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  • Writer's pictureTeam Vebo

BMW 5 Series (F10) Overheat Repair

Recently we received a BMW 5 Series (F10) with an overheat issue. This F10 comes with a 4 piston turbocharged N20 engine. The owner initially lost the confidence to continue driving the car as it is behaving erratically. Here’s what the owner experienced:

  • The car prompted an error of moderate overheat (there’s no low coolant error prior to this). Driving distance was around 4-5km before the error is shown.

BMW F10 Engine Temperature Too High
Image 1: Engine Temperature Too High

  • The car is difficult to start in the morning (cold start).

  • The car lost significant amount of coolant after a few days of driving.

BMW F10 Top Up The Coolant
Image 2: Top Up Coolant

  • Drivetrain error is triggered with the car misfiring on random pistons.

With feedback above, this sounded like an engine failure. The engine may be consuming coolant or is causing the coolant to be discharged as compression is leaked to the coolant system. It is possible that the customer experienced difficulty to start because coolant leaks into the combustion chamber overnight causing poor combustion during cold start which then leads to the misfiring. However, without solid test results, we do not want to jump into conclusion.

After the car was towed across state to reach us, Team Vebo started to perform basic tests on the F10.

  1. We performed a cold start and the car seems normal.

  2. There are no signs of low coolant or any coolant leak.

  3. Scanned the car and there are no relevant fault codes that could lead to overheating issue

The video below shows the current condition of the car's hoses after it hits 130°C.

With the results above, we have no choice but to perform more detailed tests:

  1. We perform pressure test to the coolant system with 1 bar and it passed.

  2. Performed engine compression test, all piston passed with 185 psi.

  3. Performed electric water pump test and it passed.

So with all test and all seems good, the next move is to check out the coolant pump and thermostat. We carefully removed both components and checked it physically.

  1. Coolant pump is in half cut condition and seems worn. However, it can still be activated and seems to be running.

  2. Based on a sticker on the thermostat, it was manufactured in 2019 and considered fairly new as there was no sign of heat fatigue.

So, we decided to swap out the coolant pump first with our test unit and test drive the car.

Before the test drive, we also wanted to resolve the rough engine idle. We decided to change out the injectors as the coil and spark plug had been replaced earlier when the car misfires. This time, replacing the injectors allows the car to start without misfiring and rough idling is gone.

Test 1 (Electric water pump replacement) – High Speed Test

After testing the car at the highway with speeds over 100km/h with the intention to put the engine under stress, we noticed that the temperature ranges between 72°C to 115°C, with no visible signs of overheating. However, there is a big gap between the lower temperature and the upper range temperature; which is quite abnormal. We then proceeded to a low speed test to see how the car behaves.

Test 2 – Low Speed test

The low speed test is where we simulated driving with heavy traffic in the city by driving with a stop and go pattern with speeds below 80km/h. Surprise! Surprise! The F10’s temperature now creeps up to 118°C and all the way to 120°C briefly. At this temperature, the moderate over heat sign was prompted on the iDrive. The radiator fan started kicking in at max speed and drives the temperature down to 89°C.

So with this observation, it made us wonder what causes the engine to act so last minute or slow before reaching the overheat temperature? Should we suspect the head gasket?

With a good engine compression test result, we decided to put the engine aside as the main cause of the overheat and decided to look into the engine cooling system again.

When the car triggers the moderate overheat warning, we quickly open the front bonnet to access the hoses and the expansion tank and top hose. To our surprise, the expansion tank was not that hot and the bottom hose temperature was just warm where the top hose was scalding hot. This time, we also noticed that the expansion coolant cap was running on 2 bar instead of 1.4 bar. Could this be the cause of this overheating?

BMW Coolant Cap - 2 Bar vs 1.4 Bar
Image 3: Coolant Cap Comparison

The coolant cap is a nifty little device where one of its function is release pressure from the cooling system to avoid overloading the whole system. For example, a 2 bar coolant cap will hold more pressure than a 1.4 bar cap before releasing it.

We did some math’s and here’s what we found out. With 50% coolant (ie:ethylene glycol) and 50% water, the boiling point is 106°C. With this, we calculated that the 1.4 bar coolant cap is able to hold temperature of about 121°C before releasing the pressure while the 2.0 bar coolant cap can hold until 136°C.

When the coolant temperature drops, the coolant will contract and create a vacuum. At this time, the vacuum valve opens and draws coolant from the reservoir tank back into the radiator; bringing it to the proper level. The radiator cap seals the cooling system preventing any air to enter the system.

Test 3 : After Coolant Cap Replacement To 1.4 Bar

After replacing the coolant cap, we continued to test drive the car. This time, we ran it at lower speed as the car tends to overheat faster when it is slow. We noticed that the temperature fluctuations drop to a smaller range from 90° C to 115°C. However, the issue is still not solved since the temperature still goes above 120°C and occasionally up to 130°C.

Test 4 : After Thermostat Replacement

Even the thermostat seems fairly new with 2019 manufacturing, we decided to replace it with a brand new thermostat(with manufacturing date of 2020). This time we noticed that the temperature ranges between to 88°C to 113°C and seemed more stable. We continue to test the car with higher speed and lower speed at longer range and noticed that the range seems to be stable rather than having a big range and no more overheat sign. However, as always, things are not that straightforward since there’s another important step to be done before we can release this car. If not done properly, air pockets will form in the cooling system, resulting to overheat again.

Test 5 : Coolant Bleeding

It is important to bleed the cooling system after a major coolant repair work to remove potential air pockets in the cooling system. After bleeding the car, we tested the car again and there’s no more overheat and coolant related issues. The car passes with flying colors when we performed both short and medium range driving tests. The car was driven for around 50KMs without any sign of overheating.

Test 6 : Long Range Drive

With all tests completed, the final test is to drive continuously for more than 100KMs. During the initial drive, everything seems fine with no signs of overheating. Unfortunately, the car starts to overheat again at the end of this test upon entering low speed city area after a 1 hour highway drive. At this point, the car’s temperature shot up to above 120°C and fails to cool down. This time, it became apparent that we need to target the head gasket. It’s also right on time as our shiny new head gasket test kit had arrived.

The chemical in the test kit will turn yellow when exhaust gas is detected in the coolant, which is a sign where the head gasket is blown. From the image below, it is evident that the N20 head gasket is blown.

BMW Exhaust Gas Leak Head Gasket Test
Image 4: Exhaust Gas Leak Test

With this test result, the team started dismantling the engine and guess what we found?

Image 5: Head Gasket Blown

Image 6: Head Gasket Blown

Image 7: Head Gasket Blown Closeup

Yup, no surprises here since the test result was positive. The head gasket between cylinder 1 & 2 had indeed blown. Team Vebo then proceeded to replace the head gasket with a brand new one.

Image 8: Before and After Head Gasket Change

Now the N20 engine seems to behave much better. In idle, the temperature ranges between 104°C – 112°C.

As a conclusion, this F10’s N20 engine was overheating due to multiple reasons. We were led to believe that the head gasket is good since the initial compression test was ok. Earlier test drive also shows a smooth engine after the injector replacement. At the point where this blog is published, this F10 travelled more than 100KMs without any overheat issue. It's safe to say that overheat is a thing of the past for this F10.

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1 Comment

Aug 05, 2023

Has A/C compressor stopped working during overheating? It works fine when engine cold. My compressor seems to get short circuit each time engine gets hot so I suspect overheating issue but it's F11 520d N47 engine and don't get any overheating errors so far but there is bad leak on the back of the engine it might be from valve cover gasket. Temperature on the dash shows stable 100C only slightly goes above under heavy loads

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