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I have an Audi 100 C3 from 1985 (66kw). It has a carburetor, which is tuned automatically.

On random occasion engine stops working mid-drive. This usually happens 2-3 minutes into the drive- engine just stops and electric-generator light comes on to indicate that its not working. When trying to start the car again it starts a couple of times just to stall immediately after. After this only starter cranking the engine can be heard.

When this has happened to me on a road, dripping fuel directly into the carburetor has helped me to get the car running again. Its by no means a sure fire process has taken from 5 minutes to an hour. But eventually the fuel dripping and cranking gets the motor running. Once its running it does not stop and allows me to complete the drive.

Diagnosing the reason behind the problem has been very hard as this occurs randomly and I have not been able to reproduce the problem when I have had diagnosing equipment at hand. Given that dripping fuel directly in to the carburetor gets the car running, I suspected that problem may lie in a faulty fuel-pump (its not electrical, it gets its power off the camshaft). Alas, the problem still persisted after changing the fuel-pump. Now I have a dilemma on what do suspect next, should the entire fuel-system from the tank be under suspicion or should I try exchanging the spark plugs (they were changed a around 5 years ago, but the car has driven 10k max since).

Have you experienced/seem such a problem before? Whats your educated guess on what might cause such an issue?

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  • Have you checked that you are getting a spark at the plugs when this happens?
    – HandyHowie
    Aug 26, 2022 at 8:19
  • Are you sure that adding the fuel fixes it? It you just kept trying to start it without adding fuel would it eventually start?
    – HandyHowie
    Aug 26, 2022 at 8:20
  • @HandyHowie I have not checked for spark when this happens. And I am not 100% sure that its the dripping fuel into the carburetor. Its also a fact that once i drip the fuel, engine is able to burn in, so its seems that least some spark is being created. Originally what drove me down the fuel system diagnostics path was that when I acquired this car it had not been started for a couple of years and fuel-pump was not able to suck the fuel from the tank anymore. Once I was able to get fuel flowing to the pump, everything started working. I thought it might be the same kind of problem now.
    – Kaarel Allemann
    Aug 26, 2022 at 8:39
  • So when you drip the fuel in and crank the engine, the engine fires a little, but it wont start?
    – HandyHowie
    Aug 26, 2022 at 9:32
  • Sounds like a blocked jet or, less likely, sticking float. Have you tried blowing the jets?
    – Chenmunka
    Aug 26, 2022 at 10:51

1 Answer 1

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In one comment you say, "It is an instantaneous stop, same as you would get when turning of the ignition key." That means you should concentrate on an electrical cause, not fuel. It's a carbureted engine, so any fuel supply failure will include stumbling, power loss and coughing -- not an instantaneous stop. The dribbling fuel "fix" is a red herring.

By any chance, do you have a huge key ring with a big mass of keys hanging from the ignition key slot? This can cause mysterious turning off, especially when you hit a bump. The excessive weight of keys wears out the ignition switch. Attempting to start the car after failure re-connects the worn contact, but not every time.

You don't need a big mass of keys to have a defective ignition switch. As a test, once you have the car running normally, while parked, try pushing, pulling and generally abusing the ignition key in its slot, but without turning fully to the OFF or START position. See if the engine quits. If so, you almost certainly have a bad ignition switch.

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  • I got a new ignicition switch and a starter a year ago. It used to be that ignition switch would not make contact sometimes. However I will try the ingicion abuse. But your point on it turning off instantly leads me to suspect faulty spark plugs.
    – Kaarel Allemann
    Aug 27, 2022 at 6:35
  • @KaarelAllemann No, not faulty spark plugs, no way. There is no way that a bad spark plug can suddenly turn off an engine like a switch. One or more faulty spark plugs could cause misfires, power loss, rough idle, poor acceleration. Not sudden stalling. And the fact that you replaced the ignition switch 1 year ago is an especially good hint that you have a bad ignition switch again.
    – MTA
    Aug 27, 2022 at 14:30
  • But why would the bad ignition switch stop the car from starting once the engine has stalled. Btw- no large key chain, just the key and remote for after-market central lock.
    – Kaarel Allemann
    Aug 27, 2022 at 15:14
  • @KaarelAllemann There are multiple contacts in an ignition switch. Only one of those contacts engages the starter (actually the starter solenoid). Other contacts provide power to the ignition system and computer, still other contacts may be there to cut power to accessories such as radio, headlights, blower, A/C compressor during engine cranking. If the contact that powers the computer and ignition system is intermittent but the contact for the starter is good, the car will crank OK, but only start sometimes. And after it starts, it can quit at any time.
    – MTA
    Aug 27, 2022 at 15:36

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