I have an older small block 350 Chevy pulled out of a 1990 Chevy pickup that ran before we yanked it out. I plan on using it in a 1950 Chevy pickup conversion I’m building. Since this is budget deal (aren’t they all?), I don’t want to rebuild the engine but I would like to make it nice on the inside. I could use some help with removing a lot of sludge and stuff from inside the engine. When I removed the oil cooler adapter, it was full of sludge and gunk so I’m assuming the rest of the engine looks that bad on the inside. What would be a good way to clean the engine internally without taking it apart?


We’ll offer some suggestions on ways to clean the engine on the inside, but as with most “mechanic-in-a-can” remedies, the solution may not necessarily do the job to your satisfaction.

Since the engine is already out, we would recommend spending a little extra money for an engine gasket set and at least remove the intake manifold, valve covers, oil pan, and timing chain cover. Replacing all of these gaskets and seals offers a wonderful opportunity to reduce oil leaks once the engine is dropped into your cool ’50 Chevy pickup project.

As a 1990 engine, this is a one-piece rear main seal engine and it also uses a one-piece style pan gasket. But besides replacing the gaskets, this also offers you a perfect opportunity to clean the oil pan, valve covers, and the lifter valley of sludge that may have accumulated in these areas. Oil pans are famous for collecting thick gunk at the bottom of the pan. Cleaners may eventually remove most of this but that may only work if the accumulation is light. Heavier sludge buildups will require physical removal.

Now that we’ve offered our recommendation, there are ways to clean the inside of the engine. The classic old school technique was to drain the original oil and remove the filter. Add some cheap engine oil with high detergents but substitute a quart of automatic transmission fluid (ATF) and run the engine with this for about 20 to 30 miles of basic driving and then drain the oil, remove the filter and add fresh oil and a new quality filter.

When we asked oil tribologist Lake Speed, Jr. for his opinion, we were surprised to discover that he does not recommend this technique. He says that ATF does not use the same high levels of detergents used in engine oil and as such should not be used in an engine . He says that he knows many enthusiasts have used this technique, but he does not recommend it.

Another common technique is to substitute diesel engine oil in place of regular gasoline engine oil because diesel oil generally has much higher detergent levels. Speed, Jr. has a wealth of experience with high performance lubricants and he suggests that a diesel engine oil would be a much better approach. An example of this would be diesel oil that contains much higher levels of calcium, which is a very popular detergent additive.

According to Speed, the key is a balance of low zinc and phosphorous with a high level of detergents like calcium. Diesel oil, like gasoline engine oil has undergone some substantial changes in the past few years and current zinc and phosphorous levels are much lower level and more or less even with SN engine oils for gasoline engines.

Speed, Jr. suggested using Chevron Delo 400 LE which has lower levels of zinc and phosphorous while maintaining decent levels of detergents that will help clean the engine more efficiently.

Other additives that in the past have also worked include Seafoam internal engine cleaner or Marvel Mystery Oil. In each of these cases, we would add a quart of this additive to complete a normal engine oil change, run the engine for roughly 30 minutes to get the engine up to full operating temperature and then drain the oil and replace the filter. These processes may need to be repeated more than once and our estimate would be that the only way to know for sure if the engine is cleaner is to first remove at least one valve cover to establish a baseline for cleanliness. Perform the cleaning session(s) and then again pull the valve cover to check for progress.

Frankly, I don’t think that these steps will be nearly as successful as taking the engine apart and cleaning (at least) the oil pan and valve covers the old fashioned way with solvent and a brush.

Once you’ve completed the sludge cleaning another suggestion would be to use a high mileage oil in the engine instead of a typical SN oil. There are several viscosities and brands to choose from. We found a couple of choices from Pennzoil and Mobil 1 that offer mineral-based oil as opposed to the more expensive synthetics that offer higher detergent levels also with other additives that can be helpful especially if you do not change all the gaskets and seals in the engine.

Internal Engine Cleaning Parts List

This is an oil pan from a small block Chevy with 180,000 miles that we recently disassembled. The bottom of the pan has about 1/8 inch of nasty, black sludge that would be difficult to clean just by changing oil. Doing the job right means removing the pan to get it clean. (Image/Jeff Smith)

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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.