blow by with good compression

[jtrepasso]

I am currently working on my 1948 dodge D24 sedan and I am trying to figure out why the engine is smoking before I decide to pull it for a complete rebuild. The motor smokes out of the exhaust and fill tube but always starts right up and runs good. I did a compression test with the plugs removed and got between 135 and 142 for all of the cylinders. I have researched this and I am wondering if my oil rings could be stuck? The only history that I know about the engine is that it is from a 1951-1952 dodge

car. I put some pb blaster in the cylinders to see if it helps but other ideas would be greatly appreciated.

[55 Fargo]

I am currently working on my 1948 dodge D24 sedan and I am trying to figure out why the engine is smoking before I decide to pull it for a complete rebuild. The motor smokes out of the exhaust and fill tube but always starts right up and runs good. I did a compression test with the plugs removed and got between 135 and 142 for all of the cylinders. I have researched this and I am wondering if my oil rings could be stuck? The only history that I know about the engine is that it is from a 1951-1952 dodge

car. I put some pb blaster in the cylinders to see if it helps but other ideas would be greatly appreciated.

Those compression numbers seem high, is your tester accurate? Your engine should be 110-120 psi in a new or rebuilt engine.

You have blow-by, smoking oil, hmmm, You say you have good compression, how about worn valve guides?

[JerseyHarold]

If your draft tube is clogged you could be getting pressure build-up in the crankcase that is causing blow-by.

[jtrepasso]

I tested the number one cylinder with the other plugs in and got a psi reading of 90. Then removed all of the plugs as my manual stated and It jumped up to 140 on the same cylinder. After that i tested all of the other cylinders with the plugs removedand got the readings that i reported. Should I re test the rest of the cylinders with the other plugs in? I couldn't figure out why the compression would jump 50 lbs so I re tested the number one cylinder with the other plugs in and I got 90 psi again. I was planning on a full rebuild anyway but I wanted to learn a little more about the state of the engine before I did that. I am not made of money an don't want to do a full rebuild on a decent engine.

[jtrepasso]

I did notice some steel scrubbing pad looking material in the draft tube I assumed the po put in there as a filter. I will remove that and clear the tube and see if it helps. Thanks for all of the quick responses.

[jeffsunzeri]

Your compression readings are likely high because of the engine speed while taking the readings. With the plugs removed, you will get higher readings as the engine turns over faster.

 

Yes, your oil rings could be worn or gummed up and this may be causing the oil smoke.

 

Before you go there though, you need to answer a couple of questions about the smoking:

 

- Does the most smoke occur when accelerating?

or

- Does the most smoke occur when decelerating?

or

- Does the most smoke occur when the engine is completely warmed up, or even when slightly hot after a hard run, and it smokes on both acceleration and deceleration and a bit from the oil fill tube?

 

- Has an oil change made any difference in smoking behavior?

 

The answer to these questions will lead you to the most likely condition and culprit.

[Andydodge]

My understanding is that you do a compression test one cylinder at a time, ie, remove one spark plug at a time, attach tester, turn over then reinstall spark plug and do next cylinder.......andyd

[Don Coatney]

By the book instructions once again.

 

[jtrepasso]

Jeffsunzeri - It smokes on acceleration and I assume on deceleration also. It will smoke out of the draft tube when it is first started and the smoke seems to get worse after it warms up. So it looks like your third scenario. What do you suspect that means. I haven't changed the oil since I bought the car so I can't say if that made a difference.

[Tom Skinner]

All these Old Flat Heads smoke a little out the Filler Tube when you turn them off - fact of Life.

They didn't exactly build engines to real close tolerances back then.

Back in the day they did the same thing. Fully aspirated means fully aspirated.

They Breathe - Vintage MOPAR

Edited May 12, 2015 by Tom Skinner

[Jeff Balazs]

I think you need to remove the road draft tube and make certain it is clear before doing anything else. You should also make sure the breather is not stopped up too. I would suggest you put a vacuum gauge on it and see how it reads after these items are checked.

