No/Very Low Compression While Cranking

[blackneck94]

I have an engine that is cranking over very quickly and it’s evident that it’s doing so with little to no compression in all eight cylinders. It’s audibly fast to the point where it’s not even worth doing a compression test. No rythym or skips while cranking, just a quick and undisturbed crank as if all eight injectors were removed. It’s also exhibiting a popping sound back through the intake consistent with the speed of cranking, as if multiple intake valves are remaing open on the compression stroke. It quit running when I was driving through a parking lot and gave me no prior warning signs.

My initial and current thoughts are that the cam is not rotating. It has a *keyed* SoCal 3388 cam. I cannot find it, but I was made aware of a post on here awhile back where someone had an issue with a keyed cam that actually experienced his cam key falling out. To the best of my knowledge, I believe this to be my issue.

Has anyone else experienced this with a keyed cam? Or if anyone else has any input on what could cause this, by all means I would like to hear it.

 

[56taskforce]

Probably sheared a timing pin and the cam is out. Hopefully no valves have had contact with any pistons.

 

[DAVe3283]

It does sound like the cam timing is off. Could be the cam key came out, or the pin on the crank sheared and the gear moved. You'll probably just have to pull the front cover and see what you find.

 

[Bdsankey]

It does sound like the cam timing is off. Could be the cam key came out, or the pin on the crank sheared and the gear moved. You'll probably just have to pull the front cover and see what you find.

His truck is built motor with 3388 and keyed durastar. My money is on one of the keys came out and I'd venture to say its the cam. From the video he sent me it doesn't at all sound like the cam is moving. Even if it was say a bent pushrod the truck would try to do something, his doesn't.

 

[Chevy1925]

Key cant come out on a cam, the washer behind the bolt keeps it in place. i would be well more inclined to say the cam bolt came loose.

either way, there is a whole bunch of damage inside....

 

[blackneck94]

Key cant come out on a cam, the washer keeps it in place. i would be well more inclined to say the cam bolt came loose.

That makes a lot more sense. Thanks for point that out!

 

[blackneck94]

Key cant come out on a cam, the washer behind the bolt keeps it in place. i would be well more inclined to say the cam bolt came loose.

either way, there is a whole bunch of damage inside....

I’ll be boroscoping the cylinders, I’ve prepared for the disappointing findings.

 

[Bdsankey]

Key cant come out on a cam, the washer behind the bolt keeps it in place. i would be well more inclined to say the cam bolt came loose.

either way, there is a whole bunch of damage inside....

What's odd is it happened at low throttle in a parking lot and the CP3 still moves.

 

[RMJ_LB7]

What's odd is it happened at low throttle in a parking lot and the CP3 still moves.

So then the cam key came out allowing the cam gear to still spin the CP3 gear?

 

[blackneck94]

So then the cam key came out allowing the cam gear to still spin the CP3 gear?

Have not had a look behind the front cover to see what’s going on, but I removed the oil fill tube and the CP3 is still rotating, so the cam gear is turning.

 

[Chevy1925]

Doesn’t mean the cam is turning.

Again, loose bolt that then sheared the key or broke cam or something else that locked up causing the cam key to shear will still turn the cp3 becuase the cam gear is still riding in the bolt.

There isn’t enough force on the cam to shear the key by its self. Marcus isn’t running ridiculous rpm and valve springs. Something else has happened that caused the problem.

Pull it apart and start taking pics. Literally just blowing smoke until the front cover and valve covers come off

 

[Bdsankey]

Doesn’t mean the cam is turning.

Again, loose bolt that then sheared the key or broke cam or something else that locked up causing the cam key to shear will still turn the cp3 becuase the cam gear is still riding in the bolt.

There isn’t enough force on the cam to shear the key by its self. Marcus isn’t running ridiculous rpm and valve springs. Something else has happened that caused the problem.

Pull it apart and start taking pics. Literally just blowing smoke until the front cover and valve covers come off

Agreed. My thought process was like you stated. Something killed the cam key, what was the culprit I haven't the slightest.

 

[RMJ_LB7]

Sorry that this happened to you Marcus, however I am very excited to see what the culprit is. take lots of pictures!

 

[blackneck94]

Sorry that this happened to you Marcus, however I am very excited to see what the culprit is. take lots of pictures!

Truck should be getting towed to my house tonight or tomorrow, I’ll definitely document my findinds whenever I feel like working on it. Arizona heat is degrading my motivation currently.

 

[blackneck94]

Doesn’t mean the cam is turning.

Again, loose bolt that then sheared the key or broke cam or something else that locked up causing the cam key to shear will still turn the cp3 becuase the cam gear is still riding in the bolt.

There isn’t enough force on the cam to shear the key by its self. Marcus isn’t running ridiculous rpm and valve springs. Something else has happened that caused the problem.

Pull it apart and start taking pics. Literally just blowing smoke until the front cover and valve covers come off

Here’s my plan,

Scope the cylinders and see what I find. If there’s anything detrimental at that point, I’m removing the engine altogether before tearing it down any further.

If everything looks okay up until that point, I’ll remove the valve covers and have a look at what’s going on as far as valve movement and the pushrods.

If everything checks out there, I’ll proceed with the front cover.

Trying to do it systematically in a way that I would find any major damage first so I can decide on keeping the engine in the truck or not. My luck tends to sway towards worst case scenario so I wouldn’t be surprised if this engine will be coming out soon.

 

[56taskforce]

Sorry man I hated seeing that question and knowing what happened... How built was it? If the heads were stock perfect excuse for SoCal stg 2 you'll need new valves and guides anyway. I know not really a bright side but that's the best lemonade I could come up with for those lemons...

 

[blackneck94]

Sorry man I hated seeing that question and knowing what happened... How built was it? If the heads were stock perfect excuse for SoCal stg 2 you'll need new valves and guides anyway. I know not really a bright side but that's the best lemonade I could come up with for those lemons...

Heads are stock LBZ castings with Wagler dual valve springs. I’m sure whatever happened is going to be an expensive fix, and if that’s indeed the case, it will get parted out. I’m at the point where if the heads have to come off, I’m calling it quits.

 

[RMJ_LB7]

Heads are stock LBZ castings with Wagler dual valve springs. I’m sure whatever happened is going to be an expensive fix, and if that’s indeed the case, it will get parted out. I’m at the point where if the heads have to come off, I’m calling it quits.

Darn man, I was thinking you might of found your boost leak while checking out the cylinder heads!

 

[blackneck94]

Darn man, I was thinking you might of found your boost leak while checking out the cylinder heads!

Truck got a new intercooler, repaired Y bridge stud thread, and a .9 exhaust housing... It was driving damn well before this happened.

 

[blackneck94]

Good call James!

Discovered the cam bolt had backed out. Pulled the gear off and voila. Cam is broken and the key is still intact! Chewed up yes, but it did not fail. All exhaust valve pushrods are bent. Hoping this is the extent of the damage, will know soon enough when the heads come off.