Interpreting compression test results

[MattNelson]

I have been getting some oil and white smoke coming out of both the valve covers, getting into the line from the oil cap to the air filter housing and probably into the carb. So, to start, I did a compression test. This is an I6 170 here, so don't be surprised by a couple missing cylinders:

1 - 170 psi
2 - 170 psi
3 - 170 psi
4 - 170 psi
5 - 150 psi
6 - 85 psi

It's an all stock 1969 Wagon with 26,000 original miles. I know - I thought that was unbelievable, but the factory cylinder pressures are 175 psi, so the 170 psi readings pretty much validates the low mileage of the Bronco. I've driven it over 100 miles and had a sustained top speed of 62 mph. Considering the factory top speed was 85 mph, I say it's not too bad.

Some other troubleshooting I've done:

• Looked at the PCV valve - it was filled with oil. Replaced with a new one, it quickly filled with oil.
• Captured some of the white smoke with a water bottle to see if it condensates (leaking head gasket) - no condensation.
• Looked at the oil and coolant - no sign of cross contamination.
• Smelled the exhaust - no sweet smell signifying burning coolant.

So, my question is what's next? I read this article about compression testing http://classicbroncos.com/forums/showthread.php?t=87199 and I'm really hoping the answer is something other than bad rings and a full rebuild, because frankly, I'm wondering if that's even worth it on a 170. The technical article says that any of the following can be causing low compression:

• Burned Valve
• Burned Valve Seat
• Physical Engine Damage (Burnt valve, Hole in piston, etc…)
• Blown Head Gasket
• Worn Rings or Cylinder wall
• Valve Train Troubles (Incorrectly adjusted Valves, broken valve spring, etc…)
• Jumped timing chain

How do I verify it's a valve problem or something else I can remedy without pulling the engine? Are there any other troubleshooting steps I can take to narrow down the issue or confirm it's a bad piston or piston ring?

I know this might be a basic question to some of y'all, but this is my first real engine work and I'm new to this. Thanks for the help! ;D

 

[Scoop]

First pull the valve cover to check for a broken valve spring. If it's not spring then pull the head. Very easy on a 6 cyl. Whatever is wrong should be pretty obvious with the head off.

 

[blubuckaroo]

Try putting a tablespoon of oil in the low cylinder and check that one again. If the compression increases substantially, your problem is rings.

A couple of burned valves is pretty common after running an old engine on unleaded fuel.

Either way, this would be a good time to upgrade the head for unleaded fuel.

Good finding a car with that low mileage!

How about a few photos?

 

[cldonley]

May be more trouble than it's worth but here's an option. Make sure the low cylinder is at TDC with the valves closed. Pressurize the cylinder with compressed air through the spark plug hole. We use 80psi on aircraft reciprocating engines but we have a cool tool that screws into the spark plug hole and has a fitting installed for the air hose. If the cylinder is exactly on TDC the piston will stay in place, or just put it in gear. With the cylinder pressurized, listen at the carb throat, the the exhaust and the oil filler. If you hear air at the oil filler it's getting by the rings, at the carb its the intake valve, exhaust is the exhaust valve. Remember-there's no shame replacing the rings on only the faulty cylinder as long as the cylinder wall is in good condition.

One thing you can do for valve issues is called "staking" the valve. If you pull the valve cover and find that you don't have a broken spring, take a ball peen and a drift and tap the valve stem so that the valve basically opens and slams shut. This operation can cause carbon build up to loosen from the valve seat or valve head that may be holding the valve open and causing the low compression. And yes, this is an acceptable maintenance practice we use on recip airplane engines when necessary. Done it many times with great results, haven't killed any pilots yet. Just remember that the valves have to be completely closed. You can remove the rockers to make sure.

Since this engine has obviously been doing a lot of sitting over the last 46 years, you may have gunk built up in a valve guide keeping a valve from seating properly too. From what you're describing I'm betting some simple carbon issues causing your low compression, although a cracked ring is a possibility.

Another, easier way to check rings only is to squirt enough oil in the low cylinder to help seal around the rings and no more, then re-do your compression check. If the compression comes up, it's a ring problem.

Doing the above can keep you from being forced into a new engine or a rebuild before you're ready. Good luck!

