Newbie with fuelling problem

[JamesDaniel80]

Hi,

I know it's been discussed many times before but I've tried just about everything to stop my engine surging but without success.

I'm the proud owner of a MK1B which I've had for about four years, three of which have been spent in the garage.

I'm in the process of getting it back on the road but it's developed an annoying issue where it won't stop surging on idle.

The symptoms:

It starts fine and for about a minute or so, the idle speed is steady at the fast idle speed.
Then as it gets a bit warmer, the idle just goes up and down.


Eventually the fast idle will cut out and the revs will drop as you would expect, but even though the revs have dropped, it still surges at the same rate.


What have I tried?:

I've bled the coolant and I know there is no air in there.

I've also changed the coolant temperature sensor and cold start sensor.

I've changed the MAP sensor.

I've checked for vacuum leaks and I'm pretty confident that there are none.

I've checked the TPS which is working fine.

I'm running out of ideas!!! Can anyone help?

Thank you in advance,

James

[GazRev1Tubby]

By surging, do you mean that it is 'hunting'?

If so, it could be a sensor, perhaps throttle position or idle speed control valve?

Sent from my GT-I9505 using Tapatalk 2

[jimi]

It's almost certainly caused by your AAV. I'm on the train at the mo, when I get home I'll explain fully

[jimi]

Here you go

Either you have air in the cooling system ( you seem to have eliminated that ) or you have a vacuum leak on the inlet manifold that is allowing air past the throttle butterfly when its shut, this confuses the ECU which results in idle "bouncing" between 2/2500 rpm and 800 rpm. Check all the small vacuum hoses going to the inlet are in place and not leaking.
The other thing that could cause it is the waxstat for the AAV ( auxiliary air valve ) not closing properly, there is an easy way to check this, warm up the engine until you get the "bouncing" idle remove the main inlet hose from the throttle body if you look at about the 5 o'clock position you will see a small hole (about 5mm dia ) block off the hole with your finger, if the idle returns to normal the the waxstat is not closing properly, if it stays the same then its something else possibly a vacuum leak.
The hole you want to block is marked C in this picture.



hth
Jimi
Thanks to Paul Woods for the picture :

And welcome to the club 8)

[JamesDaniel80]

Thank you very much.

I'll check the AAV today and see what happens. I really hope it is that...I've spent many an hour scratching my head!

Cheers!

James