I have a 98 c5 automatic. Some times when I get on it it down shifts and goes great, some times now when I get on it, it down shifts reves up and wont go. It hangs around 4 thousand or so. I'm getting no codes. I have a walbro 255 pump, cold air, headers, exhaust. I talked to the dealership and they said maby dirty mass air. I cleaned it and it didn't help. So I replaced it with one from the auto parts store. It didn't help. Also my mass air was all plastic, the stock replacement was aluminum. I guess it don't matter? When I drive some times I can feel the car Sarge a little also. I put it on the dyno and when it ran right it put 330hp to the wheels, when it screwed up it put 210hp to the wheels. Any ideas???

 

HOW,,,are you reading the DTC's?????? Are you using the DIC thru the IPC built in Diagnostic Program.

When the issue happens,, do you see the Check Engine Light (CEL) flashing ??

BC

 

there is no check engine light. I have a scan tool from Snap on so I can get in to the computer, but every thing looks normal.

 

Use the internal diagnostics program:

READING YOUR Engine Diagnostic Codes



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 OPTIONS
4) While holding OPTIONS, press FUEL four times within a 10-second period.


Initially, on-board diagnostics go into an Automatic Mode which 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. ( 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 kinds of diagnostic codes, Current and History designated with a letter suffix, “C” or “H”. A current C code indicates a malfunction is present and active 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 English/Metric (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 and hold the RESET button 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.
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 GOOGLE the DTC and also find information on it.




Here is very good site that explains the DTCs:


http://www.corvette-web-central.com/C5DTCcode.html


http://www.obd-codes.com/trouble_cod...d-ii-codes.php



Make sure to include the H or C suffix!!

 

wow, didnt know I could even get in to that. I didnt have a check engine light, but there were some codes. I removed them and I will see what comes back. thanks

 

ok no codes but still running like crap, the rear o2s are turned off, the front o2s are reading as follows... right bank is moveing between 200 and 900 but the left bank is moveing between 200 and 450? I also see that I have a small exhaust leak at the slip fit band clamp. Would this be part of my problem? I have no check engine light and some times when I get on it it falls on its face and wont go.

 

When you have wide open throttle,,,the O2 sensors are not used. You get your fuel and spark info from set tables in the PCM programming.

Your having an issue with fuel or spark or air. Just need to figure out which one is failing.

 

how?

 

Starting to sound like an air leak past the MAF. Have you verified that the air coupler is all the way on to the intake and especially on the bottom of it? Please also look at the PCV valve connection tube from it to the intake as it was age, collapse and possible holes in it.

 

found the problam..... Timing tables were messed up. It was going -5 degrees at wide open. Now its fixed and running great. Thanks for all your help.

 

SPARK! Man,,,,,that was easy!

Good job! let the forum know how you figured out the issue.

BC

 

I would still be concerned with the flatlined 02 sensor---They both should be oscilating bouncing --from .100-.950--The engine will run fine in PE or WOT as the 02's are now not being used to regulate fuel--But in PT operation or "closed loop" the flatlining side will use the back-up table called "commanded AFR in Open Loop Normal"--this table is used for start up fueling or if you have a 02 sensor fault---it will use this table exclusively during stable incoming air--But when there is a state of rapildy chaging air it will use this table along with the VE table-
Quite often the 02 heaters will blow a fuse--Or an exhaust leak will not allow them to heat up prperly---If ran too long flatlined it will evenatlly be damaged beyond rep--air----AND finaly In Open loop fault the side with the flatlined 02 won't e able to use the LTFT's to correct the AFR to the most efficient 14.68-1-- Several other resaons why a flatlined 02 will affect performnce--but I think you may get an idea on hoe important that they BOTH work as designed

 

You are correct, however I replaced both after fixing my exhaust leak and they are now working

Thanks for the help