One lean cylinder?
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Cargo Master
Joined: Aug 2007
Posts: 2,558
Likes: 69
From: New Zealand
One lean cylinder?
Has anyone experienced an intake manifold leak from underneath the manifold?
I have seven tan plugs & one grey (no miss - just the color difference), but no response from starter fluid around the intake.
I'm thinking I might have to lift the intake manifold
Merry Christmas!
I have seven tan plugs & one grey (no miss - just the color difference), but no response from starter fluid around the intake.
I'm thinking I might have to lift the intake manifold
Merry Christmas!
Thread Starter
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Cargo Master
Joined: Aug 2007
Posts: 2,558
Likes: 69
From: New Zealand
Pulling the plug wire kills that cylinder.....it's not mis-firing at all.
Some extra air is getting to that cylinder somehow.
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
What about a burned valve? A burned intake on the prior cylinder would upset the charge some as well as decreasing compression. An exhaust valve would just do the latter.
And, by the way, Merry Christmas!
And, by the way, Merry Christmas!
Cargo Master
Joined: Oct 2005
Posts: 2,295
Likes: 25
From: NJ
Yes, they can leak from below. I've had trouble sealing my 460, though hads have been shaved. I used the thickest intake gaskets I could find on Summit and sealed alright. Plugs on leaking cylinders looked like burning oil from sucking oil vapor from below.
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Posting Guru
Joined: May 2013
Posts: 1,122
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From: Haysville, KS
You could do a leak-down test. If it was a bad intake valve, you'd hear air escaping in the intake. Bad exhaust valve, air into the exhaust, bad head gasket, air or bubbles there
If the leak-down test looks (sounds) OK, I suppose you could have an intake manifold leak..
No functional issues other than the plug color?
Normal plug colors range from grey to light brown (tan) so I'm not sure you even have an issue here.
Maybe I missed it, but is this carburetor or EFI? If EFI, you could easily have an injector that is a little off, but again - runs fine, plugs within the normal range, whats the issue?
If the leak-down test looks (sounds) OK, I suppose you could have an intake manifold leak..
No functional issues other than the plug color?
Normal plug colors range from grey to light brown (tan) so I'm not sure you even have an issue here.
Maybe I missed it, but is this carburetor or EFI? If EFI, you could easily have an injector that is a little off, but again - runs fine, plugs within the normal range, whats the issue?
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Cargo Master
Joined: Aug 2007
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Likes: 69
From: New Zealand
No functional issues other than the plug color?
Normal plug colors range from grey to light brown (tan) so I'm not sure you even have an issue here.
Maybe I missed it, but is this carburetor or EFI? If EFI, you could easily have an injector that is a little off, but again - runs fine, plugs within the normal range, whats the issue?
Normal plug colors range from grey to light brown (tan) so I'm not sure you even have an issue here.
Maybe I missed it, but is this carburetor or EFI? If EFI, you could easily have an injector that is a little off, but again - runs fine, plugs within the normal range, whats the issue?
The issue is just that seven plugs are identical to each other, but this one alone is different (even with a carb).
You're right that grey isn't radically off on mixture....but something is causing it, & I'm just not happy to work the truck hard knowing one cylinder is burning hotter.
Although the one plug is grey now, it has sometimes shown oil.......which I'd previously assumed to be a valve seal because there was no noticeable oil consumption or smoke.
I'll do a compression test anyway to have some numbers, & go from there.
Thanks everyone
Cargo Master
Joined: Oct 2005
Posts: 2,295
Likes: 25
From: NJ
I also had a leak under the intake on a 454, that one was a torn/shifted gasket and was intermittent noticeable at idle and just off idle. That gasket was installed by previous owner not me, and I know no history of that engine other than it was rebuilt a few years before I got it.
Anyhow, if you don't have any signs of a vacuum leak and just one plug a different color, you may not have a leak. Both leakers I had under the intake were hard to tune at idle/off idle and accelerator pump shot. Plug color will normally vary between the end cylinders and the inner cylinders due to the longer and more complex runners to the corners fuel will drop out of the charge and result in the 4 corners running slightly leaner. Whether this is an issue or how much mixture varies between different cylinders varies based on the particular intake manifold. Jet so your leanest cylinder is where it needs to be.
