HOW TO: Diagnose and tune your MK2 Digifant GTI 8v

May 13, 2020, 09:01:18 PM

(thanks to rubjonny)

Too many people complain about flat performance and 'digi-lag' on their 8vs. Not many people know that this isn't actually how the car is supposed to run, and is the result of the car not being set up properly. After spending an afternoon setting the timing, idle & CO properly my GTI is like a rocket with instant throttle response and blistering performance [:Y:]

First thing to do is check the ECU temp sensor, the resistance curve is as per this chart:

If you need a new sensor then try to buy a genuine one, either by crafty ebay searches or straight from VW. On the early digifant engine its a threaded type white sensor part number 035919369M, and on the later cars it's a push fit blue one part number 025906041A. Don't bother with cheap pattern ones as they have been known to fail prematurely or read incorrectly, good makes such as Hella will be OK though.

If you have an early car you can retrofit the late sensor if you also change the front flange to late plastic, and fit the late 2 pin black dash temp sensor. The white sensor plug should fit I believe, if not just splice later plug on. Likewise with the late dash sensor you need to splice the plug to your old sensor wire, the 2nd wire just earth to the head somewhere and you're done.

Next check all the vacuum lines from the inlet manifold including the brake servo hose and don't forget the blanked off line behind the throttle body its easy to miss. The vacuum line to the FPR goes on the nipple closest to the inlet manifold, the one nearest the throttle body should be blanked off. Also one of the nipples on the brake servo hose goes to the vacuum sensor on the back of the clocks, the other should be blanked. Pull the PCV valve out of the rocker cover and check the rubber bung it sits in, take it off completely so you can check inside the lip for splits. Cheap from VW and GSF so no worries if you need another. I would take the inlet boot off too so you can properly inspect in the folds, and while you're there you may as well clean out the throttle body till its shiny. repeat for any other air hoses to the ISV and brake servo etc to be sure everything is airtight. the oil filler cap is worth a look too, as is the plastic dipstick guide tube!

Now check that the throttle closes firm to the stop when released, and isnt notchy or sticky. If it wont close fully or isnt smooth remove the accelerator cable and try again, if that fixes the problem replace the cable. if not check the throttle butterflys are clean and fitted correctly, and the the idle switch underneath isn't set too tight. Next make sure the idle switch clicks when the throttle is opened and closed just off the stop, if not adjust it till it does. Finally check the wiring to the back of the throttle body is ok, its common for the wires here to snap. Once all is well here test the function and wiring by holding the throttle just off idle so the rpm is around 1500-2000 rpm then press the WOT switch on the top, the revs should fall then rise again when release. If not, go back and check the wiring and switches with a multimeter.

Next make sure the ISV buzzes away happily ignition on, they are usually pretty reliable units but can sometimes fail. You can try cleaning it out if you like, some have reported this made a difference but it didnt do anything for mine. The 2E code 2.0 8v ISV is a replacement part for the original mk2 unit so a source for slightly newer ones if you suspect your ISV is bad. Often though bad idle is down to a setup issue or a problem with the ignition switch.

Have a look at the knock sensor next, its bolted to the engine block near the alternator. If the sensor or insulation is cracked bin it and source another, try to get genuine, Hella or bosch makes. The ones off 2.0 engines found in 90s VW and Audi are compatible, I had an Audi 2.0 8v one on mine worked perfectly. Be sure the block face where it mounts is clean as is the rear of the sensor, ideally run a tap and die up the bolt and block threads then torque the bolt to 20 Nm (15 Ibf ft).

If you suspect the injectors may be bad pull the rail off and find 4 equally sized containers, one for each injector. drop it in then crank the engine over on the starter motor. make sure the spray pattern is good on all 4, and the volume delivered is roughly equal. There are various places who can clean up old injectors, or you can search out another set from ebay or wherever. Dont bother trying to upsize you'll only mess up the fuelling without a remap of the ECU, and finding someone who can do that isnt easy these days!

The Air Flow Meter can be checked electrically, for this you need a digital volt meter with a resistance setting. First pop the probes between pins 1 and 4 to test the inlet air temp sensor, the resistance vs ambient temp should match the chart above for the coolant temp sensor. Next put it between pins 2 and 3, watch the resistance as you move the flap inside the unit, it should go up and down nicely without fluctuating all over the shop. Last you check between 3 and 4, you should see between 0.5 and 1k ohm. If anything seems off you can carefully cut the silicone sealant holding the top on to have a look inside, make sure the springs are all good and the track is clean. Here you can also mess with the spring tension, but from my experiments I found this didnt really offer any advantages but YMMV as they say. The 2.0 8v 2E code AFM is compatible, just make sure there is a CO adjuster screw under the anti-tamper cap! I was lucky enough to find a NOS bosch 2E unit on ebay, the VW part number was different but the Bosch part number matched my original MK2 part number, and it worked well.

