Fast Idle

When I first had the car, a whole three weeks ago now, (posted 10/10/00) the fast idle problem was there and from scanning all the FAQ's and Tech notes from MR2 sites around the world, it seems to be a very common problem.

One of the common fixes is to plug the air intake hole visible just in front of the main throttle butterfly (with the hose between the air filter and the TVIS/throttle assembly removed). For the last couple of weeks, I have used a cut down tyre valve as suggested on one of the MR2 sites, and although this worked it's a bodge and not a true fix and when starting the car, it has been necessary to give the accelerator pedal a simultaneous blip that usually results in initial unnecessarily high revs.

The basic problem is caused by the auto choke mechanism not working. The main element of this choke is the engine water temperature sensor that allows air flow to bypass the throttle butterfly when the engine is cold thus increasing the idle speed. As the engine warms up, this air flow is reduced and the engine speed drops back.

A pair of water pipes immediately under the throttle end of the TVIS are teed into the main engine cooling system. If the temperature of this pair of pipes is felt after the engine has been running for say ten minutes, both should be at the same temperature and should be almost too hot to hold. In my case I found that one pipe was much hotter than the other and even the hot one wasn't that hot! It was clear that water wasn't circulating through this part of the system. It could have been an air lock but repeated disconnection and reassembly did not fix the problem. There had to be blockage. The following procedure was adopted to sort out the problem.

Make up a pair of temporary pipe seals to block the water system connection tees when the connecting hoses are removed. You could of course drain down the whole system, but that is a real pain and to be avoided unless you have a good type of rusty brew installed. The easiest way to make these temp sealing stubs, is to cut a couple of inches off the polythene hose used for the cooling system level checks, and block one end by screwing in a suitable bolt, as per the pic.. A true engineer would have at least cut equal lengths!

With the cooling system cold and the radiator cap still in place remove each small hose in turn and block the connection to the cooling system with the stubs as described above. If you are fairly quick at this you will only lose a little coolant.

Use the garden hose to flush water through the throttle heater assembly, and you will be amazed at how much brown muck appears. Reverse the hose and keep the water flowing until it comes out with a decent flow and of course perfectly clear.

Reassemble the hoses to the main cooling system and before starting the engine, remove the radiator cap and check the cooling system level.

Start the engine and feel the temperature of the two connecting hoses. They should both get warm within a few minutes and the engine revs should drop back to about 750 rpm after about 10 minutes.

In my case the revs dropped back too far and I had to bring them up again using the idle speed adjuster on the right of the throttle assembly looking in from the air filter end.

From cold, and I mean overnight, the initial engine speed is now about 1700 rpm and after about 1.5 miles driving, drops top below 1000 and finally when really warmed up,  to about 750. I think this must be close to the normal characteristic of the car when new.

(6/10/00)

Copyright (c) Bob Dubery 2000