1969 280SEL runs erratically when hot
 

Hi guys. It's been awhile and I'm stuck with the same problem but I think I've narrowed things down a bit. THe car runs like a champ when cold but when it warms up it won't idle and it wants to stall and usually does. Funny thing is, it runs OK when the water temperature gets hot, let's say for twenty minutes then it starts to act up when the underhood temp gets hot. It seems to me that if it was something involving the mixture it would act up as soon as the water temp reached it's operating temperature. But it doesn't. It waits a little longer when the underhood temp goes up. Could it be that supplementary air valve? Or something else I haven't hit on just yet? Something temperature related is going on here. Otherwise the work goes well. Many thanks in advance.
Thom

Sounds to me like it's getting so hot under the bonnet that the fuel is boiling off which is causing your problems.

Matt,
I just thought of something while looking through the shop manual. A while back I replaced the fuel pump with a generic model. The new one didn't have the second hose which syphons off the air bubbles in the fuel line. Could that be the problem here? Air bubbles in my fuel which are culminating in some sort of vapor lock in the engine compartment? Maybe some asbestos around the fuel pipes would be a good diagnostic test? Hmmm.....
Thom

Does it do it if you leave the hood up?
Vapor lock was common because the fuel filter canister is mounted to the motor mount arm. That transmits engine heat to the filter . The remedy is to make an insulator type filter mount.
And a fuel return line....

Arthur,
Good to hear from you again. You raise a really good point and I'm going to devise a solution ASAP. There are some other things I can do as well. Insulate the fuel pipes with asbestos tape, perhaps find a large piece of asbestos and place it between the exhaust headers and the air intake plenum. Also, I failed to mention that my car does not have its insulating blanket on the inside of the hood. Since the car is a dark color it may be radiating heat when the sun is beating down on it. To work.........
Thom

Arthur,
Do you know of anyone who has modified the fuel filter mount and if they have a procedure that will work? Thanks very much.
Thom

Tom,
The vapor lock problem with these is usually after turning off and trying to re-start after a short time. That is why there was a start valve modification.
But , in your case, [ while running on the highway], I suspect
a fuel problem because you changed the fuel pump and went
with a generic one.
The system requires the correct feed pressure or the pump will starve at higher loads and the return line to the tank will not funtion. This return loop is what keeps the fuel line both full and cool. If the pump you used does not have the pressure to get past the return line dampers [ there are 2], then you can experience vapor lock problems.
As a test on the fuel filter container needing an insulator, I suggest you first remove the two bolts that mount it to the motor mount , pull it clear and take it for a test. If that helps , an insulator can be fabricated by remounting with longer bolts and an insulator block in between the mount and canister. [ non-metal,of course]. I have also seen guys wrap the canister in f-glass insulation. A good one can be had at the dump from the side panels of a discarded oven or dishwasher. They are a glass construction, but are compacted to hold shape and usually have a foil back. Put the foil out to reflect outside heat.
But fuel pressure is more than likely the problem. Ck the pressures and compare to specs.
Arthur