5.3 intermittent cranks but no start
07-19-2017, 01:23 PM
#1
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5.3 intermittent cranks but no start Anybody else experience this issue? It's a 2006 K1500, 5.3, intermittent problem. Turn the key and it cranks but no start, turn key off and remove, wait 30 minutes, it fires right up. Not every time, mind you, just about once every few days or week. No CEL, no codes. Has fuel pressure to the rail, can hear fuel pump kick on thru the cap when key turned, not sure if it's spark or injectors themselves not firing yet, won't be able to rule out either until it does it again. I'm hoping it's just a loose or shorted wire from the ignition, and not something major like the ECM going bad. Any suggestions what to look at?
07-19-2017, 01:59 PM
#2
If it has a fuel pressure diagnostic port on the front of the rail its pretty easy to just tap the needle and see if there is fuel pressure in the rail after you initial prime the system.
Next, if you turn the key ON and turn the motor by HAND (ratchet) you should hear the fuel pump relay click on and off (and detect fuel pump). If it does not, the crank sensor may be bad.
The engine looks for crank signal -> turns on fuel pump and fires injectors/spark. Some V8 engines will start in "waste spark" or "batch fire" mode, not sure if the LS does though.
From those two easy checks, you will start with basics,
compression test
fuel pressure gauge
pull a plug for the initial start attempt and check for spark
look at all the plugs just for reference since doing compression test anyways
check all the fuses
then if nothing seems obvious,
replace Crank sensor might be a next step. its not that hard to get to usually.
Next, if you turn the key ON and turn the motor by HAND (ratchet) you should hear the fuel pump relay click on and off (and detect fuel pump). If it does not, the crank sensor may be bad.
The engine looks for crank signal -> turns on fuel pump and fires injectors/spark. Some V8 engines will start in "waste spark" or "batch fire" mode, not sure if the LS does though.
From those two easy checks, you will start with basics,
compression test
fuel pressure gauge
pull a plug for the initial start attempt and check for spark
look at all the plugs just for reference since doing compression test anyways
check all the fuses
then if nothing seems obvious,
replace Crank sensor might be a next step. its not that hard to get to usually.
07-19-2017, 03:31 PM
#4
Well, in most ecu, when the crank trigger senses movement it primes the rail. Then, if its able to go batch fire mode, it will light the first landmark it sees on the wheel (either cylinder could be on a compression stroke). So if you are certain the pump is going on, pressure in the rail, the next easy thing to do is pull a plug.
the issue here is that you have an intermittent problem. It might be better to just start changing parts (easy cheap parts) than to diagnose it further. If you pull a plug and see spark, for example, you might not be getting it other times.
you would think the ECU would throw a crankshaft code, though. The no codes is a clue. A cranking engine, with a good crank sensor, priming fuel rail, syncs with camshaft sensor etc... should throw fuel and spark just fine. You can't lose fuel to all cylinders by coincidence just like you can't lose spark to all cylinders unless the ECU plug was loose or some ground pathway was severed (check your grounds and ECU plug).
the issue here is that you have an intermittent problem. It might be better to just start changing parts (easy cheap parts) than to diagnose it further. If you pull a plug and see spark, for example, you might not be getting it other times.
you would think the ECU would throw a crankshaft code, though. The no codes is a clue. A cranking engine, with a good crank sensor, priming fuel rail, syncs with camshaft sensor etc... should throw fuel and spark just fine. You can't lose fuel to all cylinders by coincidence just like you can't lose spark to all cylinders unless the ECU plug was loose or some ground pathway was severed (check your grounds and ECU plug).