Surging at idle

[zadumbreion]

Today my car was doing something odd. When I pulled up to a junction, and pushed the clutch pedal all the down, the engine was surging or hunting. The revs would rise up to about 1800, then drop instantly to 1000, then climb to 1800, drop to 1000 over and over again. Each cycle lasted about a second. This was happening even when the engine was completely warmed up.

Any ideas?? It's never done it before!

Edited by zadumbreion on Thursday 21st February 20:08

[E1DJH]

Have you checked the stepper motor?
I think this sometimes happens if it is allowed to get dirty - simply fix unclip the wires, unscrew and wipe clean.
HTH

[zadumbreion]

OK, sorry - looks like this kind of thing has been asked a lot so no worries - I will read the other threads.

[Pablo T]

Another point to make. What fuel have you been using. I tried saving money once by putting cheaper fuel in and it ran like a sack of sh't .

[zadumbreion]

Minimum 97 RON (super unleaded) and optimax when I can.

It's just weird the way it cycles up and down. It's not just running roughly or anything - it cycles repeatedly between 1000 and 1800 revs. Even when hot.

[Parrot of Doom]

I had a similar thing on mine, usually when cold. I even replaced the stepper motor (the end was shot), it didn't solve it.

The only thing I can think it might have been was a leak on the plenum (on the air screw) but I had that repaired and sold it, so never knew if it was solved.

[dogbucket]

mine does that occasionally if really cold, but stops when it has warmed up.

I think the ECU is trying to lower the revs to normal idle speed but if it sees the engine revs drop too low it winds the stepper open bump it up to fast idle again. Cycle then repeats. I stand to be corrected though..

Edited by dogbucket on Friday 22 February 08:57

[blitzracing]

Thats spot on, problem is, there is no "control loop" on the stepper, The ECU senses the RPM is too high, so it applies enough pre programmed step pulses to reduce the RPM to where it should be if the engines running correctly and then waits. Problem is, if the engines not spot on, like an air leak, failing plug, mucky injector- (the list goes on), the RPM drops too low in its wait time. The ECU senses this and winds the stepper back again, and the process repeats. Unplug the stepper motor when the RPM is at its lowest, and see if the idle is stable and the engines running smoothly. If its not find the reason for this first.

Mark

[FlipFlopGriff]

The temp sensor is something else to look at if this doesn't cure the problem.
FFG

[im]

Air flow meter buggered?

My car was 'surging' recently and it turned out to be the sensor in the AFM was covered in a film of oil.

Replaced the AFM and all was returned to calmness and light...

[RIP_puffsorted2]

Look possibly at where the Vacuum advance housing is screwed to the side of the distributor. Mine was doing exactly the same and I cleaned the stepper and tightened up the housing (is was virtually falling off) and all's well now.

Edited by RIP_puffsorted2 on Friday 22 February 16:05

[zadumbreion]

I removed the stepper motor but it was pretty clean already, almost no black crap on it. What there was I removed, screwed it back in but it's the same.

So I waited for it to settle down and then disconnected the stepper motor and now it's not doing it any more, so I guess it must be related to it somehow.

Anything else I can do / check with the stepper motor?

[V8 Animal]

zadumbreion said:

Today my car was doing something odd. When I pulled up to a junction, and pushed the clutch pedal all the down, the engine was surging or hunting. The revs would rise up to about 1800, then drop instantly to 1000, then climb to 1800, drop to 1000 over and over again. Each cycle lasted about a second. This was happening even when the engine was completely warmed up.

Any ideas?? It's never done it before!

Edited by zadumbreion on Thursday 21st February 20:08

I had exactly the same problems last year at rockingham actually.
I replace the stepper and cured instantly.
Paul

[blitzracing]

If its not a sticky stepper motor, you are still back to the same problem, something is making the engine rpm drop faster that it should in the stepper motors wait time. If the engine ticks over ABSOLUTELY smoothly,(ie its not HT or spark related) with no trace of a misfire then look for air leaks. An aftermarket stepper I fitted, let air past its motor with the same effect.Problem is as the airflow drops, a small leak has a greater effect,(more so after the throttle plate, ie the plenum chamber area) and the mixture becomes weaker, so the RPM drops faster. Look at AFM hose, rocker cover pipe work, distributor vacume pipe (a split distributor diaphram can let air), plenum base seal etc etc. This is just a starting point as the whole lot is pretty finely balanced, and this is only one possible cause.

Mark