97 5.7 runs on 7 cylinders but won't on 8

[Jrivers0688]

I have a 97 k1500 with 5.7 with maybe 220k on the truck ran fine then sat for about year then wouldn't start throws a p1345 code sounds like a intake valve is stuck open when cranking so I pulled a valve cover off and was cycling the motor and it started it valves look to be fine won't start with all plug wires hooked up but unplug #3 and starts right up I checked the plug wire to make sure it wasn't grounding out not it has new cap rotor cam sensor and I'm kind of at a loss on what to look for next any advice would help I'm a diesel mechanic and forgot about ignition systems years ago

 

[L31MaxExpress]

WAG, if it is not a defective new cap, it has a blown head gasket between 3&5 or a stuck valve in #3. Obviously #3 has a problem, diagnostics should start there. A bad cap could be causing #3 to spark at random times, similarly a blown head gasket between 3 and 5 could ignite #5 during the intake/compression stroke.

 

[Sean Buick 76]

Not a bad guess. It should start even if one plug wire was dead. Something else is going on in there.

 

[1998_K1500_Sub]

won't start with all plug wires hooked up but unplug #3 and starts right up

When you say "won't start" what exactly do you mean?

Does the starter fail to turn the engine continuously?

Does the starter crank continuously but the engine never fires on any cylinder?

Or... what?

Reflect on @L31MaxExpress post and read-on.

SB Chevy firing order is 1-8-4-3-6-5-7-2

@L31MaxExpress suggested that a bad head gasket could ignite #5 as the combustion blows-over from #3.

Considering the firing order, #3 would be approaching TDC and #5 would have sucked almost a full A/F charge but is not yet on the compression stroke; in fact it's approaching BTC just prior to compression.

Igniting #5 (from #3) at this stage of its cycle, near BDC, might cause enough pressure to stop the engine from turning, e.g., the starter can't overcome the downforce of the (unintended) combustion in #5...

... maybe

 

[L31MaxExpress]

When you say "won't start" what exactly do you mean?

Does the starter fail to turn the engine continuously?

Does the starter crank continuously but the engine never fires on any cylinder?

Or... what?

Reflect on @L31MaxExpress post and read-on.

SB Chevy firing order is 1-8-4-3-6-5-7-2

@L31MaxExpress suggested that a bad head gasket could ignite #5 as the combustion blows-over from #3.

Considering the firing order, #3 would be approaching TDC and #5 would have sucked almost a full A/F charge but is not yet on the compression stroke; in fact it's approaching BTC just prior to compression.

Igniting #5 (from #3) at this stage of its cycle, near BDC, might cause enough pressure to stop the engine from turning, e.g., the starter can't overcome the downforce of the (unintended) combustion in #5...

... maybe

I was thinking the result would be more like pops, bangs and fire blowing out of the throttle body, more than stopping the starter.

Then again the information posted was minimal and I was grabbing straws as to how unplugging a plug wire might effect an engine running. I have seen the burned through cap and even a defective new cap multiple times. I have also seen the roasted head gasket which was also a roasted head and block from flame tunneling. That TBI engine would start, but constantly backfired into the throttle body and sounded like a diesel when it ran.

When the last cap burned through on the van, it started misfiring and lost power at highway speeds, then stalled. Coasted off the highway and it was just spinning over with the occasional random pop out of the exhaust and throttle body. I pulled the engine cover, pulled a plug wire to check spark. Fired right up, went to plug the wire back in and immediately stalled out. Yanked the plug wire again, fired right back up, I threw it in gear still on 7 and took off toward the house, I was only about 2 miles away. It ran until I was about a block from the house at which point it also burned through the rotor causing it to quit running with a massive backfire out of the exhaust, so loud the police were called.

 

[Jrivers0688]

When you say "won't start" what exactly do you mean?

Does the starter fail to turn the engine continuously?

Does the starter crank continuously but the engine never fires on any cylinder?

Or... what?

Reflect on @L31MaxExpress post and read-on.

SB Chevy firing order is 1-8-4-3-6-5-7-2

@L31MaxExpress suggested that a bad head gasket could ignite #5 as the combustion blows-over from #3.

Considering the firing order, #3 would be approaching TDC and #5 would have sucked almost a full A/F charge but is not yet on the compression stroke; in fact it's approaching BTC just prior to compression.

