Extremely Low Fuel Mileage Issue

Discussion in 'SN95 4.6L Mustang Tech' started by stangr5oh, Jul 21, 2014.

  1. Hello all. Been a member for a long time but recently was forced to medically retire from the military and now have time to search and read on here about mustangs. A short bio of me is I have had 10 or more mustangs in my time ranging from 67 fastbacks to now my 99 gt. I was always deployed over my long career in the military. Hence the reason for lack of participation to this site. But no longer will I be a silent member.

    Now for my issues with this 99 gt. I recently moved to Puerto Rico from my home state of Florida as to help take care of my mother-n-law and her health issues. When I moved here less than 30 days ago I had the desire for another stang. Was in the market for a fox body though but searched the whole island and all I kept coming up with was crap. So I quickly turned my attention to SN 95's. After roaming the internet and setting up meetings to find a gt that would suit my needs, I came across a 99 chrome yellow gt with the 5 speed ( can't stand auto's), and the one thing that set it apart from the others was the low miles (58k).The body is in excellent shape, sounded nice and the only hiccup to the car was it sat on 20's all around and the car was slammed to the ground causing havoc on camber and rough riding (Fred Flintsone style). But there was potential otherwise. I purchased it on the spot, the ride back was approximately 75 miles, the car had less than a 1/4 tank of fuel. First thing I did was fill up before I headed back home. What I quickly realized was that 75 mile hike back home sucked up 3/4 of a tank of fuel. I thought maybe the sending unit was going out or something was horrifically wrong with the cluster. Knowing these cars with mild upgrades should get roughly 18-22 miles on highway.

    After getting it home, I quickly went online and purchased from American Muscle a set of AM SR springs for all 4 corners, a set of staggered rims with tires, and a BBK CAI setup. After receiving these items and installing them I began to drive the beast around town, as prior to the new parts, my spine/back could not handle the rough ride the car produced due to the previous owner cut 2 coils off each spring and 20's having no sidewall to take the impact of street imperfections.

    But after 300 miles or more on the odometer by me, I have filled up 3 times now, and have witnessed the check engine light throwing the same 3 codes out, I have reset the OBDII each time with my scanner adn the same ones keep coming back after about 50 or so miles.. PO420/PO430 and PO455, the first 2 are most likely a result of the previous owner installing a O/R H-pipe with all 4 O2's going crazy without the cats present. The 3rd being a PO455 could be a hand full of things from simpliest being a lose or weathered fuel cap to vent lines or hoses from the EVAP system and so on and on.

    But being that the O/R H-pipe throwing havoc to the O2's, would that really cause the drastically bad fuel mileage, or is there something else I should be looking at? I do not smell fuel odor in cabin, or do I hear or notice any vacuum leaks or the like. The car idles fine, and throughout the RPM range there is no noticeable short comings in throttle response or power lose. Now that being said, this is my first SN95 4.6, but not my first 4.6, I owned up until I moved a 97 Mark VIII that I tricked out.

    So, any help or direction would be appreciated with why so poor gas mileage. That being said also, I am not a NOOB as most would say on forums. I have for the past 3 days been reading on this website all different possibilities and reasons, but for alot that I have read, the threads tended to end with opinions being swapped and criticism going knowhere. I am not the smartest mechanic on the block, but I have been around the block a few times in my 45 years. And like to think I am a mechanic with common sense. So any direction would be appreciated in advance.

    I am contemplating on installing a catted H-Pipe to replace this O/R H-pipe. Or utilize a MIL eliminator system. But would the MIL eliminating set-up work with having no cats at all? Or would I still get the CEL?

    One more thing, just so you all know what I have under the hood. The engine is in original condition with the exception of the new BBK CAI, stock manifolds, O/R H-Pipe, Flowmasters, all piping is 2.5". No other upgrades to mention to the drivetrain. Thanks again.
  2. Has the MAF been cleaned with a cleaner designed for the job?

    Is there any evidence of gas in the oil?

    Consider having the alternator tested for excessive AC ripple (bad diode). Most auto parts stores will do the test for free.

    Bottom line. Today's cars simply will not run right without a strong battery and charging system. Cut corners here at your own risk. (don't think that excessive AC ripple couldn't cause your issues. Excessive AC ripple messes up the cam sensor signal and other sensors).

