Lean Fuel Mixture: the Silent Killer

[FreeBeeTony]

Think I understand why my engine destroyed itself......

At the end of last season I replaced a lifter which obviously required the R/R of the intake manifold. From what I hear these manifold/gaskets are prone to leaks.

After looking at the tops of the pistons again and comparing last years plugs to this years I think I can defintely say this was running lean.........I actually took note of the plugs I took out at the end of last season and they were the desired tan color. When I dis-assembled the engine after the failure the plugs were near white.

The timing was 8* BTDC and the vacum was ~19 last time I checked at idle.

Is the only way to tell if you're running lean to read the plugs?

 

[John_S]

Re: Lean Fuel Mixture: the Silent Killer

Tony,

If all plugs were showing lean mixture, that would lead me to a carb or supply issue. An intake gasket leak, usually effects only a runner or two. I'd think you would pick it up the fluctuation on the vacuum test, too.

Plug reading, I'm told it is very hard with todays reformulated fuels. This season you probably ran all gas with alcohol content. The season before, that cancer causing additive that starts with "M" and is a mile long. I don't know if white plugs are a result of the alcholol, but we'll hear more as others start checking/changing plugs for next season. Its just another difference at this point.

From your prior post and pictures, I am probably the only person who isn't convinced it was a lean/detonation problem. It could be, but not 100% convinced. Things that bug me:

- Could not see clear errosion or pitting vs denting on pistons. Might be just the pictures

- Rest of the intake valves look fine, no cupping or other destructive effects

- Valve appears to have broke off in pieces, vs melting, or errosion. To me, it had to hit something to break. The remainder of valve does not look cupped.


One theory that was going through my mind was, a small screw or nut was inadvertently dropped down the carb or intake (or possibly fell off the carb -check butterfly and air door screws). It sat on the top of the dual plane manifold until it vibrated and got sucked into the intake runner. It jammed under the intake valve and held it open. The piston then hit it on the top of the compression stroke. Now we have schrapnel bouncing around the cyl. Some of this schrapnel would get blown back up into the intake and sucked down into other cylinders, especially the three other runners that are shared with #3.

Kinda wild, yes?

 

[JustJason]

Re: Lean Fuel Mixture: the Silent Killer

tony check your "rebuild or replace thread" I answered your question there.
And sadly... it wasn't running lean...

 

[Limited-Time]

Re: Lean Fuel Mixture: the Silent Killer

Tony,
Things that bug me:

- Could not see clear errosion or pitting vs denting on pistons. Might be just the pictures

- Rest of the intake valves look fine, no cupping or other destructive effects

- Valve appears to have broke off in pieces, vs melting, or errosion. To me, it had to hit something to break. The remainder of valve does not look cupped.


One theory that was going through my mind was, a small screw or nut was inadvertently dropped down the carb or intake (or possibly fell off the carb -check butterfly and air door screws). It sat on the top of the dual plane manifold until it vibrated and got sucked into the intake runner. It jammed under the intake valve and held it open. The piston then hit it on the top of the compression stroke. Now we have schrapnel bouncing around the cyl. Some of this schrapnel would get blown back up into the intake and sucked down into other cylinders, especially the three other runners that are shared with #3.

Kinda wild, yes?

Almost the same thoughts here, the real failure mode has yet to be determined IMO.

 

[FreeBeeTony]

Re: Lean Fuel Mixture: the Silent Killer

I looked at the carb and piston again last night...........

Nothing missing from the carb..........and the piston doesn't look steam cleaned as if I had water intursion..........just looks beat to he!!.

Might never know exactly what caused it.........I decided to replace it with a long block and just use my intake/carb.

 

[Limited-Time]

Re: Lean Fuel Mixture: the Silent Killer

I looked at the carb and piston again last night...........

Nothing missing from the carb..........and the piston doesn't look steam cleaned as if I had water intrusion..........just looks beat to he!!.

Might never know exactly what caused it.........I decided to replace it with a long block and just use my intake/carb.

