Lately I see a lot of starting issues in the forum and am experiencing them myself.

Looks like maybe some may be traced back to the crank position sensor.
I'm in the midst of a course an have no time for the next couple weeks to dig into the issue but wondering if someone else has some experience with this.

I work for a car restoration company part-time and his engine shop guy had me a hint which may help.

He said: You may want to check your crank position sensor. They come in a plastic fixture which over time loosens a bit and the distance between the magneto and the crank wheel increases just so much when warming up. The magneto also looses some of it's strength due to the exposure to hot and cold.

When cold the gap is smaller and the cars usually just start fine. Once running, the crank wheel spins at a speed which keeps the signal strong enough for ignition and ECU to work fine. When you shut it down and then try to restart when hot (or warm) the starter is not able to turn it at a speed high enough for a proper signal to ECU or ignition (depends on models) and it won't start.

Makes sense to me and is also the reason it does not throw any malfunction codes.

We may have to figure a way to measure the signal output from the sensor to see if this could be the problem. I think it may fix a lot of peoples problems for little money.

 

The PCM continuously monitors the CKP signal when cranking and running.

CONDITIONS FOR SETTING A CRANKSHAFT POSITION SENSOR DTC:
The measured change in engine speed is more than 1,000 RPM during a 125 ms time period, or the measured engine speed is 0 RPM and 4 or more camshaft position sensor pulses have occurred over a period of 2 to 3 seconds

Basically if there is a problem, it will set a DTC.
If the crankshaft has excessive end play, it can set a DTC too.

You can check CKP duty cycle while cranking, and you can check for negative pulse at the PCM and at the ignition coil & module.

 

I have to check on that one. I still think that the CPS is a cause of a lot of headaches when it comes to no-start or dies in idle if vacuum is not concerned.

Unfortunately just started a formation course and won't be able to check until mid February.

 

Found the time to fix it. Hopefully for good now.

INFO
The specific vehicle 4.3 V6 4x4 with TBI injection has no crank position sensor as it derives all positioning info from the ignition distributor as far as I found out. This is applicable to all V6's on the first gen. with TBI as well as with MPFI Engine versions VIN Z and VIN W.
The ignition module provides the correct sensor input for ignition and injection timing. ONLY the 4I engine has a crank position sensor (CPS).
Despite this, at two different supplieres for spare parts the CPS is listed in the 4.3 l V6 engine parts which can be confusing.

The specific problem I had was developing over time. One day the engine would not start again when hot. Let it cool and it would start just fine and for a couple weeks the symptom didn't come up again. Then it started again, this time it would over a couple weeks develop into a consistent behavior. Starts fine when cold but not when warm or hot. Then it would simply refuse to start altogether but would eventually start and run fine intermittently. Drove me nuts and as the vehicle was at my body shop for repairs I gave them a generous tip as they had to push the Blazer nearly all the time.

Description of the problem and solution.

SYMPTOMS
Key to RUN - Fuel pump would pump until pressure is achieved and stop. (Correct behavior)
Key to START - Engine would crank fine (new battery) every 2 - 3 seconds it would start and die right away while continuing to crank.
MIL stays ON during cranking which is normal as some parameters have not been reached yet.
Engine would try to start every 3 - 5 seconds and die right away again.

CHECKS
Ignition fired - INFO - check ALL spark plugs! This in the end was my problem.
Fuel was injected during cranking. (Spray from injectors)
Now it seemed we had all three: air - fuel - ignition but obviously something was still wrong.

EXPANDED CHECKS
Removed spark plugs - all black (heat rating 6) Two of them showed arching at the tips where they connect to the ignition wires which could indicate bad connectivity from the ignition wires.
Removed distributor cap - Central pin had quite a bit of buildup, the 6 electrodes in the cap showed quite a bit of wear and buildup consistent with the mileage of 170'000 and that still being the first distributor cap.

REPLACEMENTS
Distributor cap with a new one
All spark plugs
Ignition coil

CLEANINGS
Distributor finger - gave it a thorough cleaning to remove all the excess carbon deposit from the central pin, sanded the fingers tip avoiding to shorten it.

PROBLEM SOLVED
Running fine now. I'll run it for a while again to see if any errors pop up again.

ADDITIONAL MEASURES
I'll order and replace all ignition wires and plugs.


Good luck to all with the same problem.

 

I assumed from your first post that you had a 2nd gen with a crank sensor, woops. Crank sensors were used on the 4.3L from mid 1995 through 2005. Prior to that they had a pick up coil and ignition module mounted to the distributor.


The pick up coil and the ignition module often fail intermittently, especially when hot. The pick up coil generates a pulse and sends it to the VCM. The VCM calculates injector and ignition timing and sends a modified pulse to the ignition module, which in turn fires the ignition coil, (injectors are fired separately). If/when the pick up coil fails, there will be no spark, and no injector pulse. If/when the ignition module fails, there will be no spark, but the injectors will continue to squirt fuel. Some of the auto parts stores have machines that will test ignition modules, but the results are not reliable, and can not be trusted.

 

Captain Hook,

I've been quite busy lately and only found the time to tinker with the machine two weeks ago. Thought coming back an posting the experiences made will save somebody else some headaches. Lesson learned - RTFM - then start working your way through the simple stuff as always. Measure, check, take apart, clean, fix, reassemble - hopefully - run...

The trouble with these non-start issues is that you will have the MIL "on" anyways while cranking, depending on vehicle and electronics. The first gen. has the MIL "on" as long as the system is in "starting" so you would need a TECH I or OBD scanner to actually see the DTC's thrown.