very high engine idle when hot

[mac2]

My 89 se has a curious issue. On a very hot day especially in traffic the engine idle goes up to 2000-2500rpm. I first noticed it 3 yrs ago when in a market town on a very hot day. The engine temperature was at 90 degs whish is normal top temp. When I got out of the town and had a better airflow 50mph the problem ceased. It did it once again last year during the MOT, again a hot day with engine running whilst stationary. The tester turned it off, and when started again some minutes later no problem. Yesterday, again very hot same thing happened in same market town whilst 'crawling in traffic' but took very much longer to come back to normal after driving on open road.
The car does not have A/C but the service notes refer to a number of components in the engine management section which govern and increase idle speed, eg engine rpm relay and other temperature related components. I can assure you it is not a sticking throttle cable as throttle linkage is on the idle screw. Also there is no sign of overheating by engine temp gauge.

Anyone got any ideas?

[MetBlue]

Does your car have the throttle jack. It's a solenoid plunger on the carb bank at the front.
I have one, but it's disconnected. And causes me no issues at all.

[bash]

Sounds like it might be going weak by letting air in somewhere, check all the vacuum hoses under the carbs one might be letting air in when the rubber gets warm / soft.

Bash

[Lotus-e-Clan]

Also check with a strobe that your ignition hasn't moved to full vacuum advance during the issue. Full vac advance for an SE is an additional 20 odd degrees and if operational at 0% throttle opening (idle) the revs will float upwards.
Culprit could be a green thermal vacuum valve port malfunction. The green valve should direct full manifold vacuum to the dizzy when cold via the red black hose, and weak throttle edge port vacuum from the DHLA when engine is warm via the white black hose.
Or maybe the vacuum capsule on the dizzy itself is sticking ajar during the issue?

[mac2]

Many thanks all for your replies.
I shall check these issues. Yes there is a throttle jack fitted.
I shall report back but as the fault is intermittent can't say when.

[MetBlue]

. Yes there is a throttle jack fitted.

Next time it does it, just pull the negative wire off the throttle jack. Very quick and easy check.
If revs drop, I'd just leave unplugged for now. My car has had this wire disconnected since I bought the car. If I connect it, revs rise significantly, but car runs fine without it connected (there's diodes in the feed circuit that I suspect have failed, but I've never got round to checking).
Tony