Another misfire above 3000rpm ...puzzled

[Peter Cobbold]

It's pointing to the MU leaking fuel internally into the overflow pipe back to the tank. How fast is fuel coming out there?

 

If you had to turn down the prv's set pressure to get William's correct flow then that does not mean the same 1L/2min flow is happening when the prv is set to 105psi.

 

To check for a MU leak, stop the end of the supply pipe with the gauge ( ie remove the T ) so the the MU is not given fuel. See if the prv can be adjusted to 105. If the psi drops when the flow is readmittd to the MU then suspect a MU leak.

 

Peter

[RobinTR6]

If youre bored of this join the club......

 

1. Pump and PRV independently tested and OK last week. Reconnected it all up and NO change pressure still at 80 psi at idle.
2. all pipework to and from tank, filter, PRV, pump, MU including return tested and clear
3. Tank emptied many times no debris and free flow of petrol
4. Fitted larger tank outlet and larger inlet to new petrol filter with 11mm hose
5. checked vacumn hose from MU to inlets
6. still cannot get more than 80/90 psi at MU
7. running a bit better but still missing above 3k revs. Will run smoothly after that if full throttle applied then backed off slowly, engine then settles . Notably pressure drops to 60psi when WOT applied, then increases again to 80 ish as you back off accelerator
8. Have tried direct connect from seperate fuel supply (ie not petrol tank) no difference
9. dribble of petrol returns from MU as expected (if MU return discoinnected) no difference to PSI
10. tried connecting another battery as well as the one on car to pump, no change
11. PRV adjustment does not raise the pressure at MU above the 80/90 psi

My thoughts are still stuck at PRV/Pump given its got the correct electrics(12.5 when idling), petrol flow to pump etc all tested and OK, pressure drops to 60 psi when driven at above 3k revs when accelerator floored, this seems to be the cause of the misfire, ie not enough petrol getting thru to injectors, car revs fine when not being driven (I believe this excludes ignition issues)???? Is it possible for an ignition issue to cause the fuel pressure to drop....I dont think so but ????

 

Run out of ideas .....

[John L]

You have had the PRV and the pump tested, but what value was obtained when they did it, you don't say?

There are 2 types of PRV, I remember Neil saying, the Bosch ones that moss supply for a Bosch pump were only set to 90psi at one time.

The sump isn't filling up is it? The seal on the pedestal?

If there is very little fuel returning from the MU, then the pump must be slipping flow in the gears, I suppose the in and out are correct on the pump?

John

[openroad]

Morning Robin, if you hav`nt got 106/8 psi on the guage, its not going to drive properly over 3000 revs.

 

So your dilemma is why wont that pressure go up, the same as we both had last year. Mine was down to the PRV.

 

Of out to a classic car show now, Have fun,

Conrad.

[RobinTR6]

Neil tested pumps to 140 psi, and set the PRV which is the Lucas type for my Lucas pump(s). Pump is wired out and in correctly, there is a dribble of petrol returning to MU (which is correct I believe). As noted even if the return is disconnected no change. I have tried three PRV's all tested by Neil, and 2 different pumps, again no change. I assumed it must be pipework but thats all now tested , all clear and increased flow from tank with 11mm petrol pipe and larger outlet from tank and inlet to filter.

[John L]

Do you have braided pipe up to the MU?, some of these have had problems, can you fit your gauge directly to the end of the steel pipe, and not connect it to the MU, (dead end the steel pipe with your gauge) That should then eliminate the braided pipe.

John

[astontr6]

Dear Robin,

Gravity fuel supply ONLY to the Lucas pump. I have rechecked my original figure I stated both documents state 1+ litre/min. This info came from a 1990's document by a well known west country TR Tuner and Practical Motorist 1970's?

