cant get TPS to reset to 0

[79 z28]

Ok what I found was that my base air flow table was not correct. Fixed that and it seems to idle a heck of a lot better. It doesnt die anymore for the most part but did 1 out of ten stops .. So .. the throttle cracker and follower are something to look at maybe though with the 1% tps showing ...

The throttle follower TPs step size is 1%.. can this account for a studder or idle issue when rolling to a stop.. Also as I read someones thread about it .. its there to smooth out the herky jerky .... how do you impliment that ?

[jason k]

what about the the stall saver adjustment under idle rpm?? that might help

[RonSSnova]

I'm actually going through this same thing on a 99 big single turbo Camaro with a TH-350. It's annoying as hell. If I figure it out, I'll post up what I did.

Same issue. Did you ever figure this out Dsteck?

99 Camaro PCM TB and TPS
Big single turbo.
0 it as described, unplugged key on.
Then key off it reads 0%
TPS V is .57.

Start the car and the tps reads .39% in HPT. The volts also seems to increase to .59v.

Car runs and drives fine, so maybe this is a don't care?

Ron

[bigfatls6]

I never figured out the issue with the camaro the owner picked the car up. He took it to a local performance shop and they could't figure it out either.

Im pretty sure if you have any thing more than 0% tps while ideling its going to effect you idle with fueling and timing causing an idle issue. With this car it caused a serious surging issue. I could get it to zero the tps and it would idle great for a few minutes but if you put the car in gear or turned the engine off and restarted it it would jump back up to 1-3% and would start surging again. We tried swapping in a known good computer from another perfectly good running car and that didn't fix the issue. I tried several different TPS sensors. reflashed the computer several times and nothing. Im pretty sure the shop that did the original work on this camaro fubared something on the car. It had a nitrous kit wired into the tps sensor. Im pretty sure this camaro is still sitting in the garage with the same issue. So if any one comes up with any thing let me know.

[RonSSnova]

Mine only jumps between 0 and .39%. It will actually jump between the the 2 key on engine off. I might just have a worn TPS or throttle shaft, although it feels tight.
It's not messing up how the car runs, it just bugs me.

[01 MidMet WS6]

Does anyone have any answers for this one? I have a 98 Camaro that is doing the exact same thing. KOEO the tps voltage is .55V, turn the key off and it drops to .45v when you turn the key back on it reads 3% every time. The .45v is only present as the pcm is powering down 5-6 seconds but the computer learns it as the lowest voltage. I have checked all three wires with a dual trace graphing multimeter and they all read the same. When the key is turned off the voltage on the signal wire drops to .45v until the 5v ref goes away then it drops to zero.

This has nothing to do with shaft bushings, min air rate adjust, etc. This is a pure electrical problem, if 98 F-body computers weren't so hard to find and expensive I would bolt in a new computer and retest.

Case of beer to anyone that can shed some light on this topic!!

[01 MidMet WS6]

Looks like I just bought myself a case of beer!

Alright here is what I found, hope it helps someone save some time. The Traction Control/Throttle control module was drawing my .55v key on signal down to .46v when the key was shut off. The PCM takes about 5 seconds to power down and the traction control module powers down immediately causing the drop. I would assume that the failure is in the traction control module but the customer did not want to diagnose. Regardless with the module disconnected the issue is no longer occurring. Hope this helps others.

[RonSSnova]

Nice of you to post your solution. It just might help someone else.

I made mine a bunch better by realizing that my system volts were hosed because the batt in the trunk wasn't seeing the full alt voltage. Once I put a 6 ga wire from the alt to the batt, my system voltage came up 3v.
TPS now only does the .39% thing for the first minute or so and goes to 0% from then after.

Ron

[insane8]

Hey guys , I may have another solution for anyone else with this problem , well it worked for me that .
I have an ls1 , heads and cam before , Never had a problem with it. Got my tuner to put a fast 92mm manifold and matching throttle Body . There it started going down hill. A week later it would hold its revs real high when idling and putting the clutch in , then it would ghost idle , up and down between 500 - 1500 rpm, checked any intake leaks or vaccum leaks couldn't find any, alternator was faulty , replaced that , still didn't fix the problem , went back to my tuner he noticed the tps was set a 5% when at idle , was meant to be at 0 % , he reset it , took it for a drive then it started playing up and said 5% again , he pulled the tps sensor off thinking it must be faulty, not realizing the tps was the old gm one , the. Throttle body didnf come with a señsor , the gm tps sensor was to big to fit over the shaft . Packed the inside of the sensor where the shaft goes with a bit of plastic so the shaft doesn't spin it around , was for a quick fix. Haven't had a problem since ! You could silicone/salastic it over night too . But I will be ordering a fast tps sensor to ensure this never happens again. Was a nightmare for months , hope this helps anyone . Tps sensor has a lot to do with it

[GPGTP]

I had this same issue today where the tps would reset to zero at key on engine off and jump up when you start the engine, it would go as high as 1.5% tps, it almost seemed vacuum related. Strange, I tried everything. Got the volts down as low as .290 on the tps, nothing would work. Swapped in a tps for another throttle body and the problem went away.... Not sure if it was defective. Anybody else ever have this problem with a nick Williams 102mm?

