New PCM - now bad gas mileage

[Black91SHO]

If its running cooler it could be that the thermostat stuck open, that may hurt your mileage. Or if the temp sensor is bad and the pcm thinks its cold that may do it also.

Thermostat works fine. But even of it were stuck open, the cooler running temps would help gas mileage in the summer as long as it doesn't get too cold. And wouldn't account for such a sharp drop. I've had temp sensors go bad, as well as thermostats on other cars, and I've never seen this harsh a fuel mileage drop. It's bizarre.

 

[Black91SHO]

I just noticed that my other '91 SHO's suddenly getting the same gas mileage as this one. But it was because I didn't secure the gas cap last time. It wasn't even screwed on, hardly. I suspected that was the cause of the bad gas mileage before, but I thought I fixed that problem. And last time I popped the gas cap on the car, I heard it release a LOT of pressure (or suck in a lot of air...I'm not sure which). I typically hear a little release of pressure, but this seemed extreme. The gas mileage was always a little bit low on this car, but it was extremely low once the computer went in. I need to check and see if I replaced the check valve at the front of the car connected to the ventilation hose from the gas tank. If I did it within a day or two of replacing the computer, then that might be the problem.

 

[rubydist]

on my red 94, I had just removed the purge solenoid and let it purge all the time to keep from having pressure buildup in the tank. doing that also got rid of the gas smell at the front of the car on warm/hot days.

 

[Black91SHO]

If I did that, it'd be venting all the time, which would be the same as the gas cap not being on, which kills gas mileage. A good seal on the gas tank can increase gas mileage 100 miles per tank on these cars.

 

[rubydist]

that is certainly not true based on my experience and I've had at least 12 of them.

the only value to the cap allowing pressure buildup is that it keeps the aromatics from being released into the atmosphere. by venting into the engine, those are burned up, keeping the tree huggers happy. it has nothing at all to do with fuel mileage. btdt.

 

[Black91SHO]

I track my gas mileage at ever fill up. Always have. I've seen a big difference when the gas cap isn't sealing. One of our Dallas members was getting bad mileage. He replaced his gas cap because of a bad seal. Afterward, he got 100 more miles to a tank without changing driving habits. I've seen this a few times over the years. Im not saying I'm right. I'm just saying I've seen it a few times over the yesrs. Also, I just read it again a few months ago regarding a guy's Z28.

Try it yourself. Go fill up and leave the gas cap off (or just barely screw it on, maybe half a turn). Test your mileage for a few tanks, then put you cap back on and do it again. It's worth a shot, right? Can't hurt, and might help.

What's your typical mileage right now?

 

[rubydist]

if the gas cap doesn't seal, then gas evaporates out of the tank. that is completely different from having the purge solenoid removed so the vapor vents all the time. I had that venting all the time on the red 94 for years, and did not have any adverse effect on the fuel mileage.

 

[Black91SHO]

Ah, I assumed they'd be the same and that it was some pressure effect. Well, I'm guessing the ventilation hose is stopped up in my car. Hence, the extra pressure. I wonder if that would mess with mileage. I'll have to remove the hose and see what happens. Thanks.