On starting my 1.6 Mk4 the idle revs have become very erratic. When I start the revs shoot up to about 2500 then down to about 1000 and then shoot back up again.

Once I'm on the move in 2nd or 3rd (and sometimes 4th gear) the revs shoot very quickly between about 1500 and 2000, making the journey very jerky.

I have recently disconnected the battery for a few hours while doing some other repairs and have also cleaned out the throttle body in the last few days. Am I right in thinking that the problem could be solved by using VAG-COM to do a reset of the throttle?

I know it's not very clever but when the engine is idling and I disconnect the throttle potentiomenter the idle revs settle down to their normal level of around 1000rpm. Reconnecting the tp then makes the revs go all crazy again.

Edited 4 November, 200817 yr by Blitzdonkey

Bumped....

Now with a video I made this afternoon illustrating the problem:

http://www.youtube.com/watch?v=id3sPBb-XN8

This video was made while the car was stationary but it has the same jerky rev problem when travelling around.

Edited 12 October, 200817 yr by Blitzdonkey

You're absolutely right. You need to reset with VAG-COM - and your test confirms this.

PoloD.

Cheers for the reply PoloD, I appreciate your input.

Looks like I'll have to get the Beast hooked up to VAG-COM sometime this week.

I ran VAG COM on my Polo and got the following error messages on the engine:

VAG-COM Version: Release 409.1-US

Control Module Part Number: 032 906 030 D

Component and/or Version: MARELLI 1AV 1984

Software Coding:

Work Shop Code:

7 Faults Found:

00519 - Intake Manifold Pressure Sensor (G71)

31-10 - Open or Short to Ground - Intermittent

00518 - Throttle Position Sensor (G69)

30-10 - Open or Short to B+ - Intermittent

00530 - Throttle Position Sensor (G88)

30-10 - Open or Short to B+ - Intermittent

00516 - Closed Throttle (Idle) Position Switch (F60)

27-10 - Implausible Signal - Intermittent

00523 - Intake Air Temp Sensor (G42)

30-10 - Open or Short to B+ - Intermittent

17978 - Engine Start Blocked by Immobilizer

P1570 - 35-10 - - - Intermittent

00282 - Throttle Position Actuator (V60)

36-10 - Open Circuit - Intermittent

(ignore the engine start one, I just knocked a wire connecting the immobiliser when I was plugging the VAG COM cable in)

I did a TB realignment as outlined on the ross tech website but it didn't seem to adjust any of the values and needless to say the fault with the erratic rpm still persists.

Can any advise me re: the VAGCOM faults listed above? I way too much of a noob to make sense of them.

00516 - Closed Throttle (Idle) Position Switch (F60)

00516 - Closed Throttle (Idle) Position Switch (F60) - Symptoms irratic idle speed

Possible Faults

-F60 out of adjustment

-Throttle body sticking

-Throttle cable in need of adjustment

-Open circuit between F60 and J220

-J220 faulty

- Check & adjust F60

- Check F60 potentiometer and switches

- Adjust throttle cable

- Repair open or short ciruit

- Check electrical connections

This relates to the ABY engine... Not sure whether this is any use for AEE...

Edited 15 October, 200817 yr by polo_phil

the throttle position sensor open or short to ground will have been caused by you disconnecting the throttle pot with the engine running.

Best bet is to clear all those faults, drive the car for a bit and then re-read and see what faults (if any) have come back.

Thanks for the advice both of you.

I've driven around a bit today and still the same problem with the revs although VAGCOM is showing no faults.

I decided to remove the throttle body to give another thorough clean and when I did I noticed that there were only 3 bolts holding it in position instead of 4 and that where the 4th bolt should have been there was an oil leak so I'm guessing there was a slight leak in the tb vacuum which could explain the massive over rev when the ignition starts, the high idle revs and the erratic revs in between.

I'll hunt down a bolt and then do a tb reset and see if that helps...

Update...

I managed to locate a replacement screw and now the tb is connected in correctly but the rev problem still persists.

It is no longer intermittent and happens 100% of the time like so:

1. On ignition the revs shoot up to 2500 and stay there for several seconds.

2. Revs then move down to idle between 1200-1500 rpm.

3. After a few seconds the revs climb again and jump between 1500-1700rpm. This continues indefinitely.

4. When in motion the jerky revs continue and the car jerks every time I lift my foot up off the accelerator.

5. The problem first started after I had disconnected the battery for an hour or so.

Attempted solutions:

1. I've run a VAG-COM diagnostic twice in last 24 hours after a lengthy driving session and each time it has said no faults found.

