My car was recently inactive for one month. Since then, I'm getting 20-22mpgs with no change in driving habits. It's a 00 civic si cpe with 122K, and no engine mods except an AEM CAI. Here's what I know so far:

1. Pulled my plugs and wires and they're in good condition. No unusual wear on cap and rotor. Plugs 2&3 had oil residue on top. Probably the valve cover gasket and plug seals, but would that cause a sudden loss in gas milage?
2. ECU is not reading a problem.
3. Engine idles normal at 750 rpms. No dips or spikes even when cold.
4. Car accelerates smooth - even at WOT. No apparent loss of power, no feeling of hesitation.
5. No visual vacuum leaks.
6. Throttle body and fuel injection cleaned about 1 yr / 20K miles ago.
7. Car always maintained at the usual intervals. Timing belt replaced and valves adjusted at 105K with no problems.
8. Unknown when fuel filter was replaced. I'm assuming the last one was at 60K. Would failure cause a sudden decrease in milage?

I'm running out of diagnostic options here. Perhaps a compression/leakdown test? Anything else I'm missing? Any ideas, suggestions or comments appreciated.

Also, please let me know if this would be best posted in another forum. Thanks everyone.

Kent

EDIT: I've tried a few search topics. Feel free to link me if there's already an answer.

Modified by Design at 5:43 PM 1/4/2005


Modified by Design at 5:46 PM 1/4/2005

 

Additional things being considereded:

Dirty air filter
Dirty O2 sensor(s)
PCV valve

Anything else it could be?

 

A faulty 02 sensor would give you drivability quirks at part throttle, so that's probably not it

take the valve cover off and replace the spark plug seals and VC gasket (cheap and easy) and replace your PCV valve if it doesn't rattle when you shake it. A leak down test would be a good idea, as well.

I would do this if the results of the comp/leak test are good
-Fuel filter replacement
-New spark plugs (ngk)
-New cap and Rotor (even thought it looks OK, its hard to visually tell)

Also, if its been sitting for a month, you can bet your *** the tires are going to low, causing crappy gas mileage- I just assumed you inflated them, though.

 

Great info - thanks.

Gasket and seals on the radar..... One question - if I'm not noticing a power loss, why would the fuel filter cause bad milage?

 

Usually the loss in power would be so small it would be unnoticeable when a fuel filter is old. If its clogged, then thats a different story.

 

So I cleaned the CAI filter and it got me an extra 10 miles to the tank. PCV rattles. So the NGK platinums are next. BTW, when the car was inactive at the body shop, the engine bay was exposed to the elements - maybe 4-5 days? So I'm trying to think what else could have been affected so suddenly. I think we've covered it all, but anything else is appreciated.

Thanks again. Good info.

 

One more question. The fuel filter assembly is attached pretty tight to the lower fuel line. Any suggestions on loosening the connection?

By looking at the replacement filter, I can tell the it twists counter-clockwise relative to the lower fuel line. Thanks.

 

air tools help with those banjo bolts. I tried to turn mine by hand- ended up nearly tearing it off in anger.