This time last year, I replaced the radiator and all hoses in my MZ3 due to the vehicle's mileage (over 165K miles at the time). After I completed the work, I noticed a slight weeping of coolant from the thermostat area. Since it was such a minor trickle, I opted to delay the repair.
Earlier this summer, I happened to inspect the coolant reservoir and discovered the level to be below the MIN marking. Not good! I topped off the level and included a thermostat gasket with my order for other parts (in an upcoming How To thread). I figured I would tackle the thermostat gasket replacement over the Thanksgiving holiday; however, that was expedited due to the weather.
First, the symptoms of a faulty thermostat gasket:
- Corrosion of one or more of the bolt heads which secure the thermostat
- A visible leak of coolant around the thermostat housing
- Low coolant in the reservoir
- A sudden spike in engine temperature in cool weather
- Low/no heat
Newer cars are better designed than older vehicle wherein parts like the thermostat and water pump use a flexible gasket which fits in a notch to seal the system. This is superior to older vehicles which rely on cardboard type gaskets which require sealant. The drawback to the former is they tend to harden as they age, the signs of which are barely visible leaks. Still, they're easier to fix as the part is removed, all areas of contact are dried, the gasket is replace, and the part is reinstalled.
The last two symptoms are what caused me to expedite the repair of the gasket. I had driven the car over the weekend in which the ambient temperature was in the 60s. Overnight, the temperature had dropped to the 30s due to a cold front. The temperature had not warmed much that day and, as I headed home, the needle on the temperature gauge had spiked almost to H. I quickly pulled over while turning the heat high in an effort to use the heater core as a secondary radiator. The air exiting the vents was lukewarm at best while the needle fell slightly on the gauge. This told me I had an air pocket in the system. I left the engine idling, popped the hood, and carefully opened the cap for the reservoir taking care not to open it fully, but rather only to the point I could hear the rust of air exiting. Liquid gushed into the reservoir and, after a few moments, I observed coolant pulsing into the reservoir with evidence of air pockets. I exercised care opening the cap again, carefully playing the dance of opening it to the point air would rush out as coolant rapidly filled the reservoir. Once I felt I had enough air bled, I returned to the cabin and held the throttle ~2,500 RPM. It took a few minutes before the heater was finally putting out very hot air. I resumed the trip but had to stop again in a few miles to bleed the air before finally being able to return home (a 30-mile drive).
The next morning, barely four miles into the drive to work, the same behavior occurred with the temperature gauge. Once again, I had to bleed the air. While mulling the reason why and air pocket was forming in the cooling system it dawned on me the leak from the gasket was the cause. The gasket had hardened and had now become the weak point in the cooling system. As we drive, the coolant heats and the level in the reservoir drops while it is circulating. Once the vehicle stops (engine off), the thermostat closes, the coolant cools and slowly refills the reservoir. The problem in my case is, as the coolant cooled, cold air was being sucked into the system due to the shrunken gasket.
I didn't feel like working on the car late at night in a cold garage, so I repeated this air bleeding dance to work and home for a couple of days. It wasn't until Thursday afternoon when I realized I'd been shooting myself in the foot the entire time. The best way to handle this until I could effect the repair over the weekend was to remove the cap from the reservoir once I returned home. That made the reservoir the weakest point of the system, it would remove the air ahead of time, and it would create an additional point for the coolant to cool. I removed the cap once I got home and replaced it about three hours later when I knew the engine was cold. Friday's drive to work was uneventful. Once I arrived at the office, I repeated the procedure from the night before. The trip home yielded the same results.
Before starting, I checked the records for my MZ3. The thermostat had been replaced around 2012 under warranty (I remember it was throwing a code at the time). So, it makes sense that after seven years the gasket would have dried and shrunk, thereby causing the tiny, almost invisible leak.
Naturally, this procedure can be used for replacing the thermostat itself if such a repair is warranted.
The breakdown for the repair:
- About two hours
- 1.5 beers
- Four f-bombs
- One mother f-bomb
- Tools needed:
- Band clamp pliers
- A clean bucket, preferably one with a cover
- ~1.5 gallons of fresh coolant
- 8mm socket
- 10mm socket
- 12mm socket
- 13mm wrench or Crow's Foot
- Two jack stands
- Socket for removing wheel lugs
- Torque Wrench
- One floor creeper (these are a lifesaver)
- Replacement thermostat gasket
- A sharp pick or utility blade
- Utility gloves
- Disposable shop towels
- A small wire brush or 0000 steel wool
- Optional - Dish washing detergent & scrub brush
- Optional - Q-tips
- Optional - Bench grinder with a wire wheel
- Optional - Anti-seize
- Optional - Infrared thermometer or a plain thermometer
**Observe all safety procedures regarding the usage of a jack when lifting a vehicle and using jack stands.
NEVER WORK UNDER A VEHICLE SUPPORTED SOLEY BY A JACK. DEATH OR BODILY HARM COULD OCCUR.
NEVER USE JACK STANDS ON SOIL OR IN ANY OTHER MANNER NOT CONSISTENT WITH THEIR PRESCRIBED USE.
Do this at your own risk.
- Ensure the vehicle's engine is cool before starting work. Sweep out the garage before starting.
- Open the hood and secure it with the prop rod (if applicable).
- Remove the engine cover.
- Open the reservoir cap slowly to relieve pressure.
- Remove the reservoir cap once the pressure has been relieved.
- Lift the vehicle using the jack.
