I've read anecdotal accounts here and there that cleaning various parts of older engines can cause various parts to fail, or be pushed over the edge.

Basically, people say that as the engine ages and wears down, the built up "crud" of various sorts kind of fills in the wear gaps and keeps things functioning, but that cleaning can both release so much gunk that it clogs things up, and loosens worn parts of making things fail.

I've heard this about just about everything that can be cleaned without taking the engine apart, the air intake, the pistons and valves, the crank case and the cooling system.

In my case I've got an older vehicle ( 97 Mazda 323 ) which I've visually verified is extremely dirty in just about every possible way. There is think oily sludge in the intake manifold, think carbon buildup in the crank case under the valve cover, and heavy rust in the cooling system.

I'm worried that if I start cleaning things out things might start breaking as a result.

Is this something I need to worry about, and if so are there any precautions I should take, or is this really just an urban legend?

Here is an example of one anecdote I've read regarding flushing the cooling system for example:

Search under my name...Ive done the citrus flush twice and both times other components have been taken out, like the radiator, water pump etc. Im not saying dont do it, just be ready - anything that is on the edge of failure will be pushed over. Have your credit card warmed up and parts supplier ready forewarned :-)

In my opinion, if your cooling system is functioning properly, I would not do the citrus flush.

  • 1
    if it does break something, it was already well past replacement wear anyway. it needed replacement anyway.
    – Leliel
    Commented Jun 4, 2015 at 22:48
  • Never use citric acid anyway as it destroys aluminum which a fair share of engine components is made of!
    – AnyOneElse
    Commented Nov 14, 2016 at 15:37
  • Well I can say that caked on sludge and oil was holding some parts of my engine together. When I went to do my timing belt and water pump I discovered that the timing belt cover was basically toast and was just being held together in place by layers of old sludge and oil. Commented Nov 14, 2016 at 17:48

1 Answer 1


My car's engine was in very bad condition, and had issues similar to yours. I've since repaired the original engine and years later also installed a new engine.

External Cleaning: I wouldn't say that the dirt holds everything together. Rather, when cleaning off the dirt, you might pull off parts of frail seals or break old plastic connectors, etc. In the end, cleaning it up externally can be very beneficial, since it can reveal problems that you weren't aware of before and makes troubleshooting leaks and other issues significantly easier.

Internal Cleaning I never trusted putting cleaning agents into my engine, rather, I would just drain the old fluids and refill with new ones. This way, I'd let the engine naturally flush the system each time the fluids were changed. If overtime the fluids became dirty again, I would troubleshoot to find the cause and fix the problem. Once again, I wouldn't say that the dirt holds everything together, the problem only comes when (for example, by flushing fluids) you loosen significant buildups that may clog channels, or cause problems with sensors. The oil sludge I'm sure would make it very hard for the engine to pump oil through the narrow passages.

Recommendation For you, I would minimally:

  • Make sure that all the fluids (oil/coolant/brake/steering/...) are fresh. (Are you using the proper coolant/mixture?)

  • Make sure all the filters (Air/oil/fuel...) are clean/new.

  • Clean the major external parts of the engine that have buildup (taking care to be gentle in areas where there are seals or small plastic parts that might be frail)

    Then, if you're feeling very comfortable with your abilities, you can (by hand) clean as much of the sludge/buildup inside the engine as you can, making sure to avoid sending it deeper into the engine. Only do what you're comfortable with... small projects can easily turn into huge one's... and for a car thats in that state, it might not be worth it.

Note: I'm not a professional mechanic, nor have training, but I have done 10 years of significant car repairs/rebuilds.

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