Bosch fuel pump problem

[Trevors]

My 1971 fuel injected TR6 has started cutting out when everything gets hot. My reading suggests this is a known fault caused by the fuel pump overheating and causing the fuel to evaporate. I believe this fuel pump dates to 1985.

When I leave it to cool the car runs normally.

I live in the North West of England .

I believe this requires a specialist to sort out.

Please will members point me towards someone who can do this.

Many thanks

Mark 

[stuart]

Not necessarily, could be ignition related, does the pump make funny noises?

Stuart.

[Mike C]

"started cutting out when everything gets hot"- also often  caused by a blocked fuel tank vent.

[RobH]

It sounds as though this is a recent thing, which surely makes it unlikely to be due to the pump suddenly starting to overheat as it would have been doing that since day 1.  

Are the fuel filters clean? 

Have you done any diagnostic tests?  e.g. when the engine stops is there still a spark at the plugs? 

[Jonny TR6]

Plenty of specialists in the North West, TRBitz in Warrington and Stuart at Montadore Motors in Birkenhead.

[Ernest]

I was having a similar problem - and thanks to a range of advice  made available on this site, I fitted a new red rotor arm - And this seem to have cured the cutting out - as Stuart noted '...possible ignition-related.'

So a cheap (£14.99 'Distributor Doctor') and very easy to fit.

Ernest

[Jules TR6]

…for me this ended up being a relatively new resin filled coil (resin-filled seems to be chosen for it's vibration resistance, oil-filled for cooling effect).

I’ve even got stainless steel braided fuel lines over the engine - and they cause no issues.

I put an old oil filled coil in, and bingo, no problem once hot.

/

The fuel pump issue you may have been reading about could be related to the old Lucas pump, the newer Bosch pumps should be ok, and I wouldn't point my finger at it immediately - especially if you can hear it pumping after the engine cuts out. Just bung another coil in (easy DIY job), oil filled, and see what happens.

Edited October 11, 2023 by Jules TR6

[Andy Moltu]

Back in the day, when the PI cars were current, an awful lot of problems were caused or compounded by the assumption that any and every rough running of a 6 was the result of a PI problem. 

The result was a lot of Lucas PI systems meddled with unnecessarily when the fault was elsewhere, just making it so much harder to identify the real cause.

Unless the cutting out coincides with a sudden change of the tone of the fuel pump, the sensible starting point is with the ignition system. 

[Trevors]

Many thanks to you all for all that input.

I'll take this forward and see if I can cure it once and for all.

[john.r.davies]

No one has mentioned the fuel line coil around the pump, that has been a popular way to keep the pump cool.   Rimmer's even sell one!   https://www.google.com/search?client=ms-android-vf-gb-revc&sca_esv=573265106&sxsrf=AM9HkKm5PdsV8G_0O0TEMWDor4CeOqSe4g:1697226316418&q=lucas+pi+fuel+pump+coil&tbm=isch&source=lnms&sa=X&ved=2ahUKEwiZ08Xd5POBAxX-hf0HHXUFD7sQ0pQJegQICRAB&biw=360&bih=628&dpr=2#imgrc=l6GIveIadFaIUM

[Mike C]

1 hour ago, john.r.davies said:

No one has mentioned the fuel line coil around the pump, that has been a popular way to keep the pump cool.   Rimmer's even sell one!   https://www.google.com/search?client=ms-android-vf-gb-revc&sca_esv=573265106&sxsrf=AM9HkKm5PdsV8G_0O0TEMWDor4CeOqSe4g:1697226316418&q=lucas+pi+fuel+pump+coil&tbm=isch&source=lnms&sa=X&ved=2ahUKEwiZ08Xd5POBAxX-hf0HHXUFD7sQ0pQJegQICRAB&biw=360&bih=628&dpr=2#imgrc=l6GIveIadFaIUM

As far as I can see, the coils just put the pump casing  heat back into the fuel, exacerbating any fuel vapourization problem.

[john.r.davies]

Yes, Mike C, but it depends on how you plumb it.    If the coil is in the return line then heat gained is diluted in the mass of fuel in the tank.

John

[Mike C]

48 minutes ago, john.r.davies said:

Yes, Mike C, but it depends on how you plumb it.    If the coil is in the return line then heat gained is diluted in the mass of fuel in the tank.

John

Doesn't matter, the pump heat still ends up in the petrol- that's why car manufacturer's use fuel/air heat exchangers to solve this problem.

[roy53]

1 hour ago, john.r.davies said:

Yes, Mike C, but it depends on how you plumb it.    If the coil is in the return line then heat gained is diluted in the mass of fuel in the tank.

John

Yes every little helps

[Waldi]

I never believed the effectiveness of the coil around the pump-motor: 

Is it to cool the petrol in the coil with the motor? No.

To cool the motor with the coil? No

To cool the petrol in the coil by the surrounding air? Yes, maybe, but just a tiny bit. It is a poor location if that was the intend.

Modern fuel coolers, as discussed on here are far more efficient in cooling fuel.

Waldi

[john.r.davies]

My TRansit has one as standard.

