Mustang GT horrible gas mileage after tune up

Mustang5L5 · Feb 2, 2010

My first guess would be the filler neck gasket. They usually tear when the tank is dropped.

Other than that, check the fuel line hoses and make sure they are clipped in well. Also check the evap canister line that runs to the front of the car

TDUB · Feb 9, 2010

Ok so I think I got the fuel situation taken care of but now it's acting a little sluggish on start up. From what I have read my guess is that it is the TPS sensor but I wanted your guys input before I start spending my money blindly. As it is right now it takes a second to start and then it seems to make a single loud pop once it starts. It takes longer than usual for the idle to settle down, it likes to sit at 1100 and then I have to rev it a couple times to get it to settle down to the 8-900 range. If I try to rev it within the first minute of starting it it'll wait a second after I hit the pedal to rev. My idle is surging but it only does it after I've been driving the car for a bit and it won't always do it either. The other thing is that when I'm driving if I up shift before I'm past 3000 rpm it will bog really bad until it gets higher up in the rpm range. Oh, and it's still not throwing a cel light.

HISSIN50 · Feb 9, 2010

Did you actually pull codes? They can be stored without ever illuminating the CEL.

91StangGT5.0 · Feb 9, 2010

How do you check codes on these mustangs? Because I too think I'm not getting good mileage. I refill my tank at 1/8th of a tank every two days only driving 20-25 miles a day..but I do have quite a bit of mods to the engine and bolt ons.

TDUB · Feb 10, 2010

I just realized we didn't pull the spout connector when we adjusted the timing .... Would this be causing my problems?

TDUB · Feb 10, 2010

91StangGT5.0 said:
How do you check codes on these mustangs? Because I too think I'm not getting good mileage. I refill my tank at 1/8th of a tank every two days only driving 20-25 miles a day..but I do have quite a bit of mods to the engine and bolt ons.

Lol mine's getting a total of about 6 miles every 1/8th right now... Any ways here's how you check your codes: http://*******.com/index.php/topic,2471.0.html

HISSIN50 · Feb 10, 2010

TDUB said:
I just realized we didn't pull the spout connector when we adjusted the timing .... Would this be causing my problems?

It would cause problems running with the timing that retarded. No one can say if it is causing all of your issues however.

Good luck.

TDUB · Feb 10, 2010

HISSIN50 said:
It would cause problems running with the timing that retarded. No one can say if it is causing all of your issues however.

Good luck.

Damn it wasn't it.... I was hoping I'd get lucky. Don't you mean it would cause problems with it advanced or am I missing something? I guess I can try retarding it but it doesn't make sense to me that I would need to retard it back when it's at exactly the same mark before I upgraded the pump. What are the chances it's throwing a code without a light?

HISSIN50 · Feb 10, 2010

TDUB said:
Damn it wasn't it.... I was hoping I'd get lucky. Don't you mean it would cause problems with it advanced or am I missing something? I guess I can try retarding it but it doesn't make sense to me that I would need to retard it back when it's at exactly the same mark before I upgraded the pump. What are the chances it's throwing a code without a light?

You normally set the timing to ~10* with the SPOUT out. If you left the SPOUT in and set the timing to 10*, you're timing is retarded. In other words, you'd have had to retard the timing a lot (SPOUT in) to get it to 10*. Normally SPOUT out 10* equates to about 20* SPOUT in.

There is a decent chance there are codes stored without the CEL. Best to spend 5 minutes and retrieve them.

TDUB · Feb 12, 2010

Ok I went ahead and did a little investigating under the hood and found this:

Do you guys know where I can find a diagram as to what these are and or where they go to?

1992MustangGT · Feb 13, 2010

Are those vacuum lines? If so, that could be some of your problem. You may have a bad vacuum leak. But did you do anything to the car before it started running crappy? Did you disconnect some vacuum lines and forget to reconnect them? If I remember correctly didn't you say the car suddenly started running crappy and getting bad mileage and you hadn't done anything to it?

I also have a 92 Mustang Gt and I recently had bad fuel mileage issues and after talking to a bunch of guys on here, I think I have my problem solved. Some of my issues you might be experiencing also, but it also sounds like you have different problems too. I was using a half a tank of gas and only going 100 miles. Long story short....I had put in NGK platinum plugs instead of good ole copper plugs. Someone on here advised me to use Motorcraft or Autolite copper plugs, since copper is a better conductor of electricity. So I took their advice and yanked out the platinum plugs and put in some Motorcraft copper plugs. Also I pulled the codes on the computer and I found a code that told me my EGR Vacuum Solenoid was bad. So I changed my EGR vacuum solenoid (the top solenoid (there are 3, not sure what the other two are for) mounted behind the passenger side shock tower). After changing the plugs and EGR Vacuum Solenoid, my problem was solved. However, I didn't have the bucking issues and bad driveablilty you're having, just the bad gas mileage. If I remember correctly, I think you mentioned you tuned it up. What kind of plugs did you use? Also, you're really causing yourself alot more stress by not pulling the codes off the computer. You might answer your own question and solve your problem if you pull the codes. Do that before you do anything else and then get back to us and tell us what codes the computer had. Just trying to help....

TDUB · Feb 21, 2010

I pulled the codes and got a code 14... Which from what I could find is the TFI module on the distributor. Would it have gone bad this soon or is it possible that the code is from before when I was having problems with the distributor? Also from what I found it said those lines are egr lines. Is that what is effecting killing my gas mileage? I'm getting about 40 miles per quarter tank at the moment.

TDUB · Feb 25, 2010

I replaced the broken emission lines that I could find but idle is still surging. I'm trying to follow the surging idle checklist but I could only trace where one of the lines went and can't find anything online about it. Also how do I test the egr? I'm going to try a set of copper plugs to see if it helps the bogging problem. I appreciate the help you guys have given me thus far. Another thought I had was the o2 sensors. Would it be worth it to replace them?

dave181760 · Feb 25, 2010

TDUB said:
Ok I went ahead and did a little investigating under the hood and found this:

Do you guys know where I can find a diagram as to what these are and or where they go to?

mine are broken also what are those

TDUB · Feb 25, 2010

dave181760 said:
mine are broken also what are those

They're emission lines, ours are 1/8". I found them at Pep Boy's for $4 a pack but they aren't color coded. Here's the best diagram I could find: http://www.stangnet.com/mustang-forums/789790-1990-mustang-5-0-vacuum-emission-line-routing.html

dave181760 · Feb 26, 2010

TDUB said:
They're emission lines, ours are 1/8". I found them at Pep Boy's for $4 a pack but they aren't color coded. Here's the best diagram I could find: http://www.stangnet.com/mustang-forums/789790-1990-mustang-5-0-vacuum-emission-line-routing.html

thanks bro im going to pick some up tomarrow after work

srtthis · Feb 26, 2010

jack up just the driver side of the car and inspect the filler neck seal... im willing to bet thats the issue. i had to replace the one in my 95 and my buddys 96 and a few of them when i worked at amazon back in the day.

TDUB · Feb 26, 2010

srtthis said:
jack up just the driver side of the car and inspect the filler neck seal... im willing to bet thats the issue. i had to replace the one in my 95 and my buddys 96 and a few of them when i worked at amazon back in the day.

I already replaced it. The car is no longer leaking gas but is still having running problems. I did find that it is A LOT easier to drop the tank if you remove the passenger's side wheel.

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