If you’ve wondered why there’s a bit of oil in your throttle body, also known as the manifold, then you’re not alone.
Modern engines have positive crankcase ventilation (PCV) built into them since the environmental effects of crankcase fumes have become a cause for concern. This leads to oil deposits inside the throttle body (TB).
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Is It Bad When Oil Gets Into The Throttle Body?
A little oil inside the throttle body is not a bad thing and most modern engines have a positive crankcase ventilation system that directs fumes from the crankcase into the engine to be burnt as part of the combustion process. Excessive oil build-up is a cause for concern and should be investigated.

Crankcase Fumes
A small quantity of the gases from combustion, also known as blow-by, can pass between the gap made by the compression and oil rings and the sides of the cylinders.
These gases would normally pass into the exhaust pipe and then on to the catalytic converter to be eliminated.
But now, as engines become more complex, the PCV must keep pace and become a more complex system.
The reason that more fumes are passing into the crankcase in modern cars is due to the high-performance nature of the technologically advanced modern engines.
The pistons and rings slide more easily up and down the cylinders.
The piston rings exert a lot less pressure against the walls of the cylinder, allowing more gas to find its way into the crankcase.
This gas would cause a pressure build-up that could damage the engine if a way was not available for it to vent.
Previously, crankcase gasses were vented into the atmosphere but environmental legislation forbids this.
Engine gases must be processed before they can enter the exhaust pipe, so PCV systems were introduced to burn off as much of the crankcase fumes as possible.
The Positive Crankcase Ventilation System
To balance the pressure between the crankcase and atmospheric pressure, a crankcase ventilation system is designed to draw exhaust fumes out of the crankcase and into the throttle body.
Reducing the pressure protects the engine from the damage that contaminants could do to the oil system and helps extend the life of the engine.
It also has environmental benefits due to the fumes being directed back into the combustion chamber for incineration and onward expulsion into the exhaust and catalytic converter.
Blow-by consists of hydrocarbons, nitrogen oxide, water vapor, aldehydes, sulfates, and carbon dioxide.
For obvious reasons, environmental and emissions laws have become stricter, requiring more complex PCV systems to deal with the gases.
Turbocharged Engine Challenges
With the advent of more turbo-charged engines, systems had to be found that could introduce fumes into the low-pressure system and the high-pressure system, depending on the design of the engine.
To avoid reducing the life of spark plugs, damaging the turbocharger and aftercooler as well as preventing engine detonation that damages pistons, a complex series of valves, pipes, and venting are used.
The point I’m making is that it is common for mechanics to remove parts of the PCV system to reduce oil entering the intake air stream.
This can temporarily fix the problem of excess oil in the throttle body but ultimately does damage to the engine.
Air Filters and Air Pressure
On more complex PCV systems, some filters clean the crankcase fumes before they are injected into the intake manifold or turbocharger.
These filters can get clogged and require regular maintenance and replacement.
Likewise, the air filter must also be replaced at the stipulated service intervals to avoid problems surrounding the intake of fresh air into the intake manifold.
Various sensors take care of fluctuations in temperatures, quality of fumes, and engine load, so you may not notice when the oil starts to build up in the throttle body until you perform a visual inspection.
This means your engine will have performed poorly, usually with less power and greater fuel consumption.
Water in Your Oil
A large component of combustion is water vapor. If this water vapor enters the crankcase, it can mix with the oil and reduce its efficiency. This causes excessive wear and tear on the metal surfaces where the oil is supposed to provide lubrication.
The oil and water mixture creates an emulsion that can clog oil filters and, when mixed together with other contaminants, forms a sludge that collects in the crankcase.
This water/oil mixture is cooler than the circulating oil and can form acid compounds that cause accelerated corrosion inside the engine.
These problems are not restricted to petrol engines as there are similar problems associated with turbocharged diesel engines too.
Check Your PCV Valve
You’re unlikely to see the PCV valve appear on any regular maintenance schedule at a dealership. This is because the systems usually work fine for many years and seldom cause problems.
However, if you experience a rough idle, increased oil consumption, or slower than normal acceleration, it may have its roots in the PCV valve.
The valve can become stuck. When this happens, it does not allow the fumes to move from the crankcase into the inlet manifold.
The valve is normally found on the valve cover, usually mounted inside a rubber grommet. You can remove the valve from the tube and shake it. If you hear the sound of a piece of metal rattling inside, then it is probably still working fine.
If you, however, hear nothing, then the valve is gummed up and needs replacement.
You can usually obtain the valve cover and valve as a unit rather than trying to find a valve to insert into the grommet.
The high temperatures and oily deposits make trying to replace the valve difficult and the valve cover is a relatively inexpensive part to purchase.
After replacing the valve, your car should run a lot smoother and there’ll be less oil in the throttle body.
Frequently Asked Questions About Is It Bad When Oil Gets Into The Throttle Body
What does it mean when you have oil in your intake?
Oil in your intake is normally caused by a PCV valve that has become clogged or is damaged. To fix the problem, check and replace the valve if necessary. You can also make sure that you replace your air filter, oil filter, and engine oil regularly.
Can oil in the intake manifold cause a misfire?
Too much oil mixing with your fuel can cause a misfire. It also fouls your sparkplugs and leads to your valves becoming caked with carbon deposits. This will cause your engine to lose power and run inefficiently, and use more fuel.
What happens when the PCV valve is clogged?
When the PCV valve is clogged your car doesn’t perform well. The oil will travel up from the crankcase and end up in the combustion chambers of your engine. You’ll see evidence of this as blue smoke coming from your exhaust. This should not be confused with smoke caused by worn piston rings.
Conclusion On Is It Bad When Oil Gets Into The Throttle Body
It is not bad when someoil gets into the throttle body. Excessive oil build-up in the throttle is a problem that needs investigation.