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Hi there,

New to the forum was hoping could get some help.

I have a 99 Honda Civic DX 1.6L. Over the last few months I have been noticing some fluctuations in my temp gauge. After about 10-15 mins of driving the gauge will go over the mid-point and appear to look as if it is overheating, it does not get to the H mark but it will get close as I continue to drive. While driving it will bounce from 1/2 to 3/4, then back down. I have done a few things already:

- Replace t-stat, the rubber had disintegrated in it, so needed to be replaced anyways
-Coolant flush, flushed radiator, heater core, and block.
- Pretty sure there is no air in the system, I refilled according to the manual specs: Let car run until fan came on twice with rad cap off, filled to top of radiator neck

I have a temp gun and have measured the temp around different points to see if there are abnormalities in temp. Thinking is, it is a sensor or the gauge. Here are the temp readings I am getting around certain parts of the engine, at this point the gauge is showing right below the white line before "H" so technically overheating, there are no visible signs of overheating:

- At thermostat measuring 195-196 (before fan kicks on)
- Where upper rad house connects to engine, about 205-207
- Where sensor is located for gauge (hard to get a good reading because under distributor), looked to be about 207

Are these normal readings, is it something with gauge or gauge sensor, trying to rule out the car is in fact overheating, are there other places I should measure temp to know for sure?

 
Rep Power: 143

Temperature guns can be very misleading due to factors in how they operate. How shiny a surface is can affect the accuracy of the guns. Then there are factors like how large the actual measured zone is. The gun doesn't measure a spot the size of a laser pointer even if it has a laser pointer built into it.

Having said that, your supplied numbers don't seem amazingly high. Where did the thermostat come from? Dealer or parts store? I would not trust a parts store unit, had too many bad experiences with them.

Do you have access to a scan tool that can show what the engine coolant temp sensor is displaying? They make add on temp gauges that use the vehicle data link connector to display data. Haven't used them myself, but I know people that have. Here is a link to something that a coworker has in his 2012 civic. That model year didn't even have a temp gauge. http://www.ultra-gauge.com/ultragauge/

Be aware that your car has two temp sensors, 1 for the gauge and 1 for the engine computer. After you determine if the car is or isn't overheating you can progress from there. But the D series civic engines had a history of blowing headgaskets and running the AC will only increase the thermal load on the engine.

 
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Thanks for the reply! My dad has a OBD 2 scan tool I may ask him to borrow it this weekend. I do remember there was possibly some functionality on it to be able to see the vehicle temp. I'm assuming anything between 190-215ish is normal, at least that is how it on my other vehicles?

The thermostat did come from Napa, it could still be the issue, if the vehicle really is getting as hot as the gauge is saying I would expect some more visibility, like coolant pouring out of the overflow, etc..

 
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Last year the same thing happened to my 2000 civic dx auto, It started to happen when I would be in start-stop traffic, If I were in one spot for to long I would notice the temp gauge rise, then it would fall back to its normal place after I picked up speed. After the initial time it occurred, I checked my coolant level and made sure that my fan would properly engage and sure enough everything seemed fine. This went on for about a month, where the gauge would occasionally rise and then fall once air started moving through the rad.
I did the same as you and decided to perform a full coolant flush, using a radiator cleaner, as well as swapping out the thermostat and replacing about half of the hoses(only because they were corroded). This didn't solve the issue so I went looking elsewhere for an explanation. Since this was only happening at low speeds, under about 25mph, I figured it probably has something to do with the radiator fan not turning on early enough.
To test this, when I was sitting in a drive through with the ambient temp at about 80F, I waited for my temps to rise. Once my temp got to about 3/4s(it only took about 45sec) I shifted my tranny into neutral and revved the engine to about 2500rpm. As soon as I did this my coolant gauge shot down, bringing it back down to its normal level. This method worked everytime to bring my temp down, even though id figure increasing revs would put more heat into the block. What I found out for my situation was that the radiator fan would not engage until the needle was about to hit the red mark, and then the fan would turn on to bring it back down. The revving was simply tricking the cars computer to turn the fan on.
My solution to this was simply to install a cheap 12v fan from amazon to the rad, getting rid of the old one. I then wired it up into my cabin with a little switch, so I can turn it on and off whenever I need. This isnt a perfect solution but I was not willing to figure out how rewire the stock fan to turn on earlier. Unfortunately I never actually got temp readings from my engine, since my OBDII reader doesn't tell engine temps and the stock gauge has no numbers on it, so who knows maybe all that work was for nothing and my engine wasnt actually getting hot, maybe just had a sensitive gauge lol. This setup is still working a year later and now I can have my fan always on when sitting in traffic, so no worrying about overheating at all!