Cranks, but no start in the morning

Tiny
GZ01VJ
  • MEMBER
  • 2002 CHEVROLET S-10
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 156,000 MILES
I have a 2002 Chevy S-10, 4WD, 4.3L engine and occasionally it cranks, but will not start in the morning. I used a spark checker and showed good spark. So I put a fuel gage and found the fuel pressure went to 62 psi when I keyed the ignition on and the engine tried to start briefly, but I observed the fuel pressure starting to drop during cranking and the engine would not try to start. This engine has the poppet valve injectors inside the intake manifold and needs at least 55 psi to start. The fuel pressure hold good (62 psi) if I key on with no crank and may drop 10 psi in ten minutes. I also looked inside the throttle body when it would not start and could not smell any fuel. Once the engine does start it generally starts fine the rest of the day. Not sure how the PCM powers the fuel pump during cranking (crankshaft sensor?), Because the oil pressure sensor only provides a signal to the gauge according to the wiring diagram. No code are in the PCM, so could it be the fuel pump, fuel pressure regulator or cranshaft sensor?
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Wednesday, August 24th, 2011 AT 1:41 AM

8 Replies

Tiny
RASMATAZ
  • MEMBER
Next time it refuses to start don't wait for it to make up its mind-do below immediately to determine if its fuel or spark problem

http://www.2carpros.com/articles/car-cranks-but-wont-start

Get a helper disconnect a sparkplug wire or 2 and ground it to the engine atleast 3/16 away from ground-have helper crank engine over-do you have a snapping blue spark? If so-you have a fuel related problem, Do you hear the fuel pump come On when you turn key on? If not check fuel pump fuse and fuel pump relay if okay-check the fuel pressure to rule out the fuel filter/fuel pump/pressure regulator and listen to the injector/s are they pulsing or hook up a noid light. No snapping blue spark continue to troubleshoot the ignition system-power input to the coil/coil packs,coil's resistances,cap and rotor,distributor pick-up coil, ignition control module, cam and crank sensors and computer Note: If it doesn't apply disregard it and keep testing
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Wednesday, August 24th, 2011 AT 1:49 AM
Tiny
GZ01VJ
  • MEMBER
I do have good spark that will jump my tester and from what I stated my fuel pump comes on when I key on. The poppet injectors do not have a electrical connector for the noid light and are in the intake manifold and can't listen to them. Everything above you mentioned I answered already in your generic answer
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Wednesday, August 24th, 2011 AT 2:08 AM
Tiny
RASMATAZ
  • MEMBER
When the fuel pump relay de-energizes after the 2secs and engine cranking the fuel pump and oil pressure sending unit powers the fuel pump-
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Wednesday, August 24th, 2011 AT 10:21 PM
Tiny
WRENCHTECH
  • EXPERT
Try unplugging the oil pressure sender. I have seen them short when the pressure builds.
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Wednesday, August 24th, 2011 AT 11:43 PM
Tiny
GZ01VJ
  • MEMBER
As I stated my oil pressure sender only provides a signal for the gage. It is located under the distributor and only has one wire attached. I confirmed on a wiring diagram circuit for the fuel pump system the oil pressure has no effect. I wondered if the PCM powers the fuel pump when it sees rpms from the crankshaft position sensor. Maybe the sensor does not work right when cold?
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Thursday, August 25th, 2011 AT 1:25 AM
Tiny
WRENCHTECH
  • EXPERT
Yes, it does power the pump when it sees tach signal but since you said you have spark, that means it's seeing a tach signal. The relay has a prime connector coming off of it. Monitor that when cranking and see if you have power. If you don't, try applying power and see if it runs. The symptoms you are describing really don't make sense. If the computer is priming the pump and you have spark, it should be powering the pump during cranking also.
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Thursday, August 25th, 2011 AT 1:33 AM
Tiny
GZ01VJ
  • MEMBER
Not sure where the prime connector is, but I will see if I can find it. The fuel pump relay is in a underhood box with the fuses, maybe it would be around this area. Your point about having spark so the crankshaft sensor is working is a good point
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Thursday, August 25th, 2011 AT 2:24 AM
Tiny
WRENCHTECH
  • EXPERT
The diagram shows it has a prime connector but it doesn't show a wire color so it may be an extra slot in the relay box or a short lead nearby. The wire color is usually red.
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Thursday, August 25th, 2011 AT 2:29 AM

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