it cranks but will not start until cranked for some time, intermittent.

seems like there is no fuel pressure but there is.

what i have tried;

checked fuel pressure 45 psi on cranking and in run.
vented fuel rail and checked again, same as above.
capped off fuel return from fuel rail 70 psi.
opened fuel return and removed vacuum line from regulator 50 psi.
i cannot find anything wrong with fuel.

my thoughts are that the crank position sensor is not seeing the engine rotating intermittently and when it does start, it is because the cam sensor sees the engine rotating.

some times it will start right up fine.

temperature seems to not affect this.

am i right in saying that the crank sensor must be functioning because it needs to know when to fire the right cylinder? could this be dirty or have gone weak?

_____________
1997 XK8 (added by GGG)

 

Hi John

Might help your cause if you say what model/year car you are talking about. Otherwise responses will be generic and there may be a known issue that repeatedly comes up with your model.

Sensors on older FI cars are in the early things I replace (usually CPS, temp and the key relays). I figure they arent that expensive , have had a lot of unknown treatment and maybe hard to source depending where I am. Your car may not be that old.

 

the car is a 1997 xk8,

a little background, i live in Arizona where its very hot, today i fitted the test gauge to the fuel rail to do another test as i am still not convinced that it is not fuel, this was early in the morning and the ambient temp was about 90 deg. the car started as if there was nothing wrong, so i turned it off but left the test gauge connected.

i returned to the car at about 3.00 pm, the ambient temp was 118 deg. looking at the gauge it showed zero pressure, a ha! i thought it has lost pressure but on closer inspection the pointer had gone right round and stopped when it hit the zero stop but on the wrong side.

so how do i think this happened, well one of the schrader valves in the test gauge pipe had stuck shut and the fuel had boiled into vapor and the gauge doesn't return to zero now, it rests at 10 psi.

the above means that i now have to deduct 10 psi from the readings.

at this time the car would not start so i reinstalled the gauge, this may have vented the vapor from the fuel rail as i screwed it on.

and read as follows.
KONEOFF gauge reads 50 psi KONEON, it started, and gauge read 53 psi.

all this proved to me is that the harbour fright test gauge has bad schrader valves but nothing about why it would not start.

as a side note, i have in the past had problems with vehicles with fuel injection vapor locking in the heat when parked, both return and non return systems, due to the fuel pressure being below a 40 psi threshold, the fuel boils pushing the fuel in the lines past the regulator leaving only vapor at 40 psi in the fuel rail, until this is removed either by cranking and the fuel pump replacing it with fuel or venting the pressure from the test point.

there are two methods i have employed for combating this, 1. to do as was done on my V12 XJS where a fuel/ac log cooled the fuel but not when the engine was running so boiling could still occur or 2. to raise the fuel pressure to around 50+ psi with a different regulator, remember the pump output was measured at some 70 psi with the return capped off, alas finding a suitable regulator will prove difficult.

the current pump is new.

 

John,

I've moved your question from General Tech Help to XK8/XKR forum. This is the place to post technical questions about your model.

I've also edited your forum Signature so Model & Year details appear automatically at the foot of all of your posts.

Graham

 

The starting sequence for our cars is once the ignition is turned on the fuel pump runs for 3 to 5 seconds and shuts down. This charges the fuel rail. When the engine is cranking the CPS sensor tells the ECU that the engine is cranking and the ECU turns the pump back on. I would remove and clean the crank position sensor and also remove and clean the connector to the ECU.
The start sequence should already be eliminating any chance for vapor lock.
The only other thing I could think of would be a possible problem with the ignition switch.
Try turning on the ignition to the run position count to three and go to the start position and see if the starting improves.

 

i agree with all that you have written above, however, until i get a reliable test gauge or at least change the schrader valves in the one i have got , i don't think i can go further with the fuel side.

i will need to attach a test gauge and leave it there in position to monitor what is happening in the fuel rail when the car is left out in the sun for a few hours but by attaching the gauge and the pipe to the fuel rail i have increased the amount of fuel that can vaporize and also the volume of fuel in the now extended fuel rail.

the other problem is that the gauge will quite happily measure fuel vapor at 40+ psi as well as fuel.

i am satisfied that the prime and run of the pump/ relay/ECM are operating as they should at this time, it was the first thing i did.

unfortunately i can't clean the CPS until i can get the car on the lift which will be next week when i will try the cleaning of the CPS.

thanks guys for your suggestions but i feel this would also apply to other types of Jaguar and indeed fuel injected cars in general which is why i posed the question where i did but i am fine with being moved.

i will report as soon as i know more.

i am not totally in the dark here, i am a certified master tech and also English with a vast amount of experience with jaguars but was hoping to "short cut" the problem if someone had said "oh yes the crank position sensor, they always go wrong" but no one did.

 

I will mention that in reading this forum mostly daily for 3 years I have not seen many if any people have problems with CPS that posted. I didn't do a search but do not recall this being a major issue. Also there are many members who live in high temp areas and haven't reported this type of issue. just a FYI