richrd0001

I thought I would pass on how an engine loping at idle ended up being a thermostat problem.

I'd taken my '89 Si FI Prelude to the dealer years ago with the same idle problem and they fixed it by replacing the idle air control valve. I figured it was the same problem. Ran fine otherwise.

Recently was on the interstate shortly after leaving the motel (loping worse) and the heater wouldn't work and the engine temperature gauge would mysteriously drop to completely "cold" position upon acceleration. WTH? Then the coolant started boiling.

Minimal coolant loss and was able to get it back to town, where I installed a new thermostat, which remedied the problem.

I think this is worth posting because of the odd collection of symptoms. I think the stuck thermostat was causing high coolant temps that did not cause boil over while in town and trips were short. There must have been some air pocket that had formed around the engine coolant temperature gauge sensor near the thermostat which grew larger at higher rpm thus causing coolant to not touch that sensor. The coolant sensor for the ECU is a little higher and thus would have affected operation at lower RPM. I speculate.

I can't figure why the heater wouldn't work all of a sudden. If the thermostat is closed with a cold engine, that's the purpose of the thermostat and water still circulates thru the heater core. Maybe the air pocket I'm speculating about affected the input hose for the heater and prevented it from picking up hot water. I'm just not sure about this coolant flow idea.

The loping returned immediately after installation of the new thermostat but quit as soon as I bled the cooling system (bleeder screw near the thermostat housing).

Take away lessons: bleed coolant system if engine lopes at idle. Heck, bleed coolant on general principles if you haven't already done so. Carry a spare thermostat. (They're cheap.) Worry about idle air valve problems if system is bled and thermostat is good.

440,000 miles on this fantastic machine. Engine rebuilt in May. Driving like new these days and I be lovin' life.

razorlude

How to tell a thermostat is bad?
Have u seen a engine without thermostat?
How a coolant overflow tank works?

richrd0001

A thermostat is bad when the engine coolant boils (as I found out). The thermostat should open at a certain temperature (180 F, 82 C for my Prelude). If it doesn't the cooler coolant can't circulate around the cylinders and the heat from the cylinders keeps heating the trapped water until it boils.

I bought a water thermometer because I was curious if I had correctly deduced that the thermostat was bad. I heated the "bad" thermostat in a pan of water and watched the thermostat to see at what temperature it would start to open up. It was at 185 degrees. I found a specification list that told me how far the thermostat should open up and it seemed that the opening was a little less than that.

Maybe you will not see the radiator boil over when the thermostat is bad. The coolant may be forced out of the filler cap slowly enough that it goes down the overflow tube (into the overflow tank) instead of boiling over in a big cloud of steam. Just guessing about this. If I am correct, another symptom may be that you are finding that you are losing coolant and having to add more to the radiator.

I believe that an engine will operate well without a thermostat. High-quality thermostats are designed to stay open when they fail so that is like having no thermostat at all. Thus, it is possible to drive the car until it is safe or convenient to replace the thermostat.

However, I have read that running the engine without a thermostat causes the engine to operate at a lower temperature than it is designed to operate at. I forget what problem it causes but I seem to recall that it is lower fuel efficiency and the fact that the cylinders don't expand as much as they should causing wear (?). Even so, the temperature difference is probably not that much and engine performance is not affected all that much. So . . . bottom line, the best approach is not to drive too much without a thermostat. In cold weather, of course, a thermostat allows the coolant for the heater to warm up more quickly.

I think the overflow tank supplies coolant to the radiator because coolant loss results in air in the radiator. As the radiator cools when you turn off the engine, the air contracts more than coolant and creates a vacuum, which sucks replacement fluid from overflow tank.

Someone with more savvy on this site may be able to correct what I've said above.

I hope that is useful to you.

preludeNApower

Blown HG? Compression test it.

holmesnmanny

The cooling system expands as it heats up which is why you have a reservoir. It has nothing to do with coolant "boiling over". That shouldn't even be happening. Of course it's going to get hot but the fan should be cooling the system before your coolant ever boils over unless you're running straight tap water which doesn't include coolant, which is designed to raise the boiling point of water. I would agree with a compression test suggestion.

203BB2

Even though u somewhat fixed ur problem it would still b a good idea to flush ur coolant for fresher fluids
Not just straight up antifreeze or water
Mixing the 2 50/50 will raise boiling point