My nephew wrote off his first '88 prelude after 3 weeks. We managed to find another ('89) very nice that was smokey and had a noisy gearbox. I drove it back to his house. We removed the motor and gearbox from the wreck and had a local business fit it to the second car. The job seems to have been competently done. The engine wiring loom from the '89 car was used. All hoses are connected the same as several other cars.

It has a peculiar problem. At any speed above 5-10 km/hr it will only idle at about 2500 RPM. Below that speed the idle is a normal 800 or so. It is really annoying as the revs stay too high for upshifts and there is no engine braking. When the speed drops below the 5-10 km/hr the idle drops back after 2-3 seconds. When the car is stationary you can rev it up as often as you like and it will always drop back to idle OK. When moving at more than 5-10 you can knock it into neutral and as long as you keep coasting at more than 5-10 the idle will stay at 2500.

Neither car had this problem before. I have changed the idle speed controller in the middle of the manifold, the thermal valve behind the throttle body and the ECU with no improvement at all. I can eliminate the problem by unplugging the idle speed controller (which makes the idle too slow) or disconnecting the speedo cable. The guy who fitted the engine thought the idle speed controller was probably a bit sticky and it might come good.

Any thoughts? Anyone know of an brilliant engine management problem solver, preferably in the northern suburbs of Melbourne?
Thanks,
Hugh Venables Melbourne, Australia.

 

Wow!, I've stumped the whole forum. OK, so no-one has had this problem before or has any suggestions.

I have had a power supply on the IACV that came out of it and it works really nicely between about 2V and 10V. It is some sort of solenoid that finds a unique, repeatable position from a particular voltage which has an electronics friend of mine itching to pull it apart.

I'll clean it anyway and try it again, but I doubt if it will fix the problem. Plan B is to make a blanking plate to put between it and the manifold and control the idle with the throttle butterfly stop. The AC is cactus under the dash and we don't get any very cold weather so I hope we'll get away with that.

I'd really prefer to solve the problem but I'm stumped too.
Hugh.

 

There is a vehicle speed sensor that causes an idle up to keep from slamming the throttle plate shut during deceleration. Operation is simple... at some speed, the switch closes and bumps the voltage up on the IACV. You'll have to hunt the switch down, I'm not sure of its exact location.

 

Since disconnecting the speedo cable "solves" the problem, it might be in the dash. I already had a look there and the speedo cable fits a box like protrusion on the left bottom of the instrument assembly, which then goes to a few electrical plugs. But wouldn't it switch in the ECU after looking at the speed signal from the cable?
Hugh

 

Ok, so if the voltage on the IACV gets bumped up above a speed (5-10 kmh it seems) the problem here seems to be that it's getting bumped up too much. This wasn't a problem with either car before the motor got switched from one to the other. Why now? And how do I reduce it?
Hugh.