Wednesday, June 1, 2011

RF bleeds can be a stone cold %$@#!

Go team me! (So far....)

About 18 months ago I bought Scooter the VunderVagen, right after the wife stole my truck in the middle of the night. Scooter, otherwise known as a 1994 VW Golf, with a 2.0 liter engine and a 5 speed manual trans. It came a fair amount cheaper than market value, attributed to age, mileage, and a smattering of needed repairs. Making parts by hand to repair the door locks, overhauling the brakes, some sensors and ignition work... I soon had it in decent repair. The best part? Fuel mileage good enough to almost be scary. I know how to drive stick.

Except... one nagging powertrain management problem I have been hunting since the day I bought it. I suspect it was also the chief reason I got the vehicle cheaper.

Occasionally, the check engine light would come on, the PCM would drop into limp-in mode, and it would store a code for cam sensor signal issues.

Limp-in means light on, base timing, mileage drops, power down, and performance poor. It's perfectly drivable.... but not as fun and certainly gets crappy mileage in limp-in mode. By crappy, I mean maybe 25 MPG instead of 35+.

Well today I had a lab scope hooked to cam sensor output, and I was getting occasional weird readings. Looking for a 7 (and a bit) hertz signal, I got it... with intermittent jumps to 29 hertz... and occasionally just utterly crazy readings.

I figured.... bad hall effect sensor in the distributor, and I was glad to have finally seen something I could pin these weird symptoms on. Then.... the aging brain kicked into gear.

Hmm... expecting a 7 hertz signal at idle from a hall effect sensor with a single window trigger wheel, but getting occasional bursts up to 29 hertz on the signal wire... and 7+ a bit IS 29 when multiplied by 4 and it's a 4 cylinder engine.... and... and... and........

I took my lab scope lead and probed around the distributor cap.... and %$^$%#@ if I didn't find a 29 hertz signal right above the wiring harness going to the cam sensor!

I pushed the harness away and shoved a wadded up paper towel between it and the distributor. Both the test drive and the drive come were clean, with no more symptoms. It runs like a different car.

Tomorrow I will case the wiring harness and make a grounded RF shield to go around the cap. With luck.... I just found the last nagging drivability issue this car had.

I am.... pleased with myself this evening.


Rev. Paul said...

That's a good bit of detective work, right there. Congrats!

og said...

You should indeed be pleased with yourself. Nicely done!