Engine revs up/down on its own

[El_Camino]

I believe I had this issue from the beginning owning the car, however now its become even worse after the engine change I recently made.
The original TPS sensor was reused.

The car revs in idle cold/warm between 1500-2000 RPM.
Even worse, after hard driving its revving between 2000-3500 /ups and downs/.
Made some video (sorry for the very poor quality, but gives idea) http://www.youtube.com/watch?v=qCOS9Bc6p...=youtu.be.

Any idea what causes this or when should start?

[Dino86]

I had similar issue.
I replaced The seals between The injectors end the cyl. Head and problem was solved. The old ones didn't seal off so air passend throughout.

[El_Camino]

Thanks, mate! I will rather do it and see if it solves it.
Found them online and ordered a set!

[fax0815]

Also worth checking the air bypass valve unter the throttlebody, they normally cause the >2000rpm idle when stuck open.

Take the Airfilter rubberpipe off the throttle body, you´ll se a few holes drilled in front of the butterfly. The big one on the side, that is the crankcase vent, there are a few smaller ones , shut then carefully with your finger, one of them is the bypass (2nd biggest hole iirc). If your engine is at operating temp and you close that hole and the idle drops low, down to 700 or less its stuck open and you need to change it...

Its a thermostatic airbypass, and just like the normal thermostat the fail sometimes.

[jondee86]

El_Camino Wrote:The car revs in idle cold/warm between 1500-2000 RPM.
Even worse, after hard driving its revving between 2000-3500

The 1500-2000 rpm lope is usually the engine cycling on fuel cut, and the
typical cause is a non-functioning thermostatic idle-up valve. You can test
for that one as fax has outlined above.

The 2000-3500 rpm lope is a bit more interesting :E Theoretically. with a
closed throttle and high manifold vacuum, fuel cut will be initiated if the
engine speed is above 1700 rpm, and fuel will be restored when the engine
speed drops to 1400 rpm... +/- a couple of hundred rpm to allow for
gauge error and overshoot.

Does the hot engine rpm actually lope (move up and down by itself) or
does it just hold steady at some rpm between 2000-3500 rpm ?

Cheers... jondee86

[El_Camino]

Thanks for the replys, guys! Will check that on Sunday.

The revs are going up and down, just like in the movie, but more aggresive and up to ~3500 revs.
As it was not expected had already accident, because of this.

Could faulty TPS sensor initiate something similar?

[jondee86]

You could try unplugging the TPS, and see if it makes a difference.
And also try unplugging the MAP sensor.

With the TPS the main thing is to check that the IDL switch actually
opens when the throttle is fully closed. You need to find the correct
pins and check with a meter.

Check the idle air bypass screw... down the tube on the side of the
throttlebody... turn it all the way clockwise (closed) and see if it makes
a difference.

Check the cooling system has no trapped air... park with the front of
the car up a slope. Run with the radiator cap off for a few minutes
until the thermostat opens, to let any trapped air escape.

Look everywhere for vacuum leaks into the inlet manifold. What head
(bigport or smallport) do you have on the engine ? What inlet manifold ?

Cheers... jondee86

[Sanity Clause]

As a late addition to possible causes...

Once the engine has warmed up, check that hot water is reaching the thermostat in the Auxilliary Air Valve.

A blocked hose or pipe can result in an oscillation in the revs at idle speed.

Link to an old post, with a pic.

[Apsogos]

in the past in my case was the iscv !i replaced it with a new one..!