D15B ~13mpg - please help

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New Member
Hi all,

I'm new on the threads, please forgive me for any noob-ish mistakes (and I'll try to learn quickly not to repeat any) - I'm trying to find answers to this problem and could really use your help. I will post as much information as I can think of at the moment but if there is anything I'm missing let me know and I'll try to get it to you. I recently bought the 1995 Honda Ballade (with a D15B engine swap) so anything I can't find I may have to talk to the previous owner.

I'm getting roughly 5.5km/L (~12.6mpg) and I drive it very conservatively (have to with that mpg!).

Additional info:
-the engine has been gas-flowed
-from what I can see I think there are larger fuel injectors
-composite head gasket
-Gotech MFI X engine management system
-aftermarket induction (which doesn't allow space for O2 sensor)
-bosch super 4 spark plugs

The previous owner was setting the car up for a turbo but never finished the project. He had tuned the car for that setup with a air-to-fuel ratio of ~17.

Since I've had the car:
-taken it back to the tuners and they set the AFR to ~14.2
-cleaned the air filter
-cleaned the throttle body
-put on Huco High Performance Racing HT leads

-the fan belt is currently loose (affects alternator/spark?)
-spark plugs are covered in carbon - could this be the sole cause?
-there was some recent wiring work done on the sound system and the battery was disconnected - could this have reset the fuel maps?
-no check engine or warning lights have ever come on

When I received the car the exhaust was covered in carbon (which I don't get considering it had more air-per-fuel ratio?) and when I checked the spark plugs they were also covered in carbon soot. Unfortunately I wasn't able to sand them at the time.

So the questions I have...
1. Could dirty spark plugs be responsible for the whole problem?
2. Could the battery disconnect reset the fuel map?

Thank you so much in advance for the advice.


It's only stupid if it doesn't work
South Africa, nice. Welcome.

I have no idea how you're getting that mpg, with no O2, with no check engine light. As far as I know, you need an O2, otherwise it will run like garbage. And no light to go with it? I'd almost say, see if the bulb is out, unless they tuned it off.

How does the car actually run? Does it have a misfire or anything? Or does it seem to run right, just gets bad mpg? Has it always gotten this mileage?
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New Member
How does the car actually run? Does it have a misfire or anything? Or does it seem to run right, just gets bad mpg? Has it always gotten this mileage?[/QUOTE]

On cold start up I have to keep my foot on the peddle and slightly feed it gas till it gets warm or else it will stall out, after 5~10 minutes it idles fine and doesn't stall. In that period of it warming up I can hear some soft puttering sounds from the exhaust almost like a really quiet mis-fire but the car runs smooth. I don't know if it's always this bad as I've only had it ~2 months and for the 1st month it had different fuel maps.


Well-Known Member
If its not, I would try the correct ecu for your corresponding motor. Stock ecu, no modifications of any sort.


It's only stupid if it doesn't work
A stock ECU will throw an O2 sensor code for sure. I'm still not getting why the previous owner got rid of it in the first place. The car is running rich. That is the reason for the carbon and the poor MPG.

What is your intention with the car? Daily driver? Track car?
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Well-Known Member
13 mpg is horrendous. My D15b gets 35-40+ mpg. Something has to be really screwed up.


Honda Minion
Check mechanical timing (crank and cam gear lined up correctly at TDC)

Check electrical timing (need to jump service connector w/wire or paperclip to change timing)

Get a stock ECU, exhaust manifold, and O2 sensor. See if there are any engine codes after that.

I say one or all things are causing issues. The fact it won't idle high at cold starts is an issue. Let alone the MPG's.


New Member
Updates here --

I don't have access to stock parts and the previous owner isn't responding so we'll have to work with what we can.

That being said, I've taken it back to the tuner and he insists the numbers are correct, that it is not a fuel map issue, and there is no need for the O2 sensor anymore (I'm referring to the sensor that's right on top of the throttle body). When they did the fuel map they adjusted the timing and I had him lower the idle in case that was leading to bad MPG.

I've taken it on a road trip recently and I was getting ~30 MPG highway but my short distance driving MPG is still bad - roughly 15MPG when I run a whole tank in 10-15 mile spurts.

So...what is it about short distances killing my gas mileage? Not coming up to operating temperature? Something lying to the computer? Different cams I'm not aware of?



Mad scientist
Could just be that the range of the fuel map you're in for highway driving is just fine, and the city portion sucks...


Did they dyno tune it only for wide open throttle? part-throttle requires a lot more work to get right.

The o2 is not on the throttle body-- it's in the exhaust, usually in the manifold, sometimes in the downpipe.

Is there another turner, even far away, to go see? it doesn't like your guy really knows what he's doing. 14.2 is actually pretty scary, even all motor, at WOT.


New Member
After talking with the tuner for awhile and telling him the issues he agreed to redo the whole map for me (free of charge). He put the AFR for wide and partial as close to 14.7-14.8 as he could. These numbers come from Honda recommendations for absolute (stoichiometric) combustion.

Still getting about the same gas mileage...

When I rev the engine I can see the black soot coming from the exhaust which is evidence of it running rich or not burning completely?



The Warden
Can you get NGK spark plugs? The Bosch 4 prong crap is absolutely garbage design.
Get yourself some single prong plugs copper or platinum.


I wear stretchy pants
dang that sux man. I agree with above said. change the plugs and get an o2 sensor in that biotch. I find it hard to believe it's running stoich and blowing black smoke....just doesn't add up. If something was wrong the tuner would have picked up right away.
- change plugs
- add o2 sensor in the exhaust manifold
- change oil more frequently until you get the problem solved. too much fuel will water down the oil and wash the cylinder walls as well. I bet your oil reaks of fuel.