PI Fuel pressure.

[Paul Hig]

My fuel pressure is around 102psi with bosch pump, but drops to 0 as soon as the ignotion is turned off, so not holding any pressure in the system. means priming for cold starts and makes hot starts difficult

Any suggestions folks?

Pumps not that old, injectors all recently fitted as refurb units, new injector lines, replacement PRV and no leaks in fuel lines

[michaeldavis39]

The only part that has non return valves that I am aware of in a Pi system is the metering unit- the injector lines connections. Other responses will be along shortly.

[Andy Moltu]

That's how it was designed.

Turn the ignition on for a few seconds before cranking. You will hear the fuel pump note change as the pressure builds to PRV pressure (102 psi - give or take a few psi) and then crank it up.

Some P

Some more modern cars have accumulators to hold fuel at pressure but they tend to run at lower pressures so I'm not sure plumbing one in with suitable valving would be that beneficial on the Lucas PI set up.

[Paul Hig]

Thanks Andy

Takes over 30 seconds when cold and often 2 or 3 attempts to fire when hot

Other research says it should hold pressure for a while.  Mine going to 0 psi as soon as ignition turned off

[Waldi]

I had the same question last year about the system not holding pressure after switching the pump off.

Neil Ferguson told me it was designed like that. Mine drops to zero in less than 5 seconds.

I accepted (sort of) that mine is a”poor starter” too, until Niall (TRNiall on here) told me to let the pump running for some 30 seconds before starting. Now it starts mostly the 1st or 2nd attempt, even after a month sitting in the garage.

Waldi

[Paul Hig]

1 hour ago, Waldi said:

I had the same question last year about the system not holding pressure after switching the pump off.

Neil Ferguson told me it was designed like that. Mine drops to zero in less than 5 seconds.

I accepted (sort of) that mine is a”poor starter” too, until Niall (TRNiall on here) told me to let the pump running for some 30 seconds before starting. Now it starts mostly the 1st or 2nd attempt, even after a month sitting in the garage.

Waldi

Whats it like hot starting (eg after a lunch break on a day out)?

Also this video from Steve Denton (watch from about 40 minutes in) makes it look like it should start quite quickly 

 

Edited April 28 by Paul Hig

[Steve-B]

14 hours ago, Waldi said:

I had the same question last year about the system not holding pressure after switching the pump off.

Neil Ferguson told me it was designed like that. Mine drops to zero in less than 5 seconds.

I accepted (sort of) that mine is a”poor starter” too, until Niall (TRNiall on here) told me to let the pump running for some 30 seconds before starting. Now it starts mostly the 1st or 2nd attempt, even after a month sitting in the garage.

Waldi

+1 I thought they all do that, mine does! 

[JohnC]

The PI system is not designed to hold pressure, as NF says, except in the fuel lines (even then, it drops over time). On the upstream side of the MU, as others have said, there is no pressure retention. No air should make its way into the upstream lines, but any expansion of flexible lengths will delay buildup of pressure, hence the advice to wait until you hear a change in pump note. On the downstream side of the MU, if you get air in the injector lines then you may have to crank a bit. I see you have reconditioned injectors, but they may not be perfect. As the engine cools, even a tiny imperfection will let them suck air into the injector lines. Depending on the amount, that can require a few cranks to clear. So, you probably have noting to worry about. But do check for fuel leaks.

YMMV,
JC

[k_raven_smith]

Check the fuel pressure when cranking the engine if it dropping below 95psi Check for voltage drop at the pump. Very common problem.

[Steve-B]

The rubber rings around the nylon inserts that hold injectors can and do fail. I replaced all six of mine over the winter with E10 compatible ones as I could hear a slight hiss and now it is gone. Perhaps, you've got a bit of airleak there, but past that I'd follow the other advise TADTS.

[Waldi]

Paul,

Warm is sometimes a bit more difficult  but mostly 1-3 short attemps and it runs.

Waldi

[Jules TR6]

May I suggest your core issue here is having to crank for 30 seconds to start - and is not fuel pressure. 
 

