The Stovebolt Site Forums The Shop Area Engine Shop Blue smoke on downstroke

I recently noticed that my motor seems to smoke blue (I think) out of the exhaust but only on down stroke. It only seems to happen when it is in 1st or 2nd (it's a 3 speed) and I release the accelerator from a high rpm. Even when I rev the motor, it doesn't seem to smoke. The truck has always seemed to use oil even since the motor was built but I had always contributed it to the constant oil leaks that creep up. Most motors that I've seen smoke blue do it constantly, not just on down stroke. I figure I'm going to have to rebuild this motor again eventually but I was wondering what may be causing this. Thanks for any help.

Oil consumption during high-vacuum conditions like coasting at high RPM in the lower gears is almost always caused by piston ring problems. Was the block bored during the rebuild, or were the rings just replaced without a rebore and new pistons? Cylinders wear most in the top 1 inch of ring travel, so a ring is constantly changing diameter to try to stay in contact with the cylinder walls if the bore is even tapered a little bit. At 2,000 RPM, the rings are flexing 66 times per second. It doesn't take long for a ring to lose enough tension to let oil slip past it and into the combustion chamber. Reboring and fitting oversixe pistons straightens out the cylinder top to bottom, and makes the rings last a lot longer. I will not re-ring cylinders that have more than .005" taper.
Jerry

The block was bored .60 (or maybe .40, can't remember) over during the rebuild and fitted with new ones. The motor was experiencing what I thought was piston slap before this. All of a sudden after nearly two years the noise suddenly stopped and this started. Do you think a ring is out?

Hmmmm- - - - -was the piston slap isolated to one cylinder, or was it a general noisy condition? The pistons might have been fitted loose enough to rattle, but the noise shouldn't have suddenly stopped. It sounds like it's going to need a teardown and some very careful clearance measurements. Once I wore out a set of rings in a single cross-country trip from California to Tennessee, simply because I didn't get the honing grit out of the cylinders sufficiently, and 2500 miles of highway driving completely wore the oil rings away. By the time I got to Nashville, it was burning 3 quarts of oil to every tank of gas! The engine was a 196 cubic inch Rambler American flathead six.
Jerry

To me the noise was a general noisy one but I brought it to a mechanic and he seemed to think it was isolated to one cylinder. He didn't seem too worried about it but he wasn't sure exactly what was causing it. It definitely worried me when the noise stopped but it was a break from hearing it constantly. 3 quarts per tank is intense! I don't think I’ve gotten to that point yet luckily. If the problem is major I might just save up some money and build a new motor altogether... I was hoping to get more than 28,000 miles out of this one though.

What engine are we dealing with here? I notice both of your trucks in the gallery have V-8's in them. The usual way to isolate a noisy piston is to short out spark plugs one at a time and see if the noise changes with the cylinder not making power. A loosely-fitted piston will usually quiet down noticeably with the ignition disabled to that cylinder, while a loose rod bearing might change note to a double knock with power off the cylinder. Main bearing noises or valve lifter rattles don't generally change note when the ignition is cut off.
Jerry

The engine is a 350 in my 66'. It is bored either .40 or .60 over, has a large crank, RV cam, 66' 327 heads, an Edelbrock performer intake and a 650 Edelbrock 4 barrel carb. I wish it was still making the noise so I could try to isolate it but I'm not sure if I will ever know now that the noise has gone away. The motor still runs great and is very strong. I change the oil about every 2,000 miles and do the oil filter each time. Picture of the motor if interested but I'm sure that won't help.

Hmmmmm- - - -I can't see the front of the cylinder heads very well, but it appears you've got a set of "Power Pack" heads from a 283 or maybe a high-compression 327 on it. If they have a rectangular pad cast onto the front of the head with a single triangle shape centered on the top of the rectangle, they're not 350 heads. Those are 58 CC heads that will bring the compression ratio on a 350 with flattop pistons up over 10:1. The .060" overbore will add even more compression pressure. I've heard a compression rattle on some engines set up that way that sounds almost like a Diesel.

Do your best to plug up the external leaks, and keep an eye on the oil level. Unless you live in a place that gets anal about oil smoke like Commiefornia, just drive it, and don't worry much about a little blue smoke. Oil is cheaper than overhaul parts, as long as it's not fouling spark plugs!
Jerry

That's probably the best advice I've had so far. I'm not positive but I do believe the heads are from a 327. There aren't too many commie pollution haters here in the Ozarks of Missouri so I'll go ahead and keep on driving it. I'll just have to remember to set aside some oil money alongside my gas money. Thanks a bunch.

What was done to the heads before they were put on? Did they knurl the guides, or replace them? I have seen some engines smoke under high vacuum because it was pulling oil around the valve guide. Next time you pull spark plugs, do a leak down test. Should tell you if you are loosing compression by the rings.

I rebuilt a Pontiac engine that after about 20,000 miles started smoking a little on start up and again after coasting down from speed. I had asked for bad guides to be replaced, found out later they had knurled the guides instead.

I'm with Hotrod Lincoln about the rings, but this may give you something else to look at if the leak down test seems okay.

I'm not sure what was done with the heads honestly. But thanks for the info, it's good to know.

I always thought blue smoke under high vacuum conditions was caused by valve guides. Why it wouldn't do it in third gear is beyond me, unless you are going fast enough that you aren't seeing it as easily.

I'm sure it's doing it in 3rd as well but I haven't had it at high RPMs in 3rd yet as I've been driving around town. It doesn't smoke bad so I probably wouldn't be able to see it smoking at fast speeds in 3rd anyway.

What's a "downstroke"?

I'm not sure if that is the correct term but it's what I'm calling it for when you let off of the accelerator from a high RPM and there is backpressure on the motor.

In other words compression braking ??

Dan Bentler

there is backpressure on the motor

Sorry - what does this mean?

Over the last few days I've noticed that my motor has inexplicitly quit smoking. The only thing I did was clean and shake the PCV around and reinstalled it. I didn't think that was a cure since it was still smoking immediately after driving it after I cleaned it. I have no idea if cleaning the valve had anything to do with it but I am glad it has quit smoking.