Anyhow, if you don't have any signs of a vacuum leak and just one plug a different color, you may not have a leak. Both leakers I had under the intake were hard to tune at idle/off idle and accelerator pump shot. Plug color will normally vary between the end cylinders and the inner cylinders due to the longer and more complex runners to the corners fuel will drop out of the charge and result in the 4 corners running slightly leaner. Whether this is an issue or how much mixture varies between different cylinders varies based on the particular intake manifold. Jet so your leanest cylinder is where it needs to be.
Posting Guru
Joined: May 2013
Posts: 1,122
Likes: 0
From: Haysville, KS
Maybe it's just me, but no symptoms of a problem, other than one plug that looks different but is still normal; I wouldn't go crazy. It could always be that one plug was a little different from the start. Maybe replace it, run it for a while and see if the 'problem' comes back.
If I had the time, I'd check compression and vacuum, maybe a leak down on that cylinder. If all was within normal range, I'd let well enough be.
That's why normal has a range.
But that's just me. I have enough problems to deal with
If I had the time, I'd check compression and vacuum, maybe a leak down on that cylinder. If all was within normal range, I'd let well enough be.
That's why normal has a range.
But that's just me. I have enough problems to deal with
Thread Starter
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Cargo Master
Joined: Aug 2007
Posts: 2,558
Likes: 69
From: New Zealand
Well, I've just done a compression test -
1 - 200
2 - 197
3 - 210
4 - 200.......the lean cylinder
5 - 197
6 - 192
7 - 190
8 - 208
I also did a tow yesterday; not heavy, but all hills. Afterwards there were still seven tan plugs, but the 'grey' plug was now snow-white, so I'll definitely lift the intake & look for a leak.
I also remember when I first got the truck, it would occasionally kick back hard on the starter (caused by problems from a bad module connector).
When it did this, the smell of crankcase fumes would waft into the cab, so maybe the intake gasket has been blown from then.
1 - 200
2 - 197
3 - 210
4 - 200.......the lean cylinder
5 - 197
6 - 192
7 - 190
8 - 208
I also did a tow yesterday; not heavy, but all hills. Afterwards there were still seven tan plugs, but the 'grey' plug was now snow-white, so I'll definitely lift the intake & look for a leak.
I also remember when I first got the truck, it would occasionally kick back hard on the starter (caused by problems from a bad module connector).
When it did this, the smell of crankcase fumes would waft into the cab, so maybe the intake gasket has been blown from then.
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
Ken - I used to think compression tests reveal a lot about engines. But I no longer think that as I've had more than one problem a compression test didn't find. One engine had 3 leaking exhaust valves and two leaking intake valves, but the cylinders with the bad valves actually scored high in the compression test. Another engine had a leaking head gasket, but the compression on that cylinder was in the hunt with the others. However, in all cases the leak-down test nailed the problems. So any more I don't bother with the compression test.
In your case if you had a bad valve it might well be missed in the compression test. And, while I can't come up with a good reason for the spark plug color you are getting, I would hate for you to R&R the intake and still have same problem.
In your case if you had a bad valve it might well be missed in the compression test. And, while I can't come up with a good reason for the spark plug color you are getting, I would hate for you to R&R the intake and still have same problem.
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Cargo Master
Joined: Aug 2007
Posts: 2,558
Likes: 69
From: New Zealand
Thanks Gary. OK, I'll make an air fitting for a leak-down test, & do one next week.
But because of the intermittent oil on this same plug, I tend to think there's a gasket leak under the intake.
But because of the intermittent oil on this same plug, I tend to think there's a gasket leak under the intake.
Post Fiend
Joined: Dec 2007
Posts: 8,521
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From: Lost
Prior to lifting the intake, try this. Pull and plug the oil cap and PCV. See if the idle changes at all. If the intake has internal leak, there should be signs of vacuum at 1 or both of these locations.
I have seen cases where an intake gasket had a bad spot on the lower portion, allowing air to be sucked from inside the valley. Usually, there were signs of oil on the plug/plugs of the affected cyls, as well as signs of running lean.
I have seen cases where an intake gasket had a bad spot on the lower portion, allowing air to be sucked from inside the valley. Usually, there were signs of oil on the plug/plugs of the affected cyls, as well as signs of running lean.