High rpm after hard drive

Is the accelerator pedal and throttle linkage on the throttle body fully returning to the idle position immediately? A weak return spring/bad adjustment can have that effect...which can change somewhat with engine heat. Then there's the old carpet-pushing-against-the-accelerator-pedal trick.

 

What version? US or euro?

Also, when you say it goes from being 1500 RPM to 1000 RPM, is the change all at once, or does it slowly go down?

 

I had the perplexing issue a few years ago where I'd be driving along and I'd feel a slight hiccup of the engine followed by a jump in idle rpm from 950 or so to 1500. One of the numerous helpful and knowledgeable fellows on this list (Rifledriver) told me it was bad ignition wires interfering with the computers that are located on either side of the luggage compartment in the rear of the car. I changed ignition wires, all 10 of them, and the problem immediately went away and never returned. While afflicted with the problem I noticed that as time wore on, the idle speed change began to occur sooner and sooner after starting a cold engine. Not saying this is your problem, but it's a possibility.

 

The warm idle speed changing suddenly from 1500 RPM to 1000 RPM after a single throttle blip can certainly be caused by the throttle linkage having a mechanical issue as Mike 996 suggested so you haven't ruled that out.

Another cause can be the idle potion of the throttle microswitch not being electrically closed at idle (as it should be). The test here is to just unplug the throttle microswitch at idle:

A) If it is working correctly at idle, the idle RPM will increase when it is unplugged.

B) If it is not working correctly at idle, the idle RPM will not change when it is unplugged.

I'd suggest that, the next time it is wrongly idling at the 1500 RPM, just unplug the throttle microswitch -- if the idle RPM goes even higher = the trouble is not the throttle microswitch; if it just stays at the 1500 RPM = either the idle throttle microswitch is misadjusted (or just on the boundary of its switching point) or bad. Try a search on "121517" for prior threads with more information about how the throttle microswitch works.

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It does sound like the throttle microswitch (Throttle position sensor, or TPS). With the engine off in a quiet garage, with your hand move the throttle lever at the throttle body and you should hear a very slight click just off the idle set position. If you do not hear the click (and try it by slowly pressing and releasing the throttle, and then do it rapidly simulating the pedal blip)t he two screws that hold the TPS can be loosened, and you slightly rotate the TPS so the switch acuates just off idle. It is such a fine adjustment, that a bit of wear in the linkage it can just not actuatate as it should. Spray some WD40 on the throttle lever and spring, some gunk can also build up, it needs to move smoothly. You can also adjust the throttle cable at the lever connector to take up a bit of slack in the pedal to throttle connection. Sounds like these small checks and adjustments will solve for things. It is possible the TPS switch has failed, but more likely with the pedal blip it is the mechanical aspect of its adjustment.

 

The only other thing that I can think of is the cable getting a burr and holding up in the casing.... that happened to my 308. it was subtle for a while and got progressively more "sticking"... usually after sustained higher RPM - like highway driving, then coming out of gear the RPM would drop but not to 1000, or 900 but get stuck at 1500 or 2000 ish. eventually it just stuck like that ... when they pulled the cable out and took it out of the casing ... there was a rip, and wire has gouged the side of the throttle cable casing... put a new one in and it was silky smooth.