2. I have removed the TB, cleaned it thoroughly, and replaced it with all the correct bolts (there was one missing before) but this has made no difference to the problem.

3. When doing the TB reset I can hear the butterfly valve clicking back and forth repeatedly for several minutes - this seems to be a little too long to me. Could the valve be stuck somehow? It moves freely enough but to look at it when the throttle is in "closed" position it still seems to be a bit too open, even if I detach the throttle cable completely. It seems as though the valve could close another couple of millimetres but now when it rests it sits ever so slightly open. Could the axle running through the butterfly valve be sprained in such a way that even a TB reset won't adjust it properly?

I'm too broke to get a new TB for a few weeks so am hoping that isn't the problem. I'm very much a noob but the fact that

1. The fault started immediately after I reconnected the battery and

2. When I disconnect the throttle potentiometer the idle revs were normal

made me think it was an ECU/electronics error rather than a mechanical one. Might be totally wrong though...

Any ideas?

Interesting...

I am I right to assume the erratic idle predates the removal and cleaning of the TB? I think looking from the codes and your test the the potentiometer in the TB is fcuked. The clicking you hear with the TB reset is normal and yes, it does go on for over a minute.

A test with another TB (a working one) may help to confirm this. I'd also check the vacuum hoses carefully for leaks as this is another cause of irratic idle.

Best of luck chap!

PoloD.

Yes, the erratic idle was happening before the TB was cleaned, cleaning made no difference :(

I'll try and get hold of another TB and see if that makes any difference...this might get expensive!

Well today I installed a brand new throttle body, did a full reset with VAGCOM and still the problem persists...agh

See here:

http://uk.youtube.com/watch?v=sp1dzuOpzdA

The only difference to before is that when I disconnect the TP cable the revs surge rather than drop to idle. VAGCOM is still showing no faults....

So I'm guessing that it could be a dodgy gasket or a leak in one of the vacuum hoses somewhere? Or maybe a sensor?

I also noticed today that every time the revs bounce, the car shoots out a little jet of blue exhaust smoke which I hadn't noticed before (but this is the first time I've been able to have a good look at the car in broad daylight since this problem started.)

I dropped one of the screws holding down the throttle body and I can't find it anywhere (think it might have rolled down a drain ) Anyone know the part number for a new bolt or where I could find out from?

Edited 18 October, 200817 yr by Blitzdonkey

I see what you mean... From your other reply I see you 'got it looked at'. Did the mechanic have any other suggestions?

I'd still do a finger tip search of all the hoses to see if there is a small leak somewhere. Also check the manifold gaskets very carefully.

If no joy I'd suggest VW - they're expensive but will find the problem! Cut your losses with random internet diagnosis!

PoloD.

Just thought I'd post an update on this in case anyone has had a similar problem...

Replaced the throttle body gasket, all the inlet manifold seals and the 3.5mm vacuum hose running from the fuel pressure gauge into the inlet manifold and now the engine is as smooth as ever and has noticeably more kick to it :)

Only downside was VW charging me nearly £30 for what was basically 5 elastic bands and a bit of tubing but at least the car works again.

Worst part about this job was that when putting the fuel rail and injectors back in I accidentally snapped off the plastic connector on the side of one of the injectors so it couldn't connect up to the plug. Two weeks, one wrong part and a £130 quote from Volkswagen for a new injector later, I got a new injector from a scrappy for £15 and now the whole thing works again.

Just thought I'd post an update on this in case anyone has had a similar problem...

Replaced the throttle body gasket, all the inlet manifold seals and the 3.5mm vacuum hose running from the fuel pressure gauge into the inlet manifold and now the engine is as smooth as ever and has noticeably more kick to it :)

Only downside was VW charging me nearly £30 for what was basically 5 elastic bands and a bit of tubing but at least the car works again.

Worst part about this job was that when putting the fuel rail and injectors back in I accidentally snapped off the plastic connector on the side of one of the injectors so it couldn't connect up to the plug. Two weeks, one wrong part and a £130 quote from Volkswagen for a new injector later, I got a new injector from a scrappy for £15 and now the whole thing works again.

mate sounds exactly the same as my 1.6glx!! need to try this thanks for posting it :-)

when i go over 4000rpm it jerks and power loss and have to play with the accelerator untill power comes back.

i ran the VAG and i got

00519 -155 KWP1281

INTAKE MANIFOLD PRESSURE SENSOR (G71)

CLOSED THROTTLE

IDLE POSITION SWITCH(F60)

will let you know if it sorts it!

thanks

mark