CRANKS WON'T START

  • Tiny
  • audley
  • 2002 Chevrolet Tahoe
  • V8
  • 4WD
  • automatic
  • 240,000 miles

2002 tahoe 5.3, 48 psi fuel pump pressure, spark at plug wire, installed crank positioning sensor won't start, cranks a little faster and sounds as though it wants to start, but won't. Battery is up and there is plenty of fuel. I don't want to start putting any more sensors on at random. Audley

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Tuesday, February 1st, 2011 AT 3:19 AM

6 Replies

  • Tiny
  • Ernest Clark
  • Expert
  • 1,759 posts

You should never replace parts unless you are sure that part has failed. You began your diagnosis correctly, but you didn't finish.

First, is there a check engine light on? If the crank sensor was bad, there would certainly be a check engine light.

The procedure for diagnosing a no-start (when engine cranks) is:
Spark at plug
Fuel (including pressure, injectors firing)
Compression
All related sensors/wiring and computer (although this can be checked first with a scanner, before spark at plug)

You're computer needs to see both a crank speed reading and cam position reading for it to tell the coils to fire a spark to the plug. Because you have spark, I believe your sensors are fine. Also, your computer is good, otherwise you wouldn't have spark.

Since you've checked spark and fuel pressure, use an automotive stethoscope to listen for your injectors firing. You'll hear a distinct clicking sound. If your injectors are firing, the valve timing may be off (bad timing chain). You can verify this by doing a compression test.

If your timing chain has slipped, just replacing the timing chain wouldn't be economically sound, as that would be like putting a band aid on a gunshot wound. The rest of the motor will be just as worn as the timing chain. However, I do not recommend rebuilding this motor unless you want to keep this truck for 10 more years. There are too many low-mile used engines for dirt cheap.

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Tuesday, February 1st, 2011 AT 11:59 AM
  • Tiny
  • hmac300
  • Expert
  • 43,674 posts

Fuelpressure should be 55-62 psi minimum. Looks like you need more pressure, replace fuel filter first and see if it comes up. If not then check regulator and pump.

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Tuesday, February 1st, 2011 AT 6:05 PM
  • Tiny
  • audley
  • Member

Fuel pressure is fine. It is a Vin Z motor which is flex fuel E85and book pressure is 48to 54 psi for year and vin
.

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Tuesday, February 1st, 2011 AT 6:17 PM
  • Tiny
  • hmac300
  • Expert
  • 43,674 posts

Scan engine for codes then and see if it's getting spark. You can go to the DIY section under tune up it will tell you how to check your system. It cold be a timing isssue. It could also be your ignition switch but you'll have to check for that.

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Tuesday, February 1st, 2011 AT 6:51 PM
  • Tiny
  • Ernest Clark
  • Expert
  • 1,759 posts

It isn't your ignition switch. If that were the case, you wouldn't be getting spark at the plugs. Your ignition timing is controlled by the computer. If that's out of wack, your computer would be bad. Sometimes an intermittent fault with your sensors will fail to set a code. Depending on which sensor is faulty, the computer will have to determine it has been faulty for a certain amount of drive cycles or time. If the sensor acts good, the computer will shut off the light by itself.

If a scan doesn't produce anything, you'll need to continue your diagnoses on the systems components the computer can't always read. These would be the mechanical components of the engine.

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Tuesday, February 1st, 2011 AT 7:11 PM
  • Tiny
  • Ernest Clark
  • Expert
  • 1,759 posts

I was wondering if you've solved your issue yet?

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Tuesday, February 15th, 2011 AT 2:51 AM

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