low comprresson #6=80

[modelageek]

I am looking at a 2002 with a rough idle and a compression of 80 in the #6 cylinder, it randomly throws a P0306. it might go a month without throwing it. any ideas of what the most common issue might be. I do not know if when they did the compression test if they did the oil drops to tell if it was the rings. the owner thinks it is a valve. they seafoamed it and it seemed to help it some. any feedback would be appreciated.

 

[Big Jim M]

Most likely it is a valve. Prolly caused by an air leak into that cylinder..
Big JIm

 

[modelageek]

Most likely it is a valve. Prolly caused by an air leak into that cylinder..
Big JIm

thanks Big Jimif it is the valve itself then the head has to be pulled and repalced? How tough of a job is it to do on these 3.0? any idea what the head costs?. could it be the valve seal where you can replace it with the head on....

 

[Big Jim M]

It's just mechanical work!

thanks Big Jimif it is the valve itself then the head has to be pulled and repalced? How tough of a job is it to do on these 3.0? any idea what the head costs?. could it be the valve seal where you can replace it with the head on....

It's about the same on any engine.. Ya gotta take everything off till you get to the offending head and then take it off.
Odds are 99.9 to 1 that you won't have to replace the head.

I never do a valve job! That is more wear than I wish to have on my heads. What I do is, first look at the head and resolve to myself that all looks good except that ONE VALVE. Then I get ONE new valve and a little diamond grease. I LAP that one valve in place and install new valve seals on all the valves and then install the head and all parts removed.

BY THE WAY... Most of the world does this EXACT same process when having a burnt valve. ONLY us rich americans do "valve jobs".. The rest of the worlds mechanics understand about the excessive extra wear "valve jobs" do to the valve train.

When replacing the parts is is good to find the place where the air entered to overheat that one valve.

Big JIm

 

[modelageek]

Big jim. How would I determine where the air came in? Any chance the compression is caused by the seal that can be replaced by taking the VC off and usingthe compressed air tecnique to replace it ?

 

[Big Jim M]

Nope!

Big jim. How would I determine where the air came in? Any chance the compression is caused by the seal that can be replaced by taking the VC off and usingthe compressed air tecnique to replace it ?

No chance at all. That seal keeps oil from the spray around the rockers from entering the valve guide during the intake stroke.

Most times if only one valve is burnt the problem can be traced to a small leak at the intake or exhaust manifold nearest that valve.

Other times a slightly clogged injector can be traced to the problem. Not enough fuel and too much air makes a fire that is too hot and burns valves.. Sorta like a blow torch.

Big JIm

 

[cush]

+1 to big jim. i'd throw a new valve in it. BUT if you have the money you might look into getting both heads rebuilt. it would prob run you a couple hundred but it would be worth the money. but how many miles does it have? is it the 125k? because if you have 125k+ miles, i wouldnt rebuild the heads. if you put fresh heads on a worn block, you'll make so much compression that you'll increase the blow by on the piston rings, and increase oil consumption.

 

[Rearanger]

if you put fresh heads on a worn block, you'll make so much compression that you'll increase the blow by on the piston rings, and increase oil consumption.

 

[cush]

seriously. think about it for a minute, if the valves and everything were worn in, and then all of a sudden you throw a fresh head on that can hold a ton of compression but you have the same worn in rings as before, you will build more compression but you'll have more blowby and more blowby means more crankcase pressure, more crankcase pressure means burning oil.

 

[Big Jim M]

Wow!

seriously. think about it for a minute, if the valves and everything were worn in, and then all of a sudden you throw a fresh head on that can hold a ton of compression but you have the same worn in rings as before, you will build more compression but you'll have more blowby and more blowby means more crankcase pressure, more crankcase pressure means burning oil.

I luv ya man... But you are Waaaay wrong. Leaky valves get burnt between here and the cathouse! One short trip is all iit takes!

That said it should be apparent to you that all heads provide the same amount of pressure on the piston rings.

New heads shouldn't be in the future of any engine, unless they have been run hot and cracked. Heads should last about even with the rest of the engine.

Big JIm

 

[cush]

im not sayin new heads are the right way, but here's a quote out of my auto book: automotive engines 5th edition from tim gilles
"increased oil consumption after a valve job
the condition of the entire engine must be considered before performing a valve job. sometimes and engine starts to consume oil after a valve job. the valve job increases compression, but it also increases engine vacuum. this can result in more oil being sucked past the worn piston rings into the combustion chamber.
if the cylinder heads are removed for a valve job, look at the tops of the pistons to see is oil consumption is caused by worn or stuck piston rings. during a valve job, removing carbon from the tops of the pistons can also lead to oil consumption if pieces of carbon get wedged between piston rings and ring grooves"
i remember my teacher telling also that it will increase blow by on the piston rings and overwhelm the pcv system so you'll suck more oil molecules through your pcv system

i mean, i could be wrong, or we could just have different opinions, im not saying your wrong. its just what i learned in school

 

[Big Jim M]

Well Cush... Out here in the REAL world it don't happen like that.
Big JIm

 

[hondaxr650]

I wish I had a dime for every time I heard someone say "You can't put fresh heads on a used engine, You'll blow the rings right out of it!" I think I was even taught that in vocational school 20 years ago. Didn't believe it then, still don't. I think it's one of those old-wives tales that gets handed down through generations because nobody ever stops to think about it long enough to realize it doesn't make sense.