2002 LS1 Trans am - Random low idle/stall

[lastofthebreed_ws6]

Hey guys,

Looking for some insight into a weird intermittent idle/stall issue. Car is a 2002 WS6, LS1/T56. Had it for 8 years, mostly trouble-free, but this has me scratching my head a bit.





The Issue





Earlier this summer I drove the car about an hour, parked it at a mall. Came out maybe 15 min later to move it across the parking lot — started and ran fine. Shut it off again. About an hour later, I came back out, car started but wouldn’t idle. Had to feather the gas to keep it alive. Throttle response and power were totally normal. After 5 minutes or so it smoothed out and idled fine. That was with about 1/2 a tank of gas.



Trying to be proactive (since a lot of stuff is 20+ years old), I replaced the following:

• Delco IAC
• Delco TPS
• Delphi MAF
• New PCV
• Verified no vacuum leaks
• Fuel pressure verified at 58 PSI (engine running)

After that, car ran awesome. Smooth idle, great throttle response, no issues… until yesterday.





Happened Again





Drove the car about an hour to work. Fuel level was around 1/4 tank. When I got there, I shut it off, then restarted it about 2 minutes later to reposition it. Then it sat for ~7 hours.



When I came out to head home — same issue again:

• Started, but wouldn’t hold idle
• Full throttle response and power
• No stumbling at speed, just wouldn’t idle
• After a short drive and one key cycle, idle returned to normal

Observations

• Happens only after short restart + long sit
• Seems to be fuel level related? (1/2 tank or less both times)
• Throttle response is great during the issue — doesn’t feel like fuel delivery
• Fuel pressure is solid at 58 PSI
• No codes, no lights
• Key cycle clears it

I’m starting to wonder if this is something EVAP-related — purge solenoid sticking? Tank pressure venting wrong? Or maybe some kind of heat soak/flooding issue caused by the short restart?



Anyone run into something like this? I just don’t want to ignore it and have it suddenly turn into a no-start or leave me stuck.



Thanks in advance — any help appreciated.

 

[Marcozi]

Your car has obd2 diagnostics. Scan the engine computer.

 

[lastofthebreed_ws6]

Your car has obd2 diagnostics. Scan the engine computer.

No codes.

 

[rijndael]

I would stop replacing parts, you are likely to introduce new issues by putting in bad/untested parts.

I would remove the purge and see if it's stuck open or partially open. Your fuel trim information is very important to look at here, even if it's not the purge valve.

 

[jetman]

fuel filter

 

[lastofthebreed_ws6]

fuel filter

New last year

 

[Astro14]

Sounds like an intermittent vacuum leak. Smoke check it - pull/push/flex each hose and line.

 

[We're Metals]

Sounds like an intermittent vacuum leak. Smoke check it - pull/push/flex each hose and line.

Happy Birthday

 

[JDenyer232]

Find out what the fuel trim is.

 

[clinebarger]

Check Purge Solenoid first, If that check good....Make sure battery voltage isn't dipping low enough to reset the PCM thus losing it's learned IAC positions.

 

[Marcozi]

No codes.

That's odd. What are you using to scan the engine computer?

 

[lastofthebreed_ws6]

That's odd. What are you using to scan the engine computer?

GM's TECH II Software. Its so spotty it's hard to catch and have the scanner hooked up.

My theory currently is, only seems to occur at around 1/2 tank or less, will instantly start fine but no idle, revs ok. Key cycle corrects. based on some old LS1tech threads, and what I am seeing, I popped a new purge valve in is at 1/2 tank now, and no issues, will keep eye on it

 

[Marcozi]

GM's TECH II Software. Its so spotty it's hard to catch and have the scanner hooked up.

My theory currently is, only seems to occur at around 1/2 tank or less, will instantly start fine but no idle, revs ok. Key cycle corrects. based on some old LS1tech threads, and what I am seeing, I popped a new purge valve in is at 1/2 tank now, and no issues, will keep eye on it

I recommend OBD2Fusion. It’s available for both iOS and android. It allows you to record data generated by the engine computer. You can download the data and open it in excel or similar. There are a ton of data sources you can record.

 

[i6pwr]

How old was the fuel in the tank?

I know the plugs are a bear but are they OE AC Delco? May have slightly fuel fouled them and now they could be compromised.

If fuel pressure is good, and no codes, I would look at a few plugs.

 

[97prizm]

Hey guys,

Looking for some insight into a weird intermittent idle/stall issue. Car is a 2002 WS6, LS1/T56. Had it for 8 years, mostly trouble-free, but this has me scratching my head a bit.





The Issue





Earlier this summer I drove the car about an hour, parked it at a mall. Came out maybe 15 min later to move it across the parking lot — started and ran fine. Shut it off again. About an hour later, I came back out, car started but wouldn’t idle. Had to feather the gas to keep it alive. Throttle response and power were totally normal. After 5 minutes or so it smoothed out and idled fine. That was with about 1/2 a tank of gas.



Trying to be proactive (since a lot of stuff is 20+ years old), I replaced the following:

• Delco IAC
• Delco TPS
• Delphi MAF
• New PCV
• Verified no vacuum leaks
• Fuel pressure verified at 58 PSI (engine running)

After that, car ran awesome. Smooth idle, great throttle response, no issues… until yesterday.





Happened Again





Drove the car about an hour to work. Fuel level was around 1/4 tank. When I got there, I shut it off, then restarted it about 2 minutes later to reposition it. Then it sat for ~7 hours.



When I came out to head home — same issue again:

• Started, but wouldn’t hold idle
• Full throttle response and power
• No stumbling at speed, just wouldn’t idle
• After a short drive and one key cycle, idle returned to normal

Observations

• Happens only after short restart + long sit
• Seems to be fuel level related? (1/2 tank or less both times)
• Throttle response is great during the issue — doesn’t feel like fuel delivery
• Fuel pressure is solid at 58 PSI
• No codes, no lights
• Key cycle clears it

I’m starting to wonder if this is something EVAP-related — purge solenoid sticking? Tank pressure venting wrong? Or maybe some kind of heat soak/flooding issue caused by the short restart?



Anyone run into something like this? I just don’t want to ignore it and have it suddenly turn into a no-start or leave me stuck.



Thanks in advance — any help appreciated.

Vacuum leak? O2 sensors bad?

 

[clinebarger]

That's odd. What are you using to scan the engine computer?

Not odd at all with older OBD2 controllers. Tech 2 is the factory scan tool for this vehicle. HP Tuners would be best for OBD data acquisition on a GM/Delphi P01 PCM.