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Volkswagen Air-Cooled VW: 1952-1979 1976 Fuel Injected Beetle Misfire


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Last Post: Jul 2, 2014 9:43 PM Last Post By: FKH161
1976er

Posts: 3
Registered: 06/28/14
1976 Fuel Injected Beetle Misfire
Posted: Jun 28, 2014 1:00 PM
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I have a 1976 Beetle with a 1600 fuel injected engine. I'm new to the VW world but this Beetle has taught me a lot in an unfun way. Purchased in Feb 2014 it gave me no trouble until April. I started getting a slight misfire under load. I checked the cap and rotor to find a tune up was do. Changed cap, rotor, points, and plugs. Fuel filter had been done previously. Noticed a decrease in mpg instantly, and shortly following the misfiring started again with a vengeance. Installed new plug wires, and condenser. No change. 2nd new cap seemed to fix the problem for four hrs then back to the beginning. I adjusted valves, installed an new coil, tested the cold start valve, the temp sensors, replaced the head temp sensor, replaced the air flow sensor, fried the ECU while performing some checks replaced it with a good used ECU. Still same problem. Tested for vacuum leaks didn't find any indications. Checked spray from injectors and found bad o-rings. Replaced them still no change. Installed a Hot Spark electronic ignition complete distributor that cured everything for a day. Then we come back to the same trouble. Even though fuel pressure and flow is in spec I checked the inside of the fuel tank and found it filthy. Cleaned it all out and put on new hoses and filter but this still has had no effect. The misfire sounds ignition but I see no indications of this. Removing plug wires and injector wires will cause a cylinder to die out but the misfire will continue over top of the dead cylinder. Compression test shows 100psi hot. The misfire is there when the engine is warm and is more prevalent under part throttle to idle conditions. Full throttle it is much less noticeable. It has been known to miss when cold and to clear up after a drive when left idling only to come back in a few minutes. It even will kill the engine at an idle some times. I'm absolutely stumped please HELP.
tcovenant2000

Posts: 1,762
Registered: 12/09/03
Re: 1976 Fuel Injected Beetle Misfire
Posted: Jun 28, 2014 9:32 PM   in response to: 1976er in response to: 1976er
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You tested the cold start valve. What does that mean? Did you test for leaks/drippage? Have you check the fuel pressures? What about O2 sensor?
This very much sounds like it could be O2. Try disconnecting it and see if the issue goes away. It could also be a dripping/leaking Cold Start valve.
Also, without knowing system pressure...hard to say. Could be too high or too low of system pressure.
Thomas
1976er

Posts: 3
Registered: 06/28/14
Re: 1976 Fuel Injected Beetle Misfire
Posted: Jun 29, 2014 12:24 AM   in response to: 1976er in response to: 1976er
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I checked the cold start valve for drips and leaks first by watching it then finally by disconnecting the fuel line to it completely. Fuel pressure is staying at 34 psi. This car has no O2 sensor. A mechanic for older Beetles said that is in spec for pressure. I seemed to have cured the problem again today by running a new wire from the coil to the ECU. Drove around nice. Then I took it for another run tonight and it has returned so bad the car won't idle. I had to throw my hands in the air and walk off.
FKH161

Posts: 7,610
Registered: 12/29/05
Re: 1976 Fuel Injected Beetle Misfire
Posted: Jun 30, 2014 8:48 PM   in response to: 1976er in response to: 1976er
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Remove the rocker arms and place a straight edge across the top of the valve stems... look for a valve stem that sits higher than the others (should all be even). You might have a loose valve seat in one of the heads.

Could be other things though... need an ignition scope, smoke machine etc to confirm all other checks.
1976er

Posts: 3
Registered: 06/28/14
Re: 1976 Fuel Injected Beetle Misfire
Posted: Jul 1, 2014 10:11 PM   in response to: 1976er in response to: 1976er
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Seems like I finally found my problem. Even though I could never get repeatable results by wiggling wires I decided to pull the wiring harness to the ECM out through the fire wall. The protective cover was discolored and so I stripped it and there I found some of the wires melted together. I pealed them apart and taped them up for a test run. Three days of running now with no hiccups after three months of trouble shooting. I think my sanity may return now. Thanks for the suggestions. And maybe this thread will help some other.
FKH161

Posts: 7,610
Registered: 12/29/05
Re: 1976 Fuel Injected Beetle Misfire
Posted: Jul 2, 2014 9:43 PM   in response to: 1976er in response to: 1976er
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Nice that you found the problem!

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