MA70

Sent........I think. Its a large pdf (5.4M) and I got an error message while it was being sent. Seemed to finnish ok. Let me know if you didn't get it, or its corrupt, and I'll try again

One wire connects the 'A/C amplifier' to the 'Rear window de-fogger' button on the 'Heater control switch'. The other connects the A/C amplifier to the 'A/C ambient temperature sensor' I can send you a copy of the wiring diagram if you PM me your email, or tell me how to post it on the forum.

Since buying my Supra the heater has never worked properly, blowing air slightly warmer than outside. Having read with some horror the effort required to change the heater matrix (Great write-up Jezz !) I had been putting off fixing this for some time. After driving the car I noticed that one of the hoses to the heater matrix never got very hot, and when I disconnected it, it seemed full of air. I tried bleeding it by loosening the clamp on this pipe and running the engine. This resulted in water leaking out the joint, but did not fix the problem. Before stripping the car down I decided to try blowing water through the matrix to see if it was blocked. To do this I disconnected the two hoses to the matix, above the engine, close to where they exit the firewall. As this is the highest point in the coolant system I didn't need to drain it at all. I connected a garden hose between my outside tap and one of the hoses (The one passing through the firewall on the left when viewed from the front of the car) and a second garden hose between the other pipe and a bucket half full of water. The hose end must be below the water surface !! .......... A word of caution here ! ......... I do not yet have workshop manuals and am not familiar with the heater system in the MKiv, so I don't know how much pressure the system can take or if there are any flow control valves near the heater matrix (like the MKiii has). If you simply turn the tap on 'full bore' you may burst something and fill your car with water! I switched the ignition on and set the heater to MAX. This was to ensure any valves would be open. I cracked the tap open slightly and checked that air was bubbling into the water in the bucket. This told me that matrix was not blocked and I could open the tap further. When I did this there was a lot of bubbling noises from the heater matrix and a lot of air bubbling into the bucket. After a short while the bubbling subsided leaving only water in the system. I turned off the tap, and sealed the garden hoses to prevent syphoning. I then reconnected the heater hoses and briefly ran the engine with the clamps loose. After doing up the clamps, checking the coolant level and running the car, I was very happy to have lovely HOT air comming out of the vents. The whole process took about 45 minutes and is well worth a go if the alternative is a major strip-down, a new matrix and a air-con re-gass

The movement of pistons and valves in a piston engine cause it to act a bit like an air pump, blowing out the exhaust and sucking at the induction system. The amount of air it is allowed to suck is controlled by the position of the throttle butterfly valve. The engine management system monitors the amount of air passing into the engine, via the Air flow meter, and adds the correct amount of fuel, via the injectors. There is a small bypass on the throttle butterfly valve to allow the engine to run when it is closed. This is like another small throttle valve, (called the idle speed controller or ISC in Toyota speak). The engine management system monitors the RPM when the throttle butterfly is closed and controls this valve to keep the RPM constant, dependant on engine temperature, air temperature, load etc. If your engine runs normally except for the high idle speed, then the fuelling is likely to be correct, and the problem will be with the throttle butterfly or idle speed control. Eg main butterfly not closing fully, ISC valve stuck open or being told to stay open because the management system thinks the air-con is on etc. If the engine runs roughly at idle and just above it is likely to be a hole in the induction system (split hose etc) allowing the engine to suck in air that the management system doesn’t know about. This causes the fuelling to be very wrong (lean). At idle the engine revs will rise until the fuelling is so wrong that it cannot go faster. This problem will seem to go away when the engine is under load because the amount of air entering via the throttle butterfly dwarfs the air via the split. The fuelling is then almost correct. As the other guys have suggested, check the induction system for split hoses or loose connections, anywhere from the air flow meter to the engine block. If you listen you may hear a sucking noise from the split when the engine is at idle. The HT system is the most likely cause of a miss-fire, but I doubt this would explain the high revs at idle.