5.3L dies at idle after running at operating temperature in cold weather

[greg716]

2001 Tahoe, 5.3L with around 220k miles. I've owned it for a couple years, so no idea on the age of fuel filter, plugs/wires/coils/etc. Tested fuel pressure and had 48 PSI. Codes that have been there forever are P0300 & P0332 (multiple random misfires and bank 2 knock sensor). Codes that started when the idle problem started are P0171 & P0174 (lean system codes).

In temps above around 15F or so, it runs quite well at idle, around 550RPM. But when it gets colder than that (and I'm in MN so -20F isn't all that uncommon), it randomly dies at idle. It always is able to restart, and shifting to neutral/park does seem to help. And again, it only does this below air temps of around 15F or