What a drag,

I've been trying to track down why my '66 Toronado isn't running 100% so after screwing around with ignition and the carb for a while I decided to do a compression test today and, unfortunately, I found a low cylinder. All the other 7 cylinders were about 190 PSI dry (I forgot to open the throttle when I did this so hopefully that doesn't change the results too much) but cylinder #6 was 90 psi dry. I was super bummed to find this as this is a very, very well looked after engine. My Dad has owned this thing since 1973 and was extremely fussy over it and changed the oil religiously. The only thing is that the car hasn't seen a ton of use of the last 40 years. 80,000 miles on it, but probably less than 1000 miles has been put on this car in the last 30 years.

Anyway, I threw a little oil down the cylinder and the compression test jumped right up to 120 psi so I suspect there's something up with the rings. I put some good ol' MMO down the cylinder and cranked it over by hand a little until the MMO started to run out of the cylinder just so I know it's pretty well all around the piston and I'm letting it soak currently.

Is there anything else I can do here? It's pretty upsetting to tell ya the truth but that's it I guess......the joys of old cars!

 

You've done most everything you can and moving in a positive forward direction in evaluating the issue. Perform a couple more dry/wet tests on that cylinder to see if it comes up any more than 120. However, the wet test does demonstrate a good possibility it's rings. I mean you could additionally perform a leak-down test, and it might be warranted at this point, as well. However, do a couple more dry/wet tests after it's soaked good - see if it pops up any more.

 

After your MMO soak, and just prior to your next test, add/shoot in about a tablespoon of 10-30 or heavier oil. MMO is ~3-5 viscosity. You'd prefer a heavier oil to gain a decent seal around the rings.

 

Thanks Norm for the positive response, much appreciated.

Should I change the oil before I run it or am I good for now?

 

Oh, I should mention a few things, perhaps I screwed up this test a bit.....

First, I did the test on a stone cold engine. Should I have done it warm?

Second, I didn't hold the throttle open. After I did a little research, I realized I completely forgot that you're supposed to hold it open. Does this make much difference?

 

No, there is no need to change oil. You would "prefer" a warm engine, not a "hot" engine; but, honestly, to provide a direct comparison, you'd likely want the engine same temperature you performed the first compression test. A warm(er) engine will yield a slightly higher compression than a cold(er) engine.

 

You're doing fine. With the numbers you presented (190psi vs. 90psi dry) then an increase from 90 psi>120psi at this point it's best to just replicate your first compression test with a tablespoon of a little heavier oil.

 

Metals expand when heated; albeit, expect rings to minimally but slightly expand when heated, creating less open distance (less exposure=smaller void) yielding a very slightly higher psi than cold(er).

 

I would perform a leakdown test. Obviously you know the cylinder is low but it can help you determine what the issue is. Zip tie gloves around the exhaust pipes and fill the cylinder with air. You'll have to listen hard with a stethoscope and see where its leaking by. If the gloves inflate its an exhaust valve issue, if the intake is hissing then its the intake valve. Also pull the pcv and breather and tape one or the other. Place your hand over one hole and see if its building pressure in the crankcase. I have successfully repaired one valve several times with this method and regained compression. Most recently in an old Farmall MD tractor. One cylinder had zero compression and it turned out to be a burnt exhaust valve. If you're unfamiliar with the MD engine, its an awesome antique engine and well worth the read..

 

I mean, I could go get a leak down tester but doesn't the increase in compression after throwing some 5w20 down the cylinder point more towards the rings?

 

Lets look at the facts and disregard some faulty reasoning. 7 cylinders are 190 PSI and cylinder #6 is 90 PSI. You add MMO to cylinder #6 and it jumps to 120 PSI. 120 PSI wet on cylinder #6 is still 30% lower than the 190 PSI of the other cylinders dry. Adding oil to the other seven cylinders will only increase the PSI.

Are you following me yet ? You'll need to start looking at valves and valve train on cylinder #6.

 

As pointed out (above) there is no reason to replicate the entire compression test - only No. 6 cylinder, and truly only to validate your previously provided number. If that number remains the same it may provide more of a "clue" as to the nature of the expected issue. If No. 6 cylinder increases significantly with the addition of a heavier (more viscous) oil, there's a good chance it may only be the rings. If however, you gain no appreciable increase in psi, there's likely a better chance it's valves and/or valve train.

 

Not really. throw oil in a good cylinder and the number will jump. You don't even need the leak down tester, just figure a way to fill the cylinder with air and see whats up.

 

Sorry, I disagree w/ that line of reasoning. An engine with solid smooth cyl. walls and well seated, neither not broken, nor cracked, nor scorched nor displaced nor worn rings will demonstrate virtually no difference in psi compression between dry & wet.

 

Let's not bicker.....I'm gonna follow a straightforward process. I'll assume stuck rings for now and if I have no success, I'll move on to a leak down test and start checking the valves. It's not like I'm in a rush or anything.

