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

Discussion in 'Technical Q&A' started by dave80gtsi, Mar 26, 2005.

  1. dave80gtsi

    dave80gtsi Formula 3
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    Nov 3, 2003
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    Dave Meredith
    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
     
  2. thecarreaper

    thecarreaper F1 World Champ
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    Sep 30, 2003
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    i have no clue on the tach, but may i ask if your car has been sitting? perhaps its a fuel and or fuel injection issue. i would not drive it until the problem is solved. 20 miles leaves pleanty of time to damage stuff. best.
     
  3. matteo

    matteo F1 World Champ

    Aug 1, 2002
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    I have to agree with Mike.

    It's fuel or ignition related. Tach is just a gauge.
     
  4. Steve Magnusson

    Steve Magnusson F1 World Champ
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    Jan 11, 2001
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    In the F tech literature I've reviewed, I've never come across a reference that (missing) tach operation would reflect back into the ignition system operation (and I think your's would be the first report of it here if it did). I'd go with Daniel's hunch first -- are you sure both banks are actually firing at the plugs? Has to be the first area to check if some of your cylinders are working (since you have a single fuel supply system). If all the plugs are firing, check if the advance behavior of the ignition is quasi-reasonable or not.
    Are you catless -- any information or indication of cat overheating? One other easy thing to check (if you're "calibrated" from before) is to just put a hand in front of the exiting exhaust while trying to rev the engine to gauge if you might have an exhaust blockage somewhere (causes the same symptom as "a big wad of rags crammed into the air intake").
    Good luck with the hunt...
     
  5. Ricambi America

    Ricambi America F1 World Champ
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    ^^ Above, Steve referenced a post that I accidentally deleted...

    Plugs, wires, coil. That's there I'd start. It sounds like it's running on less than 8.
     
  6. Glassman

    Glassman F1 World Champ
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    I would start with what was the last thing you did to the car before it started doing this.
    If it was removing the Tach and Spedo then I would think the problem is related. I had an XKE that just plain would not start with the tach disconnected.
     
  7. dgrperformance

    dgrperformance Formula Junior

    Feb 28, 2005
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    Oceanside, CA
    Did you check to see if the cats are coming apart?

    Check all the C.I.S. fuses and the main relay

    Due a fuel pump volume test( specs are listed in the book) if you don't have a Ferrari book get a VW or Mercedes book numbers are usually pretty close

    If all is good pull the injectors but leave them attached to the hoses and jump the fuel pump fuse. with the fuel pump running slowly pull on the throttle plate and check the fuel injector spray pattern.

    Depending on how comfortable you are with C.I.S. there are plenty more checks.

    Ignition is pretty staight forward either it is sparking or it's not and it is either advancing or it's not. if both look good than it is a fuel problem or clogged exhaust.

    Good Luck!!!!!
     
  8. dave80gtsi

    dave80gtsi Formula 3
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    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
     
  9. tbakowsky

    tbakowsky F1 World Champ
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    Sep 18, 2002
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    Make sure none of your wires that go to the tach are touching! A bad tach has been knowen to take out a digiplex.

    I had a boxer do this excat thing to me while I was on a road test. I was driving along and noticed the tach needle jumped around a couple of times, and the the car started to run like crap. I pulled over and removed the tach and everything went back to normal.

    The didiplex sends the tach signal to the gauge. But if you have grounded out a wire, or the main tach wiring is touching against somthing, this could be your problem.

    If you have the owners manuel for the car, there is a wiring diagram in there showing you the digiplex circuit. Have a look at that, and you may beable to find you problem.
     
  10. dave80gtsi

    dave80gtsi Formula 3
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    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
     
  11. dave80gtsi

    dave80gtsi Formula 3
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  12. dave80gtsi

    dave80gtsi Formula 3
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    "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?"
     
  13. geekstreet

    geekstreet Karting

    Feb 7, 2005
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    Ooops! Now it's time for some driving to put that smile back!

    No, what you describe sounds about right. The engine is operating under a no-load condition remember, so 1/4" throttle opening would still give it plenty of air to get some decent rpm in neutral as there's very little resistance to the engine gaining speed. At 2" open you'd be collecting the valves from all around the garage!!

    Time to enjoy!
     
  14. Mike C

    Mike C F1 Veteran
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    Aug 3, 2002
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    You may have lost one bank. Could just be one of the TDC sensors (that happened to mine).
     
  15. Nick

    Nick Formula Junior
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    Oct 31, 2003
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    Pull the dist caps & make sure that you don't have a ignition rotor sitting at the bottom of the cap, I have seen that happen on 1980-1982 injected 2 valve 308's. It sounds like you did loose a bank of cylinders & its running on only 4.
     

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