Thermal Acceleration

[Blackjack]

While returning from the NW & WM monthly run this week we were in a very bad cloud burst and a lot of very heavy rain.
This weather stopped many cars, some in very deep water and 2 had hit lifted man hole covers and this had stopped them.
This made the Blackjack look more like a boat than a car, due to the flooding on the roads. The 2cv engine was magnificent running very cool with all that water around it, and the long hold ups that we found ourselves in. The problem arose when on the M5 wet road but the rain had stopped, the traffic moving very slowly then stopping only to move off almost immediately . Oil temp off the gauge, ( sender unit now duff due to water damage) cooling fans on, the engine revs slowly increased to I guess about 2000+ RPM this took about 1/2 an hour. This is not what you want creeping up RPM.
We managed to get to the next junction and pulled off the slip road into an ASDA supermarket car park. We let the engine cool down for 30 mins and when started up it ticked over normally. We were like drowns rats and looking sorry for ourselves when a guy came over to speak to us. He taught apprentices car electronics and said straight away that our engine had been subject to THERMAL ACCELERATION I have never heard of this, has any one else, if they have how high can the revs go? to blow up? The engine is fine, the brakes less so and are being checked over now. Also the oil pressure swith failed due to water ingress. What a day but what a car. Philip.

[Ken Hanna]

Is the throttle on the car controlled by electronics or by a cable?
If it's the latter, isn't it much more likely to be a 'sticky' cable, affected by an overheated engine bay?

You haven't mentioned which type of carburettor is fitted (or if it's a pair), but if it's a Solex, simply relocating the anchorage point of the return spring on the carburettor will cure most instances of a sticky throttle.

Throttle spring, alternative arrangement. by slcchassis, on Flickr

[howardjs]

I too have had a similar problem of engine not returning to idle speed when it reached max running temperature (motorway run).
As Ken has suggested, the cause turned out to be an accelerator cable overheating which was running close to the exhaust pipe (twin Dellortos so not possible to fit stronger return springs).
The cables have now been replaced and rerouted... so far the problem has not returned.
Howard

[Blackjack]

Hi Guys,
Yes, this does seem to be a good reason for "thermal acceleration" and I had not thought of it.
My carb is an SU one, I did check the dash pot oil, when I got home, it was still there. While in the traffic with raised revs I blipped the throttle (many times), the revs rose immediately and dropped back the there elevated level immediately, so I did not suspect a sticking cable. While the throttle cable appears to be very good, I do not know how long its been on the car so it will be replaced.
Just hope never to be in such floods, rain and traffic again .
Many Thanks Philip.