The Air/Fuel Ratio is stuck at 1:1 and the Engine RPM shows 0?

NOLA JAX

MEMBER
2009 GMC CANYON
2.9L
4 CYL
2WD
AUTOMATIC
151,876 MILES

Recently had to replace the crankshaft position sensor (CKP) and attempted to perform a crankshaft variation relearn (P0335). The relearn repeatedly fails. After testing/replacing the ECM, testing the sensor wiring (including the camshaft sensor/wiring), I have discovered the relearn appears to fail due to the OBDII scanner showing "Engine RPM 0" The test appears to fail because the OBDII routine senses the truck is not started and is not idling.

The camshaft and crankshaft sensors are measuring ~750 RPM each. When charted/graphed together each sensor follows the other. The speed sensor (on the transmission) correctly reports the speed of the truck on the road.

When I get to the last page of the crankshaft variation relearn special function in the OBDII, the error message returns something to the effect of 'start engine and idle' before the relearn works. But, the truck is idling.

Swapped OBDII scanner/programmers out. Both return the same error message.
After digging around, every system on the truck passes OK.

Mechanically, the truck has been fully serviced, with no major changes other than brakes, struts, shocks, etc. Etc. Etc.

However, the Air/Fuel Ratio is stuck at 1:1 and the Engine RPM shows 0 (zero).
With the truck idling (poorly) the RPM is about 750.
I was under the impression the Engine RPM reading is derived from the camshaft and crankshaft sensors. It appears that is not the case.

What I have determined is the relearn test thinks the truck is not even on and idling.

Any help would be appreciated!
Thanks,
Stan

7 Replies

KEN L

MASTER CERTIFIED MECHANIC
43,070 POSTS

I know you said you replaced the ECM but it still sounds bad, but let's re-check the wiring from the sensors to the ECM here is a guide that can help also I have the crankshaft and camshaft wiring diagrams below so you can see which wires to test and to make sure the wiring connector is correct and in good shape.

https://www.2carpros.com/articles/how-to-check-wiring

Make none of the wires have resistance to ground.

Here is more information on the code:

The P0335 code is a generic diagnostic trouble code (DTC) that indicates a problem with the crankshaft position sensor (CPS).

Check out the images (below). Please let us know how it goes

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Monday, July 3rd, 2023 AT 2:06 PM

NOLA JAX

MEMBER
13 POSTS

Hey, Ken-
Thanks for the in-depth reply. Having determined the OBDII scanner reads 750 RPM on the camshaft position sensor (CMP) and the crankshaft position sensor (CKP), I surmise the wiring has no faults. Otherwise, there would be sporadic readings. The readings are rock solid. As a side note, there is a B1000 code in the Vehicle Communication Interface Module (VCIM). There is another code, but it relates to something to do with the On-Star system. I do not subscribe to the On-Star network; however, I thought it was odd there were codes presented. I surmise the codes are related to just the VCIM module. I have come to the conclusion, as you state above, the issue is in the ECM. The one I replaced it with was an aftermarket one. I discovered it was for a 2008 4WD Canyon and I have a 2009 2WD Canyon. It was not programmed for my Canyon. Since then, I have learned more about how these ECMs need to be programmed. Below, are some reports I ran from the OBDII. The Mass Air Flow (MAF) issue was reported because I forgot to reconnect it. That sensor has since been replaced. So far, I believe what I need to do is get an aftermarket ECM programmed for my make, year and model, based upon my VIN, from a reputable company.
I welcome your thoughts and even if you have recommendations for vendors who sell programmed ECMs.
Thanks for your support!
- Stan

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Tuesday, July 4th, 2023 AT 7:43 PM

KEN L

MASTER CERTIFIED MECHANIC
43,070 POSTS

Thank you for the images. It looks like you are on the right track for fixing the vehicle. Please let me know what happens after you get the new PCM installed. The p1000 code should go away as well.

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Wednesday, July 5th, 2023 AT 12:19 PM

NOLA JAX

MEMBER
13 POSTS

Currently waiting upon delivery of ECM replacement. Will follow-up once installed.
- Stan

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Thursday, July 6th, 2023 AT 1:14 PM

KEN L

MASTER CERTIFIED MECHANIC
43,070 POSTS

Sounds good let me know, have a good weekend.

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Friday, July 7th, 2023 AT 9:52 AM

NOLA JAX

MEMBER
13 POSTS

Update:

We finally figured out the issue.

Partly due to GM manual typographical errors, poor engine design and part tolerances.

A replacement ECM had no effect.

The CKP sensor was out-of-tolerance. Approximately, 0.023" between the old one (original manufactured install) and the replacement.

We installed a third replacement, and the engine RPM was instantly read.

According to GM's service manual, engine RPM is calculated either by the MAF or the CMP.

We discovered the CKP (which was the original guess) was responsible for the ECM reading RPM.

With an Engine RPM reading 0 (zero), a crankshaft relearn could not be performed. When the relearn is started, the diagnostic/program tool read the engine not running. But, it was actually running.

Even though the ECM diagnostic read out showed the CKP was reading the proper RPMs, the Engine RPM reading was zero. Therefore, no relearn could happen.

According to documentation which shows the CKP parts included 'spacers' there were no spacers in either box of the second CKP ordered and the third CKP ordered. What are the odds?

Therefore, if you are having issues attempting a CKP relearn, look at the diagnostic tool and see if Engine RPMs are reading AND the CKP is reading incremental counts. Those two data points are required.

Even though we eventually did not need spacers, as the third CKP sensor we installed was within tolerances, if you ever run into this issue, look to 0.020 to 0.030 spacers.

Hope this helps someone.

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Thursday, July 27th, 2023 AT 5:35 PM

KEN L

MASTER CERTIFIED MECHANIC
43,070 POSTS

Yep, some aftermarket sensors cause more problems than they solve. Glad you could get it fixed, thanks for letting us know. Please use 2CarPros anytime we are here to help.

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Friday, July 28th, 2023 AT 10:43 AM