Also I notice that you have a very long pipe run from tank to pump, if this is in (6mm) 1/4" bore, this may be the problem area. OR What spec rubber hoses did you use? As they are covered by SS braid if wrong spec rubber hose is used, they will swell and as it cannot expand, because of the SS, it will close up the bore! The Spec for the latest petrol hose is SAE J30 R9 or 14., hoses to these 2 specs can take 20% + Ethanol. I have assumed that there is no problem with crud in the tank?

 

Bruce.

[Blue cedar]

So everything seems fine up to and inc. the metering unit .?????

What about a restricted flow out of the MU and on into the engine.......injector pipe with some crud or injector hesitating when pulsing over 3k

[marko]

it will not run properly unless you get the fuel pressure up to 106 psi.

 

I would be connecting the fuel pressure gauge further back in stages to find out where the problem / restriction / blockage is...

[RobinTR6]

Yikes, my head is exploding. OK just disconnected feed output from PRV to MU, its flowing 2 litres/minute , which if anything is too much. I agree, the issue to be resolved first is why I cant get the pressure up. I'm not sure I can connect the gauge anywhere other than at the MU. I'll have a look at the connections for the T piece, I may be able to connect at the pump.....

 

Frankly given hoses are clear and flow is more than adequate and pump is getting 12.9 volts at idle the everything still points to PRV. Yet have tried two others with the same result......

 

Just going out to garage to ...........................what a !"£$%^*()() waste of time, even sorting the webers out on my old alfa was considerably easier than this. Just seems to be too many failure points and or things which are difficult to diagnose. Its got to be something SIMPLES.....(that Im missing)

[Peter Cobbold]

Is the pressure gauge accurate?

If its reading low you may have the right psi........and mixture..........and an ignition issue.

Peter

 

The drop to 60psi is very odd. Suggests the flow is falling off with rpm.

Or that the demand is too high: could the MU be taking far more fuel than it should, running very rich indeed?

Is the exhaust smoky at 3000rpm misfire??

Can you measue AFR?

 

Is the excess fuel - 'choke' - lever on MU fully home?

Edited May 4, 2015 by Peter Cobbold

[RobinTR6]

No smoke, plugs look ok. Can't measure afr. I'll try measuring pressure at the PRV...methinks pump but have yet to prove. Not sure how pressure measured on test bench, but 2 litres/min to mu should be plenty.

[Peter Cobbold]

Fuel flow needed is HP x 5.6 ml per minute. ( A Graham Bell Forced Induction Performance Tuning p 199)

So 2000 ml per min = 357hp

Yes, enough !!

[Peter Cobbold]

"Notably pressure drops to 60psi when WOT applied, then increases again to 80 ish as you back off accelerator"

 

Maybe two faults at once:

 

Might it be psi gauge reading low. So at tickover you actaully have ca 105psi as set by prv. not 80

And

The psi at the MU drops to '60' because there's a restriction in the fuel line from the prv to the MU. That wont affect psi at low flows but would limit flow at higher loads. Could be bad kink in the line. Or a dodgy short hose form the steel line to the MU. Measure the flow at the end of the MU rubber hose.

 

If the psi really does drop to 60 I doubt the injectors would crack open much at all.

[ijonsson]

I might have missed something in the conversation, but couldn't it be something as simple as a restriction in the tubing between PRV and MU, a kink or local flattening of the pipe that's running along the chassi? If it's out of copper like my car, it's quite soft if hit by something.

[RobinTR6]

As noted all input and output pipework checked out OK. Just tested the pump with pressure gauge plumbed in at pump outlet to PRV....same as output from PRV, is about 80 psi, so its defo a pump issue. BTW its got an 11 mm feed in so flow has benn proved to be 2 ltrs / main which is plenty. Pump tested on bench fine but not in with in car. And its getting 12+volts at ignition and12.9 at idle so its not electrics. Back to pump rebuilder....despite all the aggro at least we have an accurate diagnosis.

