garcenw

I finally got around to doing the work to check/replace my thermostat in my 1998 VFR800. Removing the throttle body assembly required lots of lube and lots of mechanical persuasion. The thermostat was failed open (about 1/4" at room temperature). I tested the new thermostat prior to installing to make sure it worked per the spec and it did (it opened in the mid to high 170's F). With the new thermostat, it warms up a lot faster. I went out Sunday night and maintained 70 mph on US50 between Bowie, MD and Annapolis, MD. Ambient temperature was 39F. My dash gauge varied between 168F and 169 F when I held the aforementioned steady state speed, so I'm matching what "MadScientist" and "MaxSwell" reported. I guess the bypass hose to the radiator is just enough flow to allow some measurable cooling to take place which is why the gauge temperature is less than thermostat temperature (assuming the dash gauge is somewhat accurate).

KevCarver, Did you happen to test the old and new thermostats in a pot of hot water before installing the new thermostat?

I lived in Roseau for 4 years when I was an engineer for Polaris. It seems that northern MN has more than their fair share of hooved forest rats.

I did a similar idle test, but this time, I put the thermocouple in the center of the oil cooler. Ambient temperature on the dash gauge was 82 F, while my thermocouple indicted 84 F. Let's call it 83 F. My results are plotted below. As you can see, at least while idling from a cold start, oil cooler temperature asymptotically approaches water temperature and the oil is slower to warm up than the coolant. I'm not sure if this would be the case while running on the road. I was expecting warmer temperatures in the oil with all the fluid shearing going on. The up and down cycling of the coolant temperatures after ~800 seconds is due to the fan cycling on and off.

KevCarver, Sorry, I should clarify what is meant by "steady state" for the non-engineers here. The temperature indicated was steady state which means I was running long enough and in relatively stable conditions so that the temperature leveled off. In the particular run you asked about, I did about 20 minutes of city driving before hitting a flat and non-curved section of limited access highway. Temperatures got as high as 206 F (briefly) in that city driving. Once on the highway, I maintained about an indicated 65 mph (true speed would be a bit less) until the temperature leveled off at 165 F (bouncing back and forth between 165 and 166, but spending more time at 165). I repeated the test going the opposite direction on said highway on my way back and temperature also leveled off at 165 F.

RVFR, Folks get worked up over the temps over 200 F, but I don't see those as an issue, unless the temps get past the warning temp level on the gauge. Most cars run around 195 to 210 with nominal loads. I wouldn't worry much about temps in the 200 - 235 F range. My concern is on the low end of the temperature scale once warmed up. Are steady state temps below thermostat temps normal? Normally not, but unlike some other vehicles, the VFR800 bypass goes to the radiator. That bypass is not insignificant in size, so there is the potential the some heat dissipation always occurs at the radiators even when the thermostat is closed. I'm trying to establish what are the normal steady state temperatures are on cool days before I decide to pull my thermostat out and test it. The temperatures I'm seeing (a little below thermostat opening temperature) are in the gray area if one assumes the bypass allows some cooling to occur and also accounts for the acceptable error in the head temperature sensor (which is a thermistor). - Walt

sfdownhill, KevCarver, Mathematical symmetry can be a tricky subject. - Walt

sfdownhill, I don't know the spec for the Mustang, but car thermostats typically open in the 190 F to 205 F range. The thermostat is likely working fine, it's just the wrong thermostat. The thermostat is the killer because of all the hassle to get to it. You can change the thermostat and correct the fan first. BTW, turning the fan 180 degrees doesn't change the blade pitch direction and thus the flow direction is the same. All it does is reduce fan efficiency. The motor polarity musts be reversed to change flow direction. Those two changes will, more than likely, fix your elevated temperatures. If not, then you can get the radiators flushed. - Walt

