the ability to hold the valve closed until the pump creates the designed maximum system pressure. At this point the inlet pressure will begin to overcome the spring and the some of the fuel will begin returning to tank, but full pressure/flow is still maintained on the injector side. The engine vacuum is what actually controls the pressure/flow at the injectors and allow the engine to idle cleanly and still allow full power at WOT.

If you haven't left your computer to get a third cup of coffee yet, now would be the time, I'm done.

Dave

 

thanks for the explanation.
I am working on another Jaguar today. The idle at 1000 is very erratic. The idle goes from 500 to 15 rpm back and forth every 5 to 10 second, very bizare.
I checked the fuel pressure at the fuel pump and the pump deliever about 38 LBS only.
I know that these pump pushes 75 to 100 PSI. can this situation affect the idling?

 

Is your car a 6 cylinder or V12? On the V12's if the idle is too high there is a fuel cutoff at 1500rpm. With fuel cut off the revs drop, the fuel comes back on and the engine revs back up and the cycle repeats.

I don't know if the 6 cylinder has the same fuel cutoff, but it might be worth checking everything in the idle system for leaks. If you have a 6 cylinder I believe there is a hose on the underside of the manifold that is out of sight. If that has split that could be the source of extra air.

 

XJ6 according to the thread topic

That hose goes to the Aux Air Valve.

Also check the duct between the throttle body and the Air Flow Meter

Cheers
DD

 

I checked all the vacuum hoses and the vacuum sit at 30 lbs which I beleive is about righ.

I did some adjustment on the intake and the idle speed is now stable a 1500 RPM, I will turn down the aux air valve a bit and see what happen.

The engine still shakes even at 1500 rpm., there is something not going right.
I decided to remove all spark plugs and look at the tip of the spark plugs. 4 of them have a nice light brown color and spark plug 3 and 4 have black soot, so I am thinking that these two have something abnormal. The pressure in these pistons is about the same as the others.
This car has been sitting 4 years and I am wondering if i have faulty injectors on 3 and 4 which would cause too much fuel going into the piston 3 and 4. Would this create this back soot?

Feel free to chime in friends!

JP

 

It certainly could. Or the spark plug wires are not in the right order, or are not conducting electricity to the plugs.

 

Thank you "Dave&Doug-o-pedia"!!!

 

sorta:

Black plugs indicate a rich mixture. A rich mixture can induce a lope in idle. A lope is a soewhat regular uo and down of RPM's
Non existant spark at a plug would reveal a clean plug and evenn wet.

or excess oil in te cylinder. fouls ths spark plug.

wrn rigs or eaking intake vlve seals.

Smoley wexhaust???

Jumbled thoughts. coffee rationed.b

 

No black smoker coming out of exhaust.
the up and down reving is now fixed but the RPM is way to high. I checked all the vacuum hoses and it all look good.