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Til Forrer
Til Forrer, Chevy Technician
Category: Chevy
Satisfied Customers: 75
Experience:  17years experiance , 7 ASE's including L1 100%certifyed in multiple GM fields
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K2500..hard starting when it is very humid, at times after a rain

Resolved Question:

I have a 1997 K2500 with a 5.0L. It has a recurring problem not starting, or hard starting when it is very humid, at times after a rain but not when the humidity is low. It seems ignition timing related for sure. When this first happened (over 2yrs ago) I replaced the cam position sensor, no help, and then the crank position sensor that seemed to resolve the problem. It has happened since multiple times, again I replaced the crank position sensor but suspecting it was not alone the problem. It happened again a month ago, I pulled the sensor, cleaned it and the crank pulley area with the trigger tabs of oil and any grime, back together and started fine. But, again today very humid, the problem back. I cleaned and re-seated all 5 connectors to the ECM computer and sensor, no help.

Any hints?

Please Help
Sincerely,

Dean
Submitted: 4 years ago.
Category: Chevy
Expert:  Til Forrer replied 4 years ago.
hello and welcome to Just Answer, i can certianly try to help ., if its due to humidity or anything wet then the first thing i would try would be replacing the distributor cap and rotor , also check for corrosion on the ignition coil and wire ,
Customer: replied 4 years ago.
Hi Til,

The first times this happened It wasn't firing at all, I changed the cap, rotor, and wirings before I dug further since I had not til then ever changed them. When that didn't help, I pulled the coil wire and saw plenty of spark. Then I checked the spark with a dummy plug at two different cyl and saw the spark there as well. That is when I started looking for something causing the computer to trigger the spark at the wrong timing. As I detailed above, I have replaced the cam position sensor once, and the crank position sensor twice. I am very suspecting of a systemic design problem in the area of the crank position sensor or its interface to the computer. Once at Autozone I had noted some documentation about an update and improvements in the later sensor I bought. I was hoping that an "improved" better sensor was going to solve my problem, no such luck.

An history of trouble with the crank position sensors, or the pulley area it senses position from?

Thanks ....

As a fun aside, I wish the motor had real mechanical position timing at least to the distributor, my old farm tractors are more reliable than this K2500 ever.

Dean

Expert:  Til Forrer replied 4 years ago.
well im going to suggest starting with the basics to get an idea as to what is happening when it doesnt start , when it dosent start check the fuel pressure and spark at one of the spark plug wires. lets see if you have both , fuelpressure should be (56-62 psi) key on engine off .
Customer: replied 4 years ago.
Hi Til,

As I've tried to detail, the problem is intermittent and seems related to humidity. If it is having trouble, when you crank it over say a dozen times, some will have no fire, some miss fire and kick out the starter. It seems like it is setting a crank position incorrectly, almost at random. If I persist say 20 or 30 tries, I've gotten it to catch apparently close enough ro start, running rough for a bit, dark rich exhaust probably from the earlier tries flooding it. At this point it may be OK until you leave it overnight (if humid). I have checked the fuel pressure when it won't fire, and it has been between 60 and 62 duting cranking. I just checked it, and it was 62 at key on (I hear the pump) and 52 during the run idle.

Intermittents like this are the worst. I am hoping I am not alone, and that experience with the 305 will have some hints.

Thanks again for any hints...

Dean Killam
Expert:  Til Forrer replied 4 years ago.

cycle the fuel pump and watch for the pressure to drop on the guage , does it seem to go to 62 and stay there or does it drop off, this would be alot easier if you had access to the engine date in the pcm , but since we dont , if you have intermittent spark that could be a sign of a crank sensor issue but you wont have injector pulse either, have you tried a ignition module yet and check the connector on that, its located right next to the coil .

Customer: replied 4 years ago.
Hi Til,

Thanks for the quick response. The fuel pump pressure on key on, pre crank is stable at the 62, no drop until crank, and then, to 52 (if it runs.) I understand the fuel pressure issue. In an unrelated failure years ago, the engin was running rich on the rear cyls as shown with black rear plugs and the pressure was not correct, it turned out to be a bad pressure relief valve under the intake manifold and throttle body, lots of fun. THis 305 has been continued nightmares for me. I have not replaced the ignition module, but connections look OK. Since the truck is sparking, all beit out in timing I expected it would be more likely the sensor area, or control to the ign module, just guessing on my part. Parts are always expensive, looking for hints on most probable before I replace things and/or tear things apart.

Thanks again.

Dean

Expert:  Til Forrer replied 4 years ago.

well if the ignition module ls failing then it could cause a loss of spark or improper spark , since you've had problems with the crank sensor hows the wiring down there, if the terminal tension is loose then moisture will effect the sensor, same with the ignition module , without the proper tools to check terminal tension its hard to do but use a small paper clip and see if the drag of pulling it out is the same on all terminals , its very unlikely that it is a mechanical issue so don't pull anything apart just yet , check these things as best as you can and let me know

Customer: replied 4 years ago.
Hi Til,

FYI, I work in Electronics for my $ with an EE degree, so I have tried to double/tripple check the connections. I don't have a full manual, so I don't know if the connections from the crank sensor to the ECM computer are a direct wired connection in the hrness, or if there is another junction/splice somewhere. I did clean the connectors at the crank sensor and computer with spray electronic cleaner and dried them out with copmpressed air. I don't see any corrosion or damage, but have not tried to test the concact finger pressure. If it is a bad connection from the computer to the crank sensor, maybe I should check it with an ohm meter? Do you know the pin connections at the computer side?

It would be nice if the system reported some OBDII code to indicate a missing pulse or something, but nothing. Are newer GM products better in that regard?

Enough for me though for the day, ...

Thanks again,

Dean Killam
Expert:  Til Forrer replied 4 years ago.
the crank shaft sensor at the vcm is connector 1 pins28 and 31the power supply has a splice as well so you might want to check resistance from the fuse to the sensor , hope this helps Object Number: 52645  Size: FS
Customer: replied 4 years ago.
Hi Til,

The schematic and detail does help, at least giving me the connection points to check from. The cold front rolled by last night, so ... no humidity today to force the problem, but I will be measuring the resistance and checking the power connections.

Thanks, Dean
Expert:  Til Forrer replied 4 years ago.

not a problem . hope you find an issue that may have been over looked,

Til Forrer, Chevy Technician
Category: Chevy
Satisfied Customers: 75
Experience: 17years experiance , 7 ASE's including L1 100%certifyed in multiple GM fields
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Til Forrer
Til Forrer
gm auto technician
75 Satisfied Customers
17years experiance , 7 ASE's including L1 100%certifyed in multiple GM fields