Hi everyone, I keep on having plugs 5 and 7 getting caboned up. My Corvette will be running great, very peppy and responsive then all of the sudden things start going south... When I pull plugs 5 and 7 they are pretty back, number 5 more than 7. When I clean them up everything goes back to normal and it runs great. I think I have the gas mixture pretty well dialed in. Is there anything else that might be causing this. Valve seals maybe...? Thanks for helping, drives me nuts..! Note: Seems to happen when I’ve been driving in traffic for a while.

 

Your profile says “motor rebuilt to original specs”. Who did the rebuild? Have you talked to them?

What does the left tailpipe look like compared to the right? How clean is the oil? Any evidence of high oil consumption? I’d run a compression test and see where these two cylinders are at.

 

Well lets see what those two cylinders have in common:

They share a headgasket. However I would think something unusual would show up in the radiator or oil.
They share a Intake Gasket. Could be oil is seeping in from the lifter valley at that location..
Or the thin gasket between the Intake Runners has failed allowing an uncontrolled fuel mixture between the two firing cycles.
They share location area under the valve cover. In a rare event, the oil drain-back hole at the rear of the head is plugged. This allows excess oil to accumulate near those cylinders valvestem seals. But not very likely.
They share a close firing order. Quite common on Chevy sparkplug wire routing is a event called Inductive Crossfire. Follow those two cables from the dizzy cap all the way to the plug. Maintain a one inch distance between the two by using the wire looms correctly.
And last, perhaps a crack in the dizzy cap, carbon tracking at that location, misfire to both 5 & 7.

 

Don't think it’s a head gasket, oil is clean...

Lifter valley, Is that on the intake manifold? Is it something I can check without pulling the motor apart?

Intake runners, I have no idea what those are but I’ll look into it.

Oil drain back hole, I’ll pull off the valve cover and have a look...

Inductive crossfire, now this on stumps me because my 350 is very stock, nothing has been changed as far as the way it looks and came from the factory so the spark plug wires go through a aluminum housing that runs along both sides of the block. It is very tight inside the channel and the wires are on top of each other inside of it. When the wires come out they are held apart by wire looms until they reach the distributor. But even at that point they are crammed into the chrome covers leading to and covering the distributor cap.

Cracked distributor cap, don’t thinks so, the dist cap, rotor and plug wires are new. I changed them when I was trying to solve the same problem a while back. Running the wires through those channel guides was a bitch... It’s a two man job. One guy under the car to guide the wires in and another above to pull them through.

Im not a mechanic, I’m more a tinkerer but I enjoy working on the Vette and sorting out the problems that come up if I can... I know enough to get myself in trouble, as they say...

 

Post some pics of plugs.
black carbon ? Ignition ?
assumes no oil consumption and all the other great advice given.
what plug are you running ?

 

Do you have electronic Ignition or Points.

My experience on my 78 is that 5 & 7 can crossfire each other. Make sure the ignition wires are far apart as possible. Also do you have a heat reflector for the wires?

 

Maybe you just need a hotter plug, what plug are you using?

 

You could run a temporary plug lead to either 5 or 7 bypassing the normal run and see if the problem goes away.

 

It could be Valve Guides or Bad Galve Guide seals. If only a couple were bad - the amount of oil consumed would not be really bad.

When the engine was rebuilt - did the rebuilder put in new valve guides ???

When you have the valve cover off - take a look through the valve spring, and see if you have something that looks like a miniature rubber umbrella on some of the valve stems. (Not all SBC's use the umbrella seals - but they are far more effective than the other types of OEM valve guide seals.) You might want to poke them with a very small screwdriver to see if they're still nice and flexible. The purpose of the valve guide seals is to keep the amount of oil flowing down the valve guides to a reasonable level. If the seals are bad, or the guides are worn - it won't show up in a compression or a leakdown test. You can replace valve guide seals on the car - but if the guides are worn - the heads need to come off....

You can test for guide wear - but it's not a fun job - you put the Motor at TDC, and feed compressed air into the cylinder you want to test via the spark plug hole. You then remove the valve spring, and put rubber bands tightly around the valve stem - say something like 0.675" up from the start of the guide (those rubber bands are there to keep the valve from falling into the cylinder). Then remove the compressed air - and rotate the ending to bring the piston down a bit. let the valve go down 0.400" or so and feel for any ability for the valve to move side to side. If you feel any kind of slop - you have a valve guide wear problem (I think the spec is on the order of no more than 0.005" side to side play at max valve lift at the valve head).

Needless to say - check other things first.... A gasket leak on the intake gasket is somewhat more likely - especially with a rebuild engine - and especially if the block or heads were milled.

 

Dave in Spain,

I am looking for common ground here. If it were valve stem seals, why just cyl 5 & 7? Why not more or all of them?
If the issue was incorrect plug heat range, again, why just 5 & 7? Why not others or all of them?

You ruled out a headgasket.
You ruled out plug wires. Although still a possibility hidden from the naked eye. (post #8)

I have a hunch about a common problem that is also hidden from view. Bottom portion of the Intake Gasket. You can use a propane bottle on that Intake all day long testing for vacuum leak and it will never show up. Because the leak is on the bottom.
The only give-away is low vacuum or a leak that is impossible to find. With the fact that those two cylinders are sharing the gasket and those two cyl are oil fouled sure points to a bad Intake gasket. The vacuum from the Intake runners draws oil mist in from the lifter valley.

The cause of this is almost always improper gasket / improper sealant especially along the bottom edge. You can smear all the RTV you want on the Intake gasket, but if you miss a spot on the bottom, it will leak, sure as ****.

I wish there was a way of testing the gasket seal bottom. Unfortunately the only fix is to pull the Intake, clean mounting surface and install a quality gasket w/sealant.

 

Very easy to switch the wires accidently. Seen this a few times where it was done on the distributor and also at the plugs.

Dom

 

Maintain a one inch distance between the two by using the wire looms correctly.

My 68 L79 have the wires run down the sides of the block through a shield and up to the distributor. This is factory routing. No way insure a 1" spacing. This is factory. Many out there like this, no issues.

 

Swap #5 and #7 wires with others and see if the problem stays the same, moves to different cylinders, or changes in some way. Then swap #5 & #7 plugs with others and evaluate for same conditions. You could have defective plug[s], defective wire[s], or the problem could fall in the group called "other". These tests will [at least] reduce the 'field' a bit.

 

A few simple things first....swap a good plug and a bad plug and see if it follows the plug or the location. If plug check that they are the same exact version. Then try swapping good/bad plug wires temporarily and see what happens. I have a similar problem but not so bad...only a few plugs get black over time. But this is a tired engine and about to be rebuilt or replaced. Good luck!

 

There's an echo in here......

 

Great minds think alike...and don't see if another post has been sent while writing theirs.

 

.....

 

Im running Delco 1 platinum plugs...

 

Okay so I assume that is one of the fouled plugs? what does one from another cylinder look like for comparison?

 

looks the same but clean...