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Pelican Technical Article:

Cooling System Leak Test

Nick Czerula


1.5 hours






Cooling system pressure tester

Applicable Models:

BMW 525i/xi Sedan (2004-07)
BMW 528i xDrive Sedan (2009-10)
BMW 528i/xi Sedan (2008-10)
BMW 530i/xi Sedan (2004-07)
BMW 530xi Wagon (2006-07)
BMW 535i xDrive Sedan/Wagon (2009-10)
BMW 535i/xi Sedan (2008-10)
BMW 535xi Wagon (2008)
BMW 545i Sedan (2004-05)
BMW 550i Sedan (2006-10)

Parts Required:

Engine coolant. Additional parts will be determined once leak is found.

Hot Tip:

Check for leaks with engine warm and cold

Performance Gain:

Repair coolant leaks

Complementary Modification:

Replace associated hoses and cooling system seals

The BMW E60 cooling system is filled through the plastic coolant expansion tank (or coolant reservoir) at the right front of the engine compartment. There is no cap on the radiator. Other components of the cooling system consist of:

  • M54 and 8-cylinder engines: Belt driven coolant pump bolted to the front of the engine block.
  • N52 and N54 engines: Electric coolant pump bolted to the right front of the engine block.
  • Electric cooling fan attached to rear of radiator. The cooling fan is controlled by the engine control module (ECM) via an output final stage.
  • Electrically heated thermostat.
  • Automatic transmission cooler (heat exchanger).
  • Heater valve and heater core (for climate control).
  • Coolant level sensor inside expansion tank.
  • Coolant temperature sensor at cylinder head.
  • Coolant hose and lines.

Pressure testing your 5-Series cooling system is the best way to find leaks. However, not everyone has a pressure tester. I would suggest owning one if you plan to maintain your own vehicles. It can save quite a bit of time when locating a cooling system leak. You can also check for leaks without a pressure tester, but results are less accurate and the procedure is time consuming. I will cover pressure testing in this tech article, as well as some tips on checking for leaks without using one. Always start with a cool engine. If you do not find the leak, warm the engine with the pressure tester installed. At times, leaks will surface as parts expand from engine heat. Look for leaks at and around all the components listed above.

A cooling system pressure tester is used to pressurize a cooling system and hold it at a specified pressure while looking for leaks. Coolant leaks can be internal (i.e., head gasket) or external (i.e., radiator). It is important to take these things into consideration when looking for a coolant leak. If you are losing coolant, but not seeing any on the ground, this would likely be an internal coolant leak. Puddles of coolant found under your vehicle would be from an external leak. Remember to properly clean any coolant that gets on the ground and properly dispose of it. Place a drain pan under suspected areas of leaks to minimize coolant spills. Six-cylinder models can develop coolant leaks toward the front of the engine, in places like the radiator outlet sensor, water pump and thermostat. Eight-cylinder engines can have the same leaks as 6-cylinder models, with the addition of the leaking coolant pipe in the valley of the engine. A leaking coolant pipe on an 8-cylinder engine will usually leak coolant down the rear of the engine, coming down around the bell housing of the transmission. A pressure tester is the quickest way to find these leaks.

Do not remove the expansion tank cap to install the pressure tester while the engine is hot. Coolant or hot steam may escape and will scald you. To do any work on the cooling system, wait until the engine has cooled off.

Keep in mind that when your car was serviced before, parts may have been replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have, so be prepared with different size sockets and wrenches.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you're working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Do not work on your vehicle if you feel the task is beyond your ability.

Vehicle models change and evolve as they grow older, so the vehicle shown in our illustrations may vary slightly from yours. If something seems different, let us know and share your info to help other users. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

When pressure testing a cooling system, be sure that the vehicle is cool and lacks pressure.
Figure 1

When pressure testing a cooling system, be sure that the vehicle is cool and lacks pressure. Then, remove the coolant expansion tank cap.

Once the cap is removed, install the pressure tester adapter (green arrow) to the coolant expansion tank.
Figure 2

Once the cap is removed, install the pressure tester adapter (green arrow) to the coolant expansion tank.

Pressurize the system to 1.
Figure 3

Pressurize the system to 1.5 bar (21.8 psi). Pressure should not drop more than 10% over a two-minute time period. If the pressure drops right off, that is a sign of a leak. Look for coolant on the ground. If no coolant reaches the ground, you could have an internal leak. If you suspect a head gasket leak, you need to test cylinder sealing via a leak down test. If the coolant leaks externally, use a flashlight and follow the coolant to its source.

Coolant leaking down the right side of the engine could be from the water pump or thermostat.
Figure 4

Coolant leaking down the right side of the engine could be from the water pump or thermostat. These are the two most common areas. From the bottom of the engine, coolant will leak down from the cluster of cooling system components. The water pump has a weep hole (green arrow) where coolant commonly leaks from. It can be viewed using an inspection mirror. Coolant leaking down the left side of the engine can be from coolant hoses or from the oil filter stand / housing. Coolant leaking down the radiator could be from the radiator or a radiator hose. The yellow arrow points to a faulty hose O-ring I found. This vehicle had an intermittent coolant leak. I found the leak by pressuring the cooling system. Then I wiggled the hoses. While wiggling one hose, coolant leaked. When I removed the hose, I found a dislodged O-ring. When looking for leaks at the radiator, inspect the side tanks on the radiator for signs of leaks or fresh coolant. Use a mirror to inspect the bottom of hoses and connections at the radiator.

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Comments and Suggestions:
jj Comments: 07 bmw525 intermittent leak when running. I pulled the bottom cover off and can't seem to locate the source. even running, the leak isn't consistent. any ideas ?
February 14, 2017
  Followup from the Pelican Staff: Yeah I have found that its easiest to find the leaks on these cars if you pressurize the system using a pressure tester and cap adapter WHEN COLD. The thermal expansion tends to seal up the leaky areas as the car warms up. - Casey at Pelican Parts  
Rarmendariz1954 Comments: 2003 X5 3.0 I have water in the coolant but not in the oil . Then I noticed the transmission acting weird. Drained the fluid and found not much left what was there was watery. Would this be the cooler leaking or the orings that are in between radiator and cooler. Thanks
January 8, 2017
  Followup from the Pelican Staff: Could be the cooler leaking. I would inspect the seals and lines as well, but most likely the cooler. - Nick at Pelican Parts  
Darkneck Comments: 525i e60 n52
October 27, 2016
  Followup from the Pelican Staff: Thanks. - Nick at Pelican Parts  
Darkneck Comments: I have a 06 525 9n520, and I got the oil minimum level warning and I checked the. Coolant expansion tank and It was full of engine. Oil. I recently replace the oil cooler and gasket, however I did not do the oil filter housing gasket. It's leaking bad.... would I be getting cross contamination from there? Otherwise the car runs flawlessly
October 27, 2016
  Followup from the Pelican Staff: You can mix from the filter housing. - Nick at Pelican Parts  

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Page last updated: Wed 3/29/2017 02:24:11 AM