Hello,

I have 1986 XJS V12. it was running o.k. then sat for about a month and would not start. After some research, I have narrowed it down, and it seems there is to much fuel pressure. I have replaced "both" fuel pressure regulators and still have the problem. With everything hooked up it will not start. If I disconnect the fuel line from the leftside fuel rail and put a hose on it to can the motor starts right up but obviously pumping addtional fuel into a can. As soon as I connect the line back it will not start. Like I said, I have replaced both regulators, so I really don't think that is the problem. Seems like relieving this pressure is the trick but what is failing under pressure? There is a device in the side of the fuel rail with two wires, what is this? Any help would be appreciated!

Jeff

 

Check for a blocked/kinked fuel return line.

The device on the fuel rail....do you have a pics? It's probably a fuel temp sensor. If the fuel is hot it provides max fuel pressure to clear the fuel rail.

On an '86, though, I would've expected the older vacuum type fuel temp sensor. Brown plastic body, two vacuum hoses attached. Anyhow, I'm not usre of the operating principle of the electric type but I guess you could unplug it and see if anything changes.

Others will chime in.

Cheers
DD

 

My 85 had the electrical fuel temp sensor. At the moment there is nothing there because it disintegrated and I just forget about it until I open the hood. Have only had one instance of a vapor lock type hard start. Cycling the key on and off about 5 times did the trick.

However that won't cause a no start. Sounds like the return line is blocked to the tank. I would suggest removing the return line at different places and checking if it is clear. Just be careful with compressed air as it has the ever so slight possibility of building static electricity and when it discharges to ground you can make a nice spark. Like I said it is a very rare occurrence.

 

Firstly, you are NEW, so welcome to our side. ENJOY.

I agree with all said.

Check the return pipe just near the front Left jacking point, I have had many with it crushed at that point after tyre fitment, mine included. If its OK there, continue visually inspecting to the rear of the car.

My nephews was crushed at about the middle of the car, workshop used one of those drive over scissor jacking contraptions, with rubber pods against the car floor panel, sounds cool, crushed that line FLAT. Had to cut it out and repair it with hose just to get it out of the workshop.

 

that sounds like vapor lock, try hooking up a compressor hose to the return line and pushing through any potential clog.

 

Vapor lock on a cold engine?

 

if youve got a clog, you cant let the air bubble pass through from the dissconnection, not technically vapor log but still.

 

this has given me some interesting troubleshooting ideas for when i get my V12 reassembled.

 

Wow... Thanks for all the responses! This is a cool forum. Well, I still have the problem. I blew out the return line, and checked for kinks, or crushed areas, all is fine. I even added a short piece of hose to the left fuel regulator return nipple to simulate the return line. still no start. disconnect the left regulator from the fuel rail and she starts right up. I find it hard to believe the old regulator was bad and the new is too. So I'm looking at other options. I have got a pressure gauge, and plan to plumb it to the rail and watch fuel pressure. Only thing left is fuel pump putting out to much pressure? anyone know what the rail pressure should be?
Thanks

 

30psi is a respectable number, you could remove the old one and add an adjustable regulator, then it gives you more options for pressures.

 

The later he xjs runs higher than 30 its more like 37-40 according to my wideband mines

 

ok guys... need your help... Return line is not clogged. and I have attached a pressure gage on the left end of the fuel rail, opposite the input. 62 psi! Like I said I have replaced both regulators. Can the electric fuel pump output to much pressure? seems it would get weaker with age, not stronger. Your thoughts?
Jeff

 

From what I gather if you disconnect the regulator it starts? What is the fuel pressure when you have no left regulator in the system? Can you provide any numbers from the original and the replacement regulator?

We all are assuming the car used to run good with the original pressure regulator and then one day it stopped starting. Also have you installed the original fuel pressure regulator and rechecked pressure.

If you disconnect the fuel return line from the chassis and substitute a short hose into a fuel can the pressure is high, correct? Then it has to be the pressure regulator.

