What a miss
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Senior User
Joined: Apr 2002
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From: cedar rapids usa
What a miss
My number 8 plug (351M) doesn't want to fire. I can pull the plug and see the spark jumping around but when I put it in the cylinder I get nothing. I replaced the plug a few times, and the old plug did not have any oil build up on it. I read somewhere that the number 8 feeds the power-brakes. I know my booster is bad because when I shout off the truck I can hear the hiss of loss vaccum. I'll replace that in the morning, but my question is, can that vaccum leak kill my number 8 cylinder also? It seems wierd to me that ford would have designed it so that only one cylinder pulls on the booster.
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What a miss
I'll bite!
How is it MR.usually Knows the answer asks a question?
First, How did you figure out nothing is going on inside the #8 cylinder when the plug is installed ?
As far as Vacuum goes ,don't you have a vacuum can for extra vacuum?
I've always assumed the vacuum at the vacuum tree is from the whole intake manifold.
[font color=red]Dennis
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How is it MR.usually Knows the answer asks a question?
First, How did you figure out nothing is going on inside the #8 cylinder when the plug is installed ?
As far as Vacuum goes ,don't you have a vacuum can for extra vacuum?
I've always assumed the vacuum at the vacuum tree is from the whole intake manifold.
[font color=red]Dennis
Calgary,Alberta,Canada[/font]http://www.clubfte.com/users/mil1ion/canada_flag_animated.gif
[link:www.ford-trucks.com/guidelines.html|FTE Moderator]
http://www.clubfte.com/users/mil1ion/Minimorleytruck.JPG
[link:www.clubfte.com/users/mil1ion/Mil1ion.html|My Website,"North Of The 49th"]
[font color=red]
Please Don't Ask Me Any Tough Questions,
"I'm Saving My Memory For When I Develop Alzheimer's" [/font]
[font color=blue]78 F-150 429CJ,Silver,Explorer Pkg.
[/font]
Postmaster
Joined: Aug 2000
Posts: 3,312
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From: Eldorado Ca. USA
What a miss
Have you tried doing a compression test?? I am thinking you might have a dead cylinder due to a bad valve or ring. Try doing the compression test before you tear into it. I doubt that a vacuum booster that leaks "a little bit" will be the cause of your problem. I would try changing the plastic check valve that the hose attaches to before you replace it.
Tony
'77 F250, 4X4 460 transplantee, "Flamer"
'74 F250. 460, "beater" now "1 dead ford"
'73 F250, "midnight auto" now a trailer for the flamer
Tony
'77 F250, 4X4 460 transplantee, "Flamer"
'74 F250. 460, "beater" now "1 dead ford"
'73 F250, "midnight auto" now a trailer for the flamer
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Joined: Apr 2002
Posts: 482
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From: lake ariel usa
What a miss
Im thinking bad valve also. My 351m I just pulled was dead on 8 without being fouled also. Never looked into the cause as I knew 460 was going in. My brakes/booster are fine on the truck. You could try plugging off the booster to see if it makes a difference if your not ready to change it yet.
Good Luck Andy
Good Luck Andy
Thread Starter
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Senior User
Joined: Apr 2002
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From: cedar rapids usa
What a miss
>I'll bite!
>
>How is it MR.usually Knows the answer asks a question?
>
>First, How did you figure out nothing is going on inside the
>#8 cylinder when the plug is installed ?
It's not hard to tell when you can pull the wire and nothing changes in the way the engine sounds and performs.
>As far as Vacuum goes ,don't you have a vacuum can for extra
>vacuum?
I've never seen (mind you I haven't been under the hood of to many fords) a stock truck come with a vacuum can for the power brakes. That is other then the stock power brake booster.
>I've always assumed the vacuum at the vacuum tree is from
>the whole intake manifold.
Yeah I always thought so too until I was doing some research and came across a sight that showed a vacuum tree and its routing within the manifold. Seems there is a plug at the manifold that has two small holes in it. This plug "seems" to feed only off the number 4 and 8 cylinders. That is why I was asking this question.
>
>How is it MR.usually Knows the answer asks a question?
>
>First, How did you figure out nothing is going on inside the
>#8 cylinder when the plug is installed ?
It's not hard to tell when you can pull the wire and nothing changes in the way the engine sounds and performs.
>As far as Vacuum goes ,don't you have a vacuum can for extra
>vacuum?
I've never seen (mind you I haven't been under the hood of to many fords) a stock truck come with a vacuum can for the power brakes. That is other then the stock power brake booster.
>I've always assumed the vacuum at the vacuum tree is from
>the whole intake manifold.
