1999 c5 bone stock won't run! Throttle shows 114%, map too high, what's wrong?!! - CorvetteForum - Chevrolet Corvette Forum Discussion


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1999 c5 bone stock won't run! Throttle shows 114%, map too high, what's wrong?!!

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Old 06-17-2017, 04:02 PM   #1
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Default 1999 c5 bone stock won't run! Throttle shows 114%, map too high, what's wrong?!!

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Old 06-17-2017, 04:22 PM   #2
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1999 c5 with no mods other than a lame exhaust system, 128k miles. The car will sometimes run and idle, then other times it won't idle. It seems to run better in speed density mode (with maf unplugged) but it's not always the case.

When i scan the car with HP tuners, and watch it idle till it has the seemingly random event, the fuel trims go to red, spark advance shows deep negative numbers, and the map voltage goes to 99%. I have video of this but idk how to upload that to the forum.

When I give it gas semi quickily, it will start stumbling and rpm won't rise from the stumble point, if I give it gas very slowly it will rev all the way 5500+ nice and smoothly. The throttle pedal app sensor shows 114% when put to wot (is this normal?)

the car is runnjng crazy rich. Sometimes it will idle great and sometimes it won't idle at all, sometimes It will hunt for an idle. It will go into reduced power mode and throw codes, then if I stop engine/ pull key/ restart it will run again. I've never seen the car drive well.

Parts replaced:
-tpp sensor (gas pedal) I was getting codes for sensor 3 performance

-TAC module (throttle actuator control) replaced and dropped seeing the sensor 3 performance codes

-MAF - I thought that because it ran better in speed density mode with maf unplugged, that indicated a bad maf. I never get maf codes unless running it unplugged

-I did bill curlees door harness repair, I get the 3 u1064 codes still.
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Old 06-17-2017, 04:33 PM   #3
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-I replaced the map sensor, and this i believe to be the core problem; when I watch the live scan tool on HP tuners, the car will be idling ok but a little rich, then suddenly with no input from me the voltage on the map will jump to 99% and the fuel trims will dump/ bar graph turns red, car runs like
irs misfiring and stalls. Often times the map reading won't drop from 99% till I either pull key or let the car sit for awhile.

I don't know where to go from here. My biggest questions are:

is there anything else in the wiring harness that could influence the map readings?

My understanding is that map should (and does) 100% voltage key on engine off, then at wot (and it does) something randomly tells the car to suddenly add like 30% fuel, retard the spark -20*, and show 99% reading on map sensor.

Im very new to the tuning software, could this have something to do with power enrichment? Is there anything else I can do before I burn this car down? Any advice appreciated, I'll post codes
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Old 06-17-2017, 04:42 PM   #4
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P0102 (maf is unplugged)
c1276
c1278
u1016

p0108 map high voltage


the car lately wont idle at all with maf plugged in. It did this with the old maf too, I guess the new one could be faulty also...?
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Old 06-19-2017, 04:05 PM   #5
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Have you checked for a vacuum leak?
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Old 06-20-2017, 12:36 AM   #6
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There is a group of grounds on the back side of the drivers side head. All under one bolt.
You might try cleaning that connection.
It's a bitch to get to.
Make sure none have broken away from the ground lugs.

There is also a ground on the pass side near the starter.

In fact, I'm sure you are aware that all the grounds can be suspect in the C5 and should be removed and cleaned. Especially on a high mileage east coast car that got driven in the rain.
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