FWIW an oil change is often the best thing you can do when beginning to assess the condition of an engine.

 

Jeff

[jeffsunzeri]

Jeffsunzeri - It smokes on acceleration and I assume on deceleration also. It will smoke out of the draft tube when it is first started and the smoke seems to get worse after it warms up. So it looks like your third scenario. What do you suspect that means. I haven't changed the oil since I bought the car so I can't say if that made a difference.

Smoke out the tailpipe on acceleration (more than on deceleration) indicates worn rings, bore, and possibly pistons (sometimes a cracked block or head can be the problem too, but that's less likely).

Since the oil quality and age are somewhat suspect, I'd do an oil flush and change hoping that will free up any sticky, clogged oil scraper rings.

If after that your oil consumption is no more than a quart every 800-1000 miles, and the engine is running well and not fouling plugs, you could just live with it.

Let us know what happens after the oil flush/clean/change.

Edited May 12, 2015 by jeffsunzeri

[50desotocoupe]

So question on the draft tube.  Do you just pull it out of the block and clean and push back in?  Does a clogged draft tube cause oil smoke on decel?

[Don Coatney]

I don't recall ever seeing a draft tube completely blocked. And if it were completely blocked and the engine had a lot of blow by it would all escape through the oil fill tube breather.

 

Smoke on deceleration is usually an indication of worn valve guides.

[Jeff Balazs]

I don't recall ever seeing a draft tube completely blocked. And if it were completely blocked and the engine had a lot of blow by it would all escape through the oil fill tube breather.

 

Smoke on deceleration is usually an indication of worn valve guides.

Not saying that it is but it may be restricted.......and that could cause it to smoke quite a bit. I suggest a vacuum gauge ....it will tell you more about this engine than a compression check.

 

Jeff

[Don Coatney]

Not saying that it is but it may be restricted.......and that could cause it to smoke quite a bit. I suggest a vacuum gauge ....it will tell you more about this engine than a compression check.

 

Jeff

Granted a vacuum gauge will tell you a lot but it will not tell what cylinders are low on compression

[jeffsunzeri]

So question on the draft tube.  Do you just pull it out of the block and clean and push back in?  Does a clogged draft tube cause oil smoke on decel?

The draft tube is held in place by a bolt. You unscrew the bolt, and the assembly comes free. It is meant to be cleaned as a maintenance item. The filter in it is meant mostly to keep stuff out of the engine. A clogged draft tube is unlikely to cause smoke on deceleration. It would have to be blocked off almost completely to cause much of a problem, and that problem would be excess breathing through the filler tube.

[Jeff Balazs]

Granted a vacuum gauge will tell you a lot but it will not tell what cylinders are low on compression

That is true enough. When I first got my engine running I did the compression test and found  #4 and #5 were a 10#-12# lower than the other cylinders. I think that is pretty typical of one of these engines that has been sitting for a long spell. I had an initial vacuum reading of 17" with a bounce of 1" or so. I was able to get a higher and more stable vacuum reading of 18" at idle by reworking the carb and replacing the faulty PCV valve. I kept after things by doing several oil changes to flush out all the sludge and deposits. Things actually got better with this treatment. Now I get a steady vacuum reading of 19.5" when cold and once it warms up it is at 20.5" or better. The last time I did a compression check all the clyinders were within 5# of each other and had an average of 116#. I am happy with that sort of number considering I did not do a rebuild. I think these engines can respond very well to modern detergent oils. Once all the old sludge and build up is flushed out even a worn engine isn't so bad as long as there is no serious damage.

 

Jeff

[55 Fargo]

That is true enough. When I first got my engine running I did the compression test and found  #4 and #5 were a 10#-12# lower than the other cylinders. I think that is pretty typical of one of these engines that has been sitting for a long spell. I had an initial vacuum reading of 17" with a bounce of 1" or so. I was able to get a higher and more stable vacuum reading of 18" at idle by reworking the carb and replacing the faulty PCV valve. I kept after things by doing several oil changes to flush out all the sludge and deposits. Things actually got better with this treatment. Now I get a steady vacuum reading of 19.5" when cold and once it warms up it is at 20.5" or better. The last time I did a compression check all the clyinders were within 5# of each other and had an average of 116#. I am happy with that sort of number considering I did not do a rebuild. I think these engines can respond very well to modern detergent oils. Once all the old sludge and build up is flushed out even a worn engine isn't so bad as long as there is no serious damage.

