Pre-Cat Fuel Pressure

[geeman237]

My 4.0 pre-cat has started to crank over for about 5 to 7 seconds before firing into life. Hot or cold. Sometimes hot its ok though or not long after being turned off.
If it is taking several cranks when it does catch its slow to actually start running, like its not quite getting enough fuel or pressure (?)

A year ago I did a fuel pressure check and all was good.

Once running its fine.

I hear the relay fuel pump priming when the ignition is turned on, and always wait for it to stop before turning the key to start.

My thought is that its loosing fuel pressure when sat, but they don't have a fuel pressure check valve fitted as standard, correct?

Could it be the pump on the way out, ie not able to prime enough initial pressure? If so, then surely the car wouldn't run under normal driving conditions properly?

I plan on doing another fuel pressure check this evening.

Any thoughts?

[BIG DUNC]

It is meant to crank over when cold for a few seconds before it fires so that there is a build up of oil pressure before a cold start. If it now does that when hot, it may be an idea to check the temp probe. Remember the ECU uses a different sensor to the gauge.

Fuel demand under load is far greater than at idle or starting. If it is running OK once started I doubt it is a fuel pressure problem, although the only way to tell would be to temporarily fit a gauge. It could be the regulator is faulty allowing pressure to bleed away, but then you would be having running problems as well.

[geeman237]

I did a bit of testing this evening with a fuel pressure gauge fitted at the fuel supply into the injector 'rails' around the plenum. Used a brass Tee.

I also found some information on the ACT website. Linky... https://www.actproducts.co.uk/2011/lucas-14cux-fue...

Results as follows.

Vacuum hose ON, ignition ON...fuel pump primes to 39 psi immediately, then (without starting the engine) drops to about 36 psi. (in range)

Cold start, engine idling, with the vacuum line off = 39 psi (just about within range in the above tech notes)
Then refitted the vacuum line, still idling = 30 psi (still just within acceptable range)

Turn the engine OFF, pressure stays at 30 psi with a less than 1 psi drop per min following. (again, I think this was acceptable)

Engine running, and revving, the gauge momentarily rose to 36 psi then drops to 30 psi.

I think this shows the fuel pump and regulator are working OK, so maybe the longer cranking is OK.