poor gas milage in town 29.8 freeway 14.2 city

[BlooDSMeaR]

I just got this atx sho on the road. I was able to get a 28k mile motor and installed it. She runs perfect with no hesitation or rough starts. No trouble codes. I made a maiden voyage trip up to Yuba City, roughly 350 miles one way. I averaged 29.8 MPG on that trip. Now that I have been in town, I have been driving conservatively (IE no pedal mashing or stop light to stop light racing) letting it shift around 3k RPM the majority of the time driving. Now for the third tank of just in-town driving I have been averaging 14.2 MPG. I am only getting around 200 miles out of the tank. (My BMW 750 IL with a v12 got better millage) In all of my other sho's I get right about 20 MPG of just in town driving, and 28 MPG freeway. Is it just the ATX taxing the fuel economy? Does the ATX suck that bad to **** the mileage? What are some things you guys could recommend to get mileage up? I don't mind spending a little money on stuff, but I am far from rich, so economy for me is an issue.

 

[rubydist]

the atx (based on my experience) should be 18-20 in the city and mid-20s on the highway. It sounds to me like its running lean, which would both give you higher highway mileage and lower city mileage.

Check codes, look carefully for vacuum leaks and let us know what you find.

 

[BlooDSMeaR]

I have re-cheked the codes. The system passes with 1-1-1. The motor runs great and does not lack any power. One thing I have noticed is it takes about 10 miles to warm up the motor to running temperature. That might be my poor gas mileage issue right there. What I am thinking is that the thermostat is not closing during warm up, and is running rich during the warmup time. Any thoughts to that being the problem?

 

[rubydist]

yeah, it sounds like the thermostat does not close all the way - the pcm will run in cold-enrichment mode for up to 20 minutes if the coolant temp does not come up, and that is quite ******* gas mileage especially in the city. a new thermostat seems in order.

 

[BlooDSMeaR]

I will replace this weekend and advise after a couple of tanks of fuel.

 

[hawkeye18]

Something else to note is that your rear motor mount (or front one) may be bad.

My wife's 95 ATX had been getting terrible mileage for a long time. Like, 12-14mpg in 70-80% city driving. The front mount was a steel plate, and the rear mount was stock. It didn't appear to be bad. We tried changing plugs, replacing the cats (they needed replacing anyway, so no big loss there), replacing all the vaccum lines (like, all the lines), everything we could think of.

Finally, the rear mount straight failed and got to the point where driving it was scary. The front steel mount was holding it together, but every time it shifted it felt as if cargo were shifting and slamming against a wall. We replaced that with a solid mount as well, and as of her first tank of gas since the replacement (with about every third start from a stop being a flooring) it got 18.6mpg. So, a nearly 30% improvement.

It's worth considering.

 

[SHOdded]

Never heard of a motor mount being a culprit for mpg before, good addition to the ole knowledgebase!

 

[hawkeye18]

Never heard of a motor mount being a culprit for mpg before, good addition to the ole knowledgebase!

Well think about it, how much torque does a faulty hydraulic mount soak up? Obviously MTX guys don't have that problem as much, but it is something to consider.

Also, two solid mounts plus gen3 subframe mounts in an ATX = f*** your neck shifts and f*** your teeth idle.

 

[BlooDSMeaR]

A 30 min trip down the freeway told me I had a stuck thermostat, Since it has been cooler today since I got her running, on the freeway I noticed that the temp kept inching down on the gauge. All the way to the under side of the cold marker. Once in town and a handful of stoplights later the temp came up to normal. Saturday I will change it out.

 

[BlooDSMeaR]

Today, I installed the t-stat. I was able to tell right off that was an issue. just a quick trip to visit the parents, she was all warmed up to mid normal. Before I put it in, it wouldn't even hit the bottom hash mark driving the same distance. I was also able to tell about the gas millage right off. I drove about 40 miles in-town today, after topping her off and its still pegged at full. Ill give a better report as I don't have any road trips coming up. So Ill be able to do just in town driving.

 

[BlooDSMeaR]

Update- 2 tanks of fuel of in town driving has lasted a little longer. I am now getting about 250 miles out of the tank. Still not where I hoped it would be. I just installed new plug wires, noticed an appreciable gain in the seat of the pants feel. This weekend will be new vacuum lines, fuel filter, oil change and trans service. Hope some of will help.

 

[SHOdded]

Worth disconnecting the MAF sensor and checking/cleaning it off with MAF cleaner too.

 

[rubydist]

Update- 2 tanks of fuel of in town driving has lasted a little longer. I am now getting about 250 miles out of the tank. Still not where I hoped it would be. I just installed new plug wires, noticed an appreciable gain in the seat of the pants feel. This weekend will be new vacuum lines, fuel filter, oil change and trans service. Hope some of will help.

You aren't really telling us much, because the gas gauges vary so much on these cars that we have no idea how much gas it takes you to go 250 miles. Can you do a little simple math and tell us the mpg that you are complaining about?