[Bluecat]

Yeah, these are all common issues to have. Many of you have hit on the most common culprits. Let me add in my two cents.


1. Ground plane issues. Very common on older cars like 98 fbodys. You know you have this issue when you log tps voltage and see a difference between car off and car running. For example you read 0.65 key on not running, start the car and without the throttle ever being moved it jumps to 0.69. I some cases adding and/or moving grounds can help this. Ive seen this problem resolved on f cars from putting the two ground eyelets back on separate bolts after a lazy installer put them on the same bolt on the back of the head. There is a reason Gm took the time and money to have two separate attachment points. To separate digital and load bearing grounds.

Basically its comes from variations or float in the ground voltage of the system at varying points due to to flow. It changes because running you have all this flow taking place, alternator charging, coils and other devices drawing. Turning on the head lights can make a difference. While most things wouldn't notice a variance in 0.02 volts, a tps will. This is why most sensitive sensors have there own ground path back to the pcm, where the pcm references the 5v signal of the sensor not from system ground, but that leg to fight such problems, But internally many of the "seperate" grounds still tie to together and interfere with each other.

Regardless, sometimes there is nothing to be done about it short of rewiring the whole car. Especially in the professional environment when you have limited amount of time to tune a car. The only solution to the problem tune wise is to raise the max tps % the pcm will qualify as "zero". This is done in the throttle follower airflow table. Basically if the follower is not active, the ecm treats the throttle as closed and all idle air flow pid controller and timing routines are allowed to operate. If look at the table you will see the first few cells are zero. If you have a tps problem, take the highest tps youve seen with the throttle closed, lets say 2.1%, and make the table zero past that point, say all the way to the 2.5% cell. This will let the car control the idle properly again. But this can create other problems, especially if youre not actually fighting a ground plane issue, so do this as a last resort.


2. Tps voltage to high in the closed position. With a cam car you will most likely have to make a change to the static idle air so the iac isnt maxed out all the time. I like to open my throttle blades up to where the iac is only open 20-30 steps at a fully warmed up idle. That way it still has plenty of air for it to add on a cold start. But to get to that point sometimes the blade has to open so much that the closed voltage gets to high to read zero.

Now some background on tps voltage. There is no idle voltage reset procedure. I get sick of seeing this bad info out there. When your not scanning and you turn the key off, a few seconds later on a genIII pcm, they go to powersave mode so to speak and shut down. This resets any data it had on min tps voltage. After that once the key is switch on and it powers back up, it takes the lowest voltage it sees and makes that "0%". And that is not just at key on. 10 minutes after you've been driving, if it happens to see even for just a split second, a voltage that is lower that what it previously seen on the tps signal, that becomes the new zero. So making changes live to the throttle stop screw, you can go down on tps, but you cant go up. If the tps was at 0.65 closed and you make a change that moves the closed voltage to 0.75, you will have to turn the key off for a few seconds and back on again before it will qualify 0.75 as zero. And you can't be scanning with your cable while you key off or the pcm never actually shuts down. As far as the range it will accept, its usually 0.15 to 0.80. The low we can change as we now have access to the tps min and max fail values under engine diagnostics. But the max idle is somewhere else we cant get to, and is usually around 0.80. So if you key on with 1.20 volts, it truncates that and makes 0.80 equal to zero regaurdless.

Back to the high problem, many people like to drill holes in the blades instead of opening the blade more with the stop or bending the stop tab on the arm. That is fine and dandy, but its hard to back up if you go to far. Or if the car goes back to stock one day and idles at 1200rpm with a stock cam. That is why I much prefer cracking the blade open more. But if that does result in greater that 0.80 volts, you have to reclock the tps sensor. It is easly done by removing the sensor, remove the alignment tit from the back side and egg the holes where the sensor bolts on so you can dial the voltage back down with the blade in the position you want it to be in. No ill effects of doing this so long at it still reads high enough at wot, which usually needs to be 4.20V to qualify as 100%. Which unless you have to dial it back stupid far to make idle numbers, and even then I've seen only one case it didn't still make north of 4.2 at wot. In other words that's not usually an issue.


3. Bad throttle body. I'll save my comments on bad workman ship on aftermarket throttle bodies and not name names. But from a $600 premium name TB, to an $89 ebay job, I always see problems. Whether its a iac or tps problem, I've not seen a ebay 90/92mm tb I couldn't fix in 30 minutes worth of work. That being said I've seen the same problems even on high dollar ones, so I tell my customers to buy the cheap ones and I'll fix the problems when I tune it. Also not relevant to this discussion, but almost all aftermarket tb's have too small of passage ways to and from the iac chamber. LS motors have these awesome iac motors with huge plunger heads. That gives us more range on what the iac can add for cold start, a big deal on big cams. Having a 1/4 passage way for air to flow when you have a 1/2 plunger head is stupid. Again, if I have to fix it myself anyway, Id rather spend $89 on junk instead of $450 on junk.

