Hey all. A couple months ago my '95 SE was stalling a lot, and had a CEL code for an EGR solenoid which I replaced and reset (no more code). At the time, I don't recall the gas mileage being too bad. In the succeeding weeks, I also replaced the ECTS and the MAF sensor, removed and cleaned the throttle body, and cleaned all of the wire grounds I could locate. The stalling completely stopped but the idle has been high ever since the ECTS went in (that was done by my mechanic since I couldn't get the old one out).

Now the car's idle is like this - after starting, it goes from 1,600 RPM immediately to 2,100, and then stays there until warmed up. Then it goes to around 850-900 for warm idle. Before having the stall issue fixed, it was idling around 650 once warmed up.

My gas mileage has definitely declined. In the past10 fill-ups, once it was only 9.5 and the last time was around 12.5.

What are some things I can do to troubleshoot the poor gas mileage? I don't drive much so the fuel economy isn't killing me, but it does bother me that I'm wasting gas and that the car isn't running optimally.

One other thing that is going on is that the fuel gauge doesn't always go up to its current fuel level. After a fill-up for example, it may stay at 1/2 until after driving around and turning it off/on a few times during making errands. It is possible that my fuel pressure is off and that is impacting my fuel gauge and fuel economy?

Thanks for any replies.

It sounds like the Idle or Accelerator cable was adjusted.

First, check accelerator cable. With the car off the throttle body should be totally closed. If not, reset the accelerator cable so that the TB is closed.

If TB is closed, reset IACV screw. Jus screw (clockwise or counter, I forget which way) until the idle is back to 600 rpms.

If neither helps then u have a differenct issue and we can go from there. But try this stuff first. ECTS wouldnt affect high idle.

to answer the the question, yes high idle can change your mpg, you have to remember an idle car gets 0 mpg and can burn about a half gallon an hour. Its normal to have high idle when its cold and it will decrease as your car warms up. 2100 is awfully high, burning more fuel, definitely check out what cashoit said, it sounds like your idle was adjusted.

You sure? 2/2 Maxs I've had went to 2k on cooold days... Mine starts and goes straight to 2k then backs down as it gets warmer...

Not a chance, high idle would not cause any noticeable decrease in fuel economy let alone fuel economy less than 1/2 of what it should be, but whatever problem is causing the high idle, could be responsible for bad fuel economy.

Could be a sensor problem, injector problem, or ECU problem.

check the ECM, my car has the same problem and it depends on if the ECM is reading correctly on that day

check your hoses for vacuum leaks, it was the cause for my 1.5k constant idle, bad gas mileage and bad putting idle.
make sure you check the evap, thats where mine was

For me, with the stock intake my cold idle was at around 1.5K then dropped down to 800 during normal operating temp.

When I got the ram air, cold idle was at 1K and normal operating idle is at 600.

And so just like what the other members suggested check the throttle cable, idle adjuster, vacuum leaks!

mine never goes above 1600-1700 on cold days then drops down till about 750ish

Did you ask the mech that did the ECTS if he adjusted idle or anything? When he did the ECTS, he should have corrected the idle if it was so far off. FWIW, my 98 with about 130k starts cold at about 1200 (@ 10*F) and idles warm at 6-700 within 5 minutes. It's never idled over 1200 ! (down to 0* F)

Edit - I saw Aackshun's post, and wanted to add: I drive 'city' -suburbs & downtown - never go over 40 mph w/ lots of stops and only get 16mpg in summer, worse in winter. If I'm all hwy, I get 30mpg

Hrmm, maybe it's a 99+ thing, I also have to mention my car's close loop idle changes, my idle is from 650 - 900 weather pending, I can use my idle to predict weather some times lol.

Even with my 2k idle surge on cold starts I still get 27-28mpg highway and 25-30 city.

Wow, hey aackshun, I you pimping a hybrid, I didn't know Nissan made one of those back in the 90s. I don't understand how higher idle will not cause an excess of gas consumption, it takes more fuel to rev higher, so higher the rev the more fuel will be used.Even if its barely significant.

Maybe you misunderstood my previous statements....

I don't drive around with a 2krpm idle... My closed loop idle ranges from 650-900, and getting high 20's city driving with a manual is not hard at all, if you know the lights very well and roll around in neutral allot then this is easily accomplished. I get about 25 in Houston, and in Manhattan, KS (Hilly area), I get around 30.

Clockwise turn = decreasing idle (less air to allow to IACV)
Counter clockwise = Increasing idle (more air...as the orifice gets bigger)

Is slight with MT cuz in neutral there is no load on the engine. WIth AT it would be a lil more.

I agree there would be some fuel loss but is prolly negligible

*** Edited to add some more info. to see if you guys think this scenario might also be responsible for the bad gas mileage.

When I am in motion and I have the clutch in, the RPMs never dip below 1300, until I come to a complete stop, and then it drops to 850ish. I'm pretty sure it used to dip all the way down to 650-700 when I had the clutch pressed in. What could be responsible for this?

I read in another thread about a torque converter that is supposed to disengage when the clutch is depressed, but I don't remember what that was about.

Is there something that monitors vehicle speed and then drops the RPMs when you come to a complete stop? Maybe that is bad now, but I have no CEL for it.

***
Thanks for all the replies guys. I'll check back in once I've had a chance to get back under the hood. Time is scarce right now with finals coming up... (my school is on a 10-week quarter system). I don't want to screw anything up so I'm going to wait a couple weeks until I get a break.

Some additional data from the last two fill-ups makes me wonder whether I really have a problem or whether my car just needs a little air in its tires or some other fine tuning. I registered 23+ and 22+ the last two fill-ups. I still haven't put any additional air since the 9+ and 12+ mpg averages a few months ago, however, it was much colder and the driving I did in between fill-ups then was probably all heavy city traffic (e.g. downtown). My own driving conditions here in/around S. Philly and Center City Philadelphia are much different than some other people whose supposed "city" driving conditions are actually more like suburban traffic for me.

We have driven down and back to AC from S. Philly the last two weekends so I have some actual highway miles, and that is where the 22 and 23 mpg are coming in.

I still have to adjust my idle slightly (750 down to 650) but it seems like the cold idle is more the problem than the normal idle. Would that be affected by the IACV screw needing slight adjustment or is it likely something else.

Is it possible that the gas additives have changed in the past month? I always put 93+ and I generally go to Sunoco.

I have never noticed a difference between summer and winter gas mileage on any of the 4 fourth gens I've owned.