98 model, A4 w/72k miles. Vararam installed, 160 t-stat, ECM tuned for those. Plugs are 2k miles old. When going from 55 and downshifting into 3 rd, it feels like a delayed opening of a 4 barrel(for us old-schoolers). What would cause this lag?

 

It difficult to understand what your feeling. Sounds more like a trans issue to me. Did the tuner make any changes to the transmission calibration when he tuned the PCM???

Read and post your DTCs and see if any pop that could be related. If there are a LOT of DTCs, clear them and take it for a drive and see if any current DTCs come back.

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.

NOTE and a WARNING. You can read the DTCs while the engine is running. I pull mine up all the time while driving.
WARNING. Don’t become distracted while reading DTCs while your driving and cause an accident!!!!! Use common sense and drive safe.

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

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

http://www.gearchatter.com/viewtopic11755.php

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

Make sure to include the H or C suffix when you post your DTCs!!

 

Thanks Bill. I should have stated no codes. I double checked to make sure. The tranny downshifts fine, but then there is that lag just like the secondary barrels of a carb opening. ECS did the tuning and yes they did adjust trans pressure & shifting points.

 

Anyone else have an idea?

 

Yes sounds like they did not disable the "power enrichment delays" in your tune--There can be sometimes a 2 second delay in a stock tune
Also did they remove your TQ management ? your Abuse mgmt ? adjust your " ETC pedal TQ limit reduction " ??
All these things all reduce throttle response

 

I know the torque management was removed. As for the other things you mentioned, I don't know. It certainly does feel like a 2 second delay. I was hoping they would chime in here.

 

If your tuner has EFILIVE software i can tell you the exact table #'s that need to be changed--

 

Hey man.. Call me.


I've got some questions for you. Hopefully we can get to the bottom of this.

I do TONS of C5 automatics and that's a complaint that I never got before.

914-332-0049
Chuck CoW

 

If its a TUNING ISSUE,, Chucks your man! Your in great hands!

BC

 

Do you have any surging going on when moving off from a stop? If so, then with your surging on when downshifting, you might want to look at the pcv line to the intake. Still though, it sounds like tuning...unless your tune was working ok then this started happening later on.