Ever since I swapped out my old d15b2 motor to a jdm zc sohc my car has been eating gas like crazy...I use to get about 330+mpg on a full tank and now im barely clearing 210-230mpg...I also went from a DX tranny to a Si tranny but just don't get why it's eating up too much gas other than the si final drive...With my DX tranny cruising at 65mph I would be at 2100rpm but with the Si tranny in the same speed is about 3300-3400rpm obviously.

I also noticed that it does smell like gas at times and when I release the fuel cap there is like no pressure at all even after priming the motor...I am getting no CEL or any ecu codes and the car idles and drives perfect besides the bad mpg...Still have the no start main relay garbage at times but starts up when I swap it out for another one.

Following has been changed & is new unless otherwised described:
jdm zc sohc motor
91 civic si tranny (bad isb only)
exedy oem clutch kit
resurfaced oem flywheel
ngk spark plugs
ngk wires
ngk o2 sensor (10k miles worth)
10w30 oil
oil filter
fuel filter (5k miles)
fram rear main seal
timing belt
water pump
pcv
cap & rotor

I did a mpfi conversion using a d16a6 complete intake manifold with a pm6 ecu and everything is hooked up correctly running obd-0 accessories...The timing has also been done and is spot on and the catalytic converter has less than 7k miles and is a magnaflow installed from back in 2008...The idle at a complete stop is steady at about 1000-1100rpm...Im also using the stock d15b2 headers with the stock down pipe.

I had a zc dohc motor in my old ed3 using a pm7 ecu with dc headers and running a test pipe and my mpg was still better then than what im getting now...If it's because of the Si tranny I have a spare DX tranny that I could swap in but I don't have a spare fuel pressure regulator and im using the old one mounted on the d16a6 intake manifold with unknown mileage.

 

I've heard that sea foam thing helps. I think that's what it's called

 

Bet money you have a problem with your injector seals. Same thing happened to me with my old EG, very very bad gas mileage. Everything got a lot better once i got new seals for the injectors.

No CEL for that ****! Plus, I smelled fuel also until i replaced the seals. Also make sure the injector tips are clean and intact.

 

i took the a6 intake manifold apart before putting it on and cleaned the crap out of everything from top to bottom...the seals were still bueno and the fuel injectors were cleaned up good as well...i guess ill pick up some new seals when i pick up some radius rod bushings from the dealer

 

fuel pressure regulator.

If it fails, it can push out too much fuel, or it could restrict fuel. No cel for that either. I forgot to mention this, but i swapped over injectors, seals, and FPR at the same time. I dont know which fixed my problem, but the gas milage was a hell of a lot better.

Something is making you run rich. go to pick a part and pocket an FPR.

 

damn. you're still getting shitty MPG? well atleast its running lol.

 

i'll give the FPR once i find it.

 

let me know if your willing to trade for them obd-0 injectors also...i have chingaderas to trade in the red link below...i basically just need everything minus the manifold itself

 

so you need obd0 injectors?

 

just in case mine are no bueno...i like to have spare parts on hand when needed...well im gone for the weekend...gonna head out camping for 3 days so i be back on early monday

 

Get a digital multimeter and check the o2 sensor. + the si tranny has a higher final drive, so that's a contributor right there. I would suggets a new fuel filter and to clean injectors and u should see a small improvement. The dpfi is made for fuel efficentcy remember that.

 

^ you're wrong about the si trans

 

DX/LX
1st - 3.25
2nd - 1.894
3rd - 1.259
4th - .937
5th - .771
Final - 3.888

SI
1st - 3.25
2nd - 1.894
3rd - 1.259
4th - .937
5th - .771
Final - 4.25

Just incase people dont know...this means these are the ratio of which the tires spin with the crankshaft.

 

wait............after reading his post again i had first thought he meant that the si trans was a gas saver MAH BAD! lol..

but now i see that he means that its adding to his horrible MPG.

 

brakes binding maybe?

I had super bad gas mileage when 1 of my caliper slides was rusted