Digifant systems

[DarnPB]

This is my first thread on this site so hi to everyone reading. I live in Spain and recently bought my second Golf II 8v GTI. It is a 1991 model with the Digifant engine management system.
When I bought the car, it was running very badly. Idle all over the place, bucking on the over run and initial power take up, expensive to run although it had plenty of power. I did what I normally do when buying an older car, replaced the plugs, dizzy cap and rotor, leads, then I noticed that the mass airflow sensor cap had been tampered with. On a closer look, the cap fell off! So the mass airflow sensor I changed for one from the scrappers. This changed the car. The idle became stable and the engine ran alot smoother, but was still bucking.
I put the car into a Bosch specialist over a weekend only to be told that they had done their best, but the engine was low on compression, so it was impossible to tune properly. I found this very hard to believe. The car did however run alot better, but still no cigar.
Now I am not a novice when it comes to engines, and although I do not have a compression tester, I don´t believe my car has a compression problem. There is no smoke from the exhaust to speak of, even after an over run, it does not use oil or water and the plugs are not oily. The engine oil is also a respectable colour and the car has plenty of power (more than my last one.).
But can anyone tell me if compression can affect the emissions? The car failed the Spanish equivilent of the MOT on emissions today with a reading of 9.3. Now I intend to get a second opinion on the compression diagnosis before I start thinking about tearing down the engine. Has anyone else had similar problems and what was the final diagnosis that put the car right?
Any feedback would be greatly recieved.
Thanks.

[rubjonny]

have a read through my digifant tuning guide in the MK2 maintenance + info section, it tells you everything you need to know to make a digifant engine run right

[DarnPB]

I have just completed a load of testing on the engine with reference to the Haynes manual and everything was in limits. I then went to set up the CO level with reference to the same manual and the info on this site. But, I have found on the MAF sensor that the adjusting screw is missing!! I don't know if this would have been removed by the garage or it was just missing. Would there be any need to remove the adjusting screw and what would be the result??
Do I just replace the screw or the whole top of the airbox?
Bemused!

[turbodub]

With regard to the mixture screw: I have checked parts catalogues in the past such as ETOS and ETKA, and it does not seem as though the mixture screw be purchased seperate to the AFM. There is a good possibility that the adjustment screw is a common metric size which could be replaced with a metric allen head bolt.

[rubjonny]

The MAF has an allen bolt not a screw, if it was missing you wouldnt be able to get the mixture & idle within specs since it would be allowing air to flood past the metering unit, in effect giving you a massive air leak.

[DarnPB]

I thought as much. The CO is off the scale. I´ll take my meter to the scrappers tomorrow, er, today, and pick out another one that´s complete. I´ll let you know how I get on.
Thanks guys.

[DarnPB]

Right. I got a serviceable second hand airbox from the scrappy today after testing it with the meter. I fitted it to the car and commenced round two of tweeking the CO. It is alot better but still high, ranging from betwen 2.8 and 4. It was 9.3. (I´m using a friends GUNSON CO meter which has seen better days!) But I have run out of adjustment on the allen plug. I´ll go for the retest tomorrow with tongue firmly emplanted into left cheek and see what happens. Hopefully they will allow me to make adjustments if I am too far out.
Any more ideas on why the CO is so high and why it cannot be brought back into the playing field?
Not out of the woods yet!!

[turbodub]

You could try replacing the coolant temperature sensor. It is the blue sensor located in the plastic coolant flange on the front of the cylinder head. These can often fail and cause the Digifant ECU to overfuel. Buy a genuine one from Volkswagen, the aftermarket products are not very good.   Cheers

Ben

[rubjonny]

I assume you've warmed it to 80 degs, disconnected the blue sender & breather & reved the engine over 3k 3 times

I've had this when the throttle body was stuck too far open, due to missalignment of the idle switch (too far in, holding open the throttle) and the same when the throttle cable was too tight.  Make sure it hits firmly up against the throttle stop.

Check for air leaks too, I'd replace all the vacuum tubing, and don't forget the blanked off connection round the back that is often overlooked

[DarnPB]

I did all the above and all I can think of is that the engine management computer is getting the wrong info, which indicates a bad sensor. I will replace the water temp sensor tomorrow. But what would happen if the lambda probe was not up to scratch? Still can´t believe its poor compression.