TR2/3/3A low cylinder compression

[Donald1107]

My engine started running slightly rough. I pulled the spark plugs and tested the compression, concluding that I have a problem. What the cause is, I’m not exactly sure yet. Here are the facts:
1) Engine has been running somewhat rough, but not smoking
2) Compression results are 147, 150, 65, and 151 for cylinders 1, 2, 3 and 4.
3) #3 spark plug is badly fouled while 1, 2 and 4 are clean (all plugs replaced 900 miles ago).
4) When I had a professional British sports car garage tune the engine, they found one push rod stuck, and unstuck it; don’t know which one or how. But the engine has run fine for 900 miles … until recently.

Question: could it be a stuck push rod? And if so, how would I tell?

If not a stuck push rod, would a burned valve cause the spark plug to foul with black crust?

Don

 

[charlie74]

I’m a TR6 guy so I stand to be corrected but I can’t see how a pushrod can get stuck as there not fitted in a tight tolerance like a valve is in it’s guide. You might want to check your valve lash because if it’s too tight then either valve in that cylinder wont be able to close fully.
If you can do a leak down test it will tell you if your losing compression through the intake or past the piston rings.
You might be able to tell if it’s rings with a wet compression test though by adding a little bit of oil to the offending cylinder (not too much, just enough to wet the rings).

 

[Donald1107]

Thank you, Charlie74. These are helpful ideas. How do you do a “leak down” test? I can do a wet compression test with a little bit of Marvel Mystery Oil. I used this oil to preserve the cylinders and rings while this car was laid up for 30 years, turning the engine over weekly with the crank. It had new rings installed during my restoration in 1987, and ran well for two years before I moved to Virginia in 1991. Then it sat idle until 2019, when I retired and found time during COVID to get it running again. I have put only 900 miles on it since 2021 when I got it running again. This loss of power and somewhat rough running came up quickly in the past, say 100 miles of driving.

Do you have an opinion why the spark plug was fouled? The gap was virtually filled with black, hard gunk. Would that indicate that the exhaust valve was not opening?

Don

 

[Donald1107]

Okay, so I did a wet compression test and it came out 100. Still much lower than the other cylinders (147-151), but higher than a dry test (65). If this indicates bad rings, that is a job to fix. But perhaps easier (by dropping the piston under the engine) than repairing a bad valve. I’ll look at the rocket arm action to see if a valve is stuck.

 

[sp53]

Sounds like a valve could be stuck open. You can pull the valve cover and turn the engine over and watch to make sure all the valves are opening and closing. I would probably pull out the plugs so the motor will spin easily and turn it by hand, although you could spin the engine with the key. If one of the valves is not moving on # 3, smack it with a block of wood and a hammer. It should pop and close. If that is the case, it could be your compression problem also.

Steve

 

[Joel Lester]

I would simply pull off the valve cover... then rotate the engine by hand (easier if the spark plugs are out) until you are on the compression stroke of cylinder #3 and wiggle the rockers. Adjust the valve as needed. Seems to me it would be pretty obvious if one of those valves is out of whack compared to the rest of them... It might be that easy!
Joel

 

[JHaydon]

How do you do a “leak down” test?

A leak down test requires a special tester (appropriately known as a leak down tester or leak down gauge, looks like a regulator for welding) and a compressed air source. It's like a compression test, except you're not asking the starter to provide the (compression) pressure. You connect the rig to the spark plug hole, bring that cylinder to top dead center, and connect the air. The leak down gauge will show you how much pressure is being held in the cylinder, often as a percentage of pressure being put in.

The advantages to a leak down test include making it easier to trace exactly where the pressure is escaping (if you hear/feel air escaping via the exhaust, it's a problem with that cylinder's exhaust valve; bubbling coolant means a blown head gasket or cracked casting; whistling in the crankcase points to a holed piston or broken ring).

Since your compression test reading bumped up somewhat with MMO coating the rings, that suggests a problem with the rings in just that cylinder. I *think* that MMO is thick enough that it could prop up compression even with a cracked or broken ring. My father used MMO religiously and only found out after several years and several dozen thousands of miles that his Packard had been running with a burned exhaust valve. The oil went a long way towards masking the problem. The engine ran smoother after the rebuild but it was honestly hard to tell a difference!

I do agree with the group though, it's worth checking the valve clearance adjustments and valve action. If some of the MMO got on the seat of a burned or bent valve, that could also bump up the reading.

One last thought, you can also get a nice borescope from Amazon for less than the cost of a good leak down tester (never mind the added cost of the compressed air source). Insert the scope into the spark plug hole and see if there's scoring in the cylinders, a hole in the piston, a wedge out of either valve... Also useful for inspecting the ring & pinion, checking the transmission internals, and finding the errant fry your kid dropped under the seat.

 

[charlie74]

Thanks JHaydon.

 

[CJD]

Right off, I would say your description points to stuck or broken rings. Before tearing down the entire engine, though, here is what is easy to check:

+1 check valve lash.
+1 Check leak down...it will fail, but you will listen for where the air is leaking. Intake or exhaust is a bad valve. Out the crankcase breather is rings.

If you confirm rings, the first step would be to use SeaFoam, to see if you can free the rings. If that does not work, then you will have to pull the piston to repair.

Another note, if you look for borescopes online, you can buy a decent one now for just over $100. That sounds like a lot, but it allows you to look into the spark plug hole and look over the condition of your valves, piston, and cylinder. Not to mention, it allows you to look in any small hole around the house you may need to. It really pays for itself in one use.

 

[Donald1107]

Thank you, fellas. This is all good advice. What I’m going to tell you now might blow your minds. When I bought this ‘59 TR3 off a gas station lot In 1987, the engine didn’t run well at all. It started, but wouldn’t stay lit in idle. I checked the compression and it was basically zero. I dropped the pan and pulled the pistons from the bottom. Here’s the punch line: none of the pistons had any rings. My guess is that the prior owner was going to rebuild the engine but gave up and just slapped it back together to sell. It had other serious problems, but not valves, e.g. the rocker arm shaft was so clogged with black funk that you couldn’t push a coat hanger through it. I ended up buying a new shaft. So anyway, I did put new rings and new connecting rod bushings in all four pistons.
I’m beginning to fear that I’m going to end up pulling the head. But I’ll try some of your ideas first, and maybe get lucky with an easier fix.

thanks again!
Don

 

[mezy]

If the rings and or pistons were the issue, you would have had blow by, into the sump, (oil Pan).
there would have been a lot of smoke out of the breather or oil filler cap had you taken it off while the engine was running.
Pushrods cant stick but cam followers can, if you have had an issue in the past it is possible that it is happening now.
If you take the rocker box off, turn the engine over using the nine count check clearances, chances are one of the valves on number 3 will not have clearance.
Remove the rocker shaft and then do another compression test, but chances are the valve will be burned by now.

 

[Donald1107]

Okay, thank you. You are right: without rings there would be serious blow by. I didn’t run the engine with the bonnet up and breather cap off, so I learned of the missing rings right away when no compression.

Possible burned valve is bad news, but driving season in Northern Virginia is about over, so I have a winter to get all this corrected.

 

[mctriumph]

The usual problem is valves suffering from no lead fuel. You may have a valve that has started to
"pocket out " its seat. No real loss the seats need to be cut out and replaced with hardened seats.
If the guides are only slightly out knurl them and install new valves.
Mag the head for cracks first, I have lots of good spares if you need one.....
Mad dog