Damn, 213F!? It gets hot in Arizona. 🤣

My temperatures might seem OK, but what I haven't done is rigorous observations with ambient temperatures in the 50's , 40's, or 30's. It is rare here in DC in July/August, but I might see mid 50's in the evenings if I'm lucky and we get an unseasonably cool day. 40's will have to wait for the fall and 30's perhaps December. In these cooler temperatures, I suspect I will see cooler dash gauge temperatures. I don't want temperatures to get so low that condensation can settle or that parts don't reach there design intended sizes due to thermal expansion. The other piece of evidence I have is that I don't get the sudden warmup of the right radiator once the gauge temperature approaches nominal thermostat opening temperature. My right radiator starts warming up roughly instep with the dash gauge readout. I collected some data on that. My procedure: 1) Insert type T thermocouple in the center of right radiator. Thermocouple is touching it's nearest cooling channel. While this wont give you exactly the coolant temperature, it will be close, as the thermal resistance across the metal of the cooling channel is low and the channel to coolant thermal resistance is relatively low. In radiators, the big major thermal resistance is on the radiator-to-air side. In addition, I'm not so concerned with the exact coolant temperature. I'm more interested in the temporal thermal response of the radiator. 2) Thermocouple was connected to my multimeter which has a built in T-type thermocouple voltage to temperature converter. 3) Bike was at ambient temperature when started and I let it idle in my driveway. At periodic intervals, I noted elapsed time, dash gauge temperature, and radiator temperature. Note, at time = 0, my dash coolant temperature gauge obviously didn't give me a temperature (ambient was 80 - 81 F) as it doesn't read until the coolant hits 97 deg F. For that data point, I used the ambient temperature as the bike had been sitting for some time. Below is a pic showing thermocouple placement. The data is shown in tabular form and is also plotted. As you can see, my radiator temperature increases roughly in step with the dash gauge. The radiator reads a little warmer than the dash gauge. That could be temperature sensor/thermocouple error, heat coming off the headers, or a combination of both.

Terry, I thought about that as test, but the presence of the bypass circuit indicates at least some waste heat should be going to the radiators even with the thermostat closed. Perhaps the amount of waste heat being dumped is too small to notice until the thermostat opens. - Walt

Terry, How old is your thermostat? - Walt

Maxswell, I normally see temps in the 200 - 210 F range in DC summer stop and go traffic. If I sit stationary for any great amount in time in DC summer traffic, my fan will come on at temperatures slightly above that and I ahven't seen anything as warm as 230 F. I think the highest I've ever seen was 226 F and that was idling in my driveway. I'm more concerned on the low end as mid 160's F seems a little low for steady state cruising at any temperature. I'm trying to determine what is normal for this bike before digging in to do a thermostat replacement. Are your low end readings (i.e. 165 - 181 F) taken with a recently replaced thermostat? - Walt

I’m likely beating a dead horse here, but I can’t find a true consensus on older postings. On cool days, say 75 F (22 C) or less, what does your 5th gen VFR coolant temperature gauge settle at once you reach steady cruising on the highway or interstate? With the temperature sensor in the front head, one would expect something a little above thermostat opening temperature. The issue is that our VFRs have the bypass circuit in the thermostat housing that always allows some flow to the radiators irrespective of thermostat state. That flow path deosn't seem insignificant in diameter. It’s hard to know how much cooling that accomplishes without applying some engineering rigor. I figure the best way is to ask the members what they are seeing. Probably the most useful responses would be those stating ambient temperature along with coolant temperature at that ambient temperature once you reach steady state while cruzing. Given the high failure rate of our thermostats, the highest confidence responses will be from those who recently replaced (say in the last few years?) their thermostats. I haven’t replaced my thermostat since I’ve purchased the bike (this March) and being the third owner, I don’t know the history of what has occurred regarding thermostat replacement. I suspect I’m in that grey area temperature-wise and suspect mine isn’t stuck, but doesn’t fully close (it reaches approximate thermostat temperature in ~3.5 miles of low traffic conditions on warm, say 80+ degree, days). Here are my steady state observations: Steady speed = ~45 mph, ambient temperature = 88 degrees F -> temperature gauge = 181 degrees F Steady Speed = 60 - 65 mph, ambient temperature = 73 degrees F -> temperature gauge = 165 degrees F Thanks. - Walt

I installed regular H4 bulbs. The harness doesn't even get warm to the touch (on an 80 degree F day).