Crappy gas mileage...

[Cogs]

I'm currently getting 13.5mpg city, no matter how I drive. Last tank of gas was all nice driving this one is a lot of donuts and fishtailing and speeding. I drove 400+mi on the highway the other day and got 14.5mpg. That is NOT right. I used to get much better. I know my exhaust is one problem, I have a major leak between the header and H pipe, but what else could be causing this?

[phayzer5]

if you have a serious exhaust leak that close to the headers you're gonna loose a good bit of torque and that oculd cause you to use more throttle that you' should need to do things.. You might not niotice the dofference since you're used to its power

[Cogs]

There is a difference, though I think it may have been leaking a little when I got the car. I'm sure that is most of my loss.

[yucatec]

Didn't you used to get 21 highway? I don't know anything about a performance engine like yours but a 6.5 mpg loss without any awful performance symptoms seems most likely to be a gas line or tank leak. Donald McCaig

[grandpaslincoln]

There are a few things that can cause excessive fuel usage. For example:

A dirty air filter.
Incorrect ignition timing.
Incorrect Idle speed.
Low tire air pressure.
Binding brakes.
A fuel leak.
Malfunctioning Transmission.
Thermostat stuck open/not installed.
Dirty or clogged fuel injectors.
Malfunctioning Carburetor.

Check these things.

[NitroNic]

I've been getting 10.5 to 11.5ish in town as long as I Can remember... I've been driving this car for almost 4 years, the worst problems I've encountered since driving it are crap starters, bad air pump, water pump, misc brake part issues (mostly master cylinders and pbb's), and driver side windsheild wiper arm.

Maybe I should pull some codes, just for kicks? :dunno:

[Cogs]

Performance engine? :haha: Lol, if you call 165-ish hp performance. Anyway, I never really figured out my mileage before, just started doing it now. I estimated it at 16mpg city before though, which seems low. I got 18-19/21-22 in my 89 Vic. I'll probably just be driving the Nissan Stanza beater when I can. :cuss:

[85crownHPP]

replace your O2 sensor and check for vacuum leaks.

wait thats right you have headders... have you made sure you have o2 sensors anymore??
That would suck if whoever did the pipes left em out.

[Cogs]

Yep, I got em. And they're new I believe.

[88grandmarq]

usual stuff here..... first verify the 02 sensor harnesses have not burned against the exhaust. then check for vacuum leaks and pull the codes.
inspect the rubber hose coming off the back of the upper intake going to the MAP sensor, make sure its not cracked, broken or kinked.

and read the list G/L posted... espcially the one about the T/stat.

[Cogs]

I just looked at the o2 wires 2 weeks ago or so, they were fine then, Guess I should check again though. Ill check most of it out tomorrow. I know I have a new 160* T-stat which doesnt help gas mileage either...but I didnt put it in.

[88grandmarq]

toss that 160 stat in the trash. with the cold climate you live in you should run only a 195. install a fresh, 195 T/stat and check for codes.

let me know what codes you find, if any.

[grandpaslincoln]

I agree. Whoever installed that shoulda known better. Having a 160* is about as bad as not having one at all.

[Grand Marquis GT]

160 Thermostat? Whoever installed that was a fool.

Wow. Even in FL I run a 180...

[Cogs]

Yep, thats what I thought too, when I got the car. He said "I just installed a 160* t-stat, cause I didnt like how hot the car was getting..." I'm assuming it want THAT hot considering it NEVER has gone over 160 while I've owned it. At those temps the fuel probably isnt even being completely burned. Which means less gas mileage and probably less performance. :coocoo:

[Lincolnmania]

running a 160 deg stat is a sure fire way to build up sludge in the engine too

[SleepyMerc]

I can't entirely agree with everyone here. Form a performance standpoint the 160 is the way to go. Hell, boats run a 143. But I do agree it will cost mileage. Around here, in Columbus, I'd go back to the 195 in early October and then to the 160 around mid-April.

