Mustang GT horrible gas mileage after tune up

TDUB

New Member
Feb 15, 2009
25
0
1
My 1992 Mustang GT started bucking under throttle a few months ago, at first only doing it once in a
while but continued to get worse. I decided to replace the fuel filter, plugs, wires, cap, and rotor but the problem continued. I then took it into the shop for diagnosis and they told me it was the distributor so I replaced that as well but the problem continued and then the car completely died. I found that I wasn't getting spark so I changed out the ignition module and coil which fixed the spark problem but the car still wasn't getting fuel. At that point I replaced the fuel pump relay but same thing. I then replaced the fuel pump with 255lph unit because I figured if I have to do the job I might as well upgrade... Long story short it now runs faster than it ever has but is getting horrible gas mileage (like 4mpg tops). My timing is set at 13 which is exactly what it was at before. It idles fine (around 1500) but seems to have a slight back fire at the low end when you initially accelerate. I checked for for fuel leaks and can't see any, although when I was filling up, it didn't warn me it was getting fuel and gas started to spill out so I got under her and re tightened the rubber piece that bolts up to the car where the pipe initially comes down from the hole to the tank... I can't see that any gas is leaking from any of the lines and I followed the write up on american muscle to the t. Am I missing something here or is it the pump?
 
Check or replace your tps sensor and also check your 02 sensors also you may want to check or replace you fuel pressure regulator with an adjustable one. Wow! 4mpg also turn your idle down too because the car will burn more gas when it is set at 1500rpm.Try turning down it to 800-850 and clean you throttle body/idle control valve and also check for vacuum leaks because the idle should not be that high and if you have a vacuum leak the computer will add more fuel causeing it to run rich and idle high.Good luck.
 
idles fine around 1500 huh? 4mpg? Its not the pump and if your car is getting that bad of gas mileage then it's not running well. Any codes poppin up?
 
Sorry.... I was guessing off the top of my head on the idle, went and looked at it right after I made the post. It idles right at 800 so nothing out of the ordinary there. Also thought I should mention that I cleaned the throttle body and maf sensor as well as the air filter when I did the plugs. The car also has straight pipes with bbk shorties as well as under drive pulleys, bbk fender wall intake and 75mm bbk throttle body and plenum. I don't know how the other mods would be effecting it though since they were on the car before and up until it started bucking ran great and got decent gas mileage.
 
it didn't warn me it was getting fuel and gas started to spill out so I got under her and re tightened the rubber piece that bolts up to the car where the pipe initially comes down from the hole to the tank... I can't see that any gas is leaking from any of the lines and I followed the write up on american muscle to the t. Am I missing something here or is it the pump?

Sounds like the gasket that connects the fuel filler neck to the tank is leaking. Usually you have to replace these when changing fuel pumps. It usually only leaks when the fuel sloshes around...ie driving.
 
idles fine around 1500 huh? 4mpg? Its not the pump and if your car is getting that bad of gas mileage then it's not running well. Any codes poppin up?

Agreed. Asking again about the codes.

And try to calculate exact mileage (no guessing).
 
Agreed. Asking again about the codes.

And try to calculate exact mileage (no guessing).

It's not throwing any codes whatsoever... I can't see that it's leaking from the gasket but I think I'm going to replace it just to be safe. After looking at it further I'm thinking it has to be a fuel leak from one of the lines on top of the tank. Is it possible to buy the connectors that connect the hose to the tank (the plastic piece)? I don't know the exact mpg but it seriously went from having a full tank to 3/4s in about 10 miles.
 
Or just an inaccurate sender (which could have been disturbed during the pump install).

If it's the sending unit would I be able to smell fuel while running? I'm going to drop the tank again... I'm going to buy brand new fuel line clips as well as the gasket just to be safe but I want to cover all of my bases. I found the clips online but is it possible to get them at the parts store?
 
I really doubt its the sending unit gasket, if that was bad and you were spilling fuel on the road you would not be able to notice the strong smell of gas spilling all over the road. If anything I would start by setting your idle properly by hooking a meter up to your TPS and adjusting the idle screw, then ensure that your IAC valve is working properly. Then since you put in a more high volume pump I would put in an adjustable fuel pressure regulator to ensure that you are running the same fuel pressure that you should for the stock 19lbs injectors and see what happens from there.
 
If it's the sending unit would I be able to smell fuel while running? I'm going to drop the tank again... I'm going to buy brand new fuel line clips as well as the gasket just to be safe but I want to cover all of my bases. I found the clips online but is it possible to get them at the parts store?

No it would not. I didn't see previously where you had noted a strong odor of fuel.
 
No it would not. I didn't see previously where you had noted a strong odor of fuel.

Well I took it out last night for a drive and could smell fuel while running but not other wise. I can't see that it's leaking from the gasket and it's not dripping any where so I'm guessing it must be where the line connects to the sending unit on top of the tank.
 
Fuel evaporates rather quickly, especially when the right temperature. I would suggest replacing the seals since your exhaust might be providing that ample heat to evap.. :eek:
 
Fuel evaporates rather quickly, especially when the right temperature. I would suggest replacing the seals since your exhaust might be providing that ample heat to evap.. :eek:

Ok so what all should I replace? So far I'm going to get new fuel line clips and a gasket. As much fun as dropping the gas tank and putting it back in is :rolleyes: I really don't want to have to drop the tank for a third time because I missed something.