75 VW FI engine won't start

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I have a 75 VW beetle engine (in a '69 body, sort of, it's a MG-TD kit) fuel injected. My problem is that it quit on me (it was running fine) and won't start. It turns over just fine. The ignition checks out. The fuel pump runs with good fuel pressure. I'm going through the trouble-shooting process now. I have a used ECU that is supposed to be good, and changing that out results in now change. Likewise with the double relay for the fuel pump.

A specific question I have is on the thermo time switch. I pulled off the connector and checked the resistance between the two terminals on the switch; I get an open circuit. The garage temperature is about 60 degrees F, so this doesn't seem right, it seems that the switch should have some resistance as this is how the fuel injection allows gasoline through the cold start injector to give extra fuel when the engine is cold.

Any advice out there; shouldn't that switch be closed at 60 degrees?

[busdaddy]

You should be able to start it without the cold start system, that's meant for days when it's close to freezing or below. The TTS shouldn't show continuity between terminals, but it should always show it between G and the body of the switch, it should also show continuity between W and the body when it's cold, as these units age the temp range sometimes changes, luckily we never drive these in winter now so a non functional cold start system isn't really a big deal.

But why isn't yours even trying to fire?, have you done any maintenace or work in there recently?, could a wire or hose have been disturbed or knocked off?, does it show any signs of life if you pour a tablespoon of gas into the S boot?


Here's the manual in case you don't have it: https://www.thesamba.com/vw/archives/manuals/afc_fi_11_1974.php
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[[email protected]]

Thank you for your help on this. I injected a little gas and tried again to start; no pops or anything! I had done a pretty good job of trouble-shooting the ignition and everything checked out except that the spark seemed pretty weak.

I got a new condenser and tried that just now; no change. I'll check that out to verify that I've got spark, then try again. I think you put me on the right track; if the ignition is working right there will at least be some firing, even if something else isn't working right.

I'll re-post when I get this figured out.

Thanks. Ed

[busdaddy]

Ignition sounds like a good suspect, don't rule out a failed ignition swtch as well, OK when static but cuts out when cranking.

But there's also a possibility it's horribly flooded, a leaking injector, blocked return line so the fuel pressure is off the charts, or even a poor contact between the temp sensor #2 (TS2) and the head, or in the wiring between it and the ECU.
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[[email protected]]

Fuel pressure checks out good.

I tested the ignition while cranking; sparks from the high tension wire to ground.

I can usually smell a trace of gasoline after cranking but not starting; pretty sure it's not flooding.

Temperature sensor has the proper resistance.

It might be a couple of days before a have to to continue.

Ed

[dopeboat]

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I was able to spend some time on the engine today. Following advice, I concentrated on the ignition. I squired started fluid into the intake and... nothing. This is after new points and condenser. I could get a good spark by opening up the points manually, but it did seem a little weak, and it didn't spark every time I opened up the points.
I have a timing light with an induction pickup, so I tried that on the spark plug wires. The light flashed OK when I put the pickup on the wire out of the coil, but it was irregular, so it wasn't sparking every time the points opened. I had pulled out one plug and had that sitting on metal, and that had a very weak spark, and with the pickup on that wire I got some light flashes, but again, intermittent. I put the pickup on another cylinder, and... nothing, no flashes.

It seems clear to me that the coil has failed; some sections of the coil shorted out so the output voltage is reduced. It seems odd to have a partial failure, but it also seems possible. I went to the local parts store and they didn't have anything in stock, and what they offered to order in didn't seem right, so I ordered the new coil on-line (from JBugs).

BTW, my compression gauge is too short to get a good seal, but every indication is that the compression is good.

Have you seen a coil failure like this? I'll update as soon as I get it in.Thanks for your help.

[busdaddy]

Anything's possible considering it might be 50 years old, fingers crossed a fresh one makes a difference.
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[Root_Werks]

If you squirted some fuel down the intake and not even a sputter or fart, seems like you're on the right track looking deeper into spark.

Hopefully the coil is the culprit.

-Dan
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[dopeboat]

[[email protected]]

I'm happy to give an update. I got the new ignition coil today and installed it. The engine popped a couple of times when it first turned over, and started right up! It's running a little rough, but I can handle some tune-up.

The lesson is that a coil can fail and yet give some spark, enough that you may think it's working. The failure must be a shorting out between some windings of the coil inside; reducing the number of active windings reduces the output voltage. Note that it takes a higher voltage to arc in the air under compression that it does to arc in atmospheric air.

The induction pickup on a timing light is a good indicator of whether or not there is an actual spark at the spark plug.

I thank you again for your help and encouragement.

[busdaddy]

Great news!, thanks for following up!
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