I have a 97 A4 and it only gets about 18 in town but only gets 22.5 on long freeway runs. It has new plugs, new wires, gets only the highest octane gas. What else should I check or do. Thanks in advance for your input.

 

That's about normal. MPG will be a little bit lower than what was advertised back in the day before E10:
http://www.fueleconomy.gov/feg/Power...te&srchtyp=ymm

 

Hmm I dont know... thats pretty low to be normal. How many miles on the vehicle? Also how fast do you go on the freeway?
My 99 A4 with 3.42's gets 27-28mpg at 80mph. I used to get 30+ before the gears.

Did you check tire pressure?

 

59000 0n the clock and usually about 70/75 MPH. On occasion a bit faster. Tire preaaure is alwasy from 33 to 35 PSI

 

that's very low compared to my 01 6 speed. I never get less than 500 miles on a tank. the computor has not been reset for 2 years and the average is reading 30.2 MPG.
most of my driving is to and from work, 20 miles each way and mostly highway.

 

lower than my a4 on 3.15 gearing. if you have any data reader that can tell you what the injectors are doing, or a wideband, i'd have to believe you have a sensor issue somewhere. clean the MAF, maybe replace the upstream o2's. if you arent smoking (if you are and you failed to mention that...) then i'd suspect something in that area first.

 

How many miles have your current o2 sensors been run for? I find it very common on this platform for people to run them until a DTC is thrown, even though mileage can and will suffer well before the light turns on! They still have a 60-70K mile service period!

 

if you keep o2's to the point it throws a DTC, you're just taking money out of your wallet and lighting it on fire!

 

Few questions; 1) Is this a new symptom or something that recently changed? 2) Do you have any mods installed? 3)When is the last time you checked your air filter?

Start with the inexpensive stuff first. Being that many Corvettes aren't driven like our everyday cars, the car is subject to "visitors". You may have a critter nest in your filter.

 

I've had 2 C5's, a 98 and my current 04 and I get 20 mpg in town and about 30 out on the road.

 

Your pressure is too high. It should be at 30 PSI.

 

really? i run mine @ 30 cold, but it creeps to ~35, did you mean cold?

 

Yes, cold measurement. If the OP has 33-35 cold his pressure is too high.

 

Your mileage is a little low. Do you have any modifications that you have installed that you failed to mention??

Some things that I would check are:

- Air filter cleanliness and condition. (If its ever gotten wet it will have deformed pleats)
- Clean the MAF and Throttle Body with MAF cleaner.


Next thing that I would do is data log the PCM and see what your LONG TERM FUEL TRIMS are doing.. You can also see if your O2 sensors are switching properly. When in closed loop they should cycle between 100mv to 900 mv. You can use a code scanner with live sensor capability or find someone that has EFI Live, HP Tuners or one of the other hand held tuners that can data log.

That will be your real answer.

Have you examined your DTCs and see if there are any that could be relevant to this issue??
READING YOUR Engine Diagnostic Trouble Codes (DTC)

This procedure should be carried out any time you experience a problem with your C5. Most inexpensive store bought aftermarket code readers will ONLY read power train DTC’s. Reading the DTC’s with the C5 built in code reader will allow you to read ALL the modules in the vehicle.

The Diagnostic Display Mode is entered with the following procedure:
1) Turn on the ignition but don't start the engine.
2) Press the RESET button to turn off any warning messages. (i.e. door open, trunk open ect)
3) Press and hold the OPTIONS button
4) While holding OPTIONS, press FUEL button four times within a 10 -second period.

Initially, the on-board diagnostics go into an Automatic Mode which will cycle through each module and shows diagnostic codes in a pre-set sequence: PCM - TCS - RTD - BCM - IPC - RADIO - HVAC - LDCM - RDCM - SCM - RFA. All codes will be displayed for each module. ( i.e. PCM = 4 codes) If none are present in a given module, you will see No More Codes on the display.

There are two types of diagnostic codes, Current and History designated with a letter suffix, “C” or “H”. A current code indicates a malfunction is present in the module displaying data. A history code indicates a problem existed sometime in the last 40 or 50 ignition cycles. When not accompanied by a current code of the same number, it's potential evidence of a previous problem, now resolved, that was not removed by clearing the codes. More likely it's an indication of an intermittent malfunction.

