I've had about a week to run through a tank of gas on my new [used] 99 Civic LX. I did mostly freeway miles and I'm averaging 25 mpg. Is this normal? I was expecting closer to 32mpg which is the rating for the automatic tranny version of this car, but perhaps my expectations are unrealistic. I haven't replaced the fuel filter, but the oil and air filter looks new. Is there anything else I can look at or is this normal MPG?

Thanks!

 

Check your spark timing. Wrong timing will really kill mpg. Also if you find the timing is off, that could mean the timing belt is starting to stretch out and needs to be replaced. On any used Civic with more than 100,000 miles, if you don't know when the timing belt was last replaced it's a very good idea to replace it when you get the car.

 

Replace the oil (5W-30), oil filter, and air filter. This won't likely improve gas mileage much but is necessary as soon as you purchase a used car.

Regarding the low gas mileage, start by replacing the spark plugs, plug wires, distributor cap and rotor (=basic tune up).

How many miles on the engine? As mk wisely mentions, check the ignition timing with a timing gun and change the timing belt if you don't absolutely know whether it has been replaced recently.

 

The original owner told me the timing belt was recently changed so that's a plus. It helps that the original owner isn't the person who sold me the car, so they weren't trying to feed me BS. I will change the plugs, oil filter, oil (I saw the oil chart but I'm not sure if there's a specific oil from that chart that is recommended for 120k miles?), plug wires, distributor cap, and not sure about the rotor (I think that's part of the cap, yes?).

As for timing, I don't have a timing gun and I don't know how to time a car (I know you have to twist the distributor cap on some vehicles). Maybe I can get it timed at a mechanic who won't try to upsell me all the other tune-up services that I can perform on my own...

 

Have you actually seen the receipt for the timing belt replacement? You need to be absolutely sure. If the timing belt snaps or slips, then the engine could be destroyed.

A reputable 5W-30 oil should be fine.

Use the Honda-recommended basic NGK spark plugs rather than anything fancy.

The distributor rotor is separate from the distributor cap.

Regarding the ignition timing, an inexpensive inductive timing gun will do. Inquire at your local auto parts store about a rental if you you don't want to buy. The ignition timing is checked with the service connector jumped with a wire/paper clip (see CEL link in my signature). The red mark (middle in group of three) on the cranksahft pulley should align with the pointer on the lower timing belt cover. If not, loosen the distributor bolts as you mentioned and turn the distributor slightly to adjust. The engine should be warm and idling to specification (~700 rpm) when this procedure is done. Note: Check the timing after you have replaced the plugs, wires, cap, and rotor.

 

You are the man for the continuous bitchin' replies. Many many thanks. So am I understanding this correctly in my assumption that timing affects idle speed? You mentioned an idle of 700rpm when warm. From a cold start, it idles at 1500rpm until warm and drops to about 900-1000rpm.

As for timing belt... gosh, I would hate to have it break and ruin the engine. Isn't it like $3-400 to have it replaced though? Is the belt something that is rather easy to replace with minimal tools? Do you have to drop the engine? I replaced a timing chain tensioner on my Nissan Sentra and managed to gasket it back up without any leaks but that's a little more intense than I prefer to do with such limited experience.

 

Looking at the DIY Timing Belt change, that looks like something I could not perform on my own. I don't even have a decent jack, torque wrench, breaker bar, or any of the stuff used. There's no test that can tell how old the belt is huh?

 

Actually, the engine speed affects timing. The ECU (car's computer) advances the ignition timing with increased rpm and load. Thus, to properly set the ignition timing, the idle speed must be to spec, and the ignition system must be functioning properly.

Your warm D16Y7 engine should idle in the range of 700-800 rpm without a load. Unfortunately, the gauge assembly tachometer is not very accurate, but your cold and warm engine rpms seem about right (should be less than 1000 rpm when warm without load).

Yes, a good local mechanic might replace your timing belt for $400. For the less experienced, you can do the job in your garage during a weekend for much less. The belt itself costs about $30. You need a couple of breaker bars (24" and 18"), a 17-mm deep 1/2" socket, and a special hex tool to hold the crankshaft pulley. If you have a very strong impact wrench, these items are not required. Obviously, first try to get a receipt for the timing belt job or verification from the shop where the work was done before you proceed with this job.

By the way, unless there is a lot of obvious wear, it is difficult or impossible to tell the age or mileage of a timing belt. The two belts that I replaced at 100k miles looked pristine.

 

I forgot to mention that, when your Civic is properly tuned and running, your freeway mileage may jump to 35 mpg or higher, depending on your driving habits.

 

I will try to find out info on when the timing belt was changed. The original owner might think I'm creepy for calling her but it could save me tons. Maybe the ole broke college kid routine will ease her into the odd conversation.

I definitely drive lightly. I got an average of 48mpg driving the 400 mile round trip from Crater Lake and back in my 97 Sentra.

Is fuel filter worth looking at? What about those add-to-gasoline fuel injector cleaners?