    Check the fuel rail pressure sensor intake vacuum reference line to be sure it's connected and leak free. Inspect the line for raw gas. If found, replace the FRPS.

    It would be real handy to know what the Long Term Fuel Trim (LTFT) values are at idle and under load.
  3. 1- no I have not cleaned the MAF. I will look into that though.
    2-no fuel in oil.
    3-alternator is brand new.
    4-I roamed through the engine compartment and was looking for any signs of vac leaks or fuel leaks and found nothing out of the ordinary. Remember this is a 59k engine so just about everything in the engine bay is in great standing order. I just ordered the MIL Eliminators from American Muscle yesterday, going to install those and see if there is any change in mileage. I have drove it the past 3 days now after resetting the CEL and after 120 miles no lights yet, but they will come at least for the PO420 and PO430 roght and left back.. Will keep poking and prodding till I see something. But got to say the fuel mileage is terrible to say the least for a 59k engine with minimal upgrades.
  4. The rear O2 sensors do NOT affect gas mileage. There only function is to monitor cat performance.

    One of the worst auto repair night mares I ever personally experienced was once replacing a bad alternator. At the time I didn't know that the "new" alternator was also bad. I spent an entire weekend ringing out the body wiring harness looking for a problem. When everything else was checked out, I removed the "new" alternator and took it back to the auto parts store. Surprise!! DOA out of the box. Hours of sweat later, lesson learned. Just because something is "new" doesn't always mean that it's good.

    I'm not saying this is your problem. But what I am saying is don't miss the change to perform some "basic" tests than can easily rule out/in some possible causes.

    Remember to look at things like how the MAF is indexed in the CAI. Some CAI's will change the factory location/index of the MAF which can cause the MAF to read incorrectly.

    Your problem could also be anything from clogged/leaking fuel injectors to bad O2 sensors.
    #4 wmburns, Jul 24, 2014
    Last edited: Jul 24, 2014
  5. Question for all you tech guys. I came across a Ford Factory Service Bulletin OBDII Code P0455 on the web. Do any of you know if a TSB ever expires or does it remain open for anyone to go to there local Ford Dealership to have checked? Here is the TSB:

      • Date Reported MAY 12 2003
      • NHTSA Reference #10002780
      • TSB Reference #0398
      • Description:

    Attached Files:

    #5 stangr5oh, Aug 6, 2014
    Last edited: Aug 6, 2014
  6. I had a similar code, turned out my gas cap wasnt sealing due to extra dirt and grime in its threads!

    Also, how are your brakes? Are they hanging up when driving? This happened to me and my mpgs went down to 12ish city...
  7. Here is what I have done so far. Without a smoker or any other diagnostic type of machinery I have physically checked all the engine compartment hoses and lines for any noticeable cracking, breaking, or leaking. I have went to the tail end of the car to the EVAP canister area and checked all the lines back there and no luck at all. I have changed the fuel cap twice just to eliminate the possibility of a bad new cap. I still get the PO455 code within 25-30 miles after CEL reset with my handheld scanner. I am wondering though since these intakes are made of plastic if I may have a cracked manafold. I have been on the national safety website and have read one after another complaint of a cracked manafold and Ford doing nothing about it and also the spark plug issue backing out and causing people to use heli-coils to rethread. I also used soapy water in a spray bottle and sprayed all around the engine bay while car is running to visually see any bubbling on the lines or the like, nothing so far.

    I have noticed some white ash stains on the back portion of the manafold itself. I am wondering if I may have a crack in the manafold. Would a small hairline crack in the manafold cause the PO455? Ont the TSB it refers to the replacement of the purge valve if all other diagnostics come up fine. I checked on the availability of that part, it is available, but for such a small part it costs $129.99 at AutoZone. I have always been a beleiver to check and recheck before just throwing parts at a car. So I am not there yet to just buy the purge valve.

    Any other suggestions out there that are reasonable to go after? The alternator is reading fine and my brakes are not dragging. I am still getting terrible fuel mileage. I noticed this last fill up that I am at little over 50 miles so far and already at a half tank.
  8. Yes a cracked intake manifold can cause a TON of problems. However this problem is most common in the coolant cross over or around the T-stat housing.

    OBTW, Ford did offer an extended 7 year warranty repair on the plastic manifolds. But for the 1999 model year, the 7 years are up.

    For a 1999 model year car there's a real possibility that the gas tank filler neck grommet has rotted out or the filler neck has rusted through. Very common problem.

