Bad Gas Mileage [Merged 9-6]

[99GST]

I am still getting some pretty bad gas mileage.

I am only getting about 19.7 mpg with mostly city driving shifting at 4k.
I have already replaced my front O2 sensor and am looking to replace the fuel filter.

I have a 99gst with about 55k miles on it. Should my fuel filter already be bad?

What else should I check or change? I have NKG plugs and Accel wires are on the way.

Thanks,
Nathan

 

[Rick4g63]

Ivan is right. You can build your own or purchase a kit for less than $30:
http://www.machv.com/macvleaktes.html

Although it is a one time use, it can be use on almost all production turbo charge vehicle sold in america.

 

[nurseryboy]

Yeah, I made my own leak tester. It looks pretty much just like the ones on that site. I will use it again tomorrow and see if I can find any.

A question about testing for leaks.. What happens when you reach a certain pressure while testing? Will the BOV let it out, or where does the pressure go?

Thanks,

Matt

 

[nurseryboy]

Ok, I ran the leak test again today.

1. First thing that I noticed, was that after a few minutes of continous pumping (from an air compressor), that my gauge showed no pressure whatsoever. It didn't move one PSI. (The boost gauge is 'teed at the fuel pressure solenoid.)

2. Second of all, I couldn't hear any leaks. I think may have been hearing something around where the intake filter normally sits, but I wasn't too sure. If it was, it was very faint.

3. I did notice something else. When I started up the car again, I went out front and listened around. I did hear a pretty loud hissing noise from where the smic is. I had to get close to the front, but there was definitely a noise. Is this normal?

4. I also noticed that when I start up the car the pressure goes to around -20. Then, after about 30 seconds, it inches back down to around -16. When I put it in gear, it goes to around -10. Don't know if that helps.

Thanks guys.

Matt

 

[nurseryboy]

Still didn't figure out anything today.. anyone have any suggestions?

Thanks,

Matt

 

[Rick4g63]

Nursery, first, click below, scroll down and make sure you boost gauge is connected correctly.

- Remove the right side inner fender wall (plastic), to gain access to the smic. carefully pressurize the system again, and closely inspect the smic and listen for any leak. There will be noise from the intercooler as air rushes past the cooling fins, but make sure the fins aren't broken or depressed and all hose connections are tight.

- It is imperative that the line coming from the stock turbo, the "accordion" rubber intake, the boost controller, intake manifold to BOV, and to the wastegate are connected and secured properly

 

[nurseryboy]

Ok. Thank you. I'll do that today and see what happens.

Matt

 

[14.5 drift]

I also wondered about the blow off valve, is it supposed to seal once the intake is pressurized? Mine leaked the entire time, even under 5 psi. What do i do to seal the bov for boost leek testing?

 

[Rick4g63]

What bov do you have? IF you the 2g stock bov, remove it and throw it in the garbage.

I have a correctly crushed 1G bo with the Gus Mahon mod like this one but it is by Dejon tool
http://www.dejontool.com/images/DSM-Tools & Misc/BOVF.JPG

Mine will hold 25 psi before she begins to leak.

 

[nurseryboy]

So if you do the leak test, the pressure should build up until the BOV can't hold it?

 

[Rick4g63]

Click belowand read:
http://www.dsm.org/menu.epl?item=494

 

[nurseryboy]

Thanks. Last question.. for now

If you have your boost gauge hooked up to the fuel pressure solenoid, will it show boost if the car is off (if there is pressure)? I didn't know if the solenoid controls the pressure through there, and the car needs to be on for it to do that, or if it'll work with no power.

Thanks,

Matt

 

[14.5 drift]

Just to be clear, I have to pressurize at the turbo and the blow off valve? And then stick a screw driver in the throttle body to keep the plate open....correct?

 

[Rick4g63]

Nursery, you can never ask too much questions.Ask as much questions as you can. I invite everyone who has had this problem to partake in helping it be resolved.

- Remove the bov hose.
- Remove The turbo outlet pipe. Insert pressure unit to outlet pipe.
- With someone slowly and carefully pressurizing the system, check for leaks from the outlet pipe to the throttle body.
- Pressurize to 14 psi
- The bov will begin to open at 10 psui (stock bov)
- still check for leaks.
It is absolutely IMPORTANT you check ALL vacuum hoses and make sure they are securle connected

 

[nurseryboy]

Thank you very much for your help Rick. I will be trying out what you suggested momentarily. Maybe that will reveal something. If not, I'll be taking it to a friend of the family who has a garage nearby. Maybe along with the right tools, we can figure something out.

Thanks again,

Matt

 

[Rick4g63]

when did you replace the fuel filter?

 

[nurseryboy]

It hasn't been replaced since I bought it in November.

Matt

 

[nurseryboy]

Ok, did the leak-test once again..

Would you say, that if my boost gauge is reading no boost, that I have a leak somewhere? . I hooked up a compressor to it and ran it for the longest time, and the gauge didn't move at all.

Just to be sure.. a boost gauge hooked up at the Fuel Pressure Solenoid should show boost even when the car is off, correct?

Thanks,

Matt

 

[doobie1121]

Well i have a 97 gst. I have had problems for awhile with my MPG bouncing all around from tank to tank so i just installed a new clutch and fuel filter and now my MPG just sucks before i could get about 375-400 miles on a tank before the gas light would even come on but now im getten like 325 mile or so. Im lost i thought the new fuel filter would fix it right up but now its even worse. ANY SUGESTIONS?
Thanks in Advance

 

[92awddsm]

Try replacing the o2 sensor. That is usually the main cause of bad or varying fuel milage on the dsm.

Im assuming that you drive the same from tank to tank. If you boost all the time on 1 tank and then drive sensibly on another tank, mileage will be different also.

 

[doobie1121]

yep i drive the same every day back and forth to work nothing but freeway 85 mph on cruise control

 

[92awddsm]

Check this place out. Cheaper than the dealer but good products. Make sure you get the denso(oem) or you will be replacing it soon again.\


EDIT:My bad, I forgot to give you a link. her ya go http://www.oxygensensors.com/ Now I feel like more of an idiot than I really am

 

[Defiant]

now my MPG just sucks before i could get about 375-400 miles on a tank before the gas light would even come on but now im getten like 325 mile or so.

400 miles before a warning light? That's damned high. I don't know if I get 300 out of a tank, I always refill before that. However, the warning light means exactly DICK.

Fill your tank, set your trip meter to 0, and do the math next time you refill.

My warning light likes to come on at a hundred miles into a tank, just for laughs. Then it goes off again and waits for 250.

 

[Mike1992]

400 good lord I dont get that from my nt and i know my motor is in tip top shape. Realistically i get about 300-350. And I dont have boost

 

[slider85]

just switched from 87 octane getting 400km's to 89 octane getting about 500km's and running smoother with more power.
Defenitly change the o2 sensor if it has never been done.

 

[fixmy97gst]

400 good lord I dont get that from my nt and i know my motor is in tip top shape. Realistically i get about 300-350. And I dont have boost

I have a 97 GST and I avg 360 to a tank and that's combined 76mi R/T to from work..and some city driving as well