engine temp, what should it be?

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newbbewb
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engine temp, what should it be?

Post by newbbewb »

messing around with my hybrid. it's running great after some main, pilot, and needle changes. still have a really low idle dammit. BUT the engine is sitting around 150F head temp after rapping on it for awhile. is this a normal temp? I never hear it talked about during tuning, so just curious. is it not as important to the tuning process as a 4 stroke bike?
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Re: engine temp, what should it be?

Post by 6 Riders »

Mine runs about 195° when riding in 80+ degree weather. It doesn't boil at that temp and no "seen" coolant loss. Another member here, that I ride with, will boil his in hot weather, but he is heavier than I am. Pretty sure I'm at about 150° just putting around, but not positive (doesn't happen much).
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Re: engine temp, what should it be?

Post by chkdx »

I don't know what method you guys are using to measure temp, but I installed a water temperature gauge into the water line between the top of the head and the radiator, and I see big swings in temp, from a low of about 160 when cruising on dirt roads, to as much as 270 (!) when tractoring the engine on a steep climb in 1st or 2nd gear. Even at 270, it runs fine. With the stock rad cap, it would boil on those hard climbs, but after I installed a 19-21 psi cap, it never boils. I have a 220 with boyesen reeds and PC2 pipe.
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Re: engine temp, what should it be?

Post by Postigo »

160 to 190F is normal riding temp for my sleeved over bored KDX223. The coolant I'm using begins to evaporate at 259F if this happens I would stop the engine and let the bike to cool for a couple of minutes, I think is not a good idea to change the radiator cap for a higher pressure rated cap and ran at higher temperatures since you are exceeding the coolant and engine temp and pressure design. :naughty:
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Re: engine temp, what should it be?

Post by chkdx »

Well......if I stopped every time the temp hit 260, I'd be stopped a lot. I ride pretty tough trails. About the 3rd time the thing boiled over with the stock cap and blew the cap off the coolant resevoir, I said enough and installed the high pressure cap. I also moved the resevoir up behind the headlight to improve airflow thru the right side radiator and ensure the system would let coolant back in thru the cap when the bike cooled down.

Now, it never boils, the radiator cap pushes a little coolant into the resevoir during rides, then pulls it back into the radiator when the bike cools at the end of the day. The radiator is always brim full of coolant. All this is just like a modern car's system.

I do have the Devol radiator guards on the bike (came with them) instead of the stock plastic shroud slats; Fredette mentions that installing the Devol guards "makes the engine run hotter" but doesn't give specific temps. I thought 260-270 was way too high, and carefully checked everything; water pump, radiators, jetting, timing....everything I could think of. I checked this forum at the time (2 or 3 years ago), and nobody had any hard numbers. Lots of guys saying "sometimes it boils over a little, but it runs AWESOME!!!!!" or "I lose a little coolant every ride, just top it off at the start of every day, and it runs AWESOME!!!!!" Literally nobody was actually measuring what their coolant temp was.

Well, um, ok, my bike runs awesome too, but if my car lost a little coolant every time I drove it, I'd say something's wrong. A higher pressure cap solved that on my bike. I'd definitely prefer my bike to run 50-60 degrees cooler, but short of sticking to easy dirt roads, I don't know what to do to accomplish that.

I'd really like to hear from somebody who KNOWS what temps these bikes are supposed to run at, and what to look for if the temps I'm seeing are too high.
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Re: engine temp, what should it be?

Post by newbbewb »

I have a heat sticker on the head and one on my radiator. I have verified the stickers with an infrared thermometer.
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Re: engine temp, what should it be?

Post by chkdx »

Ok, that makes sense. The heat stickers are ok for a rough guide, but the outside surfaces of the radiator and head don't match the coolant temp. I'd bet that if you had a water temp gage with the sensor actually in the water path, like I do, that you'd get temps closer to mine. You've given me an idea though: next time I ride, I'll take my infrared thermometer with me and compare radiator and head surface temps with water temps.
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Re: engine temp, what should it be?

Post by newbbewb »

Postigo wrote:160 to 190F is normal riding temp for my sleeved over bored KDX223. The coolant I'm using begins to evaporate at 259F if this happens I would stop the engine and let the bike to cool for a couple of minutes, I think is not a good idea to change the radiator cap for a higher pressure rated cap and ran at higher temperatures since you are exceeding the coolant and engine temp and pressure design. :naughty:
a higher pressure cap will essentially raise the boiling point of the coolant. what you DON'T want is the water to boil, as that creates a thermal barrier(air) and you will get run away temps.
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Re: engine temp, what should it be?

Post by newbbewb »

I can't really find many people posting on this subject, anywhere. 270 seems super hot to me.
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Re: engine temp, what should it be?

Post by Postigo »

newbbewb wrote:
Postigo wrote:160 to 190F is normal riding temp for my sleeved over bored KDX223. The coolant I'm using begins to evaporate at 259F if this happens I would stop the engine and let the bike to cool for a couple of minutes, I think is not a good idea to change the radiator cap for a higher pressure rated cap and ran at higher temperatures since you are exceeding the coolant and engine temp and pressure design. :naughty:
a higher pressure cap will essentially raise the boiling point of the coolant. what you DON'T want is the water to boil, as that creates a thermal barrier(air) and you will get run away temps.
You can put a different rate radiator cap but for sure it is over the cooling system design, either way how much knowledge have the poor PhD engineer how design the cooling system :hmm: . here is an article about mx cooling systems. http://motocrossactionmag.com/news/10-t ... verheating
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Re: engine temp, what should it be?

