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Crank position sensor, aka RPM sensor, was introduced for the 1989 model year. It drives spark timing, so failure could give bad knock. Sensor produces a high frequency signal needing a shielded cable for clean transmission to the computer. Cable is similar to TV coax cable with a foil outer shield and similar outer diameter. Cable plus sensor is supplied as a single unit.
Cable deteriorates over time. First causes intermittent ignition failure and finaly, no-start. Many of that vintage are due about now. The one in our wifemobile died about 2 years ago; the ones in my own daily driver and also in our '90 was on their way out but still functioning within the past year when I had them replaced.
Read about checking it in the FAQ (700/900 cars of same vintage have the same sensor), at
http://www.brickboard.com/FAQ/700-900/EngineSensors.htm#Crank_Postition_Sensor
To see it, stand at passenger side fender with hood up. Use a flashlight to peer in at engine, behind and below the rear of valve cover, where it disappears behind the firewall lower edge. You'll see the approx. 1/4 inch diam. cable going down onto top of sensor. Wire may be frayed, shielding damaged, etc. FAQ also gives procedures for testing it electrically but in your case I'd also test with a timing light or similar, looking for variations in spark timing.
Definitely, if the cable going down into top of sensor location looks bad, get the thing replaced. I can't guarantee that's your problem but if the cable looks bad it will shut you down before too long, anyway.
My shop mechanic was able to detect ignition "anomalies" with spark timing testing gear when my own '89's sensor was on its way out. Still firing OK but timing wasn't quite right - I suspect the timing was drifting + and - of a stable value.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.
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