Been quite some time since I posted and I obviously dont post much, but I could use some help. When I first got the car, I was getting around 32mpg. It had 124k on the clock when I got it. Now, at 130k, I'm getting 20mpg on average if I'm lucky. The car is a 99 Civic DX, everything is manual and 100% stock. Here's what I've tried to do:

-New Air Filter
-SeaFoam (1/3 vacuum line, 1/3 crank case, 1/3 gas tank)
-New Spark Plugs
-Oil changed 2x, so every 3k miles
-New o2 sensors
-MAF cleaned
-Started running 91 octane


What else can I really check? I'm slowly running out of ideas of what can be replaced. I'm on a very tight budget right now, but if I can get 12mpg back these parts will pay themselves back. Any help will be appreciated.

 

1) Immediately return to using 87 octane. High priced higher octane is not doing you any good and may actually be detrimental.

2) Is the CEL ON?

3) You mention changing the spark plugs. ***What about the plug wires, distributor cap, and rotor?

4) Have you checked the ignition timing with a timing gun?

 

The idle seems to be smoother on 91, but this next tank I will be going back to 87. The CEL is not on, and no codes come up when scanned through the OBD2. Wires/Cap/Rotor were not changed. I do not have a timing gun either. So, theoretically speaking, change out the wires/cap/rotor first to see if thats what causing this? This is my daily, not my toy (already have one of those) so I dont want to start another money pit. Anything to physically check out? I know NOTHING about these cars. Thanks for your response

 

You'll be glad you did for a variety of reasons.

If you have a multimeter, you can check the resistance of the plug wires; however, if they haven't been changed in a while, just replace them. Pull the cap and rotor off and inspect the contact points. I expect that you will find them corroded and worn. If so, just replace.

Some auto parts stores offer free rental of many tools, including a timing gun.

 

I'm willing to bet they have never been replaced. The previous owner did nothing for this car. He hadn't changed the oil when I bought for 12k miles. It was basically water coming out. I'll see what I find then report back. If there's anything else I can look at please let me know. Thanks for your help so far

 

Definitely check that ignition timing. A timing light (aka timing gun) is a great investment. They are only $20-40 for a basic one (all you need). It is very simple to check and adjust your ignition timing if you have a timing light, and it should be checked every so often. Having good ignition timing is an important step to having a smooth running engine.

 

also check your alignment, bad alignment and impropperly inflated tires can cause a decrease in mpg.

 

Thank you for that advice. I just called a buddy who has one and he will be most likely helping me out this afternoon.

The alignment is pretty good last I checked. Just put air in the tires as I thought that could be one of the things.

 

A couple things you can check. Make sure that your brakes are working properly. Brake dragging can put a dent on mpg. It is a manual? Maybe the clutch is slipping?

 

So I checked a few things. Cap/Rotor were both fine. Did a light sand down and clean just to confirm there was no unobstructed contact. Found out wires were replaced not too long ago. Timing was on as well. Starting taking things apart like I promised myself I wouldn't do, and found some SMALL cracks on the stock manifold when I removed the heat shield. Not enough to really make an exhaust leak, or one that would affect much. I'm going to likely just get a long tube in there and remove the cat. Was thinking there could possible be a clog in the cat as well. I checked all the connections and everything seemed to be just fine.




It is a manual. Clutch is alright. Nothing to write home about. I'm not a horrible driver either....just to throw that out...haha. Brakes are good to go as well...well kinda. Rotor feels to be a little warped, but nothing to really be worried about at this point in time.

Anyone have any other ideas on this? I'm pretty stumped as everything seems to be checking out ok.