Post by chkdx »

Good article.........and it recommends using a higher pressure cap to avoid boiling, that the factory bikes use 26 psi caps.....hmmm, but then, what do they know? I'm using a cap that's rated at 19-21 psi and works at 270 degrees. What sort of temps are the factory bikes running that they need 26 psi caps? Maybe more than 270 degrees? If so, why did they design the cooling system to run that hot?

newbbewb, I agree that 270 seems really hot. I'd really like to know how to reduce it, and if other KDX owners have similar coolant temps. Ideally, I'd like to see it never above 200.
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Re: engine temp, what should it be?

Post by newbbewb »

chkdx: Have you ever rode with your bike without the radiator guards? whats the temp then?

postigo: not saying the engineers didn't think the system through. I know they did!

I have read from random places that a guy wants a minimum of 160degF to prevent premature wear and even cold seizure problems due to piston expanding faster than cyl wall. went on to read that this guy was talking about cast or sleeved cylinders, not nikaseal/electrofusion.

my 89kx125/200 only has one radiator and I'm observing these temps with 70/30 coolant mix, no guard. I would think a kdx with it's two radiators would be better. But, since there is no difference, it's a moot point.
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Re: engine temp, what should it be?

Post by Postigo »

That's half the statement: While raising your cooling system’s maximum operating pressure does raise your cooling system’s boiling point and therefore helps to avoid boil-over, it does not lower the engine’s operating temperature prior to boil-over. It simply allows your engine to continue to operate at temperatures higher than it is engineered for.
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Re: engine temp, what should it be?

Post by newbbewb »

So we are in agreement. chkdxs' cap could of been shot, and his gauge could be off.

so can anyone else chime in? what is your temperature while riding?

I need to start a poll, how do I do that? I want more general consensus on this.
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Re: engine temp, what should it be?

Post by chkdx »

Nope, gauge is accurate, checked it with boiling water.

Cap may have been bad, but I did find that other KDXers had similar problems with the stock cap, and that going to the higher pressure cap stopped boil over. None ever measured the temp, though.

No, haven't tried it without the guards yet, I do plan to do so, just haven't gotten replacement shroud slats bought yet, because I'm going to start on a 2001 KX125 / 220 hybrid soon. On that, I'll just use the KX radiators and shrouds. That same article says that guards similar to mine "raise the temperature 10 degrees or so".

I agree, a poll on what temps people are seeing, and how they're measuring it, is a great idea. I don't know how to start a poll, hopefully one of the mods can chime in.
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engine temp, what should it be?

Post by adam728 »

Definitely need to know "how measured". If looking at actual coolant temps, location will make a big difference. On other boards some guys use temperature strips on the jug or head, some guys point a temp gun at stuff, others read spark plug temp, etc etc. All will give drastically different numbers.

The only time my 220 could ever boil was when detonating in the dunes. Stock cooling system, ported jug, cut and reworked head (squish), rev pipe, bored carb, etc etc. Ran great in the single track where it spent 98% of it's time, but a few good dune climbs had it steaming and rattling. Ended up having to go to a 175 (!) main to keep it "fuel cooled" enough for the weekend. Didn't have any better gas nearby to try.
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Re: engine temp, what should it be?

Post by TheJackRabbit »

I don't agree with stopping when the motor reaches a very high temperature as it creates hot spots and the coolant can boil only around the exhaust port or only at the head. I have never boiled any of my bikes on steep slow climbs, I tend to get up them fast as possible then ride around and get some air through the rads while I wait for the rest of the crew to catch up
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Re: engine temp, what should it be?

Post by Jaguar »

I use a Suzuki AX100 on the street and it is without a liquid cooling system. I have a head temperature gauge with the thermistor as part of a washer between the spark plug and the head. I've noticed that above 275 degrees is when the engine starts to slow down due to resistance. I have sent a piston to Swain Tech for them to ceramic coat the top so the piston won't get so hot. Of course that will probably result in higher head temps but my goal is to be able to blast down the highway during the summer and not be limited in speed due to engine heat.
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Re: engine temp, what should it be?

Post by adam728 »

Jaguar wrote:I use a Suzuki AX100 on the street and it is without a liquid cooling system. I have a head temperature gauge with the thermistor as part of a washer between the spark plug and the head. I've noticed that above 275 degrees is when the engine starts to slow down due to resistance. I have sent a piston to Swain Tech for them to ceramic coat the top so the piston won't get so hot. Of course that will probably result in higher head temps but my goal is to be able to blast down the highway during the summer and not be limited in speed due to engine heat.
275 degC, or F?
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Re: engine temp, what should it be?

Post by Railinandnailin »

I installed a trailtech vapor kit on my 92 kdx200. It's a dualsport so I have a good mix of riding. When I'm on the road and there's good air it ranges from 130*-170* F and in tight singletrack all the way up to 250 F. I'm surprised it gets that hot but I think it's safe.
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