Misfire

[DenisMc]

I was on a drive out yesterday in the baking heat in my TR5, when whilst going through a town in some slow moving traffic the car developed an awful misfire. There was no where to pull over so I just kept going hoping that it wouldn’t breakdown. On returning to the open road the misfire cleared and the cat ran perfectly for the rest of the trip. The engine is to standard specification but it does have a Bosch fuel pump and filter assembly in the boot. Has anyone got any suggestions as to what might have caused the problem? 

Edited July 17, 2022 by DenisMc
Typo

[Waldi]

I had exactly the same problem earlier this year; in my case it was a stuck PRV. The result is almost no flow is returned to tank, so the pump sees very little cold fuel when driving at low speeds. When I drove faster, I had no issues.

You could check the pump temperature (by hand), mine was really warm, like 50 degr.C, and then cavitation is occurring in the pump.

Did the sound of the fuel pump change when you had the issue? That would be a good indication.

Waldi

[DenisMc]

Hi Waldi. It is good to hear from you and thanks for your helpful suggestion. There was too much background noise to be able to tell if the tone of the pump had changed. I am pretty sure thought that the issue was caused by poor fuel delivery. With the hot weather continuing I’ll take the car out again and do some checks. 
Denis

[Waldi]

Hi Dennis,

in my case, the result of very hot fuel in my pump due to little/no flow (while the tank was still at an acceptable temperature) was cavitation, which results in pressure loss hence I had the “no fuel” feel.

Neil (NTC) and Stuart immediately recognized the issue when I asked on here.

Waldi

[Nigel Triumph]

I had a similar issue with my CP series 6 in warm weather a few days ago, also near standard worth a Bosch pump. The car had been parked in full sun for a few hours then started and driven very slowly in traffic for 5 to 10 minutes before losing power and misfiring. Once I was able to get moving at decent speeds the problem cleared quickly. After that,  it ran faultlessly for the rest of my 60 mile trip home, including high speeds on the motorway. 

It's been fine since. I put the problem down to fuel vaporisation due to the hot weather, which cleared when the airflow at speed cooled the engine bay. I'm using 99 octane E5 unleaded. 

Nigel

[Waldi]

Some additional info:
I opened the PRV and found the valve was stuck in the guide/seat. I lapped is with some alu-polish and if moved fine than. I was surprised by the very narrow clearance between the valve and the guide.

But when I measured the fuel pressure, it did not stay stable at all speeds/loads.
Some change can be expected from a simple device like this, but in my case, the pressure rose to 8-8.5 bar (116-123 psi)at idle, and it dropped to 7.2-7.4 bar (104-107 psi) under load at 3500 rpm. It was ok during cruising at 7.8 bar (113 psi).

So I still have some more work to do, and open the PRV again.

Question:
Does anyone know the size of the dowty washers for the PRV? I want to source them overhere to avoid expensive shipping and customs cost.

Thanks,

Waldi

[DenisMc]

Many thanks Waldi and Nigel for your useful responses. I have checked the car over and ran it up and it seems to be fine. I am driving to a BBQ on Sunday that has been put on by the Y Ddraig Goch TR Register Group, a 100 mile round trip, so shall see how the cars runs.

Regards,

Denis 

[Waldi]

That’s great Dennis.

Did you find a fault or was it just the high temperature?

Cheers 
Waldi

[Nigel Triumph]

Good luck Dennis. Weather forecast looks warm but more reasonable at the weekend. 

Nigel