Is the tach -required- for the engine to run?

Greetings to all. I took the GTSi out for its first run of the spring this afternoon, and would like your collective thoughts on a situation that I now find that I have.

Basically, the engine runs quite poorly, with no interest whatsoever in revving. I shift into second, as an illustration, put my foot to the floor, and the engine -slowly- builds up revs. There are no engine pops, hiccups, backfires, or anything obvious like this, and the car idles just fine. No unusual smoke, odors, or anything like that. It is sort of like trying to drive with the emergency brake engaged, or else with a big wad of rags crammed into the air intake, or something like that. I've been all over the car, and none of these obvious extraneous causes are present. I drove the car as best I could for about 20 miles, thinking that whatever it was might clear itself once it warmed up, but no such luck. The car ran nothing like this last fall, and in the process of trying to zero in on a cause, I now have a tentative theory:

At the moment, the tachometer and speedometer are both away at Palo Alto Speedo getting tweaked, so I have temporary holes in the dash where they should be. I don't think the speedo is anything to worry about, but I am less certain about the tach. My theory of the moment is that, if the tach is disconnected, this interrupts an electrical signal loop which somehow results in the Digiplex thinking that the car is running at a much lower rpm than it actually is. And, as mentioned, I can't simply plug the tach back in temporarily so to check this idea.

After hours of thought, this is the only thing that comes to mind.

So, with this preface, does any of this ring true with anyone? Does the ignition system --require-- the tach to be present in the loop in order to properly function?

Would be grateful for any 'B.T.D.T.' comments!

Cheers - DM

 

I am a firm believer in cause-and-effect, and the fact that the car was running wonderfully last fall with the tach in place leads me to suspect that somehow this is the cause. But why? A tach is just a gauge, right, and I see no reason for it to react this way! Is this a quirk of the Digiplex?

I can guarantee that a plugged catalytic converter is not the culprit, nudge nudge wink wink. But it does now run as if that were the case.

One clue, perhaps. I removed the air cleaner so to expose the fuel distributor's "round movable plate" (not certain of the correct term for this, but you all know what I mean). I then reached to the throttle cable and pulled it open to what I would guess would be about 4000 rpm and looked at this plate. It only opened up about 1/4" or so. I then shut the engine off, and verified that it was moving free and easy and that the full extent of travel was about 3" to 4". Perhaps this is the way that they all are, but at 4000 rpm I would have thought that it would have opened up to about 1/2 of the allowable travel, right?

This one is a stumper - grateful for all thoughts and advice!

Cheers - DM

 

Guys - Thanks sincerely for all of your thoughts and comments on this one thus far.

I concur that the lack of a tachometer seems at first glance to be a bit far-fetched in order to justify the engine troubles that I have encountered (that's why I posted that hypothesis here in the first place ... in the vain hope that this was somehow a peculiar little-known quirk of the series which someone in the past might already have discovered). Studying the schematic for the tach / Digiplex circuitry brings forth neither an obvious nor plausible explanation, but on the other hand I can see no other cause-and-effect reason for my current problem. At any rate, prudence would dictate that I should first restore the tach before I spend any additional time trying to troubleshoot the situation, since this will eliminate the tach as a variable and confirm with certainty that the tach is (or is not) the contributing factor.

I am unfortunately several weeks away from being able to reinstall the tach (P.A.S. does good work, but they certainly are s-l-o-w). Since I have no immediate need to get this car back on the road, I won't be spending any more diagnostic time or effort on the car until that time. So, let's simply agree to leave this thread 'as is' for now, and when I have the chance to reinstall the tach I will reactivate this discussion and post the resultant findings.

Cheers - DM

 

"Not Very Well"

 

"How does the 308 run before you discover that, while you were focusing most of your attention on the missing tachometer question, you have accidentally disconnected the distributer-to-coil wire for the front bank?"