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Pelican Technical Article:

Cooling System Leak Test

Nick Czerula


1.5 hours






Cooling system pressure tester, flat-head screwdriver

Applicable Models:

BMW E46 3-Series (1999-06)

Parts Required:

Parts will be determined once leak is found.

Hot Tip:

Test cooling system when cold and hot when looking for hard to find leaks.

Performance Gain:

Repair coolant leaks

Complementary Modification:

Replace associated hoses and cooling system seals

BMW E46 cooling system components include:

  • Radiator and coolant overflow tank.
  • Belt driven coolant pump bolted to the 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.
  • Automatic transmission models: Belt driven fan attached to front of coolant pump. Viscous clutch controls fan speed based on engine temperature and rpms.
  • 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.
  • Radiator outlet temperature sensor
  • Coolant hose and lines.

Pressure testing your BMW cooling system is the best way to find leaks. However, not everyone has a pressure tester. I would suggest acquiring 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.

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.

E46 models develop coolant leaks toward the front of the engine, in places like the radiator outlet sensor, water pump and thermostat. Start your testing 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.

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 the vehicle is cool and lacks pressure before removing the coolant expansion tank cap (green arrow).
Figure 1

When pressure testing a cooling system, be sure the vehicle is cool and lacks pressure before removing the coolant expansion tank cap (green arrow).

Working with a cold engine, install pressure tester to coolant expansion tank.
Figure 2

Working with a cold engine, install pressure tester to coolant expansion tank. Pressurize system to 1.5 bar (21.8 psi). Pressure should not drop more than 10% over a two-minute time period. If pressure drops right off, that is a sign of a leak. Look for coolant on the ground. If no coolant reaches 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 coolant leaks externally, use a flashlight and follow coolant to source.

Coolant leaking down the front of engine could be water pump or thermostat housing.
Figure 3

Coolant leaking down the front of engine could be water pump or thermostat housing. These are the two most common areas. Use a small mirror, looking from the top of engine and try to locate source of leak. From the bottom, coolant will leak down the side of crankshaft pulley from either source (green arrows).

Coolant leaking down radiator could be from radiator or expansion tank.
Figure 4

Coolant leaking down radiator could be from radiator or expansion tank. If on the driver side, suspect expansion tank (green arrow). If on passenger side, suspect radiator or coolant sensor in lower radiator hose (purple arrow).

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Comments and Suggestions:
Mallcolm Comments: Hi just got coolant leak coming from the rear engine by the flywheel,I don't kno exactly where to look.
I got 1999 bmw 328i sedan
Can u tell me what could leak on that i can be prepared myself to kno exactly what I'm going through
July 13, 2017
  Followup from the Pelican Staff: Could be the coolant pipe or hose to the heater core. - Nick at Pelican Parts  
mckeelaw Comments: How do you replace the 2 hard heater hoses that are under the intake? Please let me know if I can do this without taking the intake manifold off. I've seen some u-tube videos where that's how they're doing it. I just want to know if I can do it without taking the intake off. I know the upper line is leaking. Thanks.
May 26, 2017
  Followup from the Pelican Staff: The intake manifold had to be removed. - Nick at Pelican Parts  
Erik Walter Comments: Thanks Nick! I think you are correct. It's all back together and seems to be working leak free again. My pressure tester screwed on smoothly on to my previous, non-BMW but still European made bottle but it is tough to get threaded onto the BMW bottle.
September 20, 2016
  Followup from the Pelican Staff: Cool. thanks for the follow up. - Nick at Pelican Parts  
Erik Walter Comments: Hi! I have a 2004 325xi. I have replaced the upper and lower hoses as well as the expansion tank - BMW parts except the upper which is Reins. I pressure test it with a 21 lb cap and it leaks from the small tube on the top of the expansion tank at 21psi. New, well fitting parts, including clips. If it was over pressure the cap usually let's go. Any thoughts? Thanks! Erik
September 16, 2016
  Followup from the Pelican Staff: Your tool may not be sealing the tank correctly. - Nick at Pelican Parts  
Minty Comments: bmw 318i e46 2000/1 manual. Radiator keep leaking at tanks and was replaced thrice in 3 months. Was at a Registered MI and now advised that vehicle is building to much preassure. They have recommended that we do a cylinder on leak test, Is this correct? I took car to for a second opinion and they advised that this is a cheap radiator even though it says BHER. can someone please assist before i blow a head gasket.
March 21, 2016
  Followup from the Pelican Staff: I would pressure test the cooling system. If the system will not hold pressure and there are no external leaks the head gasket may be faulty.
- Nick at Pelican Parts

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Page last updated: Fri 8/18/2017 02:22:20 AM