Intermittent codes are the most challenging of the diagnostics. An intermittent code may have happened once, may have happened more than once but is inconsistent or may be happening on a regular basis but not at the time the codes are displayed. History codes can also be caused by a current malfunction in a system that is not operating at the time codes are displayed. An example is the rear window defogger which doesn't operate until the Body Control Module detects engine rpm. For history codes set by a module that does not operate with the key on and engine off, a special diagnostic tool called a Scan Tester is necessary to properly diagnose the malfunction.

Once the system has displayed all modules, it goes into the manual mode which allows selection of each module using combinations of Driver Information Center buttons. Manual mode can also be entered during the automatic sequence by pressing any button except E/M. Once the display shows Manual Diagnostics, select a module by pressing the OPTIONS button to go forward or the TRIP button to go back. Once a module is selected, a code is displayed, and if more than one are present; press GAGES to go forward or FUEL to go back.
To exit the diagnostic mode at any time, press E/M. If you want to erase codes in a given module, press RESET To reset the codes once in manual mode, press and hold RESET until it displays NO CODES Press OPTIONS to go to the next module. Repeat the steps until you have reset the codes in all the computer modules.
NOTE!! Only reset the codes IF you want to - it is NOT necessary to do this. Clearing a code does not repair a problem. You are simply erasing the evidence of it in the module's memory. If you clear the code/s, and extinguish the Check Engine Light, your emissions status ready will NOT allow you to pass an emissions test until you have completed the required driving cycles. There are a few body module DTC’s that if set will prevent the module from operating properly. Once the DTC is cleared, the module will return to full function. This is not true for power train DTCs.

If you have never read and cleared your codes, there will probably be a lot of old history DTCs. It is recommended that you clear your codes and see if any come back during a driving cycle. Those are the ones that you need to concentrate on diagnosing.

Once you have the codes, the next question is: What to do with the information?
First, consult the factory service manual. Any serious C5 Do-It-Yourself owner should invest in the Corvette Service Manual of the appropriate model year. The Service Manual is really a requirement if you want to understand and work on your C5.

You can read the DTCs while the engine is running. If you turn the engine OFF and then back ON, that can clear some DTCs that may be important with identifying a problem. One example is a Misfire DTC (p-0300) If the engine is shut down, the DTC will clear.

These are some very good C5 Diagnostic Trouble Code (DTC) explanation web sites!!! They also explain how to read the DTCs

Here is a very good PDF on DTCs and what they relate to:

http://forums.corvetteforum.com/c5-t...odes-dtcs.html

Here are some very good sites that explain what DTC mean:

http://www.gearchatter.com/index10.php

http://www.obd-codes.com/


*** Make sure to include the H or C suffix when you post your DTCs!! They are important!!!

Updated 13 September 2012 :-D






Bill

 

Only a few codes and all are history. Tire pressure is 29-32 cold. I do have a cold air intake on it. Titanium exhaust from 03 Z06. I do not know when the o2 sensors were changed last.

 

History DTCs are just as important as Current ones. They also tell you a story.

Recommend that you clear ALL the DTCs and see what comes back. History or current.

BC

 

i'd say more often than anything else when a fuel injected car starts to mysteriously lose mpg, its the o2 sensors. if the DTC's dont tell a horror story, just replace them being as you arent sure.

 

OK I can do that but what's the best process to clear all the codes. I have heard a few different ways and would lik\e to do it righr. Anothrt question, is it possible to clean the throttle body without removing it. Thanks again for your and everyone else who offered help and suggestions.

 

the reset is here on the forums.. i'd have to look, i do it with muscle memory now! i think its hold 1st from last button, push options 4 times, press reset, then hold reset?

as far as cleaning the throttle body, i guess you could if you jacked up the rear or put it on a hill.. but uh.. why? unless you had something explode, its not worth the effort.

 

Use the procedure and get into MANUAL MODE. Once your there,, press RESET and HOLD it for each module (may take a couple seconds) and the codes for that module will clear.

YES,,,,, you can clean the TB on the engise.. Remove the air bridge, PUSH open the throttle blade with your fingers. Spray the opening and throttle blade with carb cleaner and use a lint free towel to wipe everything down. Repaet till its clean.

NOTE,,, The blade will NOT snap shut!! It will FLOP into a near shut position.. Its NORMAL!!

BC