 

Soak the mmo for a few days, keep adding a few ounces everyday, before starting, oil the cylinders, the mmo drys the cylinder walls, worth a shot to free stuck rings.

 

I don't disagree with that but this a nearly 60 year old engine. It has a little of everything going on. It needs more diagnosing besides squirting a little oil down the cylinder and spring boarding that into a rebuild...


Also the oil down the cylinder trick will slop around the cylinder and can splash onto the valves and coat the seats, especially if it was a large amount. So the test is far from reliable.

 

No bickering, attempting to provide helpful insight which often direct/lead to further logical avenues to pursue.

Fair enough idea, but the delta between 90psi & 190psi you'd likely agree is significant & drenching/soaking No. 6 will probably prove futile. Still, I like that idea.

I agree w/ you, a leak down test is far more informative.

 

If this beastly engine is producing 190 psi I'm highly suspicious those aren't the original rings.

 

This thing has been my family's car for over 50 years. If the rings aren't original, they were done prior to 1973 by the original owner. I've got every single service record, part receipt, and oil change ticket since Dad owned the car. He tells me the engine has never been opened since he's had it at least.

 

That is fantastic & honestly - highly remarkable. Awesome!

 

Yep, the car is pretty special. Check this out, I just dug out my paperwork and checked the milage on it. Currently there's 80,683 miles on it. I have a receipt for an oil change in 1982 and the mileage at the time was 79,864. So about 800 miles on it in 42 years!

 

Retested this morning after soaking in mmo all night, poured a little 1030 in and I got 160psi.

I guess whatever I'm doing is working so I'll keep at it for a few days

 

Great news. Based upon your driving habits e.g. how many miles you drive with the engine at normal operating temperature, how long it sits & the conditions in which it sits (dry, humid, moist) etc., etc. engines are (obviously) meant to be "run". I "suspect" (hopefully) you may be witnessing ring(s) which perhaps lightly seized w/in No.6 piston grooves. Happens from various reasons & you'll never know unless you open up the engine. But, you've arrived at a landmark bit of good news - No.6 compression has increased nicely so there's good reason to suspect the oil soak has released modestly seized ring(s). You might consider another 24hr-48hr piston soak w/ MMO basically as you did previously. Eventually, you'd like to remove the MMO from the preferred engine operating oil - you may see lots of smoke from the exhaust. After another MMO piston soak, change the oil + filter. I've witnessed others who've completely filled cylinders w/ diesel, diesel + ATF, MMO, etc. and achieved excellent results. I think (hopefully) you dodged a bullet - at least you're trending in a positive direction, eh?

EDIT: Oh yeah, take this beast out and drive it.

 

Thanks for the advice Norm, and everyone else too. I'll keep working at it and post any updates.

 

This is out of VGG, for whatever that's worth, but he'll run two stroke gas in an engine that has sat for awhile to loosen things up. They smoke some but it almost makes sense.

 

I just use marvel mystery oil......I don't know what's in that stuff.....it's a mystery

 

It definitely tastes like ****.

 

‘Wild Turkey’

 

I stole that joke from Vice Grip Garage, surprised you didn't catch it. I'm a hack, I know.

 

That went right over my head. I haven't watched his stuff in about five years. The early stuff was good. Once he developed his caveman shtick I found him to be annoying. With the early stuff he was a normal guy with a quick wit and then all the sudden he started acting/talking like a simpleton. Drove me nuts.

 

Adding oil will increase cranking pressure on any cylinder, even good ones.

 

If you consider an increase of 1psi or 2psi an increase - I'll agree. Yet, I stand by my assertion. You won't see any virtual/appreciable increase in compression psi on even a good one. You find a big increase it won't be a good one.

 

A tablespoon of oil is somewhere around 20cc. Thats more than enough to raise the compression ratio temporarily as that oil will fill the piston dish and jump the psi number. Just another reason how the oil down the cylinder test is a misleading and/or meaningless diagnosis. A leakdown test is so much more valuable.

 

You're smoking hashish, right?

 

You've taken a lesson from today's news outlets by selectively quoting me. Good job!

Oil doesn't compress. Put 20cc of oil in a cylinder and you have effectively removed 20cc of dead space which will TEMPORARILY!!! Increase the compression ratio until that 20cc is out of the cylinder. Of course the cranking psi increases. Wrap your head around that.

 

I'm thinking you've upgraded from hash>smack. "Compression ratio", is it? While pounds per square inch is a ratio of lbs per one inch squared. Best to remain within the same equivalents & discuss psi. This way we don't have to figure exactly what "compression ratio" you're referring to.

 

The conversation has gone from productive to inane, IMO. I'll grab a cookie and find my way out the door. Good luck on the MMO cleanse. I think you got it.

 

I got nothing more for you. The jabs speak volumes. You honestly can't figure out the relationship between the compression ratio and cranking psi? Keep throwing the jabs.

 

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