• Hey Guest!
    British Car Forum has been supporting enthusiasts for over 25 years by providing a great place to share our love for British cars. You can support our efforts by upgrading your membership for less than the dues of most car clubs. There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Upgraded members don't see this banner, nor will you see the Google ads that appear on the site.)
Tips
Tips

Blow-by questions

Share this page

J

Jim Lee

Jedi Trainee
Offline
Hi,
I am looking for a lesson on 'blow-by'. To the best of my sketchy knowledge now it seems to be from either leaky valves seals or piston rings not doing their thing well. Is there any way I can test rings to see if they are causing the blow-by? I have had the head rebuilt recently and if those are actually my two most likely suspects
I am more inclined to the ring theory. This is going to be a pretty lame way of describing the amount of blow-by but you can see a steady stream of smoke coming out of the breather pipe when under the car at idle.
After a run you cannot avoid seeing and smelling smoke wafting up from the oil filler/breather cap.

Is there a way to determine how much is 'normal' or, as I have read elsewhere, is any amount of smoke abnormal? The car runs really well, at least I think it does ...not having another tr3 or LBC to compare it to.
The next question is, if any amount of 'blow-by' smoke is abnormal, is there any way to do any sort of test to see what is causing the smoke? For example if I used some very thick oil, or simulated the use of the same, would I be able to be confident that my problem is actually bad rings. Whether I have bad rings or not are there any other avenues for me to follow other than an engine out full blown rebuild?

Thanks very much,
Jim Lee
1959 TR3A
TS50500L
 
Have you done a compression test? Seems to me this could tell you what you need to know. Test each cylinder, then put a bit of oil down the spark plug hole and retest. If the compression reading rises significantly (albeit temporarily), then rings likely are your problem.

Is this something that suddenly started happening, or has it been "smoking" like this for some time? If the latter, rings might just be very worn. If the former, you might have recently broken a compression ring.
 
[ QUOTE ]
...Whether I have bad rings or not are there any other avenues for me to follow other than an engine out full blown rebuild?...

[/ QUOTE ]

You can certainly replace the rings without pulling the engine. You 'just' remove the head & oil pan, undo the big end bearings and push the pistons out the top. Be sure to have the sleeves held down so they don't pop up with the piston (I use a couple of long 1/2" sockets with a big washer under them and a head nut on top:


head3.JPG



You will want to do new con-rod bearings whilst you're there. You can also slip in new mains from below though that is a bit trickier and requires further disassembly.
 
I would recommend checking the thrust washers as well. I changed mine in-situ on the barn find TR4.
 
[ QUOTE ]
Hi,
I have had the head rebuilt recently and if those are actually my two most likely suspects
I had my head rebuilt once on a 74 TR6 and it caused blow-by.
It probably did this by providing better valve and stem sealing which forced any leakage past the rings.
 
That is a very interesting idea I had not really thought of. Hence the great value of this forum. Unfortunately I cannot remember if I had as much blow-by, if any, before the head was rebuilt. Let us say that my valve seals were old and leaky, which is a pretty sure thing, how would the blow-by show itself if not as a constant stream of smoke coming out of the crankcase vent pipe that I see now? I have taken the head off when I blew an old head gasket. I even learned the
lesson of clamping the cylinder jugs down. Sort of. I turned the engine over manually a few turns without knowing of the delicacy of the figure 8 seals. My local, semi-local actually about an hour + away, told me that I had a 50/50 chance of having busted one or more of those very important seals. I remember asking him how I would know if I had. He said don't worry it will show up as water in your oil immediately. I was lucky enough to be on the side that did not bust the seal.

I have done a compression test wet and dry and was relieved to see no great difference. I cannot remember the actual numbers but I do remember they were quite reasonable. That was a while ago but definitely after the head was redone. I will defintely do it again and report back. The engine runs really well but unfortunately I really have not driven it often enough between oil changes to really know if I am burning oil. I know there is no blue smoke coming out of my exhaust. I suppose that would be another way to determine if my blow-by was excessive...to keep very close track of the amount of oil that it is using. Would blow-by burn oil off or, if it is bad rings or cylinders would it just be putting that nasty smoke that I see coming out of my pipe through my crankcase and contaminating my oil?

I am waiting on a radiator recore that I am hoping will be back later this week.

Thanks very much,
Jim Lee

ps - I have a really pathetic looking web page you are most welcome to look at at https://home.earthlink.net/~sasjzl/
I have alot of work to do on it but it is there.
 
Blow-by is gas that has escaped from the combustion chamber and found it's way into the crankcase. Usually blow-by is a result of a broken or worn compression rings. It usually has little to do with oil consumption. You can have blow-by and burn no oil and burn a lot of oil with little blow-by (you will always have a little). What you need is a leak down test. Find a mechanic who has a leak down set. This will isolate the cylinder or confirm worn rings(my bet is rings). I have never heard of major blow-by from a head or valve. $0.02 /ubbthreads/images/graemlins/grin.gif
 
[ QUOTE ]
Blow-by...usually has little to do with oil consumption. You can have blow-by and burn no oil and burn a lot of oil with little blow-by (you will always have a little).

[/ QUOTE ]Excellent point. Now that I think about it, that meshes with what a local mechanic told my dad and me years ago about the "milky" substance that tended to collect mostly on the underside of the oil filler cap of his Herald, which he described as (you guessed it) blow-by. That car didn't really burn or leak much oil and ran well for several years despite only seeming to be running on three cylinders (I was too young at the time to know how to check for problems, and we retired the car to the garage in favor of a Mk3 Spitfire. I still have that Herald, to be restored, but the Spitfire is long since half scrapped, half scattered around my barn to this day!)
 
It's not unusual to acquire a blow-by problem on a higher mileage engine after rebuilding the cylinder head.

When the engine is lacking compression, with everything worn, the compression is pretty much under control.

Now, with higher compression due to a repaired head, the rings show they are, in fact, worn out.

I HATE to spend more on an engine than is required to do a good rebuild...BUT a re-ring job is usually required when repairing a head...IMHO.

Rick
 
"Now, with higher compression due to a repaired head, the rings show they are, in fact, worn out.

I HATE to spend more on an engine than is required to do a good rebuild...BUT a re-ring job is usually required when repairing a head...IMHO."

Then comes the rod and main bearings and the oil pump and before you know it, the job is actually done correctly. The problem with engines over 30 years old is that they are just that, over 30 years old. Seals harden, oil control rings stick to the pistons and wear, carbon takes it's toll on valves and compression ring grooves and on and on.

As the man in the commercials used to say, "Pay me now, or pay me later".
 
[ QUOTE ]
I was lucky enough to be on the side that did not bust the seal.

I have done a compression test wet and dry and was relieved to see no great difference.

[/ QUOTE ]

Jim
There are two possiblitities here, one you were just lucky. Or the rings really are worn, and they just didn't create enough drag to pull the cylinder liners when you turned over the engine.

I suggest you check the leak down by hooking the engine up to a compressor. Get and adaptor to hook the compressor up to the spark plug hole. Slowly let the air in and then listen to find out if and where air is leaking from. (oil hole, exhaust manifold, carb) Adding an air pressure gauge and a valve will also alow you to see how long the cylinder holds pressure. Finally leak down should be checked with the piston both at top and bottom of the cylinder.
You should be able to get a very good picture of what is happening inside the engine this way.
Hopefully, you will not find what I did on my Herald, all the air just instantly escaped through the crank case, and there really wasn't much of a reason to continue with the tests.
Yisrael
 
Back
Top