    Consider disconnecting and PLUGGING the EVAP vapor line at the intake. Does this improve gas mileage?

    P0455 - EVAP Control System Leak Detected (No Purge Flow or Large Leak)

    • After-market EVAP hardware (such as fuel filler cap) non-conforming to required specifications
    • Disconnected or cracked fuel EVAP canister tube, EVAP canister purge outlet tube or EVAP return tube
    • EVAP canister purge valve stuck closed
    • Damaged EVAP canister
    • Damaged or missing fuel filler cap
    • Insufficient fuel filler cap installation
    • Loose fuel vapor hose/tube connections to EVAP system components
    • Blockages or restrictions in fuel vapor hoses/tubes (items also listed under disconnections or cracks)
    • Fuel vapor control valve tube assembly or fuel vapor vent valve assembly blocked
    • Canister vent (CV) solenoid stuck open
    • Mechanically inoperative fuel tank pressure (FTP) sensor
  9. wmburns, a question for you since you have given me good info. Would a PO455 allow for such bad fuel efficientcy? I mean for a car with such low mileage, no alterations to engine management or drivetrain other than my CAI, I am very concerned with this terrible fuel mileage. I mean 50 miles and a half a tank is very much extreme in my opinion. I am going out to recheck the filler neck and around fuel tank now. I do not at any time smell or notice any fuel odor anywhere while running, be it the front under hood or the rear under or around tailpipes.

    Also, without the help of a smoker to test for leaks, and using the old technique of soapy water in a spray bottle, is there any other way to detect a no flow situation on the purge valve? Or the other parts of this equation? Also how would I test for a non-working fuel pressure sensor at the tank? Thanks for any of your knowledge.
    #9 stangr5oh, Aug 7, 2014
    Last edited: Aug 7, 2014
  10. OK, I went back out and did further inspection of the filler neck, the edge of the fuel neck where the cap goes on and nothing there. I inspected the lines behind the fenderwell at the neckand all in order at it should be. I reinspected the EVAP canister lines and nothing apparently wrong there either. I put the car on ramps and reinspected the fuel line return and supply and no wetness or knicks or kinks. I went under the hood again and rewet all the lines again and also added soapy water on the manafold and around all the valleys and anywhere else that may hold alittle water and no bubbleing or suction. No drop in RPMS when spraying around the intake area or lines. I sniffed the area around the fuel cap and I do smell a faint hint of fuel, but I would assume that would be the case or normal. I do smell rich burn in the exhaust tips and that also in my mind would be normal since there is a O/R H-pipe on the car without the cats.

    I am running out of ideas here without the proper ability to test the EVAP system with a smoker kit. Any ideas you all?
  11. Here is a pic of the trip odometer and the fuel gauge. This is the actual miles since fill-up and is past a half tank already. Bare in mind this mileage reflects casual driving and not wreckless driving. Pretty crappy I have to say. That is why I would like to know if the PO455 is causing this terrible fuel mileage or not. Or should I be looking elsewhere for the problem?
  12. Unless there is raw gas leaking from the filler neck, normally a P0455 will not have and effect on gas mileage. At the very least, there would be other DTC codes as well.

    Sooooooo.....have you disconnected and PLUGGED the EVAP vacuum line? From your questions I don't think you will believe one way or the other if the P0455 is causing the low fuel mileage until the results of this test is known.

    Have you inspected the FRPS intake vacuum reference line for leaks and raw gas?

    Do you have access to an ODB2 scanner that is capable of reading operational data? It would really help to know the LTFT, MAF flow, fuel pressure, ECT, and IAT values. Could make real short work of this problem.

    Consider performing a compression test.
  13. OK, I got my MIL Eliminators in and installed them yesterday. After install, I decided to take that stang out for a long joyride on the open highway. Now before I get into this I want to mention an idea I have about this car and the prior owner. As in my other progress thread I mentioned that the prior owner had this car slammed to the ground maxing out the neg camber and putting 20's on her and the spine jolting ride it produced. So based on low mileage and the stance it carried I assume this was a glamour queen instead of a driver. So after I bought new proformance springs, tires and wheels, and alignment with caster/camber plates, the stang drove as it should.