Yeah I always thought so too until I was doing some research and came across a sight that showed a vacuum tree and its routing within the manifold. Seems there is a plug at the manifold that has two small holes in it. This plug "seems" to feed only off the number 4 and 8 cylinders. That is why I was asking this question.
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Senior User
Joined: Apr 2002
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From: cedar rapids usa
What a miss
>Im thinking bad valve also. My 351m I just pulled was dead
>on 8 without being fouled also. Never looked into the cause
>as I knew 460 was going in. My brakes/booster are fine on
>the truck. You could try plugging off the booster to see if
>it makes a difference if your not ready to change it yet.
>
>Good Luck Andy
It seems that if a cylinder is going to die on one of these engines it's going to be the number 8. I was hoping that it might be tied into the vacuum leak. I found a couple of sights that thought this could be the case, but it seems that I couldn't find one where anyone repllied as to if fixing the leak solved the problem.
>on 8 without being fouled also. Never looked into the cause
>as I knew 460 was going in. My brakes/booster are fine on
>the truck. You could try plugging off the booster to see if
>it makes a difference if your not ready to change it yet.
>
>Good Luck Andy
It seems that if a cylinder is going to die on one of these engines it's going to be the number 8. I was hoping that it might be tied into the vacuum leak. I found a couple of sights that thought this could be the case, but it seems that I couldn't find one where anyone repllied as to if fixing the leak solved the problem.
Lead Driver
Joined: Nov 2000
Posts: 6,639
Likes: 349
From: Albuquerque
What a miss
my .02, On my engine there is a vacuum tree that mounts in the runner to the right rear cylinder. If you are open to atmosphere there it might kill that cylinder. Also I have an original plastic ball vacuum reservoir on the left fender well but it's for the HVAC switch etc. The brake booster holds it's on reserve.
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Post Fiend
Joined: Feb 2000
Posts: 5,130
Likes: 8
From: Brea CA
What a miss
Damn, now I’m gonna be hesitant to ever ask a question!
Wayne, this may be too simple but does the cylinder fire with that vacuum fitting capped off and sealed? If not, then I would suspect a burnt exhaust valve. How did the compression check come out?
Barry
Wayne, this may be too simple but does the cylinder fire with that vacuum fitting capped off and sealed? If not, then I would suspect a burnt exhaust valve. How did the compression check come out?
Barry
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From: cedar rapids usa
What a miss
When I posted this it was somewhere around or after midnight my time, and I had not yet done the pressure test or tried any bandaid fixes. This morning before buying the new booster (and in the name "I got to know because I'm just that way)I pulled the hose for the booster off and capped the tree. My results were, the cylinder came back to life and fired like a trooper.
My guess is that the leak at the booster was making the mixture at the cylinder so lean that it would fail to catch (burn). Upon doing a cylinder pressure test I came up with an average of only 138psi, with a high of 142 and low of 134psi, in all 8. I say "only" because this seems rather low to me (my vette holds from 190 to 210psi depending the cam setup). Taking into account the rather low compression ratio (I believe its only 8.4/1 in a 351M correct?) then 134psi doesn't seem so bad.
My guess is that the leak at the booster was making the mixture at the cylinder so lean that it would fail to catch (burn). Upon doing a cylinder pressure test I came up with an average of only 138psi, with a high of 142 and low of 134psi, in all 8. I say "only" because this seems rather low to me (my vette holds from 190 to 210psi depending the cam setup). Taking into account the rather low compression ratio (I believe its only 8.4/1 in a 351M correct?) then 134psi doesn't seem so bad.
Posting Guru
Joined: Jul 2001
Posts: 1,613
Likes: 2
From: Gainesville, FL
What a miss
All,
This is one of the more excellent topics I've read. A bad valve and a large vacuum leak could give the same symptom; a dead cylinder.
The Solution:
1. Cap the vacuum and give it a go.
2. Take a compression check.
Since the cylinder is producing pressure close to equal of the others; we know the valves are fine/OK.
A vacuum leak allows the cylinder to pull air instead of air/fuel. Threrfore the dead cylinder.
134 psi is greater than 8.0:1 so, 8.4:1 is probably what you got.
My Way is the Highway,
KingFisher
This is one of the more excellent topics I've read. A bad valve and a large vacuum leak could give the same symptom; a dead cylinder.
The Solution:
1. Cap the vacuum and give it a go.
2. Take a compression check.
Since the cylinder is producing pressure close to equal of the others; we know the valves are fine/OK.
A vacuum leak allows the cylinder to pull air instead of air/fuel. Threrfore the dead cylinder.
134 psi is greater than 8.0:1 so, 8.4:1 is probably what you got.
My Way is the Highway,
KingFisher
