Engine coolant temp drops quickly while coasting.

[brianl703]

I've never seen this before.

1998 Nissan Frontier. 5 speed manual.

If I'm coasting in gear, with the scangauge showing open loop so I know the fuel injectors are off, the coolant temperature will drop 2 degrees in 10 seconds or less--from say 180F to 178F If I coast long enough, say 40 seconds, the temperature will drop all the way to 173F.

And it'll go all the way back up up to 182F if I'm able to drive for a couple miles without having to slow down or stop.

Other vehicles I've had where I've watched the coolant temp with Scangauge or Ultragauge are far more consistent with the temperature.

Do I have a lazy thermostat?

It seems to warm up quickly enough.

 

[2010Civic]

A couple of degrees doesn't sound abnormal to me? I'm guessing the temp gauge on the dash doesn't move?

 

[ripcord]

That amount of temperature variation is inconsequential. I wouldn't worry about it.

 

[slacktide_bitog]

Are those other cars newer? Perhaps newer engine management can keep the temperature within a tighter range. However, even the range you experienced is not a big deal at all.

What temperature is the thermostat?

This is one of the reasons most cars use dummy gauges now, or an idiot light, so people don't worry about small changes in temperature that are of no consequence.

 

[brianl703]

It's a 170F thermostat. The other cars I've watched the temp on are newer. 2004 Ford Crown Victoria and 2006 Saab 93. Worth noting that they both have the thermostat in the usual place, the engine water outlet, whereas the Nissan has it in the water inlet.

 

[brianl703]

A couple of degrees doesn't sound abnormal to me? I'm guessing the temp gauge on the dash doesn't move?

I haven't noticed it moving but I'm not looking at it since the Scangauge is there.

 

[ekrampitzjr]

2° is a normal fluctuation. 20° is a different story.

 

[yowps3]

I'm not sure about the water temps but Nissan oil temps are usually lower than others.

 

[IndyIan]

Aluminum block 4 banger? I find my Focus will drop temperature pretty quickly too on the scangauge, when you take the load off the engine. Even more if you have the heat on of course. Maybe 10-15f temperature swings and I put a thermostat in 2 years ago as the old one was sticking open. With the old thermostat it would swing 20-30f with going up a long hill and then coasting down the other side... Had a hard time getting to 180f too. New one runs almost right on 208f doing 60mph on the flats but I was surprised I didn't get significantly better mileage with a functioning thermostat.

 

[Jetronic]

I've never seen this before.

1998 Nissan Frontier. 5 speed manual.

If I'm coasting in gear, with the scangauge showing open loop so I know the fuel injectors are off, the coolant temperature will drop 2 degrees in 10 seconds or less--from say 180F to 178F If I coast long enough, say 40 seconds, the temperature will drop all the way to 173F.

And it'll go all the way back up up to 182F if I'm able to drive for a couple miles without having to slow down or stop.

Other vehicles I've had where I've watched the coolant temp with Scangauge or Ultragauge are far more consistent with the temperature.

Do I have a lazy thermostat?

It seems to warm up quickly enough.

I'm used to seeing much larger fluctuations in much shorter time frames. That's exactly why nearly every gauge is a dummy, or severely dumbed down in the temp range that's considered normal by the manufacturer

 

[bdcardinal]

I'm used to seeing much larger fluctuations in much shorter time frames. That's exactly why nearly every gauge is a dummy, or severely dumbed down in the temp range that's considered normal by the manufacturer

My Mustang has a live cylinder head temp readout that goes directly off the CHT sensor in the passenger side head. I will have it up as I go up a larger hill like Highway 154 or 101 near Gaviota and watch it get to 220+. Then as soon as I get to the downhill section where I am off throttle and letting whoever is behind me listen to engine braking, I can watch the readout drop back to 180 fairly quickly. Its fascinating.

 

[bonjo]

Long ago (1980's!), I had a lazy stat and when I tested it in boiling water compared to a new one, it did not open as fully.
Once replaced, the coolant temperature fluctuation was much reduced.
Reduced cooling system efficiency will also cause fluctuaction like you describe.
Before a coolant change, I used to use flushing/cleaner to get rid of internal rust and calcium deposit. this helped a lot too.