Timing:
1. Take out spark plug #1 and turn the engine over by hand with a spanner on the crank pulley nut until the piston reaches the top of its travel. Check by either shining a torch down the hole or putting summat long & thin down there like a dowel rod. If possible check the flywheel TDC punch mark lines up (as per flywheel pic lower down in the thread, TDC marker is smaller punch mark to the right of the big diamond) you can also check the crank pulley marker:

2. Next check the punched hole on the inside of the camshaft sprocket lines up with the head surface, with the cam cover removed. If not slacken the tensioner and move to suit:

If you have a later digifant with plastic cambelt cover, you can use the outer OT marker instead:

To verify your camshaft sprocket is fitted the right way round and/or you are using the correct marks next remove the cam cover and check the camshaft lobes on cylinder one look like this:

3. Take the dizzy cap off and make sure the rotor arm is roughly lined up with the mark on the dizzy, if not loosen the tensioner and turn the intermediate shaft. This is a little tricky with the cambelt cover in place, though I managed to do it by removing the bolt here and bending the cover outwards. Then make sure spark plug lead #1 is where the rotor arm is pointing, and the rest are in the correct order 1342 clockwise. Here's a pic of a 16v dizzy, the 8v mark is the same:

Once you've done all that the timing should be pretty close, the next step is to set the dizzy advance properly with a timing light. Factory spec is 6degrees BTDC, keep it to this for now you can always experiment with a little more later on. Don't go too far though it'll mess everything up, 7-8 degrees works well for most setups, but YMMV. If you loosened/replaced the tensioner re-tighten it so you can get a 90degree twist on the longest run of the belt with your fingers, worth checking the tension even if you haven't touched it!

Here's a nice pic of the marks you're looking for, shamelessly stolen from the vortex, link found by the legendary 'TSC' from ClubGTI. You can see the arrow on the gearbox casing, the large notch with a bit of red paint is the 6degree BTDC mark and the small round 'dent' to the right is the TDC timing mark. Before you start I'd highlight both marks with tippex to make them more visible:

1. Remove the green plastic screw in cap in the top of the gearbox, not just the little seethru cap in the middle, the hole isn't big enough to see the timing marks. This can fight you, tap it round with a punch/screwdriver and a hammer and usually it will get moving. worst case just destroy it, clean the worst of the bits out of the hole once done but dont worry any small leftover plastic will get eaten up and spat out by the clutch without doing any harm.
2. Get the engine up to temp, at least 80 degrees on the MFA oil temp display and turn the engine off
3. Hook up your timing light as per the manufacturers instructions. I recommend inductive type lights as they are much easier to hook up, there is no need for fancy ones with advance wheels or rpm displays but feel free to spend more money if you want

4. Start the engine again, switching on the 'secret' MFA rev counter if desired as per the idle/co setup info below, then disconnect the blue temp sender
5. Rev the engine between 2k-2.5k, and point the timing light into the hole, the 6 degree BTDC mark should now be aligned. if not loosen the dizzy clamp bolt and turn the dizzy as required. If the dizzy wont move switch the engine off and try to free it, see below.
6. Rev the engine to 2.5k again, then reconnect the blue temp sensor. The timing should advance by about 30 degrees
7. If the timing only advances about 20 degrees, loosen the knock sensor bolt and re-tighten to 20 Nm (15 Ibf ft) and repeat. If there is still no improvement, check the wiring and if this is ok replace the knock sensor.
If there is no advance at all, check the wiring to the blue temp sensor.

Idle/CO:
1. Get the engine up to temp, at least 80 degrees on the MFA oil temp display
2. Turn the engine off and remove & plug the intake side of the breather pipe between the 'flying saucer' on the rocker cover & air intake.
3. Hold the MFA button in and turn the ignition on, off and on again, then start the car. This puts the MFA into a 'secret' mode. Put the MFA switch to setting 2 and press the button till you get the digital rev counter, position 2 I think.
4. Unplug the blue temp sender and rev the engine cleanly over 3k rpm 3 times, letting it return to idle each time.
5. Check the RPM display, adjust the idle with the idle screw on the throttle body to about 850-900 rpm. Clockwise to decrease, anticlockwise to increase.
6. If you have a CO meter, block off one of the tailpipes (i use a load of duck tape) and shove the probe up the pipe. Follow the manufacturers instructions, and set the CO to about 1.8ish using the allen screw on the MAF, clockwise to increase, anticlockwise to decrease. NOTE: You must take the reading when the rad fan ISN'T running, once it cuts in it'll throw out the reading a little!
7. Reset the idle to 850-900 rpm if it has changed, and adjust the CO accordingly, repeat till you get 1.8 at 850-900 rpm.

If your dizzy is really stuck my method is to take the clamp off completely, then dig out all the crud round the lip in the bottom with a pick or thin screwdriver. Drown it in proper penetrating oil (not WD40!!) then leave it for an hour or so. Come back and add a bit more, then clamp some mole grips round the dizzy shaft. gently tap the grips left and right, you aren't aiming to brute force it, rather shock the dizzy loose. if its still not moving add more oil and leave it for another hour. Once freed, plug the blue sender back in and restart the timing procedure again.