Retail Motor Industry Organisation

Mail Us

Customer Support

Find an

Accredited Member


Fluid loss and intermixing

When separate engine fluid circuits meet, bad things happen

By Mike Mavrigian for Engine Professional magazine
It’s obvious that engine fluids must be isolated within their individual flow circuits. Mixing any combination of fuel, oil and/or coolant is never a good thing. Here, we’ll touch on the various issues that can occur.


If the engine is “using” coolant but you can’t find an external leak or there’s no excessive steam exiting the pipes, check the oil level in the sump (dipstick check) for an overfill condition. Also, if coolant has contaminated the oil supply, the oil will be milky or brownish and thick, appearing more syrup-like. If oil enters the cooling system, the coolant will turn a darker colour.
Coolant can migrate into the oiling system in a variety of ways, including an improperly sealed head gasket allowing coolant to seep into an oil passage at the deck. This could be caused by debris trapped between the head gasket and deck, allowing a travel path for the coolant. An erosion spot on the block or head deck can also provide a minimal or no-contact area of the head gasket. A cracked cylinder wall can allow coolant from the water jacket to enter the crankcase. In some cases, a cylinder wall can crack in an application where the cylinder wall thickness is minimal, adjacent to a head stud location, where the head stud was locked in place with a chemical thread locker compound, where the thread locker expanded as it cured and placed stress against the cylinder wall.
I don’t see this as a common issue, but it can happen. Due to stupidity or simply as the result of a sleep-deprived mistake during a long night of thrashing to prep a vehicle, let’s say that coolant (water or an antifreeze mix) was accidentally poured into the crankcase. Hopefully the mistake is caught before the engine was started. Allow the engine to sit, un-started, for at least one hour. This will provide time for the coolant to settle in the sump, below the oil (remember that oil and water will separate, with oil floating on top of the water). Drain the sump to drain the coolant, until you see several seconds of only oil draining out. Replace the drain plug and adjust the oil level if needed. Operate the engine for at least several hours (or for a day or two). Then perform an oil and filter change. This should remove trace amounts of coolant that remained in the oil circuit.

If an aged block is to be bored to an oversize, it highly advised to first check cylinder wall thickness with a sonic thickness gauge. In this example, a check was not performed. After the cylinders were bored and finish-honed, a small dark spot was observed during assembly. Upon closer examination, the “spot” was a hole open to a water jacket. The iron in this location was in fact so thin and weak that the builder was able to push it open with finger pressure, revealing a hole about 0.500” in diameter. Naturally, a sleeve installation was required to save the cylinder. If this was not caught in time, coolant in the number 4 cylinder in this old Mopar block would have immediately started to enter the crankcase, with eventual catastrophic issues.

If the coolant contamination is realized after the engine was started, immediately drain the oil and remove the oil filter (the filter must be removed and discarded because it now contains a degree of coolant). In order to give any water a chance to evaporate, leave everything open to air (leave the oil pan drain plug off, leave the filter off and remove the valve covers). This may be a waste of time, but it can’t hurt. Leave these items off for a full day before reinstalling the drain plug, filter and valve covers. Add engine oil and a new filter, run the engine for a day, then perform a complete oil and filter change.
While milky or coffee and thick creamy evidence on a dipstick is one of the traditional indicators of coolant in oil, this may not always be the case with today’s detergent oils that may tend to hold coolant in suspension. However, since glycol exudes a “sweet” odour, sniffing the dipstick can provide a clue.
Bear in mind that all engine oils may contain a small amount of water due to condensation during cold/hot cycles.
Minute amounts of water tend to boil and steam-out when oil temperature hits about 100 degrees celsius. The PCV system (if applicable) draws this moisture out of the crankcase. However, a coolant mix features glycol, which does not boil away, but forms acids that thicken the oil and reduces the oil’s ability to flow as required. The acids begin to attack metal surfaces, which, depending on the severity, can cause corrosion, which can also result in the creation of metal particles that attack bearings, in addition to the harm caused by reduced lubricity being delivered by a thicker concoction of the blended oil and coolant milkshake.


If the engine’s coolant appears milky/brownish and “thick,” engine oil has entered the cooling system. The most likely cause may be oil galleys in an aluminium head opening up into a water jacket as a result of overheating, which can result in a warped head. In this case, the head gasket may not be able to retain its seal. In addition to head warpage, other possible causes might include erosion in the deck(s), improper head gasket installation or improper deck surface finish.

Note: If you observe small dark spots in the coolant (when viewing coolant after removing the radiator cap or in the overflow tank), while the coolant appears to be otherwise clear and not milky, if you’ve added a “water wetting” agent to the coolant mix, this could be quite normal and not a cause of concern. However, there have been reports of certain brands of a water wetting agent that eventually makes the coolant look like it’s contaminated with engine oil, with a brown, sludgy appearance. This can fool someone into assuming that engine oil is entering the cooling system. A water wetting solution is intended to improve heat transfer and to reduce the presence of air bubbles which can result in cavitation. Without further research, I tend to think that in most cases where a customer claims that this additive has created the issue, pinpointing the real cause of the discoloured coolant remains important to rule out any potential for oil entering the coolant.
Another potential cause of oil entering the coolant supply is a faulty oil cooler, if the oil cooler depends on heat dissipation by running through the radiator, since oil pressure is higher than coolant pressure, where a pinhole or crack in the oil channel can allow pressurized oil to force its way into the coolant. Naturally, if the engine oil cooler is remote, featuring its own dedicated oil-to-air heat exchanger and independent of the radiator, this would not apply.
A potential cause for oil contamination into the cooling system involves cracks or pinholes in a casting’s oil galleys. Dry bench pressure testing (where a soapy solution is sprayed onto the surface during pressurization) may not reveal this issue.
Submerged pressure testing provides a much better inspection for internal flaws that would otherwise be missed.


