Losing Fuel Rail Pressure with Engine off

[rdl001]

I have a problem with excessive cranking when starting my engine after it has been stopped for more than 5 mins (great fun at petrol stations). The engine will start on the first turn of the key from cold, and even when warm it will start first turn as long I don't leave it more than a few minutes between stop and start.

When measuring the fuel rail pressure, I see 36psi with ignition on and primed, then a very constant 31psi with the engine running.

When I turn the engine off, the pressure falls to zero in about 5 minutes.

I have no smell off petrol and cannot see any leaks from fuel pipe or where the injector meet the fuel rail. Could an injector be leaking into the engine and flooding one of the cylinders which is why is struggles to start? Would I see a wet spark plug if this was the case?

Thanks in advance.

[Boosted LS1]

Maybe the check valve in the pump has failed, assuming it has one.

You could remove the spark plugs to see if a cylinder reeks of petrol but I doubt it'll be an injector otherwise you may have had other symptons. Backfire or hydrolock spring to mind.

[Steve_D]

Some pumps have a check valve some not.
In either case your 2 second prime when you turn on the ignition should give you all the pressure you need.
Some tests to try...

• Turn on ignition and wait for prime to complete. Turn off and on again and wait for that prime to complete then try a start.
• Measure the fuel pressure immediately after prime.

Steve

[rdl001]

Boosted LS1 said:

Maybe the check valve in the pump has failed, assuming it has one.

You could remove the spark plugs to see if a cylinder reeks of petrol but I doubt it'll be an injector otherwise you may have had other symptons. Backfire or hydrolock spring to mind.

I presume the check valve is in the output of the pump - do these valves fail?

I do not have an additional check valve, would it be recommended to fit one?

Where else could I be losing pressure from the rail so quickly?

[Steve_D]

rdl001 said:

Boosted LS1 said:

Maybe the check valve in the pump has failed, assuming it has one.

You could remove the spark plugs to see if a cylinder reeks of petrol but I doubt it'll be an injector otherwise you may have had other symptons. Backfire or hydrolock spring to mind.

I presume the check valve is in the output of the pump - do these valves fail?

I do not have an additional check valve, would it be recommended to fit one?

Where else could I be losing pressure from the rail so quickly?

If it has a valve it will be well inside the pump and no external valves are not used.

But back to my point...you should not need a valve as the pump will prime when you turn the key and then come on again as soon as it detects the first ignition pulse so it matters not that the fuel rail loses pressure.

In your first post you say you have full fuel pressure so I suspect your starting issues are not related to fuel pressure.

If we are talking about starting from hot then perhaps the issue is ignition amp or coil.

Steve

[Boosted LS1]

Just a point to note, my AEM pump has a check valve withinn the removable outlet fitting. I can run it with or without the check valve. It's there to keep pressure in the rail after ignition switch off.

As Steve says though, you should make pressure very quickly when cranking.

[rdl001]

After priming the fuel rail with 36 psi, I can clamp the flow and return pipes to the pump and still lose pressure in 5 minutes. I am just worried that this fuel leakage is going into the engine and flooding or am I being over worried. It just feels like its flooded when starting for the second time.

[blitzracing]

You are barking up the wrong tree I suspect- the first place Id look would be the fuel rail temp sender as this controls the hot engine restart injector pulses. Start with a simple resistance check on a cold engine between the engine water temp and fuel rail temp sender- they should be the same as the thermisters are the same. You would only flood a single chamber if an injector was stuck open, and the engine would still start.

[rdl001]

Thanks Blitz, I have measured both sensors and the coolant is at 4055 ohms and the fuel is at 4105 ohms so I am happy that they are both reading correctly at a cold-ish temperature.

Note: my engine does not need to be hot, to struggle to start.
If I turn the ignition on, prime the fuel rail then let it depressurize (5 mins), I presume the fuel is leaking into a cylinder, injector maybe?
Turn the ignition on again, prime the fuel rail then let it depressurize (5 mins)
When I now go to start the engine it will struggle to start (cranks for 20-30 secs before starting then splutters for a short while before idling normally)

If I start it straight away after not using it for an hour, i.e. purge then start, it starts in seconds.

[blitzracing]

Id drop all the plugs out to start with and check they are all the same colour, id suspect a leaky injector would soot a plug. Id also pop a colour tune into a cylinder- You will be able to check for a spark to start with under compression.You will need to be in a darkened area you wont see it in daylight. Then if you see what it does fuel wise- you should get a nice flash of yellowish flame if the mixture is enriched to start the engine. If its very pale blue its lean or you have an air leak.