- Properly place the jack stands under the driver's and passenger's side of the vehicle at the proper points. Slowly lower the vehicle onto the stands. Verify the vehicle is making positive contact with each jack stand before fully lowering the vehicle onto the stands.
- As a precaution, I always remove the jack, stand up, and then grab the vehicle by a fender before giving it a firm push. This way one can verify the vehicle is securely settled on the jack stands before getting under the vehicle. Better it fall now than with someone underneath.
- Remove the engine splash shield. Set aside.
- Bring the clean bucket under the vehicle.
- My car is an automatic, so I removed the large hose which comes off the thermostat to the cooler down low. Manual transmissions may be different.
- Remove the band clamp so by pinching it and sliding it up the hose.
- Locate the bucket under the hose.
- Slowly, yet firmly, fight with the hose to loosen its grip.
- Once the hose has broken free, be firm but go slowly to remove the hose.
- Drain the content of the hose into the bucket. It will create a siphon action and pull a lot of coolant from parts of the cooling system.
- Set the bucket aside and secure the cover.
- Reattached the hose and secure the band clamp. I put this in bold so this step is not skipped. Better to reinstall the hose at this point.
- Remove the serpentine belt using the 13mm wrench/ Crow's foot.
- Remove the belt tensioner (12mm) and set this all aside.
- Remove the band clamps from the thermostat. DON'T LOSE THEM.
- Break the bolts free from the thermostat (8mm). The top ones are easy to reach. Get to the bottom one from under the vehicle.
- Remove the bottom thermostat bolt first as it's the most difficult to reach.
- Remove the top thermostat bolts however is easiest. Use care not to drop them.
- Remove the thermostat from the engine.
- If using a pick, poke the gasket to find a weak spot. Use care not to slick and stab yourself in the hand. Stick the pick into the thermostat gasket.
- If using a utility blade, gently slide the blade between the gasket and the ridge which holds it. Use care to prevent gouging the ridge. Use the point of the knife to gently dig into the gasket. Be careful not to slip and stab yourself in the hand.
- Remove the gasket.
- Use the wire brush/steel wool to clean the remnants of the gasket from the thermostat housing on the engine.
- Wipe the thermostat housing clean of coolant and old material from the wire brushing.
- Repeat the previous two steps as needed.
- Use the dish washing detergent and scrub brush to clean the thermostat.
- Use shop towels to dry the thermostat. Letting it air dry works well.
- Use Q-tips to clean the gasket ridges on the thermostat.
- Install the new gasket on the thermostat. Pay particular attention to its shape and the little nubs which are cast into the gasket. Press firmly into place.
- If needed, use the bench grinder wire wheel to clean the heads and threads of the thermostat bolts. A hand held wire brush will suffice.
- Apply a tiny, TINY dab of Anti-seize to the lower ¼" of threads of each thermostat bolt. It's easiest to install each top bolt individually before proceeding with the bottom bolt. DO NOT TIGHTEN the top bolts. Thread each in part-way before installing the bottom bolt.
- Torque specs for the thermostat bolts: 8.0-11.5 Nm; 82-117 kgf-cm; 71-101 in-lbs. If using a ft-lbs torque wrench, divide the in-lb values by 12.
- Reinstall the hoses and secure the band clamps.
- Reinstall the belt tensioner.
- Torque specs for the belt tensioner bolts: 18.6-26.6 Nm; 1.9-2.6 kgf-cm; 14.0-18.0 ft-lbs.
- Reinstall the serpentine belt.
- Verify all cooling system hoses have been reinstalled and are secured.
- Fill the reservoir with fresh coolant to the MAX mark.
- DO NOT INSTALL THE RESERVOIR CAP.
- Verify the vehicle is not in gear. Turn the heat all the way to the maximum setting. DO NOT TURN ON THE FAN.
- If working in a garage, open an exterior door to ensure a good supply of fresh air is maintained.
- Start the vehicle and hold ~2,500 RPM.
- When the engine temperature reaches the operating point, which means the thermostat has opened, let the engine idle and check the reservoir tank coolant level.
- Maintain the coolant level at the MIN mark only. DO NOT fill to the MAX level.
- Observe for air being purged from the cooling system.
- Check for leaks around the thermostat housing and all hoses.
- Return to the cabin and turn the fan on HIGH.
- Resume holding the throttle at 2,500 RPM. Plan on doing this for 5-10 minutes.
- Using a thermometer or your hand, monitor the temperature of the air exiting the vents. The heating system is capable of producing air over 140º once the air has been expelled from the cooling system.
- Once satisfied the heating system is operating properly, ensure the reservoir is filled to the MIN level.
- Install the reservoir cap.
- Turn off the heat and HVAC fan.
- Shut off the engine.
- Reinstall the engine splash shield.
- Using care, use the jack to raise the vehicle, remove the jack stands, and carefully lower the vehicle to the ground.
- Reinstall the engine cover.
- Close the hood.
- Safely and properly dispose of all towels.
- Safely and properly dispose of the recovered coolant.
- Clean all tools and put them away.
That should cover everything. I hope these pictures help.
This is a shot of the thermostat with the belt tensioner removed. Notice the corrosion on the bolts.
Removal of the thermostat bolts with the hoses moved aside.
This is the bottom thermostat bolt. Remove this one first.
This is the thermostat after removal from the vehicle.
The new gasket.
Before I cleaned the mating area of the thermostat and housing. Notice the remnants of the old gasket.
The old gasket has flat spots and is not pliable.
The new gasket placed in a freshly cleaned thermostat.