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Engine sludge is basically a dirty,sticky, grease-like substance,formed due to oxidation/contamination of oil in the engine. During oxidation,the molecules of engine oil break down and the resultant products combine with the dirt in carbon form,metallic particles,fuel,gases,water and liquid coolant. Together the mixture forms a sticky sludge.
Stop-and-go driving in heavy traffic conditions and in areas where there are lots of traffic lights can contribute to sludge buildup. Frequently driving over short distances can also cause sludge.
The resulting material blocks the engine’s lubricating system.In the worst case this can cause the engine to seize.

Common Reasons For Engine Oil Sludge BuildUp

1)Neglect of maintenance
Since it acts as a coolant in the engine and smoothens the running of other components, it is necessary that maintenance is done in time.
When servicing is delayed or postponed,there are high chances that sludge deposits will be formed since the oil will not be in position to flow freely.

2)Use of inferior oil
While conducting service,it is wise to use engine oil,which is recommended by the manufacturer.Actually,this is made easy by the manufacturer by indicating the type of oil to be used in the engine on the dipstick holder.

3)Low quality fuels
It leads to carbon build up' i.e engine sludge formation which suffocates the engine and wreaks havoc on the fuel system.

4)Excessive idling
Idling refers to when the engine is running but the car is stationary like when stuck in a traffic jam.

SYMPTOMS
*Low oil pressure or intermittent flashing of the oil light
*The oil light remaining on when the engine is running
*Slow or no draining of the oil when the sump plug is removed
*Obvious signs of thick sludge in oil fillers,rocker covers etc.
*The engine won’t accept the designated amount of oil after an oil change
*Noisy hydraulic lifters

PREVENTION
1)When you turn the ignition on,check the dashboard for the Check Engine Light and the Oil Change notification light.
Both could indicate that the motor oil needs replacing.

2)Study the owner’s manual that your car manufacturer provides for instructions on when to replace the motor oil and also the type of oil recommended.

3)Avoid stop-and-go driving as far as possible. Walk or bike over short distances to prevent motor oil sludge buildup.

4)If the dashboard gauge shows that the car is heating up,have your mechanic check for motor oil sludge also.

5)It is never advisable to add engine oil if you see that the oil pressure is low.If the oil pressure light is on have it inspected or replace it completely.

Credits:
yourmechanic.com/,monitor.co.ug/

https://autojosh.com/causessymptoms-prevention-engine-sludge/

I think "stick to recommended oil viscosity and service interval" should be #1 on the list.

How can one detect sludge without opening the engine?

ORAGAMYFARMS:
How can one detect sludge without opening the engine?

you can peep fru where you fill in Oil to your Engine
yeh that HOLE