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FCP Euro Service Kits

FCP Euro Kits

car-cooling-system-diagramDiagnosing a cooling system on your Volkswagen or Audi is easier than you think.

Using a few tools that are cheap and readily available, your cooling system can be properly diagnosed in less than an afternoon, and save the hassle and expense of a shop.

Whether your car is overheating or has fault codes for the cooling system, the process is the same, save the use of a scan tool. You will need an infrared temperature gun, a throttle stick (or someone to keep the engine at an off idle speed while you're looking under the hood), a flashlight, a pressure tester, and an OBD scanner – but if you can't get those last two, you will still usually be able to do a good enough diagnosis.

IMG_20141007_160506_503 Dead cold engine on an '83 GTI. Diagnosis process is the same for later cars.

 

First, start with a cold engine. The idea is to run the car up to temperature and monitor it the whole time, to figure out where the problem is. Check for external leaks first, even if you don't have an overheat condition. You can put it under pressure to check for leaks, but be careful not to overdo it. Even if you do find a leak, continue with diagnosis, making sure you have enough coolant to run the engine for an hour. Check if your coolant looks like rust water. If it does, you should still continue with diagnosis and do a flush after.

Fan check

After a thorough visual inspection, it's time to check the operation of the cooling system. Start by turning the engine on and turn the air conditioning on. If you have electric fans, they should come on low speed with the A/C on. If one isn't turning on, it needs to be replaced. If both are not turning on, check to make sure the A/C compressor is turning on. Some newer cars do not run the fans with the A/C on, so make sure the compressor is running at least, and then continue diagnosis.

IMG_20141007_160344_060 Good coolant flow at idle

 

On the Mk4 Golf/Jetta, a common problem is the fuse panel melting on the top of the battery. If your fans don't come on, A/C doesn't work, and the car overheats, this should be the first thing to check. In particular make sure the 30 amp fuses aren't melted. If they are, replace the entire fuse block – being careful to put everything in the same order you took it off.

Check for water pump flow at idle. This is done pretty easily on an Audi or Volkswagen by shining a light on the coolant reservoir. If you see ripples or a jet of fluid from the top of the tank, you're good. On some Audis and the 97-05 Passat, water pump flow doesn't appear in the expansion tank until after the thermostat opens. If you don't see flow at first, don't panic, it may just need to be warmer before you see flow at the tank.

Now, check the operation of the thermostat. Hook up your OBD scanner if you have one and watch the coolant temperature. Using your infrared gun, check the temperature of the lower hose and upper hose of the radiator. The upper hose should be hot, and continue to get hotter, until the thermostat opens and you see the lower hose getting warmer. In general, the thermostat opens around the time the cluster says the engine is at operating temperature.

Cluster showing a warmed up engine - the thermostat opens around this time. Cluster showing a warmed up engine - the thermostat opens around this time.

 

Once the thermostat opens, watch the temperatures of the hoses as you watch the coolant temperature in your OBD scanner if you are using one. Typically, thermostats open at around 90 degrees celsius, though some open later (as high as 99), and some open earlier. The infrared gun is not extremely accurate, so it's good to have an OBD scanner that gets a precise reading. I still prefer the gun though, as it's more accurate than touching hoses and saying, “That's kinda hot. That's pretty cold.”

Sometimes thermostats can stick while opening and cause an overheat condition. To check this, just monitor the temperatures of the hoses with the infrared gun while watching the coolant temperature on your scanner (most accurate) or cluster (less accurate). If both hoses get hotter, but the temperatures never equalize, chances are your thermostat is sticking when opening. Make sure your coolant isn't rusty mud and that you have good water pump flow before throwing a thermostat in.

Once the engine is up to temperature, the fans should cycle. If they don't cycle on low speed, but do on high, it could be either your fans or your fan switch. To check both, disconnect your fan switch and using a piece of wire, jumper two of the three pins. If both speeds work, it could be a fan switch. If both are dead on low speed, it could be your fans. 

IMG_20141007_161419_871 Fans turn on, then off. This fan only has one speed, but newer cars have more.

 

Typically, the fans cycle around the high 90s, and turn off around the low 90s. The same is true of viscous fan clutches.

To check a viscous fan clutch, put on a glove, and with the engine running, stop the fan. Don't do it if the fan is locked up, obviously, and be very careful. Hold the fan still until the secondary fan comes on, and then let it go. Let it spin up for a while, and if it gets loud and puts out a good volume of air, it's working. If it just sits there spinning at a slow speed, it could be the cause of your overheat.

ECT Diagnosis

An ECT (engine coolant temperature sensor) can be a difficult thing to diagnose. On newer cars, they set fault codes, so diagnosis is pretty straightforward. If you're getting erratic readings, check the wiring before replacing the sensor. Make sure there's no coolant or corrosion in the connector of the ECT. If your car runs fine, with no check engine light, but your instrument cluster doesn't show the correct temperature, it could be your ECT. Volkswagen used a sensor for a number of years that was two sensors in one: one for the engine computer, and one for the instrument cluster. When either would fail, they typically fail independently, and either set a check engine light or have incorrect readings in the cluster. 

VW ECT - note that there are four pins. Two pins for the engine computer sensor, and two for the instrument cluster.

 

Most OBD-II Volkswagens and Audis monitor the cooling system, and can trigger a fault for any one of the components listed earlier. Make sure you check your freeze frame data in your OBD scanner and check what the coolant temperature was when the fault was set. If it was a ridiculously low (-40), your ECT is probably bad.

Finally, with the engine up to temperature, make sure you have good heat from the heater core. Turn it on half speed, heat on full blast. If it's not melting your face with uncomfortable warmth, it should be flushed out during whatever other repairs you've discovered you need during diagnosis.

This process is simple, and yields good results. If you're checking your cooling system due to a check engine light, then you definitely need an OBD scanner. If you're checking out an overheat, an OBD scanner is not required, but is highly recommended. I recommend doing this process every time you drain and fill the engine, and before you start any cooling system repair. It's a simple diagnostic process and an effective quality control measure. It would be a total pain to have to drain the radiator to replace a fan switch after replacing your leaking radiator hose!

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Written by :
Chris Stovall

Chris is a journeyman mechanic from Berkeley, California, specializing in late model Volkswagens and Audis. A glutton for punishment, his spare time is spent rebuilding every component of his ’83 Rabbit GTI.


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