Igniting #5 (from #3) at this stage of its cycle, near BDC, might cause enough pressure to stop the engine from turning, e.g., the starter can't overcome the downforce of the (unintended) combustion in #5...

... maybe

So with all 8 plug wires hooked up it turns over and never fires but pops out the intake it also seems to rotate after I let off the starter which is why originally I thought it has a value stuck open the cap and rotor seem to be almost new the rotor seems to be burnt a little and the top looks like the coating has started to de laminating

 

[Jrivers0688]

Compression on cylinders 1 and 5 are both 150psi cylinder 3 was 150 the first time then only 110 after that even with a little oil in the cylinder all cylinders hit top Compression on the 3rd stroke lowest Compression on cylinders 1 and 5 are 110

 

[Keeper]

Isn't #3 the one that runs across the vortec cap, and can get fried?

I'll be back. @Road Trip just did a write-up on it somewhere recently.

edit:

What makes these trucks destroy distributor caps/rotors?

I have one truck in particular (1999 C1500 Sub) that will ruin a distributor in about 6 months. Also, it seems to really have a craving to do it at the most ridiculous times. My first cap was a brass Autozone Durasomething. All the others have been AC Delcos, with aluminum. I'm taking a wild...

www.gmt400.com

From post #26

Guess what? In order to sort out the spark plug wires according to Left and Right banks, the crab cap was
created. And the failure trend that I'm starting to look for is where the #3 spark plug path crosses the entire cap
and is routed right next to the center terminal. (where all 8 sparks enter and are coupled to the spinning rotor)

This looks to be a highly stressed area from the design. If the manufacturer isn't utilizing the very best high voltage
insulator materials in the cap, it seems to burn through in the area shown in the photo below:

You must be registered for see images attach

 

[Road Trip]

EDIT: Reply collision w/Keeper -- but we're pulling in the same direction, so here it is anyway. :0)

I have a 97 k1500 with 5.7 with maybe 220k on the truck ran fine then sat for about year then wouldn't start throws a p1345 code sounds like a intake valve is stuck open when cranking so I pulled a valve cover off and was cycling the motor and it started it valves look to be fine won't start with all plug wires hooked up but unplug #3 and starts right up I checked the plug wire to make sure it wasn't grounding out not it has new cap rotor cam sensor and I'm kind of at a loss on what to look for next any advice would help I'm a diesel mechanic and forgot about ignition systems years ago

WAG, if it is not a defective new cap, it has a blown head gasket between 3&5 or a stuck valve in #3. Obviously #3 has a problem, diagnostics should start there. A bad cap could be causing #3 to spark at random times, similarly a blown head gasket between 3 and 5 could ignite #5 during the intake/compression stroke.

Hello Jrivers0688,

That is a real specific failure footprint that you have described. Guess what? Since you are driving
a '96+ Vortec 5.7 with the 'crab cap', we've identified a weakness particular to this design.

It seems that the center conductor (where all 8 sparks are delivered to the spinning rotor underneath
burns through the plastic insulator to the spark plug pathway routed closest to it.

Guess what? That's the #3 cylinder path! And if the spark intended for a different cylinder
jumps through and fires the #3 cylinder while the intake valve is still open, then boom the
charge lights up, and sneezes through the open valve and throttle body.

Please remove your cap, and see if it has any discoloration similar to the one below:

You must be registered for see images attach

NOTE: This cap was removed because after sitting for awhile the truck it was on also would not start. (!)


And here's a Bird's Eye view of the #3 cylinder spark routing from above.

You must be registered for see images attach

Especially if pulling #3 spark plug wire solves the no-start problem (as you said) ...but
pulling the spark plug wires on other cylinders doesn't make the same improvement, then
as crazy as it sounds your 'new' cap is obviously causing the sparks intended for the rotor
to take the path of least resistance to the #3 cylinder. And again, if the mistimed spark
occurs while the intake valve is open you are going to hear about it via the throttle body.

Give it a close look, and if you find anything, please take a pic & post it here.

Happy Hunting. If it will run on 7, we should be able to get it to run on all 8.

 

[L31MaxExpress]

My rotor burn through was not obvious from the top of it either.

You must be registered for see images attach

You must be registered for see images attach