Iv tried posting this before on LS1.com w/ no results, so here it goes again. No engine braking Like if I downshift into 2nd and bring the rpm's up to 4,000 and let off the gas, the car keeps going, like its coasting. If I put in the clutch while cruising at any rpm's higher than 2,000 and let off the gas, the rpm's stay there for like 5 seconds, then SLOWLY fall. When I did a burnout at the track, after comming to a stop the car idled at 2500 rpms for around 5-10 seconds, raised a couple hundred w/ my foot off the gas, then fell. The throttle is not sticking. Its annoying to drive, and I've driven several LS1's and no other one has done this. I auto tapped it, and all #'s came out perfect, and the dealers have even tried putting a new ECM, PCM, and IAC motor in at my request, but nothing solves it, and they say its normal. It started doing this 2 weeks after I got the car, and it was a once in a while thing that began to occured more and more often until it happens all the time now. And I saw this link with other people with this problem.
http://www.hoodscoop.com/cgi-bin/ult...c&f=2&t=002802

 

I got this when I would do my burnout at the track...I would do it then let off and coast up...but the rpms would hold up high...I have no idea why the car does this...I haven't tried since then....so I don't know if it will repeat...I got a new clutch in now...

The downshifting part doesnt sound right...your clutch is probably shot...thats my thoughts on it so far...have you been beating on the clutch??

[ November 18, 2001: Message edited by: BIGBOS ]</p>

 

Its done this since the 3rd week I had the car. It was a once in a while thing that it had no engine braking, then more and more often, till it was all the time. I know my clutch is fine, cuz I can bark 3rd gear with no problem. For example, If im in 2nd, 3rd or 4th gear cruising along at any rpm about 2,000, and put in the clutch, the rpms stay there for 5 sec or so, then SLOWLY fall. Or if i put in the clutch, and tap the gas, the RPMs kinda FLOAT around. Its almost as if the IAC isnt closing or something. When I come to a stop, the idle speed is around 1,200, then falls to 850 after 20 seconds or so. When it used to fall to 850 right away. I know the power is there cuz I ran 110mph trap speed w/ only a lid and catback.

 

how many miles are on the car...I've had this happen once in a great while but not all the time..it doesnt sound too serious...something is either sticking or the CPU is messed up...I would bitch to the dealer to look into it or send a tech out to look at it...

 

they tried replacing the PCU, ECM, and IAC at my request. They also tried calling TAN, and 2 dealerships can't figure the problem out. They also say its normal, even though I took a tech out in a brand new 2002 SS off the lot, and it didnt do it. The car has 15,000 miles on it now, and it does it all the time now. I can run it up to 5,000 rpms in 1st, let off the gas, and it slows down no quicker than if it were coasting down in nuetral.

 

I know you mentioned the throttle is not sticking, but have you checked the throttle body to make sure the springs and butterfly are opening/closing as they should? Could the return spring on the TB be weak? Can't hurt to pull the accordin and check the TB and TB blades etc.

 

My problem is very simalar...except now that it is cold up here it VT it doesnt seem to do it. If it is above 60 degrees it "sticks" and what i mean buy sticks its more like hanging...if you down shift coming to a light to slow down it doesnt...push in the clutch and the motor races to 2500 to 5000 rpm....GM has sent a tech to look at it...pontiac says there is nothing wrong with it ..my pontiac rep and the dealer know there is something wrong and they are taking it higher to solve this problem.Also the 3-5 second delay is normal...which stinks..my 2000 hawk count 1 2 3 and it decells ...hope they solve this soon.

 

Throttle position sensor???

I had a similar problem on my wifes neon, when it was cold, it was fine.

What really tipped me off was I could slowly work the gas pedal down about half way before it would pick up and realize the throttle was on the move.. Did it mostly on warm days, or after a very long trip, or if I was beating on the car.

I confirmed the TPS was bad by hooking a multi-meter to the damn thing(ohms) and working the accelorator. found that the feedback it was sending was all over the place.


Other than this, I have no idea what else would cause it..

Chuck

 

I checked the TB spring, and its pretty strong, with no binding. And the trottle body blade fully closes. It does it no matter what temp it is outside. If I am off the gas at 3,000 rpms in 2nd and go down a slight incline, the car will speed up. Its just getting frustrating.

 

I have had this at the track right after
I cleaned the my street tires..
Staging guys looked at me funny ..

 

A mystery indeed....let us know if you get any of it solved...