    So, yesterday I take it out on the open highway and I open her up to around 120mph on a few occasssions when the open road permitted. I drove her for over 100 miles before taking her back to her stable. Now prior to installing the MIL Eliminators, I was constantly getting 3 CEL codes, PO420,PO430 and PO455. And I would usually get the CEL after about 35-50 miles after reset. So as I mentioned previously, I went over 100 miles and no CEL. I went out to the store today and on the way back, you guessed it, CEL popped up. I take her back to the stable and perform a scan on the CEL. This time I get none of the previous codes, but now it is reading PO156=O2 Sensor Circuit Malfunction (Bank 2 Sensor 2). I was shocked that the PO455 did not come back up since last reset.

    Based on previous experience when I have received a similar code, I would go and pull the O2 sensor out and clean her up and the code would not come back. So I guess I will try this later today or in the morning and see what happens. Or could this be a bad MIL Eliminator that I received? I am still shocked that the PO455 did not come back.

    Any thoughts?
  14. Well the CEL popped back up again with 2 codes now, the first being the PO156, and secondly the PO455 again. I called American Muscle and they are going to send me new MIL Eliminators again, one of the two they sent me is bad. Still working on the PO455 though. This PO455 is giving me hell I got to admit.
  15. Well I have been reinspecting EVAP parts and still not have across anything out of the ordinary. I m getting puzzled, I took the purve valve off and opened it adn cleaned it out with condensed air, to include the internal filter. And I took the rear EVAP canister out and cleaned it up, cleaned out the external electrical valve also, to include cleaning the screen inside the valve, and the butterfly flaps inside. All seems to be in good standing order.
    20140816_135042.jpg 20140816_135116.jpg
  16. Confirm that the gas tank filler neck grommet is in good condition and there's no rust through on the filler neck. Gas vapors are very hard on rubber parts.

    I'm assuming that the gas cap is new or known good.

    Can you blow through the charcoal canister? If not, vapors can't either. If just one time in this car's life the gas tank has been over filled (or filled while the motor is running), this could draw raw gas inside the charcoal canister. Once done this turns the charcoal to jelly thus blocking the canister. The only fix is a new canister. They are not all that expensive.

    Have you double checked the EVAP vacuum line from the throttle body to the VMV in the right hand fender well? Gas vapors rot these lines out from the inside. Is your hand covered with black residue when the line is handled?

    OBTW, the P0156 is almost always certainly a wiring problem. Either the left rear O2 pigtail has been damaged (usual reason is contact with hot exhaust). Followed by water in the connector. Since the MIL's are new, it isn't likely it's the MIL's themselves.
  17. wmburms, the PO156 has been resolved. When I took the new MIL Eliminator back off on bank 2, one of the needle pins did not go in the ffemale hole and was pushed to the side. Once I identified the fault, I straighten the pin back and now it is fine. The charcoal canister did allow air to blow thru. I have not checked the EVAP vacuum line in fenderwell, did not honestly know one was there. I will check that tomorrow. The gas cap is new and I bought an extra one to make sure. I on two occassions indpected the filler neck and the gaskets, all appear to be in good standings. I am going to purchase a vacuum pump this week to check some of the vacuum lines like to the EGR and what not. If I am not mistaken the vacuum runs off the number 8 cylinder, am I coorect on that assumption?
  18. I was just listening to the Car Talk guys and they suggested to a caller that their mileage might be bad because of a bad thermostat never letting the engine get to running temp. Might be worth checking.
  19. Well just a follow up to my cleaning and taking apart the purge valve solenoid and the charcoal canister and check valeve adn reinstalling it. I have since gone almost 200 miles since that work, and still no CEL lights. Usually it would only take 30-50 miles for the codes to pop up. But 200 miles later and no CEL's. I am finally happy that the PO455 is hopefully gone. I guess cleaning and blowing out the filter on the purge valve solenoid worked.

    Took it out today and got the oil/filter changed with synthetics, and bought a set of NGK Iridium Spark plugs, the name of them are called NGK IridiumIX , I will put those in tomorrow and see if it helps with the fuel mileage. I tell you though, at 8 bucks a pop on those plugs, they better help.
  20. Good work fixing the EVAP DTC code. Sometimes EVAP problems can be very difficult to locate. I wish more PPL would tackle the problem like you did. IE, take apart, clean, inspect and replace as needed.

    Is the fuel mileage still low?

    IMO your $$ would be better spent on an ODB2 scanner that is able to read operational PID's.