A tale of 2 GTIs

[cняis]

would a knackered blue temp sender have the same effect? or does that just upset the idle when warm?

[tony_ack]

Sounds like a loose wire/dodgy earth. Best way through this is to think about what exactly could be causing the behaviour you describe, and work from there...

Don't think it will be a vac pipe, because I can't see how that would be intermittent? A pipe is either split or it's not - it doesn't keep switching from okay to split!

A fuel pump problem is a possibility, but would this cause the car to cut out altogether?

I would suspect that it's ECU related - either the ECU itself, a frayed wire to one of the sensors or a faulty sensor. The engine still runs, albeit retarded, so I'd hazard a guess that something is feeding the ECU duff (or rather no) information. Without information from all of its sensors, the ECU goes into safe (sluggish) mode and retards the timing becuase it doesn't know what's going on!

I would check the operation and wiring to the following components:

Blue Temp sender (as already mentioned...) - allows the ECU to adjust the fuel mix if the engine is cold. Note that the timing will be retarded if this isn't working, regardless of the actual engine temp (as the ECU doesn't know what the temp is!). This is on the coolant hose entering the front of the head (there's a blue one and a black one - the blue one is for the ECU, the black one is for the temp gauge in the dash). When it's working okay, there's usually an audible difference in the idle when you disconnect it with the car running. Clean the plug contacts, and check the condition of the wiring all the way back to the ECU. Replacing the blue sensor itself is always a good idea anyway.

Knock Sensor - allows the ECU to adjust the timing to account for knocking. This is on the left-side as you look at the engine, on the front - can't remember if it's at the top of the block, or the bottom of the cylinder head  :embarassed: Again, the engine will run retarded if no signal is received from the knock sensor, so check the wiring.

Airflow Sensor (AFM/MAF) - informs the ECU of how much air is being let into the engine (by measuring the position of the intake flap). Check the wiring is okay, and there's a procedure of how to test the AFM itself in Haynes. Don't think it will be this will be at fault, unless there's an intermittent fault from within the unit.

If it's none of these, check the earths for the ECU, and as a last resort, swap your ECU. When you've found and fixed the problem, it probably wouldn't hurt to get your CO set and work through rubjohnny's tuning guide.

[Whaleshark]

Sounds like a loose wire/dodgy earth. Best way through this is to think about what exactly could be causing the behaviour you describe, and work from there...

Don't think it will be a vac pipe, because I can't see how that would be intermittent? A pipe is either split or it's not - it doesn't keep switching from okay to split!

A fuel pump problem is a possibility, but would this cause the car to cut out altogether?

I would suspect that it's ECU related - either the ECU itself, a frayed wire to one of the sensors or a faulty sensor. The engine still runs, albeit retarded, so I'd hazard a guess that something is feeding the ECU duff (or rather no) information. Without information from all of its sensors, the ECU goes into safe (sluggish) mode and retards the timing becuase it doesn't know what's going on!

I would check the operation and wiring to the following components:

Blue Temp sender (as already mentioned...) - allows the ECU to adjust the fuel mix if the engine is cold. Note that the timing will be retarded if this isn't working, regardless of the actual engine temp (as the ECU doesn't know what the temp is!). This is on the coolant hose entering the front of the head (there's a blue one and a black one - the blue one is for the ECU, the black one is for the temp gauge in the dash). When it's working okay, there's usually an audible difference in the idle when you disconnect it with the car running. Clean the plug contacts, and check the condition of the wiring all the way back to the ECU. Replacing the blue sensor itself is always a good idea anyway.

Knock Sensor - allows the ECU to adjust the timing to account for knocking. This is on the left-side as you look at the engine, on the front - can't remember if it's at the top of the block, or the bottom of the cylinder head  :embarassed: Again, the engine will run retarded if no signal is received from the knock sensor, so check the wiring.

Airflow Sensor (AFM/MAF) - informs the ECU of how much air is being let into the engine (by measuring the position of the intake flap). Check the wiring is okay, and there's a procedure of how to test the AFM itself in Haynes. Don't think it will be this will be at fault, unless there's an intermittent fault from within the unit.

If it's none of these, check the earths for the ECU, and as a last resort, swap your ECU. When you've found and fixed the problem, it probably wouldn't hurt to get your CO set and work through rubjohnny's tuning guide.

Wow.  Thanks Tony, that is a massive source of information.   I bet quite a few searches in the forum would be answered in just that one post. 
In fact, I wonder if a forum glossary would be a good idea?  Much as Tony has done above? 

Anyway......
Yesterday evening I had 30 mins spare so I cleaned up the earths from gearbox and rocker cover.  I presume that these are the main two?
I also intended to swap over the ECU.  However, when I opened the wiring connection that is mounted on a bracket on the front of the ECU(not the one that goes to the ignition module) and found that there was a lot of corrosion inside.  I cleaned this up.  When i ran the car, I found that the power had been restored.   Sorted!!

I'm not sure if it was cleaning the earths or the the contacts by the ECU that that has fixed it.  It could even just be in the good phase of the intermittent problem.  I will find out soon enough if this is the case.

I would say that it is now 90% of the power of my previous Gti.  This is good as it had been about 50%.
I will tune it to Rubjohnny's guide.  I have previously done the best i can using timing light and ear, but really need to get hold of an CO meter.  I also need to replace my MFA screen as the one that I have has cracked.  To that end, if anyone has a MFA screen and circuit board for sale, please let me know.  It is the one off the single pin clocks.  Perhaps someone whose clocks are knackered or very high mileage?  I just need the IC board and display.

Thanks again to all who have taken the time to provide advice and help.  WTF did we all do before forums like these?

Cheers, Ed

[tony_ack]

Don't thank me - thank all the others who have posted advice on this forum! I also had problems with the running of my digi recently, so I guess you're benefitting from the fruits of my research 

Also, a lot of info comes from here:

http://www.vanagonparts.com/digifant.html

A bit heavy for bedtime reading, but a good wealth of information.

There's also an earth strap from the bonnet to the bodywork, on the right hand side as you look at the engine.

When you get your new clocks, try them out first by removing the lower parcel shelf on the driver's side under the dash, unplugging your clocks, then plugging the new clocks in. Obviously the speedo and mpg readouts won't work like this, but at least you'll know everything else works before you start taking the dash out. Also, once the repaired clocks are in, dont put the dash back together until you've tested everything, including the dash lights! There's nothing more frustrating than put it back together and realising a bulb is out...

And yes...i have done my clocks recently too!