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Triumph TR6 - Resolving PI challenges [dies with city driving]

Dear Team,

New here on this forum as well as with my first TR6PI I recently bought.

I would like the forums view and suggestions on how to tackle an issue with the engine, which I believe to be related to the injection system.

Problem: the engine starts reasonably well and if driven on the highway, runs smooth. Quite fast if needed actually. However, if I end up in traffic jam or enter a city (e.g. slow down), after a while the engine starts to perform worse and worse. As if someone pulled the choke. Eventually it stalls and it is difficult to start it again. A 10 minutes wait helps, but the same will happen again. When this is happening, the sound of the fuel pump varies (with seems to have a direct impact on the engine). It has a Bosch 0 580 254 996 fuel pump. It has a 'long' Pressure Relief Valve (PRV).

Steps taken:
- choke bowden cable moves the lever on the measuring unit maximally back in 'no choke' position
- tried a new Bosch 0 580 254 984 pump, no improvement. It seemed worse almost.
- got another (can't be new...) Bosch 0 580 254 996 pump, this one is currently mounted
- new fuel filter
- return line from PRV to tank is not blocked
- body of the pump measures up to 60 degrees C when the problem occurs (IR surface thermometer)
- installed a fuel pump relay and new wiring
- checked that there is no huge fuel flow via the return of the injection control unit
- fuel pump is mounted below the tank
- checked at idle speed as well as with just the pump running the amount of fuel flowing back from the PRV in the tank: next to nothing (return line disconnected from the tank to actually see any flow). When the pump is hot (and the idling problem occurs) and it starts to vary in RPM small amounts of return-fuel can be seen at the RPM peaks.

My idea (obviously not validated!):
Initially I thought that the pump pressure was too low (suggested in many fora), but since the 984 and the second 996 pump did not help, I assume the pressure is sufficient (still not 100% sure though). The poor performance at low speeds and during idling is perhaps caused by too high fuel pressure leading to the engine running way too rich. Caused (again, this is just an idea) by a non-functioning PRV leading to a too high fuel pressure and no flow of fuel at low speeds/idle via the PRV back in the tank. Which in turn leads to no cooling of the pump -> overheating -> cavitation -> engine dies. etc.

How much fuel should one roughly see flowing back from the PRV into the tank with just the pump running?
Is this an acceptable poor-mans way to first order check if the pump/PRV are working in harmony?

And since the above is just an idea and I'm just a starter, any other suggestions?

I have just ordered a fuel pressure measuring device which will certainly tell me more. In short I will also be able to test a brand new PRV.

Thanks in advance!
WJC Hoogslag

First a disclaimer- I have no real experience with fuel injection since for emission reasons, carburated cars mostly made it to north america. But as a chemical engineer I do have some experience with puimpling liquids.

I would suspect as one possible cause-

fuel vaporizing in the fuel line and causing vapour lock. After running at high speeds and slowing down the heat increases in the engine compartment and fuel flow slows down flashing/vaporizing the fuel in the line. Try to ensure the fuel lines are away from hot areas.

Michael Petryschuk

Had a similar problem back in '69 with my TR250, although carburated. would overheat in traffic and stall. Suggest you try an electric cooling fan.
Doug Baker

It could well be fuel vaporisation, but don't forget to check the injectors, a weak point on these injection units.
I don't think fuel pressure is the problem as the car runs well at high speeds where more fuel pressure is needed.
Kypros Christodoulides

All, thanks for your comments!

IMHO eventually, the resulting problem is vapor lock (I called it cavitation in the first post) happening in the pump itself due to too small flow of fuel to cool the pump, unless driving at higher speeds (= more fuel flows). If this happens, the pressure drops as the pump cannot act on vaporized fuel and the engine dies.

Michael and Doug suggest the vaporization happens in the fuel supply lines to the engine - this is indeed something to consider as well. Thanks.

Kypros suggest to look at the injectors,once I get this vaporlock under control I intend to visit a specialist to have the injection system checked (in fact, if I cannot resolve this part I might do it earlier...)

Yet, there should be a continuous flow "tank -> pump -> PRV -> return line -> tank" to cool the pump. Correct? In my case, I do not physically see this flow. Any Triumph PI owner who can confirm this? (TR6, TR520, ...)

I have made a few video clips of this, will try to upload later today (my evening).

WJC Hoogslag

Wytze-You will probably find more people that are familar with the PI system on the TR register. Us poor relatives in North America have carbs.
Good luck
BTP Price

One way to cool down the pump is to coil the incoming brass fuel line around the pump so it cools it.I've done this to a friends car and it worked. Putting it under the car exposed to fresh air is another way, but it will not work under traffic conditions where your problem is. I've seen this done, but it's dangerous.
Kypros Christodoulides

Hi Berry,

Newbee mistake, sorry. US owners typically cannot enjoy the challenges offered by the PI system :-)
Re-posted my question on the suggested forum.

