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Diagnosing diesel no-start problems

Low oil can cause a no start? It just might.
Thursday, January 23, 2014 - 08:00
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From time to time I get phone calls asking for information. I really love the calls that start something like this: “My diesel pickup won’t start. I parked it at my house last night, and this morning it’s dead. It won’t start, what’s wrong with it?”

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This scan data demonstrates how Ford can skew the data when trying to use the ICP pressure for analyzing a cranks/no start. Pay close attention to the ICP voltage, which is always correct. Until the ICP voltage gets to .8 volt, the injectors will not be pulsed by the FICM. This is common to both the 7.3 and 6.0 engines.  

I always get a chuckle from calls like this. Obviously, the vehicle owner is having a problem, but with the information supplied, there is not enough information to even come close to guessing the cause of the complaint.

My diesel experience goes back to the 1960s when things were simple. All that was needed to get a diesel engine started was the proper amount of heat in the combustion chamber and the proper amount of combustible fuel, injected into the combustion chamber at the proper time. Is it any different today with the electronic diesels? Not at all; the basics still apply and by keeping those basics in mind, a seemingly hard no start problem can be simplified.

Starting with the basics, let’s discuss the two most basic needs of diesel engine operation: proper combustion chamber heat and the proper amount of fuel injected at the proper time. I find it easier to understand big problems with this approach, by understanding the basics of how a system works. I will try to break “diesel no start” down to the small problems, then flow the small problems together to create a work flow that will be easily understood.

Using the PID data, it is easy to see the engine is cranking fast enough at 177 FPM. The FICM sync and Sync both show yes. This tells you both the CKP and CMP are in proper sync, and the FICM is in proper communication with the PCM. The IPR percentage is steady at 85 percent and the ICP voltage is only at .27 volt. Everything is here except ICP voltage. With this data only three things are left to test. Is there oil available to the high pressure system, is there a leak in the high pressure system, or is the high pressure pump not doing its job. 

The Need for Heat
When asked what makes heat in the combustion chamber, most techs will reply “proper compression” or “proper glow plug operation.” These are great answers, but when I’m faced with this question, I always start with the proper cranking speed. I might move on to the compression and proper glow plug operation, but without the proper cranking speed, the engine compression is a moot point.

Most diesel engines need a minimum cranking speed of 150 rpm. If the starting system is not capable of turning the engine fast enough, do some testing and find out why. The technician might need to start with an inspection of the batteries, which will include a load test of the batteries, some voltage drop testing of the negative and positive sides of the starter circuits and maybe a current draw test on the starter. If all these tests pass, then check for something binding inside of the starter or the engine. Until the engine will crank fast enough, any other testing is a waste of valuable time.

Proper compression is also a much needed part of the “no start” problem analysis. There is a time and a place for a mechanical compression test, but that test is way down at the bottom of my diagnostic test list. On the late model diesel engine, accessing a point to get the compression can be a time consuming task and there are easier ways.

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