[marko]

Why bother with a Lucas pump it will fail today, tomorrow or sometime....they were hardly adequate when they were new.....Bosch Pump and Diaphragm PRV for me all the way

[stuart]

Why bother with a Lucas pump it will fail today, tomorrow or sometime....they were hardly adequate when they were new.....Bosch Pump and Diaphragm PRV for me all the way

Nothing wrong with Lucas, I have replaced more duff Bosch pumps than Lucas ones.

Stuart.

[RobinTR6]

I was (attempting) to remain pretty standard/original. Ive had both Bosch and Lucas before and did have issues with both. If I cant sort my Lucas then........

[AndrewP]

Without wanting to muddy the waters and having read this and your previous adventures in all things Lucas and PI..

 

Am I right in thinking pressure and flow are generally inversely proportional? i.e as flow increases, pressure reduces and vice-versa?

 

I can understand the need to increase the supply to the pump to prevent starvation/cavitation, but what size are the supply lines to the MU? Has the PO replaced them and fallen into the 'bigger is better' trap?

 

If they are bigger than 'norm', then they would present less of a restriction to the flow, and therefore (in my head at least) reduce the pressure?

 

Is it worth confirming the fuel lines POST the pump are the correct ID?

[JohnG]

Andrew has a point.

 

Don't forget, the pump can deliver flow, it does not deliver pressure.

 

Pressure is the result of resistance to the flow, caused by all the components in the system, primary amongst these, the pipwork.

 

Friction created by the fuel passing against the I/D of the pipework will be a primary contributor to the pressure in the system

 

Increased pipe diameter will reduce resistance to flow, thus reducing the pressure generated

[RobinTR6]

They are all standard ID . My recent test , ie output from pump before the PRV shows 80 psi, ie same as output from PRV, indicates thats its the pump not generating the pressure but does have the flow. Mind you bot me and the expert cant figure out whats going on, given we have tried different (tested pumps) and 3 different PRV. Yes, I hear you say , that proves its something else.....well I think Ive checked and double checked everything so I !"£$%&^*(*)(. Key facts are it flows 2ltrs/min to the MU and its got 12volts at ignition and 12.9 at idle and I cannot get the pressure ablove 80/90 psi either at the pump or anywhere else.

 

Another muse is that if I constrict the output from the PRV to the tank the pressure rises........does this indicate the opposite logic, pump can deliver pressure (Ive not managed to get it past 100PSI with constriction but thats higher than I can get doing anything else)

Edited May 6, 2015 by RobinTR6

[TR NIALL]

2 Ltrs a Min don't seem a lot from a Pump more would Flow from the Tank with No Pump

[Peter Cobbold]

But the flow to the MU wasn't measured with 80psi back pressure, or any other backpressure. That back pressure can seriously affect the flow delivered by a pump.

However the pump expert will know that, and will have tested for the right flow at 105psi back pressure. So the validated pump(s) are losing performance badly when fitted to the TR. It's somewhere in the plumbing. AS you only get 80psi with the gauge T'd into the line from pump to prv then there's not a lot of plumbing from the pump to mess up. I suspect the feed from the tank is poor. A twisted hose inside the stainless, a blocked filter, a blocked union...whatever...

So..what is the rate of fuel drain form the tank measured by disconnecting the union at the pump inlet? It's probably miserable and needs the pump to pull fuel as well as pump, and it doesnt have to do that on the bench test.

Peter

.

Edited May 6, 2015 by Peter Cobbold

[RobinTR6]

Peter, the tank has been emptied numerous times, its free flowing and clean. Fitted a bigger outlet at tank and bigger inlet at filter, now 11 mm. New hose as well no kinks etc, again flow is good. I'll measure just in case. All the pipework from the flexible hose, is where it joins copper under rear shelf floor is bog standard stuff from Moss including the flex pipe at other end to MU. So even if the flex pipe has a bigger ID the I'm assuming that the standard copper pipework from back to front of car will not be why pressure is dropping.Its got to be something that I'm missing ......but what ? I'm hoping to get a Lucas pump known to work at 104 psi from another car to prove its the car or the pump...