Thread: Gas mileage/a little knocking noise..Please help me

I might try to subtract some more fuel, but I don't personally own a wideband right now..

-3 @ 2200 RPM
-4 @ 2800 RPM
-5 @ 4000 RPM
-8 @ 4600 RPM
-9 @ 5200 RPM
-11 @ 5800 RPM
-12 @ 6400 RPM
-10 @ 7000 RPM
-10 @ 7600 RPM

These are my notes from when I had a G20 ECU and a 2.5" exhaust, stock cams, stock MAF.

was it for low throttle settings or high throttle? or did you use only the one map way?

I'm willing to bet if you replace your O2 sensor you will regain some MPG's.
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Now that your done yelling at me in your mind (because you said you replaced it already), let me explain the wonderful world of Stainless Steel headers. They don't conduct electricity as well as steel or cast iron (stock header).

OEM SR20 O2 sensors (front), where all 3 wire. This means 2 heaters 1 signal. The ground was gotten from your header, which got it from exhaust/engine. Solution to this? Get a 4 wire O2 sensor with a dedicated ground wire. Details Here. Thanks to DC Scotty for finding out about this. I regained 6mpg highway after getting the proper O2 sensor for the SSAC header.


I would first make sure your alignment in the front is good. Especially if the car is lowered.

After that Jack up one wheel at the time and spin them by hand (in neutral for front and handbrake off for rear). This will show if your brakes are locked up or something else is creating drag to the wheels.

Look around your fuel lines, make sure they are not leaking and are healthy. Check to make sure your gas tank holds pressure. If it doesn't the fuel is evaporating.

Also your SAFC could be off too, but as you said the problem still was there before the safc.

What intake are you running?

I only use high throttle.

ah ok. also vadim, i'm not going to yell..hehehe..but I can say, this has been happening before I did the header swap. Honestly guys, this has been happening after 4 months owning the car. Like out of now where. I never did any upgrades. But I just remembered one thing. After the 4 months, I did my first tune up. The first time I went to check my timing, everything went fine.NOW I have a problem where I disconnect my tps to adjust timing and my idle stays above 1200rpm. I literally have to disconnect the IACV just to have the rpms drop.

When I do adjust timing, everything goes fine and stays at 15* even after the timing procedure, and running my car around a little, Its still 15* and the distributor is in the middle, like it should be. I did everything in the FSM to adjust idle and nothing works at all. I looked on g20.net and followed every procedure to adjust idle, timing and what not and I did everything correct. I then find another 99 p11 at the junkyard and get the IACV, when I install that, my car will bog and die unless I rev it and then the check engine light will go on. But when I disconnect the tps, the idle goes down where is should be. I did clean the IACV before installing it too. Weird..

I was playing around about yelling :-D.

BTW why are you running 92 gas with stock timing and stock ecu?

I think you might have some sort of vacuum leak. Maybe the seal for IACV is bad?

when you say seal..are u referring to the gasket? I was thinking about that too why the idle won't go down. I don't think I have a vacuum leak cause I check practically every hose, but what I can say, awhile ago, my CEL light when on for: egr,bpt,boost sensor,all those emission control stuff. Then I put some fuel system cleaner and used seafoam and the light turned off by itself. It never came on. But before, my IACV light would go on, then I would reset the ecu and it never came on again. Also, I did try 87,and 89 octane to see a difference, and the only difference I noticed was performance and it ran like crap, not mileage wise. I always used premium fuel in all my cars. we do have 10% ethenol here, but even last year it was like that when I was getting good mileage..

I would Voltage test the MAF.

Never use fuel system cleaner or injector cleaner in Nissan injectors.

Sea foam is no good either .

You shouldn't have seen any noticable performance differences between 87-93 unless the gas station uses peewater for 87/89. Unless you have advanced timing and it knocks for 87/89 and causes the ECU to retard the timing.

If the car is not designed to take premium fuel, or you didn't advance the timing you are actually wasting money. Though Premium fuels generally will have better additives. Higher octane fuel has a higher resistance to spontaneously combust but it also burns slower. Read more.

Ethanol definitely hurts MPG's, but if it was great with Ethanol, then we can rule that out.

Have you tried replacing the knock sensor? It might be having issues and not throw a code. I've had that one happen to me.