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  #1  
Old 09-03-2011, 08:18 PM
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Default hard start but once started runs fine?

2003 XKR conv

lately the motor does not catch right away....plenty of juice at the starter, it cranks and cranks....eventually it starts....guess a gas or ignition issue?? Runs fine once started so that makes it puzzling.,...anyone ever heard of or have this issue and what did you find??????
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  #2  
Old 09-03-2011, 08:45 PM
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What do you mean catch? Does the starter turn but does not engage to the flywheel to rotate the engine?
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Old 09-03-2011, 10:24 PM
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You have the same problem I have. Let me guess... if you park and restart the car within a few minutes it starts instantly, but after that it takes 5 to 10 seconds of cranking before it will start.

Here's what I've found: the fuel pump has some sort of mechanism that prevents the fuel from draining back into the engine when you turn it off. Your's is bad and you need a new fuel pump. The fuel pump on the 2003+ XK8s costs $500, and it's located in the fuel tank. The total cost to have it replaced at the dealer is over $1000. Doing it yourself is possible, but removing the tank is supposed to be pretty tough. Apparently the fuel line is a PITA to disconnect from the tank.

I've rigged up a workaround on my car, but it's not for the faint of heart. I cut the wires between the fuel pump modulator and the fuel pump and I installed a DPDT relay. The relay disconnects the modulator and applies 12vdc straight to the pump. If I run the pump for a few seconds before attempting to start the car, it starts instantly. I installed a button in the passenger compartment which I can press and hold for a few seconds before starting. The switch activates the relay and the pump then pre-pressurizes the ignition system. I also ordered a time-delay relay which should be here one day this week. I'm going to tie the time-delay relay to the door-unlock impulse. When the time-delay relay senses that I have pressed the unlock button on the remote, it will automatically pre-charge the fuel line for 8 seconds. That way I'll never even have to think about it. I'll just get in the car and start it.

If you want to verify that this is the problem on your car, do this: turn the key to the "on" position but don't start the car. Then turn it off and remove the key. Repeat five or six times. Then try to start the car. I'm guessing that it will start right up. Each time you turn on the key the fuel pump modulator tells the pump to run for a second or so. Unfortunately that brief little charge isn't enough to fully pressurize the system and get the fuel at the injectors. But doing it five times is usually enough.
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Old 09-04-2011, 07:34 AM
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Sam,

If you had described this work earlier, I kust have missed it.

Curious: heat-related? I'm thinking when it's hot the fuel wants to vaporize in the fuel rail which could be a contributor.

You, sir, are one smart cookie! Nice work.
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Old 09-04-2011, 08:08 AM
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Sam based on your writeup, it would appear that the modulator is bad and not the pump...the car rides great down the road and if the fuel pump was faulty then it wouldnt run....unless this modulator is an integral part of the fuel pump???
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Old 09-04-2011, 11:19 AM
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There's a fuel pressure regulator on the fuel rail which tells the ECU how much pressure is in the fuel system. The ECU then tells the fuel pump how fast to spin to keep the fuel pressure at the proper level. When you first turn the key the fuel pump runs for about a second to pre-charge the system, but in my car that isn't enough since there isn't any fuel in the fuel line. I have an OBD scanner that gives me a reading of the fuel pressure, and after that initial charge the pressure in the line is normal, however the fuel just isn't in the rail yet. I've verified this by pressing on the schrader valve on the fuel rail after it charges and only air came out. By forcing the pump to turn for 8 seconds or so, it forces the fuel up to the fuel rail and I get an instant start.

The fuel pump contains an internal check-valve which prevents the fuel from draining back to the tank. It's not the pump mechanism that's bad, just the check-valve. But it's an integral part of the pump. This problem occurs on more cars than just Jaguars, and there are vendors online from whom you can buy an in-line check valve to put on the fuel line. They are marketed specifically for this problem. That was my first attempt at fixing the problem. It worked, however it worked too well. It didn't allow ANY pressure to bleed off from the fuel rail. If I turned off a hot engine and tried to restart it 10 minutes later, it would not start at all. Period. I could crank it until the battery was dead and it wouldn't start. I checked the pressure with my OBD scanner and it was something like three times the normal pressure. As the gasoline in the fuel rail warmed and expanded it just had no where to go. It turned out that the pressure would build up at the fuel rail to the point that the injectors couldn't open. If I waited several hours the pressure would bleed off and the car would start instantly, or if I vented a little fuel manually by pressing on the fuel rail schrader valve the car would start instantly.

If the modulator was bad the car wouldn't run at all. The purpose of the modulator is to receive the electrical signal from the ECU and translate that into pulses to power the fuel pump. If it was going to go bad it would either fail on or fail off. It's not on all the time, if it was I wouldn't have this problem. And it's not off all the time, if it was then the car wouldn't run.

The 2002 and earlier cars controlled their fuel pressure differently. They had two fuel lines; one carried the fuel to the fuel rail and the other returned the excess fuel to the tank. The fuel pump ran continuously. There was a mechanical pressure regulator on the fuel rail that kept the pressure at the proper level. In 2003 Jaguar switched the the returnless fuel system because it was supposed to improve fuel economy and reduce emissions.
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Last edited by Reverend Sam; 09-04-2011 at 11:22 AM.
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Old 09-08-2011, 09:04 PM
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Are there any aftermarket fuel pumps and are they recommended?
I see some listed for a 2003 xk8 but not the xkr, arent they the same??
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Old 09-12-2011, 10:34 AM
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Sam....

you state "I have an OBD scanner that gives me a reading of the fuel pressure, and after that initial charge the pressure in the line is normal, however the fuel just isn't in the rail yet."

Can I use my torque application on my cell to check fuel p??
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Old 09-12-2011, 11:36 AM
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Yes, that's how I read mine. It displays the value in metric, but it's easy to convert to PSI.
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Old 09-12-2011, 11:49 AM
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It is most likely the FP, as Sam stated. But I would check the FPR (Fuel pressure regulator), it could be leaking also. It is an easy check, pull the vacuum hose and smell for gas. If you smell gas, the diaphram has ruptured and is time to replace.
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Old 09-12-2011, 12:11 PM
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Quote:
Originally Posted by randyb View Post
It is most likely the FP, as Sam stated. But I would check the FPR (Fuel pressure regulator), it could be leaking also. It is an easy check, pull the vacuum hose and smell for gas. If you smell gas, the diaphram has ruptured and is time to replace.
2003 and later (4.2) models do not have a conventional vacuum operated fuel pressure regulator (see Rev. Sam's discussion above).
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Old 09-12-2011, 07:47 PM
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Could well be worn starter motor contacts
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Old 09-12-2011, 07:47 PM
 
 
 
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2003, car, difficult, failure, fine, fuel, gas, giving, hard, jaguar, pressure, regulator, runs, start, started, starts, type



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