I've been chasing the low fuel pressure on startup issue for some time. I replaced the FPR, hoses, fuel pump, filter etc. I even added an aftermarket fuel check valve inline to make sure the pressure wasn't bleeding off through the fuel pump/supply lines but still the same problem. When I turn off the engine and it sits a while, the pressure in the fuel rail drops enough that the engine is hard to start. I use the shift to neutral trick and turn the key a to build up pressure which does work fine but I would like to fix the issue eventually.

I recently learned about a pair of valves in the return line back to the tank that I didn't know where on the Series III. They are called return valves (pic below) and are, I believe in the wheel wells somewhere behind a plate. I wonder if these valves help to maintain the fuel pressure and if they could be faulty and contribute to the dropping pressure problem I am chasing.

Does anyone know if these values could be part of the problem, where to find them and how to test them?

 

Those are on the fuel return side of the system, downstream of the regulator. By design one of them is always open. I don't think they're related to your problem. But, anyway, they're behind the rear wheels.

Can you clarify "hard to start"? Five seconds of cranking? More? Less?

The fuel pump should re-pressurize the fuel rail in 2-3 seconds. I'd attach a pressure gauge and see if this is happening.

Cheers
DD

 

Cheers
DD

 

2-3 seconds is also about the time it takes to fasten your seatbelt...

I've never tried to start my Series 3 cars immediately on putting the key into the ignition. I'll turn the key to the "on" position, then fasten my seatbelt, and then start the car. Seems to work every time - one of the side effects of combining two good practices.

The second tip I'll share is never using my left hand to open the door after parallel parking. If you use your right hand, it's almost automatic to shoulder check for oncoming traffic before opening the door in the path of an oncoming car.

 

Thanks for the clarification.

Yes, 4-5 seconds of cranking total to get it started. I usually don't crank it continuously for that long. It usually starts on my second or third attempt to crank the engine.

Naturally, I would like it start on the first crank attempt.

I got onto the idea of a fuel pressure problem because someone suggested to put the shifter in neutral, turn the key to the run position for a few seconds and then it would always start on the first cranking -- after 1 or 2 seconds.

I measure the pressure when idling and it was about 30 psi at the fuel rail. As soon as I turned off the motor, the pressure would slowly drop and I measured about 21 psi when I checked it an hour after. This is despite me have a fuel check valve installed after the fuel pump.

I have not tried leaving the ignition in the run position for a few seconds (shifter in Park I assume) before cranking to see what that would do. I never hear the fuel pump starting when the shifter is in Park so I assume this would do nothing to increase pressure.

I have replaced the FPR and had the injectors cleaned. I have replaced a lot of the fuel hoses and have never detected any sign of a fuel leak. Did a lot of other work on the head and had the engine harness replaced along with the adding a Pertronix distributor.

It really runs well once I get it started. I just want to get the cranking down to having it start on the first try if possible. Not sure what could be bleeding the pressure.

 

I wonder if the check valve you installed is working?

Although simply turning the key "on" does not run the pump it does give a moment for the ECU to recognize the coolant temp and AFM readings. Or that's what some have said.

An oddity I've noticed on some of the 4.2 engines is you can crank for a few spins, release then key, and then crank again and they'll start in half a spin. I dunno why that happens or what that means.

Fuel pressure when cranking should be 36 psi so you can test for that if you want

A long, long time ago....25 years maybe...I had an extended cranking problem with my XJ6. It was my first Jag. I tried various fixes. Long story short a new battery fixed the problem. The fuel injection system likes to see lots of voltage. The battery was slightly subpar and the draw from the starter created enough voltage drop to confuse (for lack of a better word) the ECU.

Cheers
DD

 

Second what Ken and Doug said. I was having long cranking times on my SIII and sometimes wouldn't start till I put on a charger set on 50 amp start. Now, I have a battery maintainer mounted under the hood and plug it in every few days and when ready to start, turn the key to the "on" position for few seconds and then "start" position and it starts right up. A strong battery seems to be requisite.

 

I rewatched the video I made of the pressure gauge reading while I was cranking it to start after it sat a long while. As soon as I turned the key to idle, I did see the gauge jump op to 35 psi while cranking and it jumped from 21 to 35 very quickly. I guess this kind of rules out my theory on fuel pressure.

Get the old ECU to 'wake up' seems to be a good theory. I have a fairly new AGM battery but I should test the voltage the next time I have the multi-meter out just to see if it is showing 12.6v at rest and also how much lower it goes while cranking.

I know my battery cables or original and perhaps not in the best shape. Perhaps I will replace them and check the ground straps and starter cable as well. Speaking of higher voltage/clean power, does it make sense to add one of those car stereo capacitors across the power/ground pins of the ECU ?.

 

I can't answer about the capacitor but the rest sounds good.

Might be interesting to specifically check voltage coming out of the main relay on the brown/slate wire....which is the voltage supply to the ECU and injectors. And check it again at pin 10 of the ECU.



A quick experiment would be using a jumper battery to give some more oooomph when cranking. If no change then nothing has been proven or disproven. But if the engine starts more easily then it's confirmation that you're on the right track. I dunno; just an idea.

Cheers
DD

 

Check crank signal circuit on pin 4. Looks like several components to accomplish what later motronic uses a crank sensor to do, but more room for error.
Thinking 21lbs pressure after a few hours isn't too bad for leak down and would fire the engine without delay.

 

On the subject of replacing cables, I have ordered new battery cables as a start. Is there any other heavy cable (like a starter or alternator cable) or possibly a ground strap that is commonly replaced at the 40 year mark? I am beginning to think my problem was electrical all along and I believe all my cable assembles are original except for the fuel injector wiring harness which I replaced a few years ago when doing the head.

Also, do the connections on the main relay need cleaning from time to time. Does anyone have a pic of the main relay. This seems like a critical component to check from time to time.

 

There are several ground wires at the rear of the water rail. These are the fuel injection grounds. Make sure the connection is clean and tight.

The main ground is underneath, RH side, about where the engine and transmission come together. It's a braided ground strap. Clean and tighten.

The main relay is on the firewall and, if original will look something like this:



At a glance the main relay and fuel pump relay look the same so you have to go by the wire colors. Never a bad idea to check and clean the connections.

Cheers
DD

 

Will inspect and polish all the connections. I imagine there is some corrosion at this point. Many thanks!