Truck engine will not start

JNETRICK

MEMBER
2000 GMC C3500
7.4L
V8
2WD
AUTOMATIC
190,000 MILES

Truck will start and run on starting fluid. Fuel pressure at schrader valve is 55 psi during initial key on and cranking. Power supply to Injectors, but injectors are not pulsing. Injectors will fire if grounded. Camshaft position sensor has been changed.

The spark plugs are firing and the engines runs on starting fluid.

If the crankshaft position sensor is defective so as to not signal computer to fire the injectors, would it not also prevent the computer from firing the spark plugs?

Thanks, Patrick

3 Replies

JIS001

MECHANIC
3,411 POSTS

Have you read off the Engine control module for fault codes? Using an advance scan tool, when cranking are you reading an RPM signal? If there is no RPM signal the injectors will not pulse since it does not know the engine is cranking over so it does seem like the crank position sensor is faulty.

Another way to check this is to hook up a lab scope to the cam and crank sensor and look at the waveform. But if you do not see an RPM signal on the scan tool then most certainly your crank sensor is faulty. Let us know about the RPM signal first. What are you reading when cranking over.

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Wednesday, June 12th, 2019 AT 1:00 AM

JNETRICK

MEMBER
2 POSTS

Thank you,

I found no fault codes and have no check engine lights. I will check again for RPM signal.

Respectfully, are you able to answer my initial question or did I unintentionally miss your response?

Putting this another way, if the Crank sensor is defective to the point of the computer interrupting or stopping the injector pulse, will the computer not also at the same time interrupt or stop the spark plug spark?

I have spark plug spark and the engine will run on starter fluid. It seems to me that if the Crank sensor is defective, the computer would logically take out both the injector system as well as the spark plug system.

I realize that I can test the Crank sensor before replacement, I just wanted to get your take on my question before going any further.

I'v seen the same question posted on line, read through numerous forums on this subject but have not found as yet an answer to this question posted.

Thanks-Patrick

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Thursday, June 13th, 2019 AT 12:08 PM

JIS001

MECHANIC
3,411 POSTS

Hello Patrick and sorry that I did not fully answer your question. The answer would be not all the time. Spark is controlled by the ignition control module. Attached an image below. The ICM is triggered by the cam sensor input in the distributor. Since you have good fuel pressure and there is spark but no injector pulse then the crank sensor will most likely be faulty since it does not see the engine turning over. Here are a couple of links on how a crankshaft sensor works and how to test injector pulse and what are some of the things that will cause no pulse like a bad computer or faulty wiring and so on.

https://www.2carpros.com/articles/symptoms-of-a-bad-crankshaft-sensor

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

If you have any other questions please do not hesitate to ask and let us know if you are able to see an RPM signal while cranking engine over or not and I hope I was able to give you a better explanation.

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