cylord

Hoping someone can help me out here!

Last night I drained the old coolant out of my 1999 Prelude (DOHC VTEC model, automatic 4-speed) and refilled the reservoir tank up to the MAX line with Prestone's extended life 50/50 premixed ("sillicate-free") engine coolant/antifreeze mix.

Now, when I start the car, the engine revs up to almost 2k RPM's then falls back down to its normal idle in rapid succession. I opened up the radiator cap and all I saw was a bunch of frothy bubbles.

Could this be because I didn't use genuine Honda coolant? How would I go about fixing this? Drain the system again and run distilled water through, drain again, and re-fill?

red92si

https://honda-tech.com/forums/honda-prelude-4/end-all-idle-surge-bounce-etc-problems-thread-1979500/

You have to bleed the cooling system of air properly or you will get an erratic idle.

cylord

Forgive me for being an auto-idiot, but one quick question...

I did know about the bleeder nipple on the thermostat, but I couldn't find it. Can you see the thermostat (and/or the bleeder nipple) from above the block looking down, or am I going to have to find some jack stands and go at it from below?

Any help finding the thermostat and/or bleeder nipple on it would be appreciated.

crxsleeper210

^^^^^x2 i had the same problem a few months ago after i did my coolant flush....OEM honda coolant/antifreeze is recommended of course but another brand shouldnt cause the problem by itself

crxsleeper210

follow the lower radiator hose up to the thermostat....there should be a little "nipple" with an 8 or 10 mm nut on top sticking straight up you should be able to see it from on top of the engine bay....with the motor on you loosen the nut until coolant comes out in a steady stream (and ur radiator cap must be off during this process) then quickly tighten it up and you are done....

lude_lover

Its a 12MM I believe you just loosen it a quarter turn, the best way to do it is when the engine is cold and with the heater on high

crxsleeper210

yea it could be a 12 i forget which size it is.....and yes its best when the car is cold and heater on high

cylord

Just to be sure... this would be the bleeder nipple, right? (center, circled in red)



Sounds like I should grab a couple of rags, too, for when I turn eh. lol

crxsleeper210

yes that is the bleeder.....yea go ahead put a couple rags under/around it because coolant will flow out of it....

cylord

Well after bleeding some air out of the thermostat bleeder (did indeed hear a gurgling noise then a steady flow of coolant) and topping off the fluid in the filler neck a couple of times, it's back to running smoothly now and happily idling at about 800 RPM's.

Thanks for the help!

crxsleeper210

no problem.....dont forget to fill up the overflow tank to the full mark

cylord

Will do. Originally, before I did the whole drain, the overflow tank was almost bone dry.

I've let the engine idle for ~5-10 minutes and I didn't see any leaking fluids, so later tonight or tomorrow I'll drive it around for a few minutes to get the engine nice and warmed up and check levels again then.

Thanks again!

brownsoxx

Hey Gents,
'93 Prelude Si Automatic. I had this problem "Idle Fluctuation". After running to operating temp, my Idle would rev up and down, but only in PARK and NEUTRAL. SOMETIMES when running for a bit she would tend to run hot, but other times. . . not at all. Twice the temp gauge ran into THE RED! (I pulled over immediately). Coolant levels were always full or exactly where they needed to be in both the radiator AND the reservoir. After reading this thread as well as a few others, I tried "burping" the system through the bleeder valve that is on the thermostat housing.
Started the car, opened the valve (12mm) until coolant came out (approx 5-8 seconds). I left it running to operating temperature then opened the valve again (just to be sure) to bleed/burp. Tightened it back and drove the car all around . . . NO MORE OVERHEATING and NO MORE FLUCTUATING IDLE !!
Thank you so much to all of those who take the time to leave such posts to help others!
I hope this helps someone else also. The whole process took less than 15minutes and only a 12mm wrench.