You might need 10-20secs of letting the fuel build through throughout, before you crank, this is quite normal, though the system when optimised might only need 5-10 seconds of pump time before cranking. 
 

Assuming your plugs are good, what is often overlooked is the cold start setup, namely the adjustment screw at the front right of the engine that opens the throttle bodies when the choke lever is out. 
 

The is often set to add too much air, i.e. so a relatively low fuel ratio. You can always add air with your foot, but you can’t take it away. I’d try backing this screw off a turn from its current setting, then try, if no joy, then back off another turn. There should be a clear gap between the screw and the throttle lever when the choke lever is in. I really think your issue is just too much air when trying to start. 
 

If the engine will run happily, though at very high revs with the choke lever out, this means too much air, the engine should start to stumble a bit with the choke lever out - telling you the fuel ratio is too rich for normal running - but what the engine needs for starting.

[PodOne]

Agree having the cold start cam/screw set correctly makes a difference in having the throttles set to allow 2000rpm on start with full choke. Then pushed into half for 1500rpm and you're away. It took a bit of time with a file to get the cam profile right in my case.

I allow the pump to run for 20-30 sec prior to a cold start and she generally catches after a couple revolutions on 2-3 cylinder's with the rest rapidly following. Hot starts after say a cup of tea or a beer I usually need a bit of initial choke which is pushed back in as soon as I drive off..  

All the above is secondary to having a sound ignition and plugs. Plus these are big lumps of iron which take a while to heat through fully despite what the temperature gauge says to achieve a smooth choke free idle of a EFI set up.

[Paul Hig]

Hi folks.

Thanks for your replies. Just heard from the guy rebuilding the engine. We agreed to swap out the metering unit for a refurb one and he has tested today and now starting properly hot and cold. Hell carry on testing over next day or two to make sure before I collect at end of week.

 

So looks like it was failed seals in the metering unit, so thanks for Chris Mcpheat for engine work and Martin at Fred Millturn for the parts

[iani]

My PI Saloon died on Friday, I'd seen it coming as my fuel pressure was dropping when I came off the M6 and joined local roads, once I was in some roadworks the car stopped. The cause was heat, a bag of frozen peas got me running again. On advice from Dave Pearson at Canley Classics I fitted a low pressure pump between the tank and the CAV filter, my fuel pressure is now constant and my car is starting on first crank, I wish I'd fitted a lift pump years ago. The use of a lift pump was what the factory did on the WRC saloons, so a factory mod.

[Mike C]

5 hours ago, iani said:

My PI Saloon died on Friday, I'd seen it coming as my fuel pressure was dropping when I came off the M6 and joined local roads, once I was in some roadworks the car stopped. The cause was heat, a bag of frozen peas got me running again. On advice from Dave Pearson at Canley Classics I fitted a low pressure pump between the tank and the CAV filter, my fuel pressure is now constant and my car is starting on first crank, I wish I'd fitted a lift pump years ago. The use of a lift pump was what the factory did on the WRC saloons, so a factory mod.

My car was fitted with a high suction, low pressure Carter booster pump under the tank maybe 50 years ago. The only time I've suffered from fuel vapourization  was about 20 years ago when I had a blocked fuel tank filler vent.

[Paul Hig]

Collected car this morning and 30 mile drive on lanes, A and B roads after it's 500 mile post rebuild service (20thou rebore, new pistons, crank grind, new bearings, seals, valve sleeves, recon rocker assembly, new timing gear, overhauled distributor etc, then during service replaced metering unit with refurb one).

NOTE

Old metering unit appears to have internal seals fail. My engine man said that the engine looked like it had never been out of the car since it left the factory, so at we recon after about 117000 mles, needed doing

Fuel pressure upped slightly (into the 106<110psi range), timing checked, valve clearances checked, throttle body balance checked. Plug colour spot on

Running smooth

Starts straight off

Pulls well through rev range (not pushing revs as still building these up after rebuild)

Electric fan only cut in after sitting running for a while

When I finished my journey and parked up, opened bonnet and was NOT hit by heat. temp under bonnet was only slightly above ambient, which is certainly a change from before the rebuild