[danbourassa]

rdl001 said:

When I turn the engine off, the pressure falls to zero in about 5 minutes.

When I read this, I got curious about whether there is a spec for this. Turns out, there is.

I have a portion of a Land Rover document (pages 63-78). The chapter is called "LAND ROVER FUEL INJECTION SYSTEMS". It mentions everything from 13CU to Bosch Motronic 5.2.1, so it's a newer doc as far as 14CUX is concerned. In the section regarding the return-type fuel pump system, this is stated:

 •  Used 1987- early 1999
 •  Integral fuel level sending unit beginning 1991 
 •  In-tank with external fuel filter
 •  2.4-2.6 bar  (34-37 psi) operating pressure
 •  Key off pressure drop from 2.3-2.6 bar (36-38 psi)- less than 0.7 bar (10 psi) in one minute
 •  Integral Advanced EVAP sensor from October 1996

[rdl001]

Thanks for the information Dan, very interesting.

So where would the fuel be going?
I can't believe it is going past the pressure regulator, the fuel injectors should hold when they are de-energised/closed and there is a check valve in the pump? and no fumes = no external leaks.

Is anyone else able to measure their fuel rail pressure and how long it holds after engine shut down

[Dougal9887]

My engine is out at present but I had a pressure gauge fitted to the rail all last year. I didn't take particular note of residual pressure, but remember being surprised just how long pressure remained, certainly 50% of the pressure over say 6 hours. However this was using Vectra injectors, I never used a pressure gauge with the old Lucas injectors, so may not be relevant.
Dougal.

Edited by Dougal9887 on Tuesday 18th February 07:17

[blitzracing]

Mine drops to zero as soon as the fuel pump stops. I've pulled the fuel pump to bits in the past and never found the mysterious one way valve talked about- so I assume in my case at least there is not one. It does not affect the cars running in any way, but I still think you are chasing a red herring as your fuel pressure drop is within spec anyway.

[rdl001]

Hi Blitz,
I am still also taking your advice I have ordered a new coolant sensor and a new fuel rail sensor. I will report back on the results.

[Teamred]

I'm with Steve D. I reckon it's ignition related. Try changing the coil. After/before that, try the rotor arm, distributor cap, LT lead.

[rdl001]

RECAP - my original problem, was that it took a lot of cranking to restart my engine after it has been started once from cold. Note: The engine could still be cold and not restart on the second attempt.

After changing the fuel sensor and coolant sensor I still had the same issue. Symptom - Full fuel pressure on prime and during running, but then loosing all pressure in 2 minutes after turning engine off.

FIXED - Fault turned out to be that fuel was leaking past the fuel regulator diaphragm and back into the plenum by the stepper.
New fuel regulator fitted today and totally solved.

Now when the engine is switched off, the fuel pressure drops to 20 psi in 30 seconds and then takes 3 hours to drop to zero. Engine starts on the first crank every time from cold, from warm, from hot.

This post may help others in the future.
Regards,

Edited by rdl001 on Sunday 1st March 17:07

[Steve_D]

A painful process but a good find.
Thanks for the update.

Steve

[Oldred_V8S]

rdl001 said:

RECAP - my original problem, was that it took a lot of cranking to restart my engine after it has been started once from cold. Note: The engine could still be cold and not restart on the second attempt.

After changing the fuel sensor and coolant sensor I still had the same issue. Symptom - Full fuel pressure on prime and during running, but then loosing all pressure in 2 minutes after turning engine off.

FIXED - Fault turned out to be that fuel was leaking past the fuel regulator diaphragm and back into the plenum by the stepper.
New fuel regulator fitted today and totally solved.

Now when the engine is switched off, the fuel pressure drops to 20 psi in 30 seconds and then takes 3 hours to drop to zero. Engine starts on the first crank every time from cold, from warm, from hot.

This post may help others in the future.
Regards,

Thanks for posting up the resolution, too many don't bother. It is certianly one for the memory banks. Glad you got it sorted.

Edited by rdl001 on Sunday 1st March 17:07

[blitzracing]

rdl001 said:

FIXED - Fault turned out to be that fuel was leaking past the fuel regulator diaphragm and back into the plenum by the stepper.
New fuel regulator fitted today and totally solved.

That is interesting- so id guess the non start issue as due to a plenum with a load of extra petrol vapour in it flooding the engine to first start. Not something Id ever of thought of.