The common problem relavent here on aftermarket throttle bodies, is the shaft that sticks out and keys into the tps sensor. They have a flat spot machined into them that makes a "D". That "D" is often in the wrong spot or wrong size. Wrong spot is usually easily fixed by re clocking the sensor as mentioned above. Wrong size causes slack between the sensor and shaft, which mean its bouces where it likes and wot and idle may not be the same voltage any two times you open or close the throttle. To resolve this issue you have to take up the slack. I usually trim up some tape and wrap the shaft until the sensor slides on kind of snug. Not to tight or you'll bust the guts of the sensor trying to force it on. I've seen new sensors be enough to fix this problem, but there was technically nothing wrong with the old ones. What happens, is inside the "D" on the sensor there is a little tab that functions like a spring loaded arm to ensure a tight fit. One living on a stock throttle body for years the tab looses some of its tension. Put on a aftermarket TB with a smaller shaft causes the problem that a new one will fix, but its not really the sensors fault the shaft is the wrong dimensions.


4. Bad tps sensor. Yep, it happens. You see it from time to time. But knowing the three previous issues you should now have the knowledge to deternine if a sensor is truly bad. Not always, but most times a bad ls tps sensor will do something major like be stupid inconsistent or have a low sweep range, like only make 2v at wot. Pretty easy to pick out a truly bad sensor.

[DSteck]

I used to always clock the blade, but recently I've started drilling them because I've found it results in a more stable idle. I don't know why, but hole vs. clocking, every time, at the same IAC value, the hole performs better on idle recovery. No BS... had one car that was dialed in with the blade cracked. The only change was opening the hole and dialing the blade back down with the same IAC value, and all the sudden the sagging idle went away with no tune changes.

[Bluecat]

Looks like I just bought myself a case of beer!

Alright here is what I found, hope it helps someone save some time. The Traction Control/Throttle control module was drawing my .55v key on signal down to .46v when the key was shut off. The PCM takes about 5 seconds to power down and the traction control module powers down immediately causing the drop. I would assume that the failure is in the traction control module but the customer did not want to diagnose. Regardless with the module disconnected the issue is no longer occurring. Hope this helps others.

That is something I had never caught. In hind sight I've probally seen that before. Thank you for sharing. I will add that to my bag of tricks as something to test and look out for. Makes perfect sense that that could happen knowing how they are wired.

[ElecTech]

I used to always clock the blade, but recently I've started drilling them because I've found it results in a more stable idle. I don't know why, but hole vs. clocking, every time, at the same IAC value, the hole performs better on idle recovery. No BS... had one car that was dialed in with the blade cracked. The only change was opening the hole and dialing the blade back down with the same IAC value, and all the sudden the sagging idle went away with no tune changes.

I use this method as well. I have a tapered hand reamer that allows me to make quick changes in small increments to sneak up on the desired hole size/IAC count.

[Bluecat]

I used to always clock the blade, but recently I've started drilling them because I've found it results in a more stable idle. I don't know why, but hole vs. clocking, every time, at the same IAC value, the hole performs better on idle recovery. No BS... had one car that was dialed in with the blade cracked. The only change was opening the hole and dialing the blade back down with the same IAC value, and all the sudden the sagging idle went away with no tune changes.

Buddy, I hear you there. Been a many of things that logically should make no difference, but do. I think at one point in the past I had similar feelings on the topic. But since we finally have access on all the follower/cracker amounts/decays/delays and I setup my own values from scratch, I can't tell you the last time I had issues with a GenIII car catching itself falling back to idle. Its really more of a issue in assuring the Idle pid controller doesn't go active and start pulling STIT numbers down while the motor is still falling in rpm from inertia holding it up. That and paying attention to which pcm calibrations have the delay table wrong and 160 equals 1 second as the scalar is not being applied correctly in the editor.

Plus its less work to open the blade / reclock the tps than it is to pull the tb off and drill it. And it kills me when I retune a car that a bad tuner that went over kill on the drilling. Then when I get the car lined out tune wise it idles at 1200 with the blade as closed as I can make it and the iac parked on zero. I try to just never go there. There should not be any difference in the air coming from a hole or blade perimeter. Now carbs are a different story, as blade position is very important to timing the transfer slots and such. But on a dry FI TB, there should not be a difference in either method.

[GPGTP]

When plugging the sensor in detached from the tb the voltage did not change at all between key on engine off or running. I'm convinced it was lateral movement caused by engine vacuum. Strange.

[Bluecat]

When plugging the sensor in detached from the tb the voltage did not change at all between key on engine off or running. I'm convinced it was lateral movement caused by engine vacuum. Strange.

As James Short had mentioned, if you have a sloppy or wore out TB with shaft play, you can definitely run into problems like that. Not much you can do other than raise the max tps% up to make it happy. But is those cases often the blade hits the stop at different places and you have a floating random base mechanical aiflow, nothing you can do in the tune to fix that.

[GPGTP]

Brand new nick Williams 102. Swapped the sensor and the problem went away

[DSteck]

The only ones I usually end up drilling are aftermarket throttle bodies. They tend to start with a freaking pin hole.

[GPGTP]

Ill drill if I can't get the iac at a reasonable count at hot idle.

Unless its a 3800 or LT1, no air flow tables accessible so they get the drill