Man it sure would be nice to know what exactly went wrong. It sounds as if the valve itself may have been the weak link. Wasn't that the valve that you had to re-adjust earlier this season, to correct a high speed miss??. If it was there's a tell tail there.

 

[John_S]

Re: Lean Fuel Mixture: the Silent Killer

Tony,

You might want to take it down to the head shop and get their opinion. Given pictures of two very similiar broken valves, both on vortecs, they might have more valuable insight than us theory mungers.

 

[JustJason]

Re: Lean Fuel Mixture: the Silent Killer

trust me, it was water.
you won't see rust on the plugs unless you start with new plugs and do a plug chop, and then pull them out asap!!


If you guys want.. hehe... try figuring this one out.

its not a marine engine, but try to guess why it blew. This one came out of my soobie about 6 years ago. Take your best shot... after i hear it all i'll let you know how and why it went....
And it only had 58K miles on it too =(

The proof is in the picture... you have to look very hard to see it!!!
And if you want to know where the piston is... It was in the oil pan... well.. what was left of it.

 

[JustJason]

Re: Lean Fuel Mixture: the Silent Killer

and don't just stab at it... give a reason for what you say!!

 

[JustJason]

Re: Lean Fuel Mixture: the Silent Killer

couple of "stabs" so far... but noone's close. here's a few more pics to see of a "fubar'd" engine...
if your trying to guess what happened though i'd look real hard at the first picture in the thread!!!

 

[Silvertip]

Re: Lean Fuel Mixture: the Silent Killer

Cylinder crack at about 1 o-clock. Water intrusion, hydrolock, parts go into the pan.

 

[JustJason]

Re: Lean Fuel Mixture: the Silent Killer

the crack... or both of them is a result from the failure... but not the failure itself....
nope not hydrolocked
not water intrusion either....

 

[Robj]

Re: Lean Fuel Mixture: the Silent Killer

OK Captain, what is your prognosis for the failure?? I only see two cracks and assumed that it would have been caused by water ingress.

Rob.

 

[JustJason]

Re: Lean Fuel Mixture: the Silent Killer

im going to give it a couple of days before i answer. i want to see if some of the more veteran members care to chime in

 

[Bondo]

Re: Lean Fuel Mixture: the Silent Killer

I thought this thread was about Freebe Tony's Blown Valve,..............

And,.........
Capt. Jason,....... You're Wrong....... Tony's motor did Not have Water Intrusion........
There's Absolutely NO signs of it........

 

[FreeBeeTony]

Re: Lean Fuel Mixture: the Silent Killer

I really don't think it was water either............

In any case.............I'm pulling the rest of the engine this w/e.........maybe I will see something else after it's out.

 

[JustJason]

Re: Lean Fuel Mixture: the Silent Killer

ok bondo... then how do you explain the carbon being blown off the top of the piston/cly head and the super clean spark plug??????????????????????????????????????????????????????????????????????????????
because "runnin lean" does not do that!!

 

[kay]

Re: Lean Fuel Mixture: the Silent Killer

Blew the headgasket and maybe coolant got in there?

 

[Silvertip]

Re: Lean Fuel Mixture: the Silent Killer

I suppose another cause might be a broken timing belt/chain, interference engine, piston hits valve and scatters the parts. Broken valve spring is another possibility.

 

[Bondo]

Re: Lean Fuel Mixture: the Silent Killer

ok bondo... then how do you explain the carbon being blown off the top of the piston/cly head and the super clean spark plug?????????????????????????????????????????????? ????????????????????????????????
because "runnin lean" does not do that!!

Ayuh,....... I beg to Differ,.....
There's Plenty of Carbon left, both on the Head Chamber,+ the Piston......

If it had been Water,......
There'd be No Carbon,..... At All.......

The only places that Don't have Carbon,.....
Is where there was mechanical Contact with Hard Parts.........

The pictures appear to be gone,.... But I wouldn't call those chambers Clean by any means.......
Water in the combustion chamber will leave it looking Totally Clean, as in Brand New,+ Unrun.........
Tony's Aren't..........