 

Greg,

Yes car starts right up with left regulator disconnected. this is relieving the pressure in the fuel rail. I guess I can connect the pressure gage after the the right regulator, and disconnect the left one and turn pump on to measure pressure, but not sure what this is going to show me. Does anyone know what the pressure should be straight from the pump with no regulators? I could test this, and if bad, replace fuel pump. Sure seems at 62 psi, the regulators aren't doing much. And they cost almost $100 each. maybe I need to think how to test regulator. It's just pressure in and pressure out. Or maybe google to find a adjustable regulator, but I really want to keep it orginal. sure is education working this vs. small block chevy!

 

The fuel pump is not your problem. The job of the regulator is to drop the pressure to the correct amount. The can supply pressures over 100 psi when dead headed.

When checking a fuel pump disconnected you can only measure volume. To measure pressure you need a restriction.

Have you installed the original regulator again to see if the pressure is the same?

Are there any part numbers on the replacement regulator?

Where are you connecting the fuel pressure gauge?

 

I have attached the old regulators one at a time, heard the left one is really the one that regulates, but I replaced both. then started swapping old an new trying figure problem. I connected the pressure gage at the line that would feed the left regulator. I also connected a short piece of hose simulating the return line also, but still does not start. here is link to regulators I purchased http://www.carpartswholesale.com/v5/...Listing+Page:1
and
http://www.carpartswholesale.com/v5/...Listing+Page:2

 

It appears those are the correct regulators. Your pressure does not make sense. If you remove the return line and let that flow freely the pressure is still 62 psi, correct?

Are we sure the gauge you are using is accurate? You have it connected in the right place. It has to be before the left regulator. The left regulator is the one with a hose barb rather than a threaded fitting for the outlet.

Lets try something else for a second. Disconnect your coolant temp sensor and jump the terminal together. See if the car starts now. A bad sensor or corroded terminals will give the ECM a false input and send the fueling to super rich and the engine won't start.

Fuel pressure at 62psi with the proper regulators and no restriction in the return defies logic. I would check your fuel pressure gauge. You can connect it to a compressed air line that you know the pressure and verify it is accurate. I have been burned a few times in my career by defective tools.

The failure mode of a fuel pump is lower output. If the output is too high the voltage may be high but since you are only cranking the voltage is normal. Also the regulators will drop the pressure.

The failure mode of a regulator is lowered pressure due to a ruptured diaphragm. This will push fuel out of the vacuum port. Even if there is no vacuum the pressure would not be 62psi. Besides there is little to no vacuum during cranking.

Lets approach your problem from another angle. You have determined the fueling is too rich. The other likely and easy to test culprit is that coolant temp sensor.

Feel free to contact me via email or if you want to discuss on the phone I am glad to talk.

 

The gauge is brand new, the procedure I used to check the pressure was to connect the gauge at the out line of the fuel rail. So example, I have incoming fuel line into right side regulator, then out of regulator to fuel rail, then there is a out line to the second regulator on the left of the fuel rail. I connected the gauge here bypassing the left regulator. I guess maybe thinking now, I should some how connect gauge to the output of the second regulator, showing pressure to return line to tank? Fuel temp sensor is a good option to start looking. I will test that this evening...

 

Just because the gauge is new that does not mean it is accurate. The gauge needs to be connected before the regulator on the left side. It can be connected at the feed line on the right side.

I am suggesting the coolant temp sensor not the fuel temp sensor.

If the fuel temp sensor in your car is vacuum then it just changes the rail pressure via the vacuum port on the regulator. If it is electrical it changes the fuel via pulse width of the injector.

 

Well the gauge is a dead head gage, meaning it does not measure as "flow thru". So if it's connected before the left regulator, like I orginally tested, the pressure ends there at 62 psi. I also meant "Coolant Temp" not fuel temp. miss typed. I think mine has the electric "fuel temp" senser, screwed into fuel rail with two wires. I will pull connector off coolant sensor and inspect connections, and jumper to verify as well.