Honda Civic Gas mileage just went down 5 mpg

I just had the lower heater hose replaced in my 1991 Civic Wagon because the temperature gauge was doing an imitation of a roller coaster–up down, up down–and now the car takes forever to heat up and the gas mileage has gone from 30 to 25 MPG average.

Eventually, the car heats up, but it can take 20 miles, but how would this effect the gas mileage?



“now the car takes forever to heat up and the gas mileage has gone from 30 to 25 MPG average. Eventually, the car heats up, but it can take 20 miles.”

The cooling system thermostat needs to be replaced.

The symptoms that you report are 100% consistent with a thermostat that is stuck in the open position. If you reported this symptom to your mechanic and he did not know the cause, that would be a cause for concern. This is so obvious and so basic that you should probably seek a new mechanic.

Yes! It can effect gas mileage. The longer it takes for the engine to heat up, the longer it takes for the engine managemant system to go into the closed loop mode. And until it goes into the closed loop mode, the engine will use a lot of fuel.

Since you just had a service performed that involved the cooling system, there may be air trapped in the cooling system. And if that’s the case, it can cause the problems you’re having.

To remove the air from the cooling system, drive the car until it gets as hot as possible. With the engine idling, slightly loosen the upper radiator hose clamp, then take a flat bladed screwdriver and slip it between the upper radiator hose and the radiator hose neck. Let the engine idle until all the air is purged out of the cooling system. Retighten the hose clamp. After the engine has cooled down, check the coolant level.


To be fair, I have not asked the mechanic about it yet. I’ll take it in Monday. Thanks for the quick, understandable reply.

Tester makes a good point. While I think that it is more likely to be a bad thermostat, it is also possible that your mechanic failed to purge air from the cooling system when he drained the cooling system and replaced the heater hose.

And put the heater control in Hot position and make sure the water valve controlled by that control does indeed open all the way. You want full circulation to and from the heater core.

Heater control works, when the temp gauge finally moves up, and putting the heat on full blast makes the temp gauge go down.

I’ll have the mechanic check the thermostat and purge the air from the system. Thanks for your advice.


Why did you choose to replace a lower radiator hose to cure a long warm-up time? I know your post reads heater hose but typically people don’t say “lower heater hose” If it really was a “lower heater hose” why did you choose it?

The lower heater hose was replaced to fix the temp gauge wildly fluctuating from normal to red line and back as I drove normally down the road. The radiator and thermostat had been replaced six months earlier. Then the temp gauge starting going nuts, and my mechanic said that lower heater hose was probably the cause–it was the only radiator related hose/part that had not been replaced in the previous fix, so the mechanic went after it. After replacement of the lower heater hose, as I said in the first post, my temp gauge now reads low for an extended period of time. It wasn’t until after the lower heater hose was replaced that I started having the low temp problem.

So the logic use to make the call that the heater hose was the cause of the problem was that it was the only part that had not yet been replaced? I am a ASE Certified Mechanic with 35+years experience and I cannot understand why this was done like this.

Was some defect noted about the hose like leaking,kinked,collapsed something other than it was the only part left to be replaced?

I appreciate your experience and insight. Perhaps some more facts could shed some light on my mechanics thinking.

There was some cracking and slight seepage from the head to valve heater hose (what I have been referring to as the ‘lower heater hose’). I have no idea how my mechanic arrived at that being the cause of the erratic temp gauge, but it did solve the normal to redline and back again swing in the temperature gauge–it just introduced a way longer than normal low temperature state in its place. By the way, they also cycles the cooling system at that time.

I’ve been using this shop for years, and I’ve never been steered wrong by them before, so I’m willing to cut them some slack. They could have taken advantage of me many times over. For instance, when I brought in the car for the erratic temperature gauge, it was also making an awful clunking noise in the front end whenever I went over any sort of road imperfection. I was convinced, and told them so, that I thought I might need new tie rods but it turned out to be that the front sway bar bushings were worn out–a simple cheap repair. Another constant on this car is the seeping head gasket that has been present since 55K miles (it now has 153K). It loses a negligible amount of coolant and oil between oil changes, but it’s a ‘keep an eye on it’ situation. Another shop on the other hand, told me to do a full motor rebuild from the rings on up for $4K for that same problem.

Thank you so much for your interest and concern.

Replacing the hose probably bled the system or when he refilled the system he bled it correcly thats why it appears replacing the hose fixed the temp swing. A temp swing from normal to hot fits well with air in the cooling system. It does sound like you have a second problem,one that has been pointed out,stuck open thermostat.

It was the thermostat. Now the car heats up in about 10 blocks, and the heater works super fast now too. Temperature gauge is back to warming up fast and then sitting in one place–nothing to see here folks. Thanks to all who weighed in with replies.