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Tue Sep 26, 2006
My 1991 Golf (Blue) has been developing a problem lately. As the engine warms up, it seems to lose power. The other day I took a long detour on the way back from work, and the power loss was so bad that I was worried I wouldn't make it home. This type of symptom in a Digifant VW is normally due to a problem with the spark advance functionality. The spark advance is controlled by the ECU (there is no mechanical advance system in the distributor) based on feedback from the coolant temperature sensor and the knock sensor (among other things, I'm sure). My fear was that perhaps my Digifant control unit had gone bad. Armed with my trusty Bentley manual, I started some troubleshooting the other night, and I found that the spark advance functionality from the ECU is working just fine - the testing involved a baseline ignition timing reading (which should be within factory spec) compared to a timing reading with the coolant temperature sensor connected (the baseline reading is taken with the sensor disconnected). There should be approximately a 30 degree difference, and that is exaclty what I found. I was relieved, because a remanufactured ECU for my Golf (Digifant II, non-California model) costs close to $300.

I did not suspect my coolant temperature sensor, because I replaced it last summer (but it is easy to check, in any case). My suspicion immediately fell on the knock sensor - not only is it a key piece in the spark advance/retard functionality, but mine is physically in bad shape. Unfortunately, the Bentley book does not give any tests for the knock sensor used on RV code engines (which mine is), it only gives a test to verify the wiring from the sensor to the ECU. The plastic housing around my sensor is cracked and broken, and sections of the insulation covering the wire are cracked and missing (leaving exposed shielding). It's clearly the original sensor... and after 15 years, I think it has paid it's due.

In the interest of experimentation, I ran the car all day today with the knock sensor disconnected and it was a marked improvement. I visited my buddies at Atlantic and ordered a new knock sensor, which should be in tomorrow. Blue is going to be running pretty sweet before long, methinks.

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