Hey everybody Im still fairly new here but Ill get straight to the point and Im a bit confused as I havent found a thread accurate for my situation.

I recently made the great purchase of a 1999 Corvette 6 Speed. I have been babying it while occasionally gunning it from time to time. The only mods that I am certain it has is a Borla Exhaust setup. Anyways yesterday I got in the car to start it up and it immediately revs high to about 2500 rpm for a few seconds then drops to about 800RPM and stalls out. I get the check engine light and reduced engine power and as frustrated as I was I took out a code reader and got codes:

P1120 TP Sensor 1 Circuit
P1220 TP Sensor 2 Circuit
P1514 TAC System MAF Performance
P1516 Command vs Actual Throttle Position Performance (TAC Module)
P1517 TAC Module Processor
P1518 PCM to TAC Module Serial Data Circuit
P0108 Manifold Pressure (MAP) Sensor Circuit High Voltage

What would be the best first step to do for this situation? It has had the P1516 several occasions and would idle strange time to time ans two occasions die at stop signs but I would start it up and drive again. I initially blew this off because my knock sensors were going bad and threw codes for them. I replaced the knock sensors and the car worked great but then a few of these listed codes popped up time to time so what should I do/look for first? Any help would be greatly appreciated.
-Oz

 

https://www.corvetteforum.com/forums...-pictures.html

 

http://www.corvetteactioncenter.com/...train-402.html

 

So more than likely its the TAC control module and nothing else? Thats causing all of my codes?

 

From the research, it appears. I would check with Bill C. or C5 Diag. Those guys are electrical geniuses.

 

I appreciate the help, Ill take a look at it and see what I can do, if nothing Ill ask one of them, because with all of those codes additional to the P1516 I wonder if it is because I took the throttle body off to get to the knock sensors, however it has done it before that intermittently so It might be the tac module. Thank you for the help!

 

Hey Oz !!
I see a lot of "circuit" related DTC's here...TAC module, TPS, MAP...I thinking maybe this might be a voltage related issue especially after seeing the P1518...I'm seeing a lot of PINK here...as in wire color..the PINK wire coming out of the ignition switch feeds a lot of important stuff on these engines...the PCM, injectors, coil packs, AND the TAC module.... I don't know if the engine is running right now but I would try a voltage drop test across the ignition switch...hook up the red lead of the multimeter to battery positive and the black lead to the THROTCONT (fuse 17) in the underhood fuse block....see what the multimeter reads...shouldn't even be a .1v...the fuel pump is also part of that "pink gang" so maybe that's why the engine stalled out when it did...fuel pump just quit running...this is just a SWAG !!

 

Ill give it a shot! What should the Voltage parameters be between? I also wouldn't understand why the fuel pump would quit running as you are saying? Would I need my ignition on when I do the test? I havent been able to start the car since yesterday unfortunately...well by start I mean continue running lol, It would Idle high then die. And sometimes just not crank with that reduced power. Finding a TAC module for a 1999 is pretty difficult since most places seem to have them out of stock or backorder and ones on ebay are close to 300 bucks

 

You aren't really looking at "parameters" per se....in plain English you're looking at the voltage that is coming into the switch and what's coming out...we call it a voltage drop check...if you have say 12.0 volts at the battery coming into the ignition switch and 11.0 volts coming out your ignition switch the voltage drop is 1 volt and that's way excessive !!...as far as the fuel pump yes it may slow down a bit but really there is not enough voltage to keep the fuel pump relay coil energized (pins 86 and 85 at the relay)....the fuel injectors and coil packs also may not be getting enough juice to fire correctly...the crank sensor probably is THE MOST important sensor in the car and if the PCM doesn't see the right voltage the PCM can't process the data and gets confused....garbage in...garbage out....the engine might just conk out while driving and then it probably won't start...a few members have had crank sensor issues lately...there is really no "factory data" as to what the correct voltage on the crank sensor should be but I like to follow the 90-10 engineering rule...no more than 10% off ground voltage and no less than 90% at the top end but a good crank sensor shows 0-12 volts while running....all the sensors in the car are really picky about voltage...now, the voltage drop test HAS to be done when the circuit is "loaded"...i.e. cranking or running...the saying is... "no current flow...no voltage drop"....OK, so the first thing is probably get your battery load tested and then check the charging voltage at the battery....the #1 thing !!...since the car won't stay running you can do the voltage drop check while cranking if it does start or stays running for a bit. You can try this too...select "voltage" on the DIC and with the key ON and CRANKING see what voltage it reads...it taps off the IGN1 circuit at the IPC I believe...I had to change my ignition switch a while back...when it didn't crank at all I read the DIC voltage and it was like 8 to 9 volts or so...I knew my battery was fine so I cycled the key a bunch of times and it started right up...I just couldn't get it to do it again but I just went ahead and changed the switch out..I also had a P1518...when the TAC module wasn't seeing a good voltage it threw that DTC instantly. Also, before you start it turn the ignition switch to ON...listen if you hear the fuel pump run for 2 seconds and shut off...remember that fuel pump relay...if you don't hear it I bet the engine won't even crank...my pump didn't !!...keep us advised...maybe Bill will chime in with some words of wisdom !!

