Volkswagen: misfire at cruise, air flow meter, smog inspection

Question
Mr. Wolff
I own a 1978 VW Bus, stock 2.0L FI engine, automatic trans. Recently I've developed a misfire/stutter at cruise, between 55 and 70 mph. What's odd is, at higher speeds or anything below 55, there is no problem. If I accelerate or decelerate through this range, it runs fine. It only acts up if I try to keep a constant speed within the range mentioned above. The air flow meter checks out, fuel pump, injectors and cold start valve are ok as well. Ive replaced cap, rotor, plugs and wires, and I have a Pertronix electronic ignition installed as well. I rebuilt the engine myself about 3 years ago (stock except for ignition), and though the egr functions properly, I have it plugged until smog inspection time rolls around. I'm an ASE certified master tech (auto and heavy truck) as well as L1, but this issue is driving me NUTS! Any help/advice would be greatly appreciated. Thanks

Answer
Hi Selvin,

A couple of things to check.

1) Although the Air Flow Meter checks out ok, the misfire may be caused by the contacts being worn away from the air flap contact.  I would suggest taking off the black plastic cover on top of the air flow meter, and checking the contacts for continuity.  55-70 is where the most wear occurs on the circuit board.

2) The next thing to do is to replace the #3 cyl temperature sensor.  It's less than 20 bucks and can (and does) create a myriad of problems, including intermittent signals to the ECU causing a shutter at certain speeds.

Let me know how you make out with these items!

Happy Motoring,

Ron