 

Jeff

Hi Jeff, your's is certainly a success story and one I admire. but do think you were a bit lucky too. Your engine must of had good internals, maybe dirty and out of shape from lack of oil maintenance and use.

Compression reading like yours indicate a very healthy engine, and not far from a new engine spec, anything over 100 is considered good for these engines.

I discovered I had a bit of sludge in my valve chambers, and those valves/lifters seem dirty, have not had oil pan off yet, but reckon it is sludgy in there too.

I do not think just doing oil changes will bring my engine back into fitness as yours did, but you never know....here is a pic of the valve chamber, and tonight did go fora short drive down the highway at 65 mph...

[soth122003]

Drop about 1/2 a quart of Seafoam cleaner in the crank case before the oil change and put about 100 miles on it then change the oil and filter. That should clean up a lot of gunk and put it in the oil pan, at least it did for me. 

 

Joe

[Jeff Balazs]

Hi Jeff, your's is certainly a success story and one I admire. but do think you were a bit lucky too. Your engine must of had good internals, maybe dirty and out of shape from lack of oil maintenance and use.

Compression reading like yours indicate a very healthy engine, and not far from a new engine spec, anything over 100 is considered good for these engines.

I discovered I had a bit of sludge in my valve chambers, and those valves/lifters seem dirty, have not had oil pan off yet, but reckon it is sludgy in there too.

I do not think just doing oil changes will bring my engine back into fitness as yours did, but you never know....here is a pic of the valve chamber, and tonight did go fora short drive down the highway at 65 mph...

Fred;

I brought this up because I kinda think that there are a lot of these engines out there that could benefit from this sort of treatment.

And you are correct I was lucky......but I knew it had a chance because I pulled the head and oil pan before I ever turned it over. I won't go into that whole story here. One point I am trying to make is that these engines can hold a lot of sludge and deposits and they can run better once most of this crud has been flushed out. I know I had a couple of sticky valves early on and that is no longer the case. The other point is that a vacuum gauge can be one of the most valuable tools you have in assessing the condition and state of tune. They seem to get ignored by many and for the life of me I can't understand why.

 

One thing I believe in very strongly is if you do your best to keep the oil in the engine clean ......and the fuel and air going into it clean as well you will see the benefits. This isn't as easy as it sounds if we are talking about an old engine that has not been completely torn down and rebuilt. There is plenty of areas for the crap to form and build up and one or two quick treatments are not going to get it all. And I don't think the situation is helped at all by retaining the old road draft tubes and 160 degree thermostats. .

 

Jeff

[jtrepasso]

I flushed and changed the oil and decided to do a tune up while I was at it to track down the missfire. I replaced the plugs, made a new set of copper core wires and took a look at the cap and rotor. It turned out the po put the wrong cap on and the rotor was slightly hitting the cap. I have an ignitor so there are no points. My timing light was broken so I didn't check the timing before changing the cap and rotor. After replacing the cap and rotor it seemed to run a little better so I went and bought a new timing light. I checked the timing and it was advanced around 20 degrees. I set it to tdc and it runs great with no missfire. I don't understand how it was running decently with the timing that far advanced.

[Ulu]

How sure are you that your timing marks are correct? There's an easy way to find TDC and verify the zero mark, if you aren't sure.

[jtrepasso]

To update my progress after flushing the oil and tuning up the engine it isn't smoking as nearly much. Unfortunately I removed the transmission to replace the gaskets and found six missing teeth on the counter shaft gear and rounded teeth on the first and reverse sliding gear. This explains the grinding going into first and reverse. Time to track down some gears or a replacement transmission.