2002 Silverado idle/throttle issues

[wa9njm]

Hello all, I've got a 2002 Chevy Silverado 1500, 5.3L LM7 (non-flex fuel). This one has the cable operated TB and it didn't have EGR from the factory as far as I can tell. Truck is stock.

 

Occasionally when coming to a stop the idle will drop to 400~ rpm. The engine has stalled at least once when it's in this low idle state. (It has done this both with the hvac on/off)

 

Also when the AC compressor is on (AC or defrost cycle) the throttle will 'stick' when at speed.
ie. doing 60mph on the highway, take foot off gas pedal, you'll pretty much continue at 6omph as long as you're on level ground.

 

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No active or pending codes, no check engine light
Cleaned battery cables
Cleaned ground on frame under drivers seat
Replaced pcv valve with updated ac delco part
cleaned throttle body, replaced gasket, tps, and iac at same time since it was removed anyway.
hosed down pretty much the entire engine bay with brake clean looking for vac leaks.

 

I'd appreciate any advice on what to check next.

[richard wysong]

Welcome to the forum! Does your scan tool do live data? It sounds like an IAC issue, with live data you should be able to see the IAC counts and desired idle speed as well as the actual RPM the PCM sees. Any chance the throttle cable itself is sticking or somebody been playing with the throttle stop adjustment?

[wa9njm]

6 hours ago, richard wysong said:

Welcome to the forum! Does your scan tool do live data? It sounds like an IAC issue, with live data you should be able to see the IAC counts and desired idle speed as well as the actual RPM the PCM sees. Any chance the throttle cable itself is sticking or somebody been playing with the throttle stop adjustment?

 

Hi, my scanner does do live data but seems to have a really slow refresh rate. It is a pretty old scanner, I'll see if I can borrow a better one. The throttle cable doesn't seem to be hanging up and the cruise control cable is adjusted properly.  When its holding speed on it's own the second I turn off the ac I can hear the exhaust note change and it'll start slowing down and actually coasting at that point. The little throttle stop screw looks pretty corroded, it doesn't look like it's ever been touched.

[richard wysong]

Hmm, maybe try disconnecting the cruise cable just to eliminate the possibility that that is screwing up. It may be the PCM is commanding the IAC to open too much as a result of false input data, just for giggles try clearing the codes even though there aren't any showing

 

 

 

 

Edited January 22, 2023 by richard wysong

[wa9njm]

So I cleared codes and unhooked the cruise control cable, took it for a spin in and no change.

 

As far as live data I've tried a standalone Actron scanner, a cheap bluetooth scanner w/ an android phone, and an obdlink mx usb cable with a windows laptop. Every scanner/app/program i try on it only brings back 26 available PID's.  No info on iac counts or desired idle. I'm attaching a screenshot of all the available PID's it shows.

 

Any suggestions on a scanner/app/device,etc that can pull more data from one of these trucks?

[richard wysong]

Looks like the TPS is out of whack with the RPM, The coolant temp is low as well ,T-stat maybe? but that shouldn't affect the throttle

[wa9njm]

I took that screenshot just to show the available PID's. The truck wasn't fully warm, it'd only been running for a few minutes, just long enough to get it in closed loop.

 

That was idling in park without the throttle being touched, i've seen it read between 0.0 to 0.8% in that same situation.

What should should the tps show with no throttle input?

[richard wysong]

close to 0 I believe, It is easy to check with a multi meter. The voltage should increase and decrease smoothly with throttle movement and return to the same % every time. An old analog meter sometimes picks up a spike in the voltage better than a low buck digital. Make sure the TPS screws are tight , some TPS are adjustable too