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Increasing (idle) RPM

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Hello all.

I have a 626 with 1.8 engine (GD) from 1990. It's still a bit below 100k km. For most of the last decade it is occasionally driven with intervals of up to months.

The last few times the engine's RPM when the pedal is released tends to increase compared to the idle RPM it should have. During the drive the RPM seems to gradually increase. This is awkward at traffic lights and when driving slowly, rounding corners etc.

I'm not sure why the RPM doesn't return to the normal level when the pedal is released. Friction between the inner and outer cable doesn't seem likely. I can't influence it by letting the pedal 'snap' up from the depressed position.

When the engine is started it runs normally with the automatic choke causing a slightly increased RPM but instead of the RPM decreasing when the engine warms up it starts to increase and gets gradually higher during the drive which suggests being related to the engine warming up.

Anyone recognising this?

Can it be the carburettor? I've had some problems before with the choke valve getting stuck which resulted in the engine revving down and shutting down shortly after driving away. Could this be a milder manifestation? The choke valve not opening up all the way again as the engine heats up? If so what maintenance should I perform to get it to work properly again?

Or could it be something else?

Thanks for your help.

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I recently had a similar issue with a 94 GE 4 cylinder. I noticed that the coolant hoses that go to the IAC on the throttle body were cold, unlike the rest of the coolant hoses. Due to the blockage in the coolant system, no hot water was getting to the IAC causing the cold start air bypass to remain open. This confuses the hell out of the ECU causing high idle and hunting idle. Check the hoses leading into and out of the IAC on top of the throttle body, if they are not hot like radiator and cabin heating hoses you have a blockage some where. You can either find the blockage and clear it or make a blanking plate (gasket) that blocks air flow through the cold start air bypass.

Another test is to get car to operating temp, remove air intake boot at throttle body. Start car (may take a couple of goes), if air is being sucked into both bypass orifices just before the throttle plate you have a stuck open cold start air bypass. I think the one on the right is for cold start and the one on the left is for IAC solenoid. Air should only be going through the IAC solenoid at operating temperature.

Took me ages to work this out. Replaced nearly every sensor and solenoid in the thing.

I will probably leave the blanking gasket in and just bypass the heating hoses.  

Not familiar with your model. The layout of the cold start air bypass and the idle air control solenoid may be different.

Let me know if this helps.


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