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Wed Jul 16, 2008 7:42 am

I have a question about setting the timing. The Bentley is kind of vague. It shows some special equipment to set the timing. It mentions a timing light but is it timed at 3000rpm or is it set at idle(900rpm) at the 5º btdc mark? '91 2.1L Digifant FI

Wed Jul 16, 2008 8:01 am

VW's special equipment read the engine position/tach pickup mounted in the flywheel flange (this is the 3-pin "mystery connector" that gets asked about a least once a week here by every new owner who goes poking around their engine for the first time) and the #1 plug wire and was able to pinpoint timing to a tenth of a degree, but you can use a strobe light just the same. So, nothing fancy is needed; a normal strobe, you can watch your tach from the back while you tune. Follow the specs in Bentley to the letter: engine fully hot, Temp2 plug disconected, etc. etc., time to the V-notch at 2000 rpm or whatever, blah blah blah.

I long ago dispensed with all that rigamarole, which I found gave inconsistent results because once you unplugged Temp2 there was no way of knowing what the thing would do; rev high, die completely, make me a cup of coffee, steal my beer, anything. I went back to the time-tested method of tuning for maximum advance, and let the rest fall where it will. I couldn't give a fig what the timing is at idle, I want it to be where it needs to be when I put the engine under load.

The max advance spec is 35deg +/- 5 at 3000rpm. I make a mark on my pulley wheel 2" to the right of the TDC dished mark (not the V-notch), which is 35 deg. Then with the engine warmed up and everything left connected, I just time to that mark while holding the revs to 3k.

You'l find for the most part that when you do it this way, the idle timing will fall on the 5deg advance mark anyway. Always road-test afterward, put it thru its paces, and you can try tweaking it a couple degrees more or less advance to what feels best, always listening for pinging (these engines are very hard to make ping, though). Just don't stray far from that 35 at 3k setting, and you can always come back to it if your tweaks don't seem to improve performance.

Thu Jul 17, 2008 12:25 am

excellent reply.

I also thought the Bentley was vague and/or lacked a descriptive timing procedure.

I have set my timing at 5 BTDC and checked it as the rpm increased, However I do not see the timing advance as I would expect. I had a computer problem. The symptoms were, the tach would spike and the engine would die, but would restart easily and run well until the tach spiked again.

A mechanic loaned me his ECU and the spiking and dying disappeared. I bought a rebuilt ECU, but the engine would not accelerate with that ECU and we put his ECU back in and the engine ran well with the AFM disconnected, but would not accelerate when the AFM was connected. I installed a brand new $524 AFM, but that made no difference and the engine did not run well except with the mechanic's ECU and either AFM disconnected. The engine seems to run rich, and I assume that is an AFM function and not a problem except for gas mileage, slight loss of power, and possibly fouled plugs

The bottom line is that the spark does not advance with increased RPM and i don't know why. I assume that the ECU does that, but it is not happening with two supposedly good ECU's.

Perhaps I should put the original ECU back and look for advance. I would expect tach spikes and dying, but maybe I'll see an advance.

Aloha
Tom Powell

Thu Jul 17, 2008 4:40 am

The definitive answer, thanks to Mr. Cent:

http://www.thesamba.com/vw/forum/viewtopic.php?t=261067

Thu Jul 17, 2008 7:04 am

Tom, you need to get an ohmmeter and ring out that entire wiring harness. Something's up, especially on the AFM circuits. Digijet/Digifant are pretty robust FI systems, but the engine wiring at twenty years old is their biggest weakness. If VW had used higher-quality wire we would see a fraction of the FI problems people report. That guy Kyle who does new harnesses has a goldmine on his hands once he gets the Digifant version on the market.