Odd overheating issue (and oil question)

'93 Civic Cdn CX with an original computer (P08) and a non-original engine (D16Z6 - the D15 engine died a while ago) with a new radiator (installed with the Z6 engine a little over a year ago) - this problem
showed up only in the past month.
Engine comes up to just-below-midpoint temperature reading normally. After long drives (i.e. engine warmed up) when left idling (red traffic light, stuck in traffic, etc) temperature reading starts climbing. I've seen it go almost to the red line. If the engine RPMs are brought up the temperature will come back to normal - either through normal driving, or by putting the car in neutral (with brakes on, of course) and gently touching the gas.
Any sort of engine load (RPMs up) and the temperature regulates properly.
Coolant level looks OK - there's fluid in the overflow tank and the inside rubber hose is 'wet'. Color is green, looks clear.
When the engine is warm, the 'low idle' seems to be quite low - sometimes the engine seems to want to stall. I wonder if this very low idle mode isn't conducive to proper coolant flow. Any other theories? Water pump, maybe?
Second question: my mechanic is fairly insistent that older Honda engines (> 200000 km) tend to start consuming oil, and that this isn't a big concern. Agree/disagree, anyone?
I'm about 1500 km into an oil change and noticed that the dipstick level has come down from 'full' to halfway between the dots. I also see some oil under the engine on the driver's side where the plastic of the bumper comes in under the engine compartment, so I suspect a slow leak more than actual oil burning (tailpipe doesn't show any signs of oil burning).
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snipped-for-privacy@gmail.com wrote:

not the water pump. does the electric cooling fan come on? does the thermostat operate properly? [best to replace because it's now old anyway.] are the radiator fins clogged with leaves/mud/insect debris?
low idle is another issue. ensure the ignition system is healthy, correctly adjusted and that the idle system works within spec.

some burn a little oil, yes. unless it's excessive, it's not a concern, no. and some oils burn off more than others.

that's within spec.

it won't show signs on the tail pipe unless the catalyst is completely shot. check the oil pan for bolt tightness, and try using a quality oil with proper seal conditioners in it. also check the pcv valve.
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i'll check the fan. the rad is definitely clean, as as for the thermostat, it's probably original.

i replaced the pcv valve not long ago, the old one was most likely original and was stuck open. i'll have a look at the oil pan.
thanks for the advice.
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snipped-for-privacy@gmail.com wrote:

Hi, Basic thing first. You know for sure fan comes on when it supposes to?
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i verified that the fan is working - when the temperature gauge comes up to around mid-point, the fan cuts in.
i decided to replace the thermostat - what i found in my engine is best described by a picture.
http://www.flickr.com/photos/angstful_mfg_employee/2724998691 /
needless to say, i *don't* think they're supposed to look like that!
(incidentally, the haynes manual underneath it says to pull off the upper rad hose to get to the thermostat ... uh, no, that would be the *lower* one - i'm very thankful that I have a PDF of the Honda shop manual)
a quick test after purging shows improvement. i could feel flow through the top radiator hose (it got warm) and the temperature gauge was stable. i still have to take the car for a short tour before i judge success or failure.
the rubber gasket and o-ring around the thermostat are showing signs of deterioration. i can live with a slow leak until i can get to the dealership and get those replacement parts. it appears that the coolant needs replacement anyway (looks a little cloudy).
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snipped-for-privacy@gmail.com wrote:

that's not a honda oem thermostat. for a few dollars more, it's worth getting the real deal - much better temperature control /and/ the correct vent valve. it also comes with the rubber gasket.

you should replace that stuff periodically anyway. use distilled water as the dilutant.
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no overheats yesterday. yay!
i took the car in to get the coolant system flushed and refilled. the mechanic told me that there's combustion gas coming out of the radiator, he suspects a head gasket problem (or worse, maybe a crack).
i'm not convinced that this engine is going to last much longer. considering the amount of $ involved to even find out what's going on, i'm seriously consdering getting a new car and being done with it.
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snipped-for-privacy@gmail.com wrote:

it's easy enough to check for yourself. and if this person couldn't figure out the thermostat before, you absolutely cannot trust them on a head gasket diagnosis.
simply look for bubbles in the expansion bottle. drive the car around for about 20 minutes to get it to full working temperature, then carefully unscrew the lid of the expansion bottle [NOT the radiator]. hold the lid to one side so that the tube inside is still below the surface of the liquid, but so that you can see in. use a flashlight if necessary, then observe for bubbles. if you see any, the head gasket is indeed shot. if not, then it isn't!

replacement jdm engines are seriously cheap. if your body is unrusted, it's more economic to replace the motor than to replace the car. and if you do consider replacement, be sure you're going to be satisfied with the handling of the new vehicle. one of the reasons i stick with the old stuff is that every time i try a new one with macpherson front suspension, i'm disappointed. gotta love the old honda wishbone civics.
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no diagnosis of the overheat problem had ever been made by this particular mechanic. in fact, this wasn't my regular mechanic - my regular guy isn't open on sundays, and since this should have been a simple drain/flush/refill, i trusted it to a local mass-market shop (cdn. tire)
the engine from which the incorrect thermostat was pulled was installed in the car around a year ago by my regular mechanic. i suspect that the engine had this particular thermostat in it when it was delivered to him.
should this engine turn out to not be economically repairable, i'd have a hard time coming to grips with a second replacement.

well worth checking. what concerns me is that if the head isn't shot, how could combustion gases make their way to the radiator?
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snipped-for-privacy@gmail.com wrote: <snip for clarity>

misdiagnosis.
bubbles are either improperly purged system, leaking head gasket, or cracked cylinder/head. there are no other possibilities.
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i followed the 'haynes' procedure for purging the air out of the system after replacing the thermostat:
- close all drains - open air bleed valve on top radiator hose - slowly fill rad until fluid runs clear through air bleed valve (no bubbles) - close air bleed valve, top up overfill reservoir - keep rad cover open, operate engine until therostat opens up and top hose gets hot - shut down engine, top up radiator to fill lip - top up rad to fill lip and close rad cover
the thermostat is keeping constant at slightly-below-half, so i don't think there's an immediate cooling problem.
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