5.7 Vortec intermittent long crank --> crank no start, backfires, clunking

[Schurkey]

if you do not have one of these or know some one that has one of these I would get one easy safer way to check for spark at spark plugs https://www.ebay.com/itm/374223151089?epid=1128500261&itmmeta=01HYPG6CB6ARZ0FDMNWWK35CR5&hash=item5721701ff1:gLwAAOSwLhdjAswt&itmprp=enc:AQAJAAAA4MhnnRKo/Q9JQssPrU2caYf1mFbGUf/NDYqOIElXgAVP/WgEJe9uvf19oVuMrHkMekn/bv7LVZMgCc3UwP3Qfjq5VP9CHG6Z5/4gdXz+qYnIQ1aw4e7KXGNGtA9RhOP3ZftqniDrIIpDoPgOKo89DFs3f4YxLrwzDAMekEYFj3WgBt636j1qKrLlgh+uFf7eJpbThumupfTQTspQs/MFMv0QCUmCyD6MtzvBn6o--roaHhJ/0EF1ZQBA3DFOqVhZa24QvEJQzhvKA/ZFvzY/njyQhmD8adzETaTWvZNCQh4m|tkp:Bk9SR9rFmdD1Yw

I have a VERY STRONG preference for a spark tester that forces the spark to jump a calibrated gap instead of making a light-bulb glow.

The whole purpose of the spark is to jump the gap at the end of the spark plug, when the plug is under compression pressure. A weak spark can make a light-bulb glow, but perhaps not jump the spark plug gap when the plug is under pressure.

A spark plug in open atmosphere doesn't load the coil enough. Therefore, a spark tester calibrated for HEI, where you can SEE the spark jump, is the better way to test coil power.

There's multiple designs of spark testers that have an actual spark gap, this is my favorite style:
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

 

[99xcss4]

I have a VERY STRONG preference for a spark tester that forces the spark to jump a calibrated gap instead of making a light-bulb glow.

The whole purpose of the spark is to jump the gap at the end of the spark plug, when the plug is under compression pressure. A weak spark can make a light-bulb glow, but perhaps not jump the spark plug gap.

Therefore, a spark tester calibrated for HEI, where you can SEE the spark jump, is the better way to test coil power.

There's multiple designs of spark testers that have an actual spark gap, this is my favorite style:
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

thanks I did not know that just thought it was better then sticking the plug in the plug wire and touch it to ground or stick a screw driver in the plug wire and touch it to ground

 

[slow_c1500]

I have started to also get occasional backfires while cranking it, as well as occasional clunks. No start still. Pretty sure it has to be something ignition or timing related. My neighbor mentioned a worn starter, but would that make it backfire? I’m starting to think it’s a slipped timing chain, sticking valve, or worn dist gear.

 

[slow_c1500]

So I recently found out about a trick where you unplug plug wire 3 and if it starts right up, you got a bad distributor cap. Well guess what IT STARTED RIGHT UP after I did that. Blue distributor cap on the way. I would be lost without the wizards in this forum ahahaha

 

[99xcss4]

So I recently found out about a trick where you unplug plug wire 3 and if it starts right up, you got a bad distributor cap. Well guess what IT STARTED RIGHT UP after I did that.

that is pretty interesting to me never new or heard about that before

 

[Schurkey]

you unplug plug wire 3 and if it starts right up, you got a bad distributor cap.

that is pretty interesting to me never new or heard about that before

That bit of wisdom has been floating around this forum for a year or more, but doesn't get a lot of publicity.

I'm thinking that #3 is the conductor that bends around the central coil-wire conductor in the distributor cap. The plastic insulation there is under the most stress, due to the coil wire firing four times per crank revolution while the conductors for the plugs fire once every two revolutions. The plastic fails progressively from the central coil wire button outward...until it punches-through to the #3 conductor.
Photo shamelessly stolen from...somebody.

You must be registered for see images attach

or this photo, stolen from a recent post by Road Trip:

You must be registered for see images attach

Removing the #3 plug wire opens that circuit again, so that the spark is more-likely to go where it's supposed to, except of course to #3 cylinder. An engine running on seven cylinders still runs better than one firing only #3

(Or have I mis-numbered the conductor bending around the coil wire button?)

 

[Frank Enstein]

I’ve tried priming the pump with the injectors pulled out and they didn’t seem to be leaking but I could try again at some point. How would I test the CTS?

You would test the resistance across the sensor at a known temperature vs. what the manual says.

 

[Road Trip]

That bit of wisdom has been floating around this forum for a year or more, but doesn't get a lot of publicity.

I'm thinking that #3 is the conductor that bends around the central coil-wire conductor in the distributor cap. The plastic insulation there is under the most stress, due to the coil wire firing four times per crank revolution while the conductors for the plugs fire once every two revolutions. The plastic fails progressively from the central coil wire button outward...until it punches-through to the #3 conductor.
Photo shamelessly stolen from...somebody.

You must be registered for see images attach

EDIT: I'm 99% sure that this photo was originally shared by @L31MaxExpress, and it was
related to a failure he experienced a short distance from home. I liked the dramatic failure on display,
a memorable photo!

or this photo, stolen from a recent post by Road Trip:

You must be registered for see images attach

Removing the #3 plug wire opens that circuit again, so that the spark is more-likely to go where it's supposed to, except of course to #3 cylinder. An engine running on seven cylinders still runs better than one firing only #3

(Or have I mis-numbered the conductor bending around the coil wire button?)

Schurkey,

You got it right, #3 is the run closest to the coil wire button. Here's a bird's eye view
of the Vortec distributor cap showing both the run in question and the cylinder #:

You must be registered for see images attach

@slow_c1500 , nice job coming back and closing the loop with what you discovered and
the final fix. This is exactly what should always occur in order to turn these threads from
helpful/thoughtful suggestions from the talent in the forum into no-kidding proven fixes for
other GMT400 pilots researching similar issues after the fact.

Thank you for this. Well played --

 

[slow_c1500]

Should I be worried about timing or anything being off, since i had cranked it a little bit with it only running on one cylinder? Or will i be good to go once the new (quality) cap is in?

 

[slow_c1500]

Should I be worried about timing or anything being off, since i had cranked it a little bit with it only running on one cylinder? Or will i be good to go once the new (quality) cap is in?

bump, gonna put the new cap in today