Car has big cam and vacuum pump to run brakes. Pump only comes on when first start the car and as needed while driving. It frequently throws a "Service traction control" "Reduced engine power" code when I tap the brakes or coming to a stop. Like the vacuum pump is not coming on soon enough. Does anyone know what the minimum vacuum value is? Can I get a vacuum switch that will close sooner? Its getting to be a real pain. Any thoughts?

 

How about a vacuum tank like they used to use on the C3's??

 

I think a tank would give me plenty of reserve but once the vacuum dropped to a level to close the switch the pump would not recover fast enough and make issue worse. I think the vacuum switch that I have must be too close to the value that causes it to throw the limp mode code. I don't know what that value is and if I can get a switch that comes on sooner.

 

You must be running one big cam to experience those issues. The C5 should have a small vacuum tank already on the car in the passenger side fender by the ECU. You might want to check for a leak. Below is a link for finding the tank.

https://www.corvetteforum.com/forums...voir-tank.html

I am not sure the vacuum is causing you problem. You could have some issues with your tune, ABS system, master cylinder or brakes. Have you check the car for codes?

 

I dont know what specs are on the cam but it's pretty lopey at idle, I did not do any of the mods on this car. It may not be vacuum related it just seems to be. If I tap the brake frequently and keep the vacuum pump cycling it it never throws code. But if it does its always at the wost time. It always goes into limp mode so I pull over reset code fire it up and on my way until a few miles later it does it again. Very frustrating. I'm determind now to figure it out. I'll check codes this evening. Thanks for your help

 

SIGFORTY

I had the exact same thoughts.

OP You most likely need to do some PCM light tuning and adjust the throttle air setting..

Your Brake Master Cylinder really doesn't use that much vacuum when you normally apply the brakes. If you repeatedly pumped the brakes, it could deplete the brake vacuum and have some effect on the engine.

I have seen Brake Boosters on C5s go bad.

If you let the car sit over night and the next day disconnect the booster vacuum line from the grommet on the booster shell, you should hear a large in rush of air into the booster. If you have NO VACUUM after it sits for a period of time, you may have a bad booster.

If all checks out and you really need more vacuum, a small vacuum reservoir just for the brakes would help stabilize the engine vacuum.

Bill

Bill

 

The C3's had a one way valve to hold vacuum in the tank. If you ever saw an old C3 "wink" at you it was because this one way valve on the intake was leaking and caused one (or both) of the headlights to pop up on hard acceleration.

 

Thanks Bill, I definately need a good tune. I'll check out booster tonight as well as other suggestions and report back.
Thanks again.

 

Update. The only codes I have are PCM P1571 H and P1689 H
There is no TCM codes as I reset them this morning when it went into limp mode on the way to work. I pulled the vacuum hose on booster after it set for about 5 hours and it was holding plenty of vacuum. After futher inspection I found the factory booster vacuum hose with a bolt clamped in it running around back of engine. Just for fun I pulled out the bolt and started engine and there it a ton of vacuum on that hose at idle anyway. I plugged hose again and will drive and record codes when it happens again.

 

It happened again this morning. So I got some codes. I realized today that cycling the ignition is what gets me back on the road not resetting codes.
Anyway code are C1278H, P1516H, P1571H, P1689H, U1016H, U1040H, U1064H, U1096H

 

The ONLY DTC that has anything to do with your issue (yes there all important) is:

DTC P1516 Throttle Actuator Control (TAC) Module Throttle Actuator Position Performance.. The rest are caused by P 1516 being set

Something is telling the car, your throttle is not functioning properly. I had a 98 Coupe and would get this DTC all the time. It would shut the car down and I could only limp home at 2-3 MPH on the side of the road. Sucks being passed by a VW and they beep at ya when they go by. ARRRRRRG!!

It got so bad that I would only drive the car as far as I could walk home or limp home. No BS!

Like you, I found out that I could get rid of the DTC and recover. I’m my case, I could enter the DIC routine and RESET the PCM DTCs and the car would run NORMAL.. Got real good at it!!!

Here is what I did to FIX it. I replaced the TAC Module. The 97 & 98 C5s has a Technical Service Bulletin (TSB) on this issue for 97-98 C5s.

So,,,, Im not saying that your TAC Module is bad BUT,,, something between the throttle blade and the module that control module is NOT RIGHT.

When I replaced mine the module was $230. Its a PLUG AND PLAY replacement.

Check the Throttle Position sensor connections, harness and PCM connections. Pay careful attention to the harness on top of the engine where all those wires are as it gets lots of abuse. Also check the accelerator peddle positon sensor for proper connections.

Bill

 

Something else I would do is find the fuse for the TAC Module and actually measure the voltage on top of the fuse at the test slots. Measure the test points to GROUND. Make sure that you have VERY close to full battery voltage. Yhat year C5 do you have??

Keep us up-to-date on the results.

Bill

 

I will check everything this afternoon. (I'm still at work) Thanks for your help. I guess I was hung up on the brake and vacuum because it always seem to happen when I was coming to a stop or using the brakes to slow down.
It's a 2000 C5
I do have the shop manuals for this car but I'm new to LS motors, ahem..where is the TAC module located?

 

Passengers fender well, under the fender well access door in the wheel well attached to the PCM bracket:



Littlle Black Plastic Box:

 

I checked the connections all connections all looks good. I ran out of time before I could check voltage and pedal position connections. I'll check tomorrow.

 

I checked voltage at throttle control fuse it was 14.5vdc
Have not got to pedal connection yet. Supposed to rain for new few days so I won't be driving either. I'll keep updating.

 

Hi fdeaver2,

Can you let me know what the fix was for this? I have a 99 Corvette FRC and it goes into limp mode when it rains. Only when it rains. Would like to know what the fix was for yours.

 

Rodvet,
I really didn't do anything except unplug and replug connectors on TAC module, throttle position and pedal position sensors. I've driven it about 500 miles since then (been raining a lot) with no problems yet. Maybe I had a bad connection somewhere. Maybe your TAC module connections are getting wet when it rains. Its located right behind the right front tire. See Bill Curlee's post above.