adding an upper thermostat to a Honda D15 engine?
 

I have an 87 civic with a D15a2. My motor has the thermostat located in the lower radiator hose and I believe most D15 motors are this way. Since there is no thermostat in the upper end of the engine, coolant travel through the cylinder head outlet to the top tank of the radiator is unimpeded.

My engine warm up times are quite long. I feel that this might be because of convection currents moving hot coolant from the cylinder head up to the top tank of the radiator and then cold coolant from the top tank of the radiator falling down into the engine to replace where the hot coolant was.

I am wondering if the warm up time could be shortened by either installing a second thermostat in the upper radiator hose or making a radiator hose with a u shaped bend that would keep the coolant from simply rising/circulating into the upper radiator tank.

If I installed the second thermostat, I would trim it down so that it would sit inside the radiator hose right up against the cylinder head outlet tube, then I would put an extra hose clamp around the hose only to "lock" it in place.

I wish I could speak with a Honda engineer to listen to the reasoning as to why they designed the motor with a lower hose thermostat- surely there was a good reason.

Anyone tried this? Good idea? Bad idea?

Does anyone with a lower thermostat setup see their temp needle go right up to operating temp on a 40 degree day after driving only 5 minutes with the heater off?

Should I remove the lower thermostat while I am testing the upper one?

I'm confused, and want to disagree with you on where your thermostat is, I recently replaced the thermostat on my 1985 crx engine, and from what I can tell you have a thermostat on the output end of the water pump, right behind/under the distributer, I'm pretty sure this is how my newer 1992 civic is as well, and because heat rises, any warm coolant that is in the heat and sloshes thru the radiator hose to the top of the radiator isn't likely to flow down, of course a way to check this is to feel the that top radiator hose, to see if it's warm befor the engine warms up, so this is not something that I would worry about, I had/have (rear ended at 40mph) the same car as you only the 1984 version of it, and it took a long time to warm up as well, I figured it was because the engine was reasonably efficient, if you wanted to of course, I'm sure you could fit a crx hf radiator in there, it's going to be nearly half the size, and people seem to like them while racing, but if your thermostat is good then I wouldn't worry about it.

The thermostat housing is definitely connected to the lower radiator hose- and yes, we agree that the thermostat is pretty much under/behind the distributer.

Its my understanding that impeller type pumps suck from the center of the impeller and discharge off center of the pump housing- so it seems that the tube going across back of the engine would be a suction tube (since it leads to the center area of the pump) and then the pump would force coolant into the water jackets of the block (since that is the off center passage). That's my interpretation.

That's a good idea about just feeling of the top radiator hose to see if it gets warm before the temp gauge rises much.

I assume that the VX engine has the same thermostat set up- any VX owners out there have long warm up times?

My wife's car (1992 Nissan 240 SX 4cylinder 2.4 L) has an upper thermostat and twice the amount of coolant in the block and it comes all the way up to operating temp within 3-4 minutes of gentle driving on a 40 degree day.

when they put the t-stat in the lower hose, it prevents the water pump from sucking cold coolant from the radiator. because nothing is leaving the radiator, very little coolant enters the radiator. rather, it bypasses the radiator and goes straight back to the water pump.

6 one way, half a dozen the other way. only downside I see is confusing people and possibly cavitation because the bypass hose is usually smaller than the lower rad hose. high rpm when the t-stats closed and the pump can't get enough liquid through the bypass hose so it cavitates. that's just a wild theory based on the principle. I haven't seen the honda implementation so I'm not gonna give anything too specific.

Do you have the stock engine in that car? if so then it might not be a D15A2 and insted be a EW1, as they changed the block and some other stuff when they changed body styles in 1988, was just poking at https://en.wikipedia.org/wiki/Honda_E_engine and noticed this.

Its not the original engine, but it looks the same as the stock engine and is CVCC.

I always order parts for the 87 civic 1.5 with a 3 barrel and they all have fit so far- thermostat, head gasket kit, pan gasket, front seal, water pump, timing belt tensioner, timing belt.

It would be interesting to know what the differences are between the EW1 and D15a2 blocks and why they made the changes.

I checked today and the upper rad hose gets warm even when the temp gauge is just starting to come up. I also replaced my thermostat today- the old one was closing completely.

I am having second thoughts about putting the thermostat in the upper hose (I don't want to risk overheating my engine in case the upper thermostat doesn't get hot enough to open).

I might just buy a couple of molded hoses and hook them up so there is a u shaped dip between the cylinder head outlet and the upper radiator fitting- this should put an end to hot coolant freely rising into the upper radiator tank.