Good luck! Since you've shown interest I'll see if I can dig up a complete description of what the crank sensor learn does and how the misfire detection works.

 

Thank you

 

Here's how the misfire detection works (slightly simplified)
Misfires in OBD II Engines: How to Detect

Because the misfire detection depends on minute changes in crank angular velocity as determined by pulse width of the crank sensor at specific rotational times - it needs to be calibrated for your particular crank sensor / engine combo in order for it to work correctly. This is the function of the crank sensor relearn (AKA CASE Relearn). It reads what is normal for your engine, stores it in the PCM, and if there are changes over a certain % in angular velocity under certain conditions it flags that as a misfire and turns on the light. Crank sensor relearn is done by triggering a test built into the PCM and requires bi-directional control (something cheap scanners will not do)

This article also explains why we pay so much attention to verifying fuel system and electrical system when misfires are detected. Worth a read IMHO.

The cam sensor function is to tell the PCM which cylinder is firing at the moment. In our engines this is only useful for misfire detection (which cylinder is missing), and to tell the PCM which fuel injector to pulse next (and when) in Sequential Fuel injection. If you disconnect the cam sensor completely engine will still run but misfire detection will be disabled and fuel injectors will be pulsed in two banks instead of sequentially. Cam sensor which is mounted in the distributor also has the ability to tell us if the rotor is pointing exactly at the right terminal at the right time - and this can be an important thing for ignition performance. This is the Cam Sensor Retard value (AKA CMP Retard) we talk about and it should be set to 0 +/- 2 degrees for the ignition to operate without misfire and is adjusted by turning the distributor. Note that Ignition timing of the engine is not adjustable and is determined solely by the crank sensor and the PCM - turning the distributor does nothing to the ignition timing. The distributor only distributes the spark, and a distributor is only necessary because we use a single coil ignition system. CMP retard can be checked with a couple of cheap scanners I know of. Car Gauge Pro will do 1998+ Blazers (Android, $8.95) and Dash Command will do 1996+ Blazers (Android, iPhone, $10 + $10 per vehicle for Enhanced PIDs). For the Android get the BAFX Bluetooth OBD2 adapter as sold on Amazon for $21.00. For iPhone you're on your own, but you will need a Wifi OBD2 adapter.

 

Wow Thanks, I will probably have to read this a few times
to get brain wrapped around it.

 

I think how these engine control systems operate is pretty interesting stuff. Lots of crank sensors get replaced needlessly, and without the crank sensor relearn done - additional issues are created.

In any case any time an engine is replaced, or crank sensor is replaced, or crank sensor/timing cover is even disturbed the relearn becomes necessary.

I can attest that misfire detection is so sensitive that even excessive bearing clearance can trigger it. That is what my creampuff low mile Blazer was doing when I first bought it - bearings were very loose apparently due to coolant in the oil from leaky LIM gaskets. LIM gaskets had been replaced but SES light was still apparently on all the time with a P0304 misfire code. Someone in the past disconnected the SES light wire from the instrument cluster and actually put a piece of tape on it. Lucky me. I figured this all out as my saga unfolded.

I never let the engine warm up completely and never put a scanner on it when I bought the vehicle. Will never make that mistake again. Even though bearings weren't loose enough to audibly hammer, the P0304 code was being set as pending immediately upon startup even though the engine was running very smooth. Crank sensor relearn did not fix it. Only thing possibly pointing me to a major engine problem was a lifter tick at idle on a hot engine. Still oil pressure was above the minimum spec at the back of the lifter galley (just not at that lifter in the front of the engine)

This false misfire code on my Blazer thoroughly confused and prompted me to study misfire detection and buy my own software/hardware to trigger the crank sensor relearn (I bought HP Tuners). Undoubtedly my own experiences is why this remains an interesting topic to me.

One thing I believe should be done if one has a misfire code being set but the engine runs well is to first check the function of the misfire detection system by inducing a dead miss on select cylinders and monitoring the misfire counters using a capable scanner (Car Gauge Pro, Dash Command, Torque Pro, etc will all do this). This can be done by installing a grounded plug wire into the distributor cap at various cylinder positions and starting the engine. That cylinder should immediately start counting misfires on the scanner. You can move it to several different cylinders. If it doesn't count correctly then misfire detection is simply not working and must be fixed before it can be trusted. I could do this test on mine when it had the original problem, and it would never detect and count misfires correctly on the cylinder that was being shorted out (even after doing a crank sensor relearn).

p.s. I still have my original crankshaft position sensor! Just a different rebuilt engine!

 

When I replaced my engine with a junk yard motor I didn't have any issue with the crank sensor. I just plugged it in and have had zero issues with it.

 

Goes to show you there is always an exception. Does the misfire detection actually work? Can you induce a miss and the system recognizes it? Seriously, I would like to know for my info.

 

OMG....I am Pissed to the MAX....

My Monday didn't start well for me
7am got my Blazer towed to shop

Shop guy went to pickup my engine
I paid for 3 weeks ago and he was told
they couldn't find it- in other words they
fu*k*n sold it ...so needless to say I am Livid
right now.

They said "we will refund your money"
needless to say I gave them a mouthful and said
damn right your going to refund me..!!!

Got my refund and now I am trying to find another engine ..!!!

 

Les is there a RPO Code or build date cut off I can verify this with ?
I just found a donor 2001 blazer, with a rebuilt engine -
also does it mater if it's 2wd.
Is there a picture or something of the 2 wire sensor ?

I also found a 1999 but it has 180k miles/ hmmm rather get the top one

I am waiting for both guys to get home and send the VIN #.

Bill

 

You need to look at the knock sensor that is mounted on the block just to the passenger side of the distributor next to the oil pressure sensor. You need one that has only one wire going to it. Does not matter if is 2WD as long as you are willing to switch oil pans. I am betting that 2001 will have a 2 wire knock sensor, but you should check if you have found one.

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