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TR6 High Compression in One Cylinder

Martin Secrest

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What could cause such a thing in my '72? I've got 150psi in cylinders 2-6, but 180 in cylinder 1. I've checked it cold and hot, throttle open, etc. My local LBC mechanic suspects bad oil ring in #1.
 

NutmegCT

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If only one cylinder has unusually high compression, it's often a sign of (1) carbon building in that cylinder, or (2) sticking valve.

To fix that in my TR, I removed the plug in that cylinder, dropped two tablespoons of Seafoam down into it, and used a rag to close the plug hole.

Let it sit for two (three?) days. Removed the rag, and saw there was no liquid in the cylinder (either drizzled down, or evaporated).

Then did another compression test. The psi was now nearly the same as the other three cylinders. Put the plug back in and all was well.

By the way, when you do a compression test, remove the plugs, remove the ignition coil center wire, block the throttle wide open (push the gas pedal to the floor), and use the starter to turn the crank the same number of times for each cylinder. That'll give you more reliable readings. Probably something else, but my brain has turned to jello again.

Tom M.
 

NutmegCT

Great Pumpkin
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Great. Let us know how it goes.

If there's no change, you may have a major carbon buildup problem in that cylinder (not likely if the other cylinders are ok), or a broken/jammed valve spring.

Good luck!
Tom M.
 

Rut

Obi Wan
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Martin,
If you do have carbon build up there are several things you can do, but the simplest is to spray water or Seafoam into the carb of the offending cylinder while the engine is running at high idle. Add a can of Seafoam to the gas tank as well to help clean out deposits/carbon over time.
Good luck, Rut
 
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Martin Secrest

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If I drop the two tablespoons of Seafoam in the cylinder, should I rotate the engine a turn or two, or just leave it in there?
 

NutmegCT

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I didn't turn the engine - just left it (Seafoam) sitting on the piston head. Wanted the "fumes" to reach the valves, and let the liquid slowly work its way down.

Tom M.
 

sp53

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When I was in high school we were taught to add a squirt of oil to raise the compression to see if the problem was rings or valves. If rings, the compression would rise with the oil. What does the spark plug on # 1 look like? Does it have that black burned oil on it?
 

CJD

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It is a sign of oil in the cylinder. If only one cylinder has a carbon build up, then that is still a sign of bad oil control in that cylinder. Causes would be intake valve guide/seal, piston rings, crankcase ventilation installed wrong. Seafoam will remove carbon and may help temporarily, but it will not fix the cause of the carbon.
 
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Martin Secrest

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Plug was sooty but not oily. Oil squirt did not change compression. When I went to pull the plug lead just now, the lead's interior contact came out with the plug. I might have been loose all this time, and that could have caused a soot buildup I imagine.
 

Geo Hahn

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...when you do a compression test, remove the plugs, remove the ignition coil center wire, block the throttle wide open (push the gas pedal to the floor), and use the starter to turn the crank the same number of times for each cylinder... Probably something else, but my brain has turned to jello again...

Have the gearbox in neutral. Ever run over your own right foot?
 

poolboy

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.The air valve would have to open to obtain WOT; but think twice about propping it up with something...
Regardless, there should be enough getting air sucked in to build compression without a real WOT...just be consistent that's the key.,
 

CJD

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Plug was sooty but not oily. Oil squirt did not change compression. When I went to pull the plug lead just now, the lead's interior contact came out with the plug. I might have been loose all this time, and that could have caused a soot buildup I imagine.

The key here is that adding oil did not increase compression. Even in a carboned chamber adding oil should show an increase. This again points to an oil control problem in that cylinder.
 

Jayplum

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Could just be that the #1 is carboned up due to running cooler as a normal condition than the other cylinders. All the fixes mentioned are good. Used to find a 'top end' oil to clean out old V8's; just pour down carburetors and smoke up the neighborhood. Works on valves as well as rings. With side drafts it is more problematic but some injected in spark plug holes will work.
Look into some of this type conditioner at local FAPS, Purple products also decent.
 

poolboy

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When it comes to carbon build up on valves affecting compression, it's going to depend on where it's located....On the face,the combustion chamber side the decrease in combustion chamber volume can create higher compression...but if the carbon is on the manifold side of the valve the valve may not completely close because of that and compression is lost. The opposite of your test results, I know but still worth mentioning.
A vacuum gauge can help differentiate valve problems, although not the specific valve, but if you have a valve not completely closing, the vacuum gauge needle will be very jittery...google for examples
Cleaning up a sticky valve from 112 to 140 psi within 2000 miles was accomplished with Techron gasoline additive and some spirited driving.
 

sp53

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Maybe try this; add oil to some of the low compression plugs and see if they come up. Once I added some of those split fire spark plugs with the split in the electrode deal and the oily/ sooty fuel burned cleaner. I worked with guy who made his own split plugs, but I personally did like it. The split-fire people might be out of business because I think they got sued or something.
 
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Martin Secrest

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After a few days soaking with a few tablespoons of Seafoam, the #1 compression has come down from 180 to 170lbs (compared to 150lbs for cylinders 2-6). I'm trying another soak now with about an ounce of Seafoam, and I turned the engine over to make sure the cylinder is up closer to the top of the bore. We'll see if we can get the compression lowered some more.
BTW, I made a simple (and disposable) funnel for the bore by using a socket wrench extension bar as a shaping form for some tin foil. Worked like a charm. Will report back next week, thanks.
 
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