Alright guys, needing some expert help here. I have been driving my 5.3 ls swap Silverado for over a year now. No problems with high idle till about 2 months ago. When it happens it idled at 2000 rpm and the tps says it's 4% even though the throttle blade mechanically is closed completely.

ive tried a few different tps sensors, checked my tps wires, even rerouted the harness to make sure it wasn't getting interference.

sometimes it happens from initial start of the motor and sometimes I'll start it, let it idle down drive for a few miles and it's doing it all the sudden. Shut the motor off for 10 seconds and restart it, idles normal.

any advice on where to go and what to check next would be appreciated

 

Hi, thanks for the quesiton,

is this drive by wire?

 

Sorry I forgot some details, it's a cable operated throttle body and manual transmission

 

Idle speed is controlled by airflow. You have a leak somewhere if the idle is high (is always the answer). Vacuum is created in the cylinder, presented to the intake valve area when it opens, so this is the first initial big leak. If there was nothing after this point, no intake manifold on top, the engine could take its full breath. The following compression->ignition of this gulp would yield alot of engine torque, RPM would increase. If we cover this area with an intake manifold, than any place air can move into the manifold and then to the intake valve is a leak, and/or restriction, since sometimes we want the "full leak" and nothing to "be in the way". In this way billions of dollars are spent trying to perfect manifolds and throttle bodys to "flow as if no restriction were present" and "increase flow and control of air molecules under special circumstances" which maximizes the ability of that valve area to pull on the air molecules around it and for those air molecules to move into the cylinder all by themselves under special situations, enhancing specific characteristic "power bands" of an engine. The throttle body is a leak, when we open it, it allows air molecules to enter the manifold and reach the valve area. The IACV also creates variable sized leaks to control idle. If you unplug a vacuum line and let air in, the idle will increase proportionately.


ignore the tps for now since it can say 99% or 0% and that will not affect the number of air molecules entering the intake manifold per unit time. It can change timing within some limited range. If you think the timing is being advanced, unfortunetely the LS probably doesnt have a balancer mark for you to check easily. Better just to connect the computer and see what it says the timing is being set to. If you cant do that for some reason then I would suggest disconnecting the tps for the time being.


A boost leak test (full pressure test in the air path) is the recommended procedure for finding this leak. It could also be in the PCV valve (they can break and allow too much air).

If we go by just symptoms, it sounds like the IACV valve is just going bad.
Or the park/neutral switch could be telling the engine "park" and "drive" when it wants. Is this a swap? Im asking again because I thought silverados came with an LS engine. What matters is: did you change the computer in the vehicle?

I would say a scanner is necessary if this is a swap for sure, so you can read all this info from the computer. It will give you idle control position, gear, desired idle, error codes, faults, etc...

 

My cable driven TB started to idle high after I disconnected the battery. I just performed a TPS reset which fixed it.

 

Sounds like its not entering closed loop idle. Airflow and spark correction will not occur unless TPS is 0. I've seen this on f bodys where resolution in the scanner is not high enough to pick up one bit of pedal and just rounds the TPS value down to zero. It looks like everything is fine in the calibration except for high and/or oscillating idle RPM. When that happens its never 4% TPS though, but much lower. I'd make sure the throttle isn't getting hung up instead of closing all the way. Either way, a TPS relearn is a good idea.

 

This is a swap into a 91 Silverado, with a 04 gto pcm tuned by frost. I've checked the manifold for leaks and didn't find any, I will double check with a smoke machine. I have done the tps relearn multiple times and after a day or two of driving it idled up again and if I shut it off and restart it's fine. I have checked my pedal and throttle cable to see if it was hanging up somewhere and saw no issue.

as for pcv I have the factory 04 Silverado 5.3l valve cover set up the drivers valve cover has no pcv valve just a barb at the back and some baffles built into the valve cover. This valve cover goes to the vacuum side of the throttle body and he passenger valve cover goes right before the throttle body but after the MAF sensor.

i have access to a scanner that I can monitor things next time it acts up, but the iac usually shows 160 counts when it's idling high and it says that is what it is commanding, tps is always showing 4% when it happens. I think the iac is cracking because it thinks I'm slightly on the gas pedal, but the throttle blade is closed

 

4% throttle position is more than slightly on the gas pedal. Wouldn't think this would be the issue seeing as it was running fine before, but see if you can get position to zero with the set screw on the throttle body.