 

[quote]Originally posted by TA_2001:
<strong>I have had this at the track right after
I cleaned the my street tires..
Staging guys looked at me funny ..</strong><hr></blockquote>

they probably looked at you funny for doing a burnout on street tires.... j/k

 

ttt

 

if you have a fly by wire throttle you may want to check for corrosion on the positive battery terminal. ive seen it throw a few saturn techs through a loop, different system but same principle

 

[quote]Originally posted by Red02LS1:
<strong>Iv tried posting this before on LS1.com w/ no results, so here it goes again. No engine braking Like if I downshift into 2nd and bring the rpm's up to 4,000 and let off the gas, the car keeps going, like its coasting. If I put in the clutch while cruising at any rpm's higher than 2,000 and let off the gas, the rpm's stay there for like 5 seconds, then SLOWLY fall. When I did a burnout at the track, after comming to a stop the car idled at 2500 rpms for around 5-10 seconds, raised a couple hundred w/ my foot off the gas, then fell. The throttle is not sticking. Its annoying to drive, and I've driven several LS1's and no other one has done this. I auto tapped it, and all #'s came out perfect, and the dealers have even tried putting a new ECM, PCM, and IAC motor in at my request, but nothing solves it, and they say its normal. It started doing this 2 weeks after I got the car, and it was a once in a while thing that began to occured more and more often until it happens all the time now. And I saw this link with other people with this problem.
http://www.hoodscoop.com/cgi-bin/ult...c&f=2&t=002802</strong><hr></blockquote>

I don't think anything is wrong actually. My 01 TA does the same thing. anytime I'm over 2k rpm's engine has no braking affect. I think it's supposed to be like that. I am used to it but I miss being able to "blip" the throttle between shifting. You can't do it on these cars above 2k cause the engine won't slow down fast enough. I think it's a fuel economy drivability fuction of the computor control. IMO

My 99 silverado did it too. It was an auto and when you were say, getting off the interstate hwy on an off ramp you could feel it catch at 2k and then you would get the engine brake. Before then you would think the accelerater was sticking.

Good luck, Hawk

 

Yeah, these cars don't drop like a rock to idle, that's for sure. Sounds like his and some others are a little worse than typical though. It's like the computer is anticipating the engine stalling or dropping too low in the rpms for some reason.

Throttle position sensor, where is it with your foot off the gas? Needs to be below .2 volt for 10 seconds to set a code, but "should be near .6 volt at idle" I wonder if it was around, say .25 volt, could that contribute to the problem? Is the output stable?

Something eles I thought was interesting. There's a code P0121-TP sensor circuit insufficient activity. This is a diagnostic the computer runs while driving. It "compares actual throttle position from the TP sensor to a predicted throttle position value. The predicted throttle position is calculated from engine speed and other inputs. (Other inputs would be vehicle speed and the MAP sensor) If the PCM detects the predicted throttle position is below the actual TP sensor value, this DTC sets." It takes 20 seconds to set this code. Not too much other info, doesn't say what the deviation percentage is, or what if the predicted throttle position is above the TP sensor value. At any rate, the map sensor is a big part of the testing procedures for that code. There range can get out of whack, or unstable.

I guess my thinking is, you might not have enough of a problem with these things to set any codes, but they could be off enough that they're causing this idling problem. Both of them could be on the edge, and stacking up with each other. (Too low of a throttle angle and too high of a vacuum reading being detected) causing the IAC motor to open up way more than needed.

I really don't know what the hell else it could be...if it was my car with the problem, these are the two things I'd go after first. It would be interesting to compare the A-tap readings for TPS, MAP and the IAC counts, under the conditions, between your car and one that didn't have the problem.

Thad,

[ November 21, 2001: Message edited by: Sapient ]</p>

 

Have you checked all possible sources of vacuum leaks? A vacuum leak will cause a high idle. Check your PCV system, EGR system (if you have one), and your AIR system. Also check your entire intake tract for leaks- any unmetered air getting past the MAF can cause this. I have had a very similar problem on my 99 since installing the LS6 intake manifold- haven't been able to find the leak yet, but I'm pretty sure it is a vacuum leak. My IAC counts stay at 0- what do yours read? Don't let GM tell you this is normal- its not!

 

I understand that these cars are slow to return to idle, but I'v driven several LS1's, and none displayed what my car is doing. They already put in a new computer, and IAC. I probably wont have access to Auto tap till next spring. Is there anything I can tell the dealer to check?
Like I said, its not normal to be able to run in 1st gear up to 5,000 rpms, let totally off the gas, and have the car slow down at the same rate as if it were in nuetral. Then the next time I do it, the engine braking might kick in and out during decelaration.

 

Have the dealer check for all possible sources of a vacuum or intake leak. They probably have already done this, but sometimes the most obvious things get overlooked. You could also use a vacuum guage- an abnormal reading here can give you some clues. Look for cracked or loose hoses- check PCV, AIR, and intake. I think you don't have EGR on your car. You can use soapy water and wet down all your vacuum hoses- then start the car and look for bubbling. There is a vacuum hose that connects to the back of the intake manifold which is difficult to reach- but should be checked.

 

[quote]Originally posted by Red02LS1:
<strong>Like I said, its not normal to be able to run in 1st gear up to 5,000 rpms, let totally off the gas, and have the car slow down at the same rate as if it were in nuetral. Then the next time I do it, the engine braking might kick in and out during decelaration.</strong><hr></blockquote>

Damn, this certainly sounds like a electrical or sensor problem causing the IAC motor to move irratically to me. That would be one hell of a vacuum leak and shouldn't be interminent like that. Kind of interesting what BADSS said about noticing a difference after changing his intake manifold. Sure, he could have a vacuum leak somewhere, but that LS6 intake came with a new MAP sensor already installed in it, so he has a different sensor in his car now too...