Why am I only getting 10 MPG in my SE?
04-25-04, 12:52 PM
#28
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Ok, Last week I put in a new O2 sensor. On a fresh tank of gas I made it about 300 miles, so I thought the problem was solved.
That next tank of gas I only got 15 MPG!
Anyway, I put an air/fuel gauge in my car 2 days ago, and the car is not entering closed-loop mode at idle.
It runs rich all the time it is idling, and when I accelerate It actually leans out a little bit.
The computer is entering cloesd-loop mode when I am cruising, but it bounces further off than I expected. The instrustions that came with the gauge said it should bounce back and forth in the yellow range (proper mixture) but right now it bounces from completly lean to halfway into the rich side. Is this normal?
Anyway, It Idles kind of low, and it actually died twice yesterday. (I let off the gas, it just didnt stop idling down untill it was dead)
Anyone have an idea of what is going on?
That next tank of gas I only got 15 MPG!
Anyway, I put an air/fuel gauge in my car 2 days ago, and the car is not entering closed-loop mode at idle.
It runs rich all the time it is idling, and when I accelerate It actually leans out a little bit.The computer is entering cloesd-loop mode when I am cruising, but it bounces further off than I expected. The instrustions that came with the gauge said it should bounce back and forth in the yellow range (proper mixture) but right now it bounces from completly lean to halfway into the rich side. Is this normal?
Anyway, It Idles kind of low, and it actually died twice yesterday. (I let off the gas, it just didnt stop idling down untill it was dead)
Anyone have an idea of what is going on?
04-25-04, 04:33 PM
#29
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Buggy ECU maybe? My 84 200SX used to do something similar when its ECU was cooked, and aside from the engine itself the systems in two fuel injected cars from the 80s would be very similar. Thats why I got my GSL, I got tired of dealing with the computer glitches on the 200SX.
04-25-04, 04:49 PM
#30
84SE-EGI helpy-helperton
I think you may be overestimating the effect of an O2 sensor on fuel mileage. Your driving style has more to do with it unless you drive highway miles under CC all the time - in which case you'll see a wider dispersion between City and Highway miles when you calculate it.
What you described with getting 300 miles per fillup is actually about 22mpg or just slightly better. That's better than I get with my 84SE, and I've done some things that shouldn't affect mpg that much. I would say that it's above average mileage if you're driving mixed city/highway, and I think it's about on-par if you did all highway driving.
The AF gauge that you bought may be looking at the O2 sensor signal and trying to use this to display a bar graph, but what the engine is doing at any given moment is totally controlled by the ECU - the bar graph AF is always going to be lagging by some, that's why you'll see swings in the lean/rich mixture even during idle conditions.
Also, it's not unusual for the car to run rich during idle, particularly during a cold-start. Since you have this gauge installed, have you tried adjusting the Idle Mixture trim pot located on the passenger side strut tower? You can manually change the Variable Rheostat (Mazda term) to adjust idle mix, which could make a big difference in your case. What I noticed is that this can be adjusted for best idle and smoothest idle speed to get a nice, smooth condition.
A simple turn with a screwdriver may fix more than one problem with your rich idle. The new O2 sensor certainly can't hurt. I'm getting about 18-19mpg mixed city/highway and am not complaining. When you were getting 10mpg, you had a problem, now you're just in with the rest of us.
What kind of mileage were you expecting, afterall?
What you described with getting 300 miles per fillup is actually about 22mpg or just slightly better. That's better than I get with my 84SE, and I've done some things that shouldn't affect mpg that much. I would say that it's above average mileage if you're driving mixed city/highway, and I think it's about on-par if you did all highway driving.
The AF gauge that you bought may be looking at the O2 sensor signal and trying to use this to display a bar graph, but what the engine is doing at any given moment is totally controlled by the ECU - the bar graph AF is always going to be lagging by some, that's why you'll see swings in the lean/rich mixture even during idle conditions.
Also, it's not unusual for the car to run rich during idle, particularly during a cold-start. Since you have this gauge installed, have you tried adjusting the Idle Mixture trim pot located on the passenger side strut tower? You can manually change the Variable Rheostat (Mazda term) to adjust idle mix, which could make a big difference in your case. What I noticed is that this can be adjusted for best idle and smoothest idle speed to get a nice, smooth condition.
A simple turn with a screwdriver may fix more than one problem with your rich idle. The new O2 sensor certainly can't hurt. I'm getting about 18-19mpg mixed city/highway and am not complaining. When you were getting 10mpg, you had a problem, now you're just in with the rest of us.
What kind of mileage were you expecting, afterall?
04-25-04, 05:08 PM
#32
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Ill try and adjust the Variable Rheostat to get it to idle better.
The gas mileage is fine for me now, at this point im just trying to get it to idle right. I thought it was supposed to go into closed loop mode at idle, but if its not, then I guess its running ok.
The gas mileage is fine for me now, at this point im just trying to get it to idle right. I thought it was supposed to go into closed loop mode at idle, but if its not, then I guess its running ok.
04-25-04, 05:26 PM
#33
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You may want to adjust the throttle position switch. It's a bit tricky to adjust correctly because it had multiple switches in it. Read your service manual (if you don't have a factory Mazda one - get it) on how to do it. There are a bunch of screw adjustments on the throttle body that people love to screw with and any one of those can cause idle problems too.
-Scott Tucker
-Scott Tucker
04-25-04, 05:47 PM
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Well I just tried adjusting the Variable Rheostat and it didnt do anything. It stayed rich and the idle speed didnt change at all.
I dont have a FSM, but I am going to try and fix it using this site. seems like pretty good info.
Thanks for your help, everyone! Hopefully I can get this thing running great very soon!
I dont have a FSM, but I am going to try and fix it using this site. seems like pretty good info.
Thanks for your help, everyone! Hopefully I can get this thing running great very soon!