Thanks again,
WJC Hoogslag

Hi Wytze

Sounds like fuel vaporization. Bosch pumps do not suck well like the old Lucas pumps so should be mounted as low as possible; most installations I have seen have been in the spare tyre well.

Have you checked your filter it may be reducing the flow to the pump causing it to run hot.

When this fault occurs measure the voltage across the pump.

Have you measured the output pressure yet?

C J Norcott

Hi Wytze

See the link below

C J Norcott

Hi Colin,
The pump is mounted in the spare tire well and the filter is new. Measuring the voltage across the pump prompted my to install a pump relay as well as a thicker direct wire directly to the pump.
I have not measured the pressure yet - will happen soon, I have ordered a pressure measuring set.
Thanks for the link - I had seen the interesting site from Tim Walker before. Good stuff!
My current guess is that the PRV is stuck, meaning no default return flow and hence no cooling of the in-line Bosch pump. I have x-posted a similar question with the TR Register ( as suggested by Berry and got a reply stating that a continuous return flow should be there. It is not at the moment. I have also ordered a new PRV, so with a bit of luck...
WJC Hoogslag

Hi Wytze

Sounds like you have diagnosed the problem.
Keep us posted, it is always good to here from people with fuel injected cars.

C J Norcott

I have just replaced the PRV with a new one (from Moss Europe). Now there is a constant flow from the PRV return line into the fuel tank! Unfortunately, the ordered 'universal' fuel pressure measuring kit (not Moss btw) which came with a huge collection of adapters, hoses, T-pieces, bit & pieces and whatever, except... those needed to fit to the fuel line thread on the Lucas Metering Unit... . Hence I have not been able to measure the fuel pressure 'before' and 'after'. I just need to get some other plumbing someday.

In any case: the net effect is that the engine does not stall anymore after one or two minutes of idling, the fuel pump makes a relatively constant noise, city driving is not a challenge anymore, it does not feel like I forgot to push back the choke and the temperature of the pump goes up to some 31 degrees C (the ambient temperature was about 10 lower than when I measured the 60 degrees, therefore I should probably say 'about 40 degrees'.

The engine did develop last week (still with the defective PRV) a problem in that it did not always run on all 6 cylinders. Using the 'thumb / finger' method, cyl. 4 was detected as the problem as no 'heartbeat' could be felt. Swapping injectors 3 and 4 moved the problem to cylinder 3, so I concluded that the injector from 4 was not working. This could also be seen by looking at the spray cone of the injector: no spray cone, just a bit of leaking. As per the 'TR6 Brown Book' I used my air compressor to blow some air through it. This actually improved it, though to spray cone was still not as nice as #3. In any case it's running on all 6 again, but I think I'm in for one new injector.

In summary, yes! There is a poor man's PI Fuel Pump Check: there shall be a constant flow of fuel from the PRV to the tank when the pump is running.

Thanks for the advice!

WJC Hoogslag

Hi Wytze

Thanks for the update. Its good to hear that your car is running normally again (notice I did not say properly).
As for the injector does it still dribble a bit now that you have cleaned it out? If so you may have a faulty internal seal, that is the small O ring inside the injector. You may be able to pull the injector apart (depending on the type of injector) and replace the O ring, I have done this to several of the injectors on my car.

C J Norcott

Hi Colin thanks,
replacing the O-ring depending on the type of injector: I have the newer 'push-in' types. Are these servicable?
As with anything on my TR6: it is a first time experience. What is the proper procedure to check on dribling? Now that you mention edit: I noted that when opening the supply line to a 'good' injector there (engine stopped, pump off) there was some (residual) fuel pressure in the line to the injector, there was no (residual) perssure in the line to the defective injector, hence it is not closing properly.
WJC Hoogslag

First check, then ask - sorry.
Servicable yes/no: see picture.
Circlip = yes

WJC Hoogslag

Hi Wytze

The injectors on my car have a small circlip at the end of the injector, this can be remove which allows you to push the internal components back out of the injector, i.e. towards the fuel line inlet. If the injector will not hold pressure it is most likely the rubber O ring failing, allowing fuel to bypass the injector nozzle. Be careful not damage the spindle at the end of the injector and also note that sometimes there is a small filter in the inlet side of the injector that is difficult not to damage. I know, but does not seem to affect the operation of the injector if not replaced.

C J Norcott

This thread was discussed between 28/05/2012 and 03/06/2012

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