Won't idle after 10 minutes at motorway speeds

[Inchworm]

I have a 1986 T25 with a 1.9 DG engine.
Several trips ago it stated stalling after exiting the motorway. It would start right up again but I couldn't let it idle or it would run really rough and stop. The temperature and everything else seemed normal.
If I let it sit for awhile it would be fine again until travelling hard at motorway speeds for 10 minutes or so.

So far I've done the following:
1.Replaced the original single carb and manifold with a new twin carb setup from Eurocarb.
Result - no change / same problem

2.Replaced the original distributor with a new Powerspark digital distributor eliminating the VW ignition module and idle stabiliser module.
Result - no change / same problem

3. Replaced all plug leads with new leads
Result - no change / same problem

4 Replaced the original coil with a new replacement coil.
Result - no change / same problem.

I also modified the thermostat at one point to allow the engine to run cold all the time but still the engine would not idle after pushing it at motorway speeds.

I'm at my wits end! It must not be a carburettor problem or an ignition problem. Could it be a valve sticking open after a hard run or could it be a crack somewhere that lets in air after running at elevated revs (3000rpm / 60mph)?

Any suggestions appreciated.

[Dad]

Just thinking, and I've heard it before, the fuel pump is cooled by lots of fuel at 70, then you slow and it gets hot and overheats. Certainly on some cars.

[Inchworm]

Thanks for your input.
The fuel pump is about the only thing I haven't changed so I'll take that on board. I'll let you know how it works out.

[kevtherev]

An electric pump would be ideal.
Facet pump 1-4psi

[Oldiebut goodie]

Just thinking, and I've heard it before, the fuel pump is cooled by lots of fuel at 70, then you slow and it gets hot and overheats. Certainly on some cars.

That used to happen on my Mk2 Cortina - the fuel pump was ridiculously close to the exhaust manifold. Took me ages to work that one out. Had to fashion a shield round it.

[Da dan]

Just had the same. Fuel pump. Electric ones are noisy though, but you know when they working!! Just put one on. Just about to post my problem with it though. Good luck


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[300CE]

May also be down to a failing vacuum advance unit on the distributor.

[Inchworm]

Well I've done everything I can think of:
New carbs
New distributor with new vacuum advance
New plugs and plug leads
New ignition coil
New facet electric fuel pump and new fuel lines and filter

Still...it runs fine until it warms up then it won't idle, runs rough at low rpm, and seems to have reduced power.

Someone suggested isolating the carb manifolds from the heads using a phenolic spacer. They said that it could be that the carbs absorb heat and cause problems with fuel vaporisation.
Any thoughts appreciated.

[kevtherev]

The pierburg was fitted with a 10mm thick bakerlite spacer surrounded with two paper gaskets
Has yours got this?

The inlets also have a thick bakerlite spacer each.