I drive a 98 civic and I had a b18b stock in the car and was getting 30mpg and just a few weeks ago I swapped another b18b into it with some mods, but I don't believe these mods should cause the terrible gas mileage I'm getting.

Stock head
Eagle rods
Forged pistons (stock compression)
Lightened flywheel
Stock manifold's
Stock injectors
No special tune
LS tranny
Stock exhaust
k&n filter
Cold air intake

I think that covers it. None of them are big upgrades that should affect the mileage in a really bad way. On the new motor I'm getting 18mpg!!! I Drive like a granny and only let my car idle for about a min before driving it. It's winter here and very cold which I know affects the milage but not this much. I didn't get a chance to drive the engine in good weather but I doubt it'll change a lot anyways. What could be wrong? It's a completely new rebuild with a flush of everything and such. I checked all the normal things like tire pressure dirty filter and such. Any suggestions will be nice!

 

Did you get it tuned?

 

How's your O2?

 

x2

 

I haven't checked the O2 but the check engine light isn't on or anything and I haven't reset it. And no it is untuned. Something else that I forgot to mention that I know doesn't help, but on start up when it's cold, the engine will idle around 2k till warm and drops to like 800rpm. It feels like it has a big
Cam in it but I don't. I know it's typical that rpm fluctuates on cold but that seems a lot. Is there anything I can do to keep that lower while the engine is cold? I know the high rpm idling hurts it but I still don't think it accounts for huge drop in mileage

 

Check the O2 sensor and the ECT sensor.

Pull the plugs and see if your running rich, [would account for poor milage] if so, the problem is most likely the ECT sensor. 94

 

most hondas rev up like that at idle so they can get warmer faster my prelude integra and civic all do that. only when its cold though

 

Alright I'll give it a try and drive it for a few days to see if it changes. Thanks for the help

 

Maybe plugs, my car used to have a similar problem, did a tune up and ran like a champ lol