This may be more of an issue with diesel engines, where injector cups/O-rings are bad or not installed correctly. This can allow diesel fuel to enter the cooling system. In severe cases, this can require not only injector service (or even cylinder head) replacement), but flushing or replacement of the water pump, heater core and all cooling system hoses.


All too often, we hear the phrase “head gasket failure.” Yes, a head gasket can fail, but it’s extremely rare that the head gasket was at fault. Rather, head gasket failure is a result of being acted upon by other issues. The head gasket can be considered as a “fuse” in a combustion/oil/coolant leakage issue. As with a fuse in an electrical system, a surge or failure of something in the circuit caused the fuse to fail, while the fuse itself was not at fault. Head gasket failure is a symptom that a mechanical issue is present, whether that involves improper cylinder head fluid loss clamping, a distorted block or head deck surface, improper deck surface finish, a contaminated deck surface, a crack or erosion issue in a cylinder bore, block deck, head deck or combustion chamber. Other possible sources, depending on engine design, include improperly sealed head bolts, where head bolt holes are open to water; improperly machined intake manifold mating surfaces and/or intake manifold bolts that may be open to water; and (again, depending on engine design), improperly sealed engine oil coolers, water pumps or timing covers. It’s not uncommon for many engine blocks to feature head bolt holes that are open to water. If the female threads are stripped, this poses a dual concern not only of water migrating along the head bolt threads but insufficient head bolt clamping, which is turn can lead to head gasket failure.
Considering the materials, design and quality of today’s performance MLS gaskets, it’s extremely rare that the head gasket itself caused the problem. If the head gasket has failed, “the fuse blew.” Look for the cause of the problem instead of blaming the gasket.


If coolant or an excessive amount of fuel enters the cylinders, this can potentially result in a hydraulic lock. Since fluids are not compressible, if enough fluid is displaced between the pistons and combustion chambers, this can prevent the pistons from travelling to full top dead centre, placing destructive resistance force between the pistons and rod bearings and rod journals.
This can easily result in bent connecting rods, destroyed bearings and depending on the forces involved, a broken crankshaft and a severely damaged block. If forces are significant enough this can even attempt to push the cylinder heads from the block, potentially involving failure of head fasteners, head bolt threads pulled out of the block, warped or fractured head, etc. Not a pretty sight. Depending on when hydraulic lock occurs, this can either prevent the engine from firing or, if occurring during a high engine speed event, this can spell catastrophic engine failure. Any evidence of fluids entering the cylinders requires immediate diagnosis and correction, hopefully before damage can occur.


Excess fuel entering the oil system is obviously a bad thing. In a gasoline-fed engine, a small amount of fuel entering oil isn’t uncommon, due to conditions such as cold starts, too-rich carb mixture setting, leaking fuel injectors, etc. In general, if the amount of fuel in the oil exceeds about 2%, this is enough to dilute the oil and lower viscosity to the point of potential inadequate lubrication for bearings, valve guides and all frictional areas. Even if the fuel-in-oil issue is minimal, this is yet another reason to be diligent about performing oil changes.
Personally, I never wait until vehicle maker “extended interval” mileage levels are reached. Regardless of how long, in theory, the engine is rated to run between oil change intervals, I never exceed 8000 kilometres on the oil supply. I’d rather spend more on oil and filters than take a chance of compromised oil viscosity that may be caused by the introduction of fuel.
Drag engines that run nitromethane fuel can be difficult to start until the engine spins fast enough and builds some heat. A common practice is to use gasoline or alcohol to get the engine to spin before feeding nitromethane. Failure to get the engine spinning fast enough before you have controlled ignition that a hydro-lock can occur, potentially blowing the cylinder heads off the block. Excessive fuel wash can enter the crankcase, severely diluting the oil, even compromising the heavy weight 70W oil used in Top Fuel).

An example of oil being diluted by fuel is a racing engine that runs on alcohol. Due to the lower air fuel ratio (you need about twice as much alcohol as compared to gasoline), the mixture can run very rich, potentially resulting in excess fuel washing the cylinders and entering the oil supply. Top Fuel and Funny Car nitromethane engines are routinely serviced between rounds, with an oil change being a routine part of the service. Also, alcohol is very corrosive and can cause galvanic corrosion on metal surfaces, especially aluminium. The increased electrical conductivity of alcohol, as compared to gasoline, results in corrosive action and subsequent grit particles that can contaminate fuel system components and internal engine surfaces when excessive fuel wash occurs that allows fuel to enter the oil supply.
In short, engines that operate on alcohol (methanol or nitromethane mixes) require much more frequent engine oil and filter changes.
Using alcohol as a fuel is another reason to run coated cam, rod and main bearings. The anti-friction coating (which also does a better job of surface oil retention) can help to provide added insurance against the potential damage that diluted oil poses. Coated bearings never hurt, and can help, especially when running alcohol fuel.
Alcohol (using methanol as the example) offers advantages for a race engine. Due to the higher octane rating and its higher resistance against detonation, allows you to run higher compression ratios (14:1 as an example), and allows you to run more boost in a forced induction system. Methanol requires a richer-than-gasoline fuel ratio (say 4:1). You’ll use more fluid loss fuel when running alcohol, but you can make more power. Methanol also absorbs more heat as opposed to gasoline, reducing the engine’s heat level.
Because you’re running more fuel in the fuel/air mix, during a cold start-up, when the pistons haven’t fully expanded and the rings may not be seating against the cylinder walls to the optimum degree. The extra clearance allows more fuel wash to enter the cylinders, making its way down into the crankcase and sump.
We’ve already discussed the need to be diligent about oil changes, but this caution bears repeating.