EDIT: I think I might have had maybe .02 volt "voltage drop" or so when I had checked mine cranking and running when I changed the ignition switch !!

 

"DIAGNOSTIC SOFTWARE AND CALIBRATION NOT OPTIMALLY UTILIZED" ????????????????????????????????...Huh ???...basically the Engineers are saying "we don't know WTF the problem is but if you can't diagnose it change the TAC Module"....LOL !!!

 

Oz,
Did a quick voltage drop check between battery positive and throttle fuse...cranking was .07 volts...did same on PCM fuse cranking.... about .1volts !!

 

You've been so helpful sir, I should expect at maximum .1 V on the voltage drop test from what it seems. I haven't done the voltage drop test yet but tomorrow I will get that ball rolling. However something interesting did happen to me while I was tinkering with it today. After trying to start it those times the battery started dying so I disconnected it. I didnt have my Multimeter at the time so I went through and checked the wiring and connections and even the air filter, MAF sensor, and throttle body. I took everything out past the throttle body. I have an Oil catch can and I drained out the oil from it, made sure the hose connections were tight, and cleaned the throttle body with some throttle body cleaner. After I tightened everything up, I hooked the battery back up and my friend gave me a jump. Car started up and ran fine! No codes to pop up yet but that was just one time after the battery was disconnected. As it Idled it did fluctuate a bit which still made me suspicious but only by a 200rpm variable. It maintained a 900 to 1000rpm idle. Tomorrow I will do the voltage drop test, but this kind of suprised me because I wasnt expecting a start up LOL

 

Great !!...like I said first get that battery load tested and if you can get the car running get out those multimeter leads across the battery terminals and see if the battery is charging somewhere between 13.5-14.5 volts...if it’s low ( less than 13.5-14.5) put the red multimeter lead at the alternator B+ terminal stud (big red wire) and black lead to a good ground...if the voltage output from the alternator is 13.5-14.5 there is high resistance (loose connections or corrosion) at either the alternator B+ terminal, the other end of that alternator B+ cable down at the starter solenoid or at the positive lead at the battery...you can also have high resistance on the ground side...where the alternator mounts to the engine, the negative battery terminal, or the negative battery cable at the engine block. And please don’t call me Sir, I work for a living...LOL !!

 

Update! I finally got the car running again but I am still suspicious... The other day I unhooked the battery and removed the throttle body as well as the intake and inspected the connections on the sensors and checked for any vacuum leaks. I found some oil residue in the throttle body and cleaned it out along with the MAF sensor too. I put it all back together and drained out the oil catch can. I did notice that I didnt have a PCV valve and that there is a whistling from the other hose connection (towards the back of the engine). It seems the previous owner did a PCV delete when they installed and I didnt notice until now. Ill post a picture of it. After I got everything cleaned up and put together I hooked the battery back up and she started right up. I checked the Voltage drop between the battery and the Throttle control fuse 17 and showed 0.01V a couple times and 0.00V a few other tests. As the car was running the idle still fluctuated but I didnt have any codes. Disconnecting the battery reset them, but I have started the car about 6-7 times now and even drove it up the block and back. The idle still wants to act up and dropped low enough on me and almost stalled out when I was in gear downshifting to 1st or neutral to a stop sign. I got in today and did the voltage drop again and still the same, and no new codes. I gave the throttle a couple quick blips but even as I got off the gas it held its RPMs around 2000 before settling even after my foot was no longer on the gas. Could that missing PCV connection and the PCV itself be part of the cause? Or would the TAC module still be accountable?

 

 

Without the PCV system installed the catch can is really not required now...Is the top hose of the catch can plugged now ??...I would make sure the catch can is tightly secured so you don't have a vacuum leak....as far as the idle issue since you disconnected the battery you might have to do an "idle learn" procedure.
1) turn off the A/C
2) set parking brake
3) make sure coolant is more than 176 degrees.
4) idle engine for 5 minutes
5) turn off ignition for 5 minutes
6) YOUR DONE.... or if you have a Tech 2 or bi-directional scan tool just push play...LOL !!

 

When you had the intake off did you be sure and reconnect the small vacuum line that is for the HVAC controls at the back of the intake? That line is often accidently left disconnected. Could that be the whistling noise
you are hearing? A vacuum leak will affect idle quality as well.

 

That's 100% right !!...forgot to mention about that whistle...funny no P0171/0174 DTC's !!

 

That looks like a big vacuum leak, no pcv valve, manifold vacuum directly to the valve cover and an open hose fitting on the same valve cover to the rear. Is there an open hose fitting on the other valve cover?

It will run better with the rear hose fitting on the valve cover(s?) plugged, but in my view, you need a hose to the opposite valve cover and a pcv valve.

 

Hey Jim !!...that hose is on the front of the TB...that’s the fresh air source for the PCV system...no vacuum there...the rear ones on either side tie in together and go to the PCV and that goes to vacuum...PCV is actually a metered vacuum leak...LOL !!