Page 1 of 1
What temperature does petrol vapourise in fuel rails?
Posted: Mon Jul 29, 2024 5:09 pm
by scudderfish
I was doing a 100 mile journey yesterday and my car cut out twice. I let it sit for 5 minutes and it would restart and I continued the journey. After the first time, I plugged in my laptop and caught the second conk out in the attached log at about 3300 seconds onwards.
It looks like pulse width stays normal, but the AFR pegs to the leanest the sensor can do as if no fuel is actually getting injected. Letting it cool for a few minutes (to me) suggests I was getting vapourisation in the fuel rails (or somewhere). The engine coolant temperature looks normal, but it was hot in the bay. I separately recorded that the air coming off the radiator was about 80C and the temperature near the firewall was up to 60C. I have a swirl pot in the engine bay and that was hot to touch (40-50C maybe)
Is my theory plausible?
Re: What temperature does petrol vapourise in fuel rails?
Posted: Mon Jul 29, 2024 5:31 pm
by Ian Anderson
Possible but I would look further.
What is feeding the swirl at low pressure and the return to tank. The swirl with hot fuel will see Vapor’s and possibly add enough pressure to stop a low pressure pump which in turn will cause the HP pump to suck air.
This happened a couple of times in my Gt40 made worse when first on the road and fuel filters that got clogged helping slow the LP fuel circuit. Clean filters, easier bends on the fuel hose lines and it never happened for years…..then reared it’s head again at LeMans 2016 where the day temp was up around 45 C
A stop for 5 with engine bay open and off I went again.
BAD THING is if this is the problem then the HP pump has had air when running and that will fail in short order or get very noisy.
Ian
Re: What temperature does petrol vapourise in fuel rails?
Posted: Mon Jul 29, 2024 6:10 pm
by stevieturbo
scudderfish wrote: ↑Mon Jul 29, 2024 5:09 pm
I was doing a 100 mile journey yesterday and my car cut out twice. I let it sit for 5 minutes and it would restart and I continued the journey. After the first time, I plugged in my laptop and caught the second conk out in the attached log at about 3300 seconds onwards.
It looks like pulse width stays normal, but the AFR pegs to the leanest the sensor can do as if no fuel is actually getting injected. Letting it cool for a few minutes (to me) suggests I was getting vapourisation in the fuel rails (or somewhere). The engine coolant temperature looks normal, but it was hot in the bay. I separately recorded that the air coming off the radiator was about 80C and the temperature near the firewall was up to 60C. I have a swirl pot in the engine bay and that was hot to touch (40-50C maybe)
DSCN3122.JPG
Is my theory plausible?
I've driven many hot places, been stuck in traffic for ages and never experienced such a thing. I'd say for most part it is a myth, or a reality so far away....you'd need to be in Saudi or something
And letting it sit and heat soak for a time, would only make such a thing worse unless it was long enough for a full cool down.
First and foremost, log fuel pressure. Without that info, you don't even know if there is fuel getting to the rails.
Re: What temperature does petrol vapourise in fuel rails?
Posted: Mon Jul 29, 2024 6:15 pm
by scudderfish
Thanks, that makes sense as it occurred at a time of a reasonable spell at 70 so if LP wasn't feeding, the swirl pot could have drained and then refilled when I stopped. I have a restriction in the line back to the main tank as I was led to believe the LP pump needs something to work against. It's (I think) a small diameter welding tip end.
Re: What temperature does petrol vapourise in fuel rails?
Posted: Mon Jul 29, 2024 6:16 pm
by scudderfish
stevieturbo wrote: ↑Mon Jul 29, 2024 6:10 pm
First and foremost, log fuel pressure. Without that info, you don't even know if there is fuel getting to the rails.
Woohoo! An excuse to add more widgetry
Re: What temperature does petrol vapourise in fuel rails?
Posted: Mon Jul 29, 2024 8:27 pm
by stevieturbo
scudderfish wrote: ↑Mon Jul 29, 2024 6:16 pm
stevieturbo wrote: ↑Mon Jul 29, 2024 6:10 pm
First and foremost, log fuel pressure. Without that info, you don't even know if there is fuel getting to the rails.
Woohoo! An excuse to add more widgetry
Well you could add fuel temp too.
But I've seen fuel temps on mine in the 60-70egC range, and it's never posed a problem That said, that is with heat soak and vehicle stopped, I don't think I've ever seen over 50deg with a running engine and even that is stop start traffic or queuing for racing and also with lowish fuel in the tank,
I only log it because I can. Most people who make claims about hot fuel doing this or that....have never logged fuel temps to even know if, or how hot their fuel is.
Sensors are cheap, if you have spare inputs, then go for it. A sensor in your swirl tank should be fine. Fuel pressure should be local to the rails, or at the FPR, etc etc
Re: What temperature does petrol vapourise in fuel rails?
Posted: Tue Jul 30, 2024 6:24 am
by scudderfish
I don't have any spare inputs on the ECU, but I've got an ESP32 doing logging duties and I can merge that with ECU logs with a little extra work. I haven't managed to find a fuel pressure sensor I can easily integrate (yet), but I'm going to add some temperature sensors to the fuel pump, the swirl pot and the fuel rail to get an idea of what they are doing. The HP pump could well be suspect. I replaced it a few years ago because the previous one was making noises it shouldn't. If this one is on the way out, I may have a more fundamental problem that is using fuel pumps like fuses.
Re: What temperature does petrol vapourise in fuel rails?
Posted: Tue Jul 30, 2024 4:32 pm
by stevieturbo
Pumps can make noises if they do not have a good clean supply of fuel available.
Sensors are readily available, eg
https://uk.farnell.com/honeywell/mipan2 ... dp/3364907
Unless you go China, which can be cheaper.....but if fuel leaks it can be bad.
Typical EFI pumps are very very reliable unless something with the install is causing them problems. Whether that's lack of supply, lack of clean fuel, wiring issue, etc etc.
Or if again....China pump