PO336

**96LT1355Z28** · 05-25-2010, 05:30 AM

I finished my new build and went to start the car. It runs but has very rough idle and pops and backfires if you rev it or try and maintain 2-3000 rpm. It's setting a code po336 "crankshaft position sensor circuit range performance". First thing I checked was to make sure all the plug wires were going to the correct cylinders. After doing some searching I've determined it's one of two things. Either a bad CPS or an issue with the wiring or harness for the CPS. Everything is clean and the wires look fine. The second and more likley is the opti has an issue with the low resolution pulse causing the CPS to be out of the acceptable range causing the code. I bought a NIB MSD opti from here and just to be safe I sent it to MSD to be checked and ensure it was within factory specs before I installed it. I've learned there are a few others who have used the MSD opti and it pulled a ton of timing causing all sorts of issues and similar symptoms as mine. I got a new scan tool that I'm still trying to learn how to use but it looks like at idle the car has -30 to -38 degrees of timing! My questions are:

-How do I test the harness for the CPS and what is acceptable?
-Is there a way to check the low resolution signal from the opti before I pull it off and send it back to MSD?

It looks like others have solved this by having their tuner modify the timing around the MSD, I've got a PCM4less tune making this pretty much impossible. Thanks for any help you guys and gals can provide.

**96LT1355Z28** · 05-25-2010, 12:13 PM

Well the sensor has 11.05 volts at the harness, the battery is low from sitting it has 11.75 volts at the terminals. I called PCM4less and he said he has seen this problem many times with the MSD and Accell opti's. He said the only solution he knows of is to get a GM opti. I called MSD who of coarse denies the issue is opti related and insisted if the PCM set a CPS code then that's where the problem is. So now I'm torn, spend more $ on a GM unit and take my chances spinning it to 68-6900 rpm and having a $500.00 paperweight. Or try and get a local tuner to look at it which will likely involve making changes to the tune wasting the $ spent on the mail order tune! DAMN

PS Still very open to any suggestions!

**tleed** · 05-25-2010, 08:30 PM

its not the sensor or the circuit i had the same problem it is the tune. after i had pcmforless modify my tune about 3 times it finally went away. if u have 30 degrees of timing at idle thats more than likly the problem. u should check your fuel trims to make sure your not running to rich or lean and then call pcmforless and discuss your tune. a dyno tune is the best way to have your car tuned a mail order tune is hit or miss on a highly modified engine

**96LT1355Z28** · 05-26-2010, 08:03 AM

I'm considering two options right now. Sent an email to Ed Wright to see if there's anything he can do, or buying a Dynaspark and hoping it fixes the issue.

**Robsquikz28** · 05-26-2010, 06:04 PM

I bet its the tune.

**96LT1355Z28** · 05-27-2010, 02:04 AM

Ed and pcm4less are both suggesting the opti. Both said go with a GM unit or Dynaspark. I talked to Dynaspark and expalined my situation, they're making me a deal on one of their opti's. We'll see what happens.

**96LT1355Z28** · 04-25-2011, 05:00 PM

UPDATE Ed Wright tune and Dynaspark opti fixed the problem. I also had fuel trims that were night and day difference between bank 1 and 2. This was caused by the IAC passage in my 58mm TB. A quick modification the TB IAC air routing and the fuel trims cleaned up.

FWIW I learned that Ed tuned out the CPS so I still can't say for sure what the problem was, I believe it was a multitude of small things.