 

very good point about the idle algorithm not operating when the tps is open. It does sounds like there may be a throttle valve hanging issue unresolved.

Otherwise, the only thing left is the tps sensor, the wiring, the ecu. IS the TPS wiring shielded? I had to re-shield mine for my swap to make it "silent".

The air has to come from somewhere. If you see 150 idle position that is pretty far open which shows where your "leak" is coming from. So it again, points back to the throttle valve, giving the TPS% which is causing the 150 counts. You can try taking the TPS sensor off, holding in your hand, and turn it 100% one way for it to read 0%.

 

Sounds to me like when you start your car, throttle is zero or close enough and it goes into the idle routines. Once you drive, it's not going low enough to go into adaptive idle. It's probably follower air or cracker air that's commanding the high IAC counts.

Now, are you SURE it's 4% and not 0.4%? Usually the lowest non-zero reading you'll get is 0.4.

What's your TPS voltage? If it's above 0.7, it may not reset the TPS no matter how many times you reset it. If voltage is high, back off the idle set screw to get to 0.67 volts, do TPS reset again. Then start the car and just let it idle from cold to warm. It might get a bit screwy while it relearns.

Hope that helps with the troubleshooting.

 

PCM driver could be shot. I have the same problem...

 

Test the reference voltage going to the TPS. It should be 5V. If it's higher, the PWM in the PCM has begun to fail...

 

My tps wires are unshielded, originally I had them routed under the intake with the iac wires for a year and had no problem, then I thought maybe the heat of the motor was causing high resistance and moved them across the top of the intake loosely as a test and still would occasionally get the high idle.

Today I watched some data on the scanner and it didn't act up once. But my normal readings at idle are:

Ac off

Tps .69v
tps 0%
Iac counts 45
Idle speed 850
MAF .02 lb/s
Map 6.6 psi

Ac on

Iac counts 87
MAF .03
Map 7.4 psi

this is my daily driver and seems to go days without the problem then it'll do it, and maybe even do it a few days in a row, then stop doing it again for days. Very random. Next time I get it acting up I will check the tps volts again and percentage.

ill get back as soon as that happens

 

Awesome post. That's all really helpful info. So basically you've got it set up right. If you can catch it when it acts up and get the same data that'll help.

I suspect you'll end up tracing wires to find the problem though.

 

Check the reference voltage now and it when it acts up. If the reference is reading like mine was at 5.95... it's not a short. It's the PWM failing. And then when the PCM reads the signal, it's always going to be high. So if it's .69V now with 5V reference... it'll probably be .90V with it at 6V... And then realize that almost everything is based off TPS signal integrity and you can see how that would totally F the car up on idle routines, follower, PE mode...

 

Ok, I know it's been a while but my truck finally acted up on me today.

It was idling at 1500 rpm
deaired idle 850
iac 163
maf .04
tps .69

so I don't think the pcm driver is bad, or a short on the tps wires, I've already traced them anyway. Any other ideas?

 

Is it weather related?
Did you ever check for leaks with a smoke machine?
Did you happen to see the throttle percent when acting up and when it isn't.
I have the same swap could it be a throttle cable problem?
Just curious what throttle cable did you use for your swap? I used the 1999 silverado cable.

 

it seems to only act up when it's 85+ outside, or that seems to be the pattern. I'll get as much info as possible when/if it does it again this season.

i know the cable isn't binding, the tps voltage stayed the same during normal idle and the high idle, but the iac counts were much higher, so the computer is making he idle high. I guess the question is why? Seeing as the tps sees the same voltage

as for throttle cable I used he lokar ls1 cable

 

Well, that's weird. Because I would think if the TPS shows 2-4% open, the IAC counts would go up thinking the throttle is opened.

Did you do an external test of the voltage? You still need to do that. Don't rely on the scanner.

Check the 5V reference for TPS, IAC, and IAT.