Mk2 8v digi revs fall and rise - help

[jezza16v]

Hi all, the mk2 ran fine for about 2 weeks after a rebuild but now we have a very strange problem which makes the car undriveable.
The engine starts and ticks over fine and initially revs through the full range OK but after 30secs or so of running if you hold the throttle open at around 2500rpm it drops to idle then revs up to 2500 drops to idle then back up to 2500 all by itself ad infinitum. 2500 is not a particular rpm it does the same at any held position below about 4000rpm. Weird. Changed the MAF for another & its just the same. Any ideas what it could be. Tries pressing throttle switches but no change, checked continuity on idle & full throttle switches and they operate. Thanks 

[Sgt_Lemon]

What happens when you unplug the idle control unit?

[oakgreener]

Check the throttle body switches, one on top and other underneath, and the wiring to them as well. It has a habit of breaking.

[jezza16v]

Thanks all for your replies. I managed to download a VW Digi II manual from a US web site and after a thorough reading, ran some checks. When warmed up and you disconnect the blue temp sender the engine runs a little faster and rough but the surging problem goes away, running through the specified checks, we tested the idle and WOT switches at the connector into the ecu and they are fine, the blue temp sender is working fine as well. Anyway, the problem is that the ecu is permanently stuck in 'idle' mode so it surges between open an closed throttle at anything above 1250rpm and below 4000rpm. In fact the surge test is specified in the VW manual as part of the testing of the blue temp sender and throttle switch circuits. The circuits of these two elements are interlinked once the engine temp gets above 60 deg. We have just won a replacement ecu off ebay which we hope to get installed tomorrow, I'll let you know how we get on. Regards J

[Santiago]

A link to that manual would be much appreciated, by PM if you'd rather not post it?

[jezza16v]

Ignore the 'vanagon' and oxygen sensor references, we don't have that here. Pages 108 + 105 sorted it for me.

http://shoebox-electronix.com/sitebuildercontent/sitebuilderfiles/VW_Digifant_Manual.pdf

Also this provided very useful confirmation.

http://vw-passat-mk3-35i.blogspot.co.uk/2010/09/volkswagen-passat-engine-control.html

Ignore the Passat references, its exactly the same system as the golf. This doc gives more detailed electrical information and brain damage in the first section but gets much more practical well down the page.

Happy reading 

[Santiago]

Very kind, thanks.

I'd found the first one myself earlier, but that second link is awesome, much appreciated.

If you're ever passing through Radley, give me a PM shout and I'll buy you a beer.

[jezza16v]

Hello All. Just an update on the problem. We just replaced the ecu and the problem is solved......Yay!
So analysis confirmed that the 'ecu' was stuck in 'idle mode', probably due to an internal short or break. I'll open it up when I get a chance and see if I can see anything obvioius. 

[Santiago]

Good news and well done for getting to the bottom of the problem.  Unusual one that.

I'd be happy to assist with looking at the dead ECU if you like, I work in electronics FWIW.