 

[Mike C]

Same with my Isuzu Dmax 4WD.

[michaelfinnis]

8 hours ago, Waldi said:

I never believed the effectiveness of the coil around the pump-motor: 

Is it to cool the petrol in the coil with the motor? No.

To cool the motor with the coil? No

To cool the petrol in the coil by the surrounding air? Yes, maybe, but just a tiny bit. It is a poor location if that was the intend.

Modern fuel coolers, as discussed on here are far more efficient in cooling fuel.

Waldi

It’s not fuel cooler, it’s intended to cool the motor, using petrol as the cooling medium. The temperature of the petrol through the coil is raised, but is then returned to the tank where the larger volume of petrol acts as a heat sink.

[Mike C]

33 minutes ago, michaelfinnis said:

It’s not fuel cooler, it’s intended to cool the motor, using petrol as the cooling medium. The temperature of the petrol through the coil is raised, but is then returned to the tank where the larger volume of petrol acts as a heat sink.

With fuel vapourization  the problem is not caused by a hot pump motor, it's caused by fuel boiling when its temperature dependent vapour pressure exceeds the operating pressure in the fuel piping.

The piping with the lowest operating pressure is the  pump suction and the fuel vapourises in this first, and the Bosch pump stops pumping as it can't pump vapour. Feed the pump cold petrol and fuel vapourisation won't occur.

[Waldi]

Hi Michael,

That’s why I cannot imagine it was designed to cool the motor:
In that case it was intended to fix a motor temperature problem that exacerbated the fuel vaporisation, leading to cavitation. 

Waldi

[roy53]

5 hours ago, michaelfinnis said:

It’s not fuel cooler, it’s intended to cool the motor, using petrol as the cooling medium. The temperature of the petrol through the coil is raised, but is then returned to the tank where the larger volume of petrol acts as a heat sink.

+1

[Andy Moltu]

4 hours ago, Waldi said:

Hi Michael,

That’s why I cannot imagine it was designed to cool the motor:
In that case it was intended to fix a motor temperature problem that exacerbated the fuel vaporisation, leading to cavitation. 

Waldi

I'm not sure much thought went into the design of the cooling coil.

It wraps around the motor case above the pump so only indirectly cools the pump if at all. Worse still, it heats the fuel so that it compounds the problem. So, when the tank is low and the filling pressure of the pump is low, the fuel is at it's warmest and thus more likely vaporise in the pump.

If the fuel from the coil subsequently passed through a cooler before being returned to the tank it might have been better but the siting of the  pump and PRV makes that impractical.

A number of modern vehicles do use coolers on their return flow from the fuel rail. Of course that fuel has been heated in the engine bay heat and not just by the pump. Is that to reduce the risk of vaporising or simply to restrict the expansion of the fuel affecting the mixture and emissions?

 

[john.r.davies]

I thought to investigate and educate myself a little more about this.   It's complex, as petrol is a mixture of compounds, all with different vapour pressures, but Lucas Pi operates at 105psi, when the "Reid Vapor Pressure" (measured at 38C) of the mixture is between 1 and 20psi.

The temperature at which a liquid will boil is defined as the temperature at which it's vapour pressure exceeds the ambient pressure, usually atmospheric.    That Boiling Point of the mixture that is petrol is 65 to 90C. 

 

This graph indicates that too, but a Lucas Pi system, after the pump, is regulated to 105psi, when atmospheric is about 15psi.   So the pump and post pump piping will need to be hotter than before the fuel boils, and produces bubbles of vapour in the system.  60C is more than 'hand hot' and 90 is scalding hot!   

Is "fuel vapourization" a false fear?   Next to engine, after a run and sitting in the sun, fuel lines in the engine bay might get that hot, but while running?

I'll be grateful for comments from others who know more than me!

John

[Jules TR6]

The stainless-steel braided fuel lines that are often fitted between the MU and the injectors (as a stronger 'upgrade' to the original nylon lines) have been deemed a poor choice - due to conducting the heat of the engine, over which they pass.

I pointed my finger at them while have worsening misfires, then I made sure they weren't actually touching the top of the engine, and that they weren't all 'clamped' tight to each other. I'm not sure this helped much, but worth doing. My misfire finally was identified as the new coil - not firing when hot. Since that was was replaced, I've had no issues with misfire or 'hot fuel lines'.

I did buy some Design Engineering foil wrap, that I intended to place gently around the fuel lines directly above the engine, but wasn't required.

I think coils getting too hot are a bigger issue than fuel lines. Some though put a lower temp thermostat in to run the engine slightly cooler, I've not needed that either, but is an option.

[john.r.davies]

Jules,

Blaming braided injector lines is almost certainly misplaced!    Because;

1/ they are silvery, the worst finish to pick up heat.   Standard lines by contrast are Matt black, the best finish for the same purpose!

2/ they have a small diameter, and so surface area.   Heat transfer is directly connected to heat pick up, or loss.    If you have central heating at home with 10mm pipes, or microbores, you will notice they are fitted without insulation - adding that increases the surface area and so heat loss!

John