The thing is, the lower temp, COMBINED WITH THE PROPER TUNE, is worth maybe 5-8 rear wheel horsepower. Since it takes 10 horse at the flywheel to reduce quarter mile ET by one tenth of a second you will never feel the difference in the Seat Of The Pants dyno!

For the guys with bumped up compression and other mods who are looking for the last hundredth of a second it's very worthwhile. If you're bombing around the street with a stock engine I'd rather get the extra .5-2 MPG the 195 gets.

[Mercracer]

I can't entirely agree with everyone here. Form a performance standpoint the 160 is the way to go. Hell, boats run a 143. But I do agree it will cost mileage. Around here, in Columbus, I'd go back to the 195 in early October and then to the 160 around mid-April.

The thing is, the lower temp, COMBINED WITH THE PROPER TUNE, is worth maybe 5-8 rear wheel horsepower. Since it takes 10 horse at the flywheel to reduce quarter mile ET by one tenth of a second you will never feel the difference in the Seat Of The Pants dyno!

Combustion chamber heat makes power. Intake manifold latent heat costs power due to the heated air charge. With a FI car, in some cases, a 160 can cost you horsepower and mileage. You are better off bumping up fuel pressure than trying to trick a ACT sensor.
You are better off with ice cooling your intake between rounds and running a phenolic spacer on FI carbs than running a 160 stat.
If you want to run a cooler stat, go with a 180 instead of the 195.

[SleepyMerc]

I can't entirely agree with everyone here. Form a performance standpoint the 160 is the way to go. Hell, boats run a 143. But I do agree it will cost mileage. Around here, in Columbus, I'd go back to the 195 in early October and then to the 160 around mid-April.

The thing is, the lower temp, COMBINED WITH THE PROPER TUNE, is worth maybe 5-8 rear wheel horsepower. Since it takes 10 horse at the flywheel to reduce quarter mile ET by one tenth of a second you will never feel the difference in the Seat Of The Pants dyno!

Combustion chamber heat makes power. Intake manifold latent heat costs power due to the heated air charge. With a FI car, in some cases, a 160 can cost you horsepower and mileage. You are better off bumping up fuel pressure than trying to trick a ACT sensor.
You are better off with ice cooling your intake between rounds and running a phenolic spacer on FI carbs than running a 160 stat.
If you want to run a cooler stat, go with a 180 instead of the 195.

Just to clarify here, I did put a heavy emphasis ON THE PROPER TUNE. If you just stick a 160 in an otherwise stock engine with factory programming you will PROBABLY lose power and mileage. If you have an SCT programmable chip or you get the car dyno tuned and have your timing and injector pulse widths optimized and a/f ratio correct and all the other things that go along with a good tune you should- not WILL, but SHOULD -gain power. Just as the optimal fuel ratio for power is richer than the optimal fuel ratio mileage, the optimal engine temp for power is lower than mileage. In the early smog era there were some cars set up to routinely run 210 and higher to reduce emissions and get better mileage. But they certainly didn't make good power, they usually didn't even run well, they were leaned out so much.

Mercracer, you sound like you have a lot of 5.0 experience and your suggestions sound like they should work well. Most of my experience is with carb'ed Chevies and now some with the 4.6. That's why I'm here, cuz I want to learn all I can about my Merc's 5.0. But I may become a PITA because I do have a good understanding of the basic engine theory and how mods affect an engine. Besides real world experience in playing with my own cars and helping a buddy run a stock class stock car in '80 & '81, I've been reading Hot Rod and Car Craft and PHR and the like for over 35 years. So when I see something that doesn't quite make sense I tend to flog it to death until it does make sense.

In reality, back when carbs were king, the optimal engine temp for power was 180, because 160 wouldn't get the intake warm enough and fuel would drop out of suspension and 'condense' on the intake manifold. With the advent of sequential FI the fuel was atomized just before the valve so it didn't have to stay suspended throughout the entire intake path. Hypertech started selling 160 t'stats as part of a stage 2 or 3 tune and a lot of people have jumped on the band wagon since, without really understanding what they're doing. The bottom line is COMBINATION, COMBINATION, COMBINATION!! You can't count on any single alteration working if you don't optimize everything it affects.