Where is the thermostat located on a 1987 Honda Prelude SI?

Answer 1

The thermostat is located in the engine compartment on the passenger's side near the distributor cap. The thermostat is located in a cylindrical housing containing three or four wired sensors. The housing is connected to the cylinder head to the left of (behind) the distributor (looking in from passenger's side). There is a 1-1/4" black hose going from the end of the thermostat housing (housing cover) to the upper radiator. That is the easiest indicator to locate the thermostat housing. The labor time is about an 90 minutes if all goes well. The housing cover can be removed with the hose attached by removing the two hex bolts with a #10 socket. I broke a "stuck" bolt on this housing trying to get it out. NEVER use a drill-like device called an "E-Z out" because they don't work. I had it so botched up, I had to buy a whole new housing ($80) just because the last guy didn't coat the screw threads with thread sealant. Believe me, use thread sealant. 1.)Buy a new thermostat (~$30) before starting and notice only one side has a spring. 2.)Buy a new "upper" radiator hose/hose clamps (~$20). (Optional-it's pretty easy and no leaks this way.) 3.)RTV Silicone Gasket Maker (ie. Permatex Item # 22071) 4.)Anti-seize Lubricant (ie. Permatex Item # 81343) 5.)Penetrating oil to loosen screws. I noticed that the aftermarket thermostats have a smaller hole for coolant to pass through than the original/dealer "stats" do. So I bought an original equipment thermostat (~$45) because I HATE temperature problems -- they affect everything from the engine to the tranny to the A/C. But it's definitely OK to go to Kragen or Autozone if you want. Before beginning, spray the thermostat-cover bolts with penetrating oil about an hour before starting the project. Best if you can spray them just after the engine is turned off and they're still hot/warm so the penetrant will work better. Next, you MUST drain some of the coolant first or you'll have a BIG MESS. After you drain some of the coolant (1/2 gal./couple of qts.), put some rags under the thermostat-cover/hose junction and remove the two bolts. Pull the housing cover off. After the housing-cover/hose assembly is off, the thermostat is easily seen -- hopefully you bought a replacement "stat" before starting so you know what to look for. You'll see the "pointy side" sticking out and you just pull on that and the stat will come out -- look out 'cause there will be some coolant leaking out from behind the thermostat when you remove it (that's why you stuff some rags underneath the work area). Carefully scrape the ridge the "stat" sits in to remove any deposits interfering with the new "stat's" ability to seat correctly. Wipe out and clean the area. Install the new thermostat with the spring side in and the pointy side out towards the radiator hose. Clean off the thermostat-cover of old gasket material and apply the RTV Silicone to the interface area of the thermostat-cover. Install the housing cover/hose assembly with the two bolts treated with thread sealant (i.e. anti-seize lube). Don't crank the screws down hard (10 ft-lbs.) and refill the radiator. Check for leaks. If you decide to change the upper radiator hose before refilling, the hoses can be a little stubborn as they get stuck on the metal nipples. You may have to twist/rotate the hose back and forth with a large pliers (gently -- don't tear up the rubber hose) to loosen it and then you can pull it off. Use new hose clamps and put them on the new hose before sliding the hose over the radiator or thermostat-cover nipple. I like to put a THIN FILM of multi-purpose grease on the nipples to prevent the hose sticking and it's easier to slide on. The hose clamps should be not be located on top of the nipple's "shoulder". Rather, the clamps should be positioned just behind the nipple's shoulder to get a leak-resistant grip on the nipple. Don't overtighten the clamps (if the clamp can't be rotated, it's about right) -- tighten later when the hose is hot, if leaking. The hose's fit and the clamp are enough to prevent leaks. You'll know the thermostat is working properly by checking the upper radiator hose after the engine is running about ten minutes. The radiator fan usually comes on twice during this period. The thermostat will open when the engine coolant is getting hot (~135F) and the thermostat will allow coolant to flow into the radiator through the upper hose. You'll feel the hose get hot (careful). This also happens to be the way to check if the thermostat is not working properly. Sometimes a defective thermostat will allow water all the time (engine takes a long time to warm up) OR it never opens and the hose is cool and the engine stays hot.