Misfire Diagnosis: Dirty CPS

My little blue buzz bomb has been driving like it’s up to it’s bumper in peanut butter. This has been going on for 2 or 3 weeks, but generally driveability has been declining for the last 2 months. Initially I thought it was a fuel problem which wasn’t totally out there but my mechanic drove it and was 90% sure of an ignition problem on the primary side.



  • initially idles rough, runs strong once the temp. gauge starts to move (“normal” for this car)
  • runs great for 5, 10 minutes
  • at some point driving or idling CEL (1221 o2 sensor) comes on
  • at the same time sounds and acts like a cylinder is dropping
  • loss of power especially at low RPM, poor fuel economy
  • let it sit for 20-30 minutes
  • it will initially start up like the cylinder is missing with CEL still lit
  • after 20 seconds the CEL clears and it drives fine for another 5, 10 minutes
  • non-conductive spark tester thing seems to indicate it’s cylinder 5 that is misfiring

The happy ending to this is that it’s finally fixed! But I’m going to relate the story in 3 sections: what I’ve learned, what fixed the problem, and then I’ll talk about everything else I tried to fix the problem.

4 Easy Lessons

  1. Start At The Ends: To experienced trouble shooters this is probably so obvious that it’s stupid. But the “ends” of a system – where stuff is input and where stuff is output – are the best places to look for solutions. If you turn on your tap and no water comes out you wiggle the faucet, check for hot and cold and then you check to see if the rest of the house has water, right? The problem is likely at one of those two places. More complicated systems tend to be the same: check the ends, then work your way backwards from the output end.
  2. Start With The Easy Fixes:┬áIf something just needs to be cleaned, adjusted or can be eliminated with a visual or low-tech inspection then start there. Because isn’t it awesome when something is an easy fix?
  3. Diagnostics Over Parts: It’s better to run tests than throw parts at a problem. It’s cheaper and doesn’t introduce new variables. You’ll always have people telling you to replace this or that part but that’s wasteful. Save you money and energy for the really hard problems.
  4. Cleaning Is Maintenance: Cleaning your car is a chance to inspect stuff. Typically I stay out of my car’s underside because I don’t get it onto jack stands very often but I will do better in that area now.

Problem Solved: Clean the Crank Position Sensor (CPS)


Above: CPS location and closeup.

So I’ve heard of the CPS before. Heard people talk about:

  • the wires being frayed
  • adjusting it (early model e30s only, so I’ve learned)
  • it can test o.k. and still not work
  • if you have spark and fuel it must be working (confused yet?)

Generally I was confused and unsure of what to do with it, though I knew it was in scope for the problem I was having. Here’s the layman’s explanation of what the CPS does (source):

Any one of the things I’ve replaced could cause the symptoms I’ve noted, but replacing parts did nothing because the parts weren’t actually failing or bad. Other things that were blamed by people I asked:

  • CPS
  • DME (it’s a computer, almost untestable without the right equipment)
  • oxygen sensor
  • cap and rotor (even though I just replaced it)
  • HT wires

Most people want to throw parts at this kind of problem, so I’m glad it didn’t come to that for me. That’s the end of the story, so here’s some photos taken during the various fixes:

  1. Replaced the plugs because I was checking them anyways. This was #6 which looks bad but would still work in that condition.
  2. Checked the plug wire set and found that the #5 wire had a cut in it.
  3. Off to the Pick n Pull to find another wire set. This one was not perfect but it was a match and the only one available. e30s are not appearing at the dismantlers very often, right now.
  4. Dirty but nothing I can’t clean up. Discovered that some of the wires were not in good condition but had what I needed. The entire harness was only $10.
  5. Quick solution to see if the new wire makes a difference. It did not.
  6. New cap & rotor and cleaned up that harness. The wire cover is now black!
  7. Tested the coil. Seems out of spec.
  8. Borrowed a coil from a friend. Of course it made no difference.

Image Gallery

One Comment

  1. […] values so far out of spec it can’t be interpretted. I suspect that driving around with a nagging misfire for almost a month fouled the sensor. The sensor is only a couple months old, which is a shame. […]

Leave a Comment

You must be logged in to post a comment.

February 8, 2013 mechanical, repairs