Cuts out when hot

[JohnGT6]

Hello. Wondered if anyone could help with a problem I have with my MKIII GT6 please?  Essentially, it runs fine when started from cold but then cuts out when it gets warm. If I leave it for 30mins it will restart fine but then cut out again after a while. I initially changed the fuel pump, the coil, and reverted back to points from an electric ignition but the problem still remained. I then noticed that the fuel filter generally empties before the car cuts out. If I detach the fuel pipe between the fuel filter and carbs immediately after the car cuts out, there appears to be some back pressure in the fuel pipe. If I turn over the car with the fuel pipe detached after the fuel filter, the fuel is delivered fine. I suspected a problem with the float valve in the carbs and replaced both on the Strombergs but that didn’t help (one was sticking so I thought I’d nailed it but alas this was not the case). I now suspect a fuel lock it generated - possibly due to some potential heat escaping from the exhaust manifold gasket but I’m clutching at straws now. Before I start taking the exhaust manifold off, does anyone have any ideas or suggestions for me please? Thanks. 

[glang]

Its not a case of air being unable to enter the fuel tank is it? Try opening the fuel cap when the engine has cut out to see if a vacuum has been pulled in there....

[RobPearce]

Is your filter after the pump? Normal recommendation is to fit it before the pump. Anyway...

Some pressure is to be expected in the fuel line between pump and carbs, since the pump produces that pressure using the (passive) spring, while the (active) cam operates the draw from the tank. The carb float valves will normally be closed because the chambers are normally full.

If the pipe between pump and carbs is "empty" then it's got full of some gas. That could be air or it could be fuel vapour. The former is normally down to failed pump or a leak on the pre-pump plumbing (or an empty tank!). The latter is heat soak, probably combined with sticky float valves or an excessively large cavity in the pipe (like a filter). Either way, if it's been empty for any time while running, the fuel level in the carbs should have dropped and the valves opened to relieve the pressure.

A test that you can do: when it cuts out, pour a cup of cold water over the filter and pipe work. If it fills up then there's too much heat in there. Pour some water over the pump. If the car then starts, you've got too much heat in the pump. I had that on my Vitesse when it had a Herald pump fitted - putting a proper AC glass-dome pump on fixed the problem.

[PeteStupps]

Echoing Rob's point, it sounds a lot like fuel vaporisation to me. Try removing the filter entirely if you've got enough petrol pipe handy.

If the filter is between the pump and carbs, it's usually close to heat sources and high up. Even when the engine is running, fuel moves much more slowly through the big filter than it does through 1/4in pipe. Hence it'll absorb heat and evaporate quite readily, and the vapour displaces all the liquid fuel in no time.

Had the same issue on my 1300 FWD, took me ages to realise what was going on. Ethanol exacerbates the problem!

In the end I still had problems with the filter moved upstream so had to blank off the mech fuel pump and go electric. But think that's mostly due to poor airflow through the FWD engine bay. 

Pete

[JohnGT6]

Thanks for the very helpful suggestions folks. I tried opening the fuel cap but that unfortunately didn’t help. I like the idea of pouring cold water on the filter and the pipe work and I will give that a try. The filter is currently situated on the drivers side towards the front of the engine near the water pump.  It has been there for as long as I have had the car (20 years+) and never caused me any problems. These days the car spends most of the time gathering dust in the garage and this problem has only recently occurred. I’m left thinking that perhaps a gasket has degraded as there seems to be quite a bit of heat around the exhaust manifold (particularly behind the left hand carb) - although I don’t notice any noise that would suggest a leaking gasket.  I’ll try the cold water approach and perhaps an electric fan to keep things cool and see if it will run for longer when ticking over on the drive. I’ll keep the forum posted in case anyone else has this problem.  Thanks

[glang]

Can I ask for how long and why youre keeping the car ticking over on the drive? It is quite an onerous activity as obviously when moving the heat generated is dissipated better and also fuel flow is faster both of which should help a lot.

Does it do it with the bonnet raised? If so there must be quite a large direct heat transfer somewhere.... In the end a reasonably powerful electric fan like I have fitted will definitely keep more air moving through the engine bay when stationary but of course must cut in before the problem occurs😆  

[JohnGT6]

Apologies - I should have replied earlier. In answer to your question, I found that the fault is replicated when on the drive rather than taking the car out for a drive and it breaking down at random points - less stressful because when it starts to splutter it doesn't take long to cut out completely! I also found that the car seems to run a little longer with the bonnet up and the air filter off (again possibly pointing to heat being concentrated between the air filter and the engine). 

Holidays etc have got in the way but I intend to do the cold water test this weekend. Thanks.

[Guppy916]

John any news yet, Glang did you fit a Kenlow type fan ? how about a photo please