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Smoking 1948 DeSoto

I have a 1948 DeSoto Custom four door sedan with the M-6 hydraulic transmission and the engine smokes excessively nearly all the time. The engine sounds good with no knocking sounds and it idles perfectly. The smoking is greater while taking off from a complete stop after the car has been idling for a while. I have tried Lucas oil supplement and it is not helpful. The car sat for many years until I got it in 1997 and the restoration has been a slow process, so it has also sat a lot since I have owned it. Some of the local old car collectors have told me that the problem is likely that the lower set of piston rings have corroded from the long time of inactivity which does not damage the upper rings as badly. This is why the engine runs and sounds fine and passes a compression test. Others have told me that it is likely that the valve guides are allowing oil to get where it does not belong. Others have told me that this is less likely the problem since defective valve guides smoke the worst when the car is first started and then the smoking decreases. This is not the scenario with my DeSoto since it smokes all of the time.

I would appreciate any opinions or advice on this smoking situation with my DeSoto. I noticed when I registed to post on the website that DeSoto is not listed among the choices of makes of cars. DeSoto was made by Chrysler from 1928 to 1961 and was comparable in cost to a Buick or Oldsmobile of those years. Many taxi companies used DeSotos during the 1940s and early 1950s because the M-6 transmission, which was coupled with a Fluid Drive unit made the cars much easier to drive in city traffic. Thanks for your help

“Fluid Drive”

It would help to know which engine you have. Is it the flathead 6, OHV 6 or a V8? If it is any type of flat head engine, then it is very unlikely that it is the valve guides, it would most likely be stuck oil control rings. Does it smoke a lot more when you go downhill with your foot off the gas?

The problem is probably with the oil control rings. The oil control rings are much thinner than the compression rings so if they become rusted to the cylinders from the engine sitting idle, they’re more prone to breaking when the engine is first started.


I believe your DeSoto has the 6 cylinder 236 cubic inch flathead 6. I had a similar problem with a 1948 Dodge. The car sat for 6 months without being run and I had quite a bit of smoke out the exhaust pipe. It eventually cleared up by itself. I would go along with the problem with the lower piston rings. I would not use detergent oil in this engine nor would I use a multiviscosity oil. These engines seem to do better with 30 weight non-detergent in the summer and 20W-20 non-detergent in cooler weather.
You may want to get some Casite “Motor Tune Up” (not Casite “Motor Honey”) or equivalent and pour it through the carburetor with the engine running. That may help the piston rings.

Some more info on the compression test would help also along with affirming what engine it has.

The actual numbers involved, did they perform both a dry and wet compression test, etc, etc. might shed some light on it.

The reason I ask about the compression tests is that quite often the numbers are misinterpreted even by mechanics who may be relying on erroneous info published in a shop manual.
(Regarding the latter, several Chiltons manuals I have state that 150 PSI on one cylinder is fine and so is 115 PSI on another. That statement is horribly incorrect. In another manual, it’s stated that a 75% variation in cylinder pressures is acceptable and that premise is equally flawed.)

Hello Fuid Drive:

What exactly are you running for oil?
Second have you scoped the bores and performed a cylinder leakdown test?

Before starting her up did you let the cylinders soak in Marvel?

I don’t believe this engine has valve seals? and if the old iron guides are cracked or worn she’ll smoke till the cows come home.

Typicaly yes the engine will tend to smoke more at idle when the vacuum is at it’s highest if the guides or seals or both are worn out.

If you are restoring this car, pull the engine and rebuild it…Having done that, use 10W/30 modern, high-detergent oil…Make sure all the parts you need are available before you start…Those 235 flathead sixes were used in millions of Chrysler vehicles, including Dodge pick-ups through the 1950’s…

When you say that the engine is “smoking”, is the smoke coming from the exhaust pipe or from the road draft tube? Cars in those days did not have positive crankcase ventilation. If you are getting excessive smoke from the road draft tube, you have a ring problem.
While the M-6 transmission was available, DeSoto built many specific duty cabs that had a manual transmission and a conventional clutch with no fluid coupling. These cabs were on a longer chassis because New York City and possibly others required taxicabs to be of limousine dimensions. When this law was rescinded in the eary 1950s, the DeSotos and Packards that had specific cab models disappeared from the scene and the Checker was redesigned. There was a non-commercial model on this longer chassis that DeSoto called the suburban. It had three seats, a roof rack and was quite a plush automobile.

What are the compression readings wet and dry ?? looking up info, this engine only had a 6.6 to one compression ratio ( rated at 109 gross hp ), so you should have around 135 psi per cylinder minimum give or take 10% between them.

\If the oil rings are coked up you can always try adding a quart of an engine flush or a quart of automatic transmission fluid ( any brand ) on top of the oil in there ( one quart overfill will cause no harm ). Just let the engine idle for about 10 to 20 minutes…do not drive the vehicle putting a strain on it on the engine.

Change the oil and go with a straight 30w and see if it helps, other than that you may be looking at a rebuild.

I have a complete '47 DeSoto engine with a stuck valve that was removed for a Chevy small block conversion with a Camaro subframe. The SBC makes my coupe a whole 'nother car. I also have the fluid drive transmission and another complete 4-door parts car. Send me a personal message if you’re interested in any of the above. Just click on my name in this post, and you’ll see how to PM me.