 

[MattNelson]

Awesome. Thanks for the help! Here's what I'm going to do this week (as soon as I finish some critical home repairs) in this order to continue the diagnosis:

• Perform a dry compression test on the bad cylinder then again with the plug tightened on the neighboring cylinder to double check there is no leakage in the head gasket.
• Perform a dry compression test on the bad cylinder then a wet compression test (with oil in the cylinder) and see if anything changes - if yes, then piston / ring issue; if no, then valve issue.
• Pull the valve cover and check for damage - broken spring.
• While valve cover is pulled, "stalk" the valve.
• Do the pressurized cylinder test by cldonley to verify where the issue is coming from.

blubuckaroo - I'll post some pictures asap! I just want to do a clay bar, glaze, and wax first. It still has the original paint and it's in pretty good shape!

 

[MattNelson]

A couple of burned valves is pretty common after running an old engine on unleaded fuel.

Either way, this would be a good time to upgrade the head for unleaded fuel.

Do you know approx how much it costs to have the hardened seat inserts put in? Is that all that's needed to make the head good for unleaded fuel? It's either that or put in the additives, right?

 

[cldonley]

Talk to some of the guys in your local area about a reputable machine shop. A friend of mine just had his whole 240 in a 67 pick up done by a local shop here in central Texas for about $1500 (we pulled and delivered to the machine shop) so I imagine that valve job with new seats would be a fraction of that.

 

[MattNelson]

Talk to some of the guys in your local area about a reputable machine shop. A friend of mine just had his whole 240 in a 67 pick up done by a local shop here in central Texas for about $1500 (we pulled and delivered to the machine shop) so I imagine that valve job with new seats would be a fraction of that.

Haha - sometimes I do need to venture outside the forums to get answers. I called a local reputable shop in Houston - Texon Motors - and the valve job including new hardened seats is $225. Not too shabby!

Now, fingers crossed this is not a bad piston ring...

 

[MattNelson]

Well - bad news. Looks like the piston rings are shot.

Results from dry compression test:
1 - 170
2 - 170
3 - 170
4 - 100 (looks like this one has a problem too!)
5 - 150
6 - 65 (it dropped 20 psi in a week)

Results from wet compression test (squirting in a couple tablespoons of oil into cylinder) on cylinders 4 and 6:
4 - 180 psi
6 - 110 psi

Question - I drove it 100 miles or so in between compression tests. Is that really enough to damage the rings further and damage cylinder 4? The PO did say that the engine did use to run hot, so I'm guessing that's what caused the failure.

So, for sure the piston rings need replacing. I'm hesitant of putting in much more money in this engine because I think I should swap it with a 200 6 cylinder. The 170 ci produces 105 hp / 158 ft-lbs and the 200 ci produces 120 hp / 190 ft-lbs. I want to add A/C, power steering, and power brakes to the bronco, so I think I need to start with the 200 and do some upgrades - 2 barrel carb, DSII ignition, maybe new heads and intake down the line.

So - what are y'all's thoughts on me pulling the engine, honing the cylinders, replacing the rings myself, and then putting it right back in? If I send it to a shop for a full rebuild, we're looking at $1100-1200. I'm thinking if I can replace the rings and get it back running again, that can last me while I look for my 200 ci and build it up.

Granted - the cylinders could be gouged. In that case, its machine shop or engine swap.

So - should I try to just hone and replace rings or am I wasting my time?

 

[roundhouse]

Could be broken or bad rings. But usually the cylinder walls are worn as well

 

[Broncobowsher]

Low mileage old vehicle. It has sat for a long time. Cold have easily rusted a bore and when brought back to life ate up some rings.

As low (and dropping) as #6 is, now is a good time to get the head off.

 

[Izzy]

It's hard to tell just over the forum. It truly will all depend on how the walls look. If they are not badly gouged, then you can hone and replace rings.

If the gouge is deeper, you might be able to bore and put a larger piston in but then I think you'd have to do them all. I can't imagine it's ok to have 5 pistons of one size and then a 6th of a different size.

At this point, best bet is to open it up and take a look.

 

[blubuckaroo]

^
Yea, pull the head for a look. If the bores look good, do an in-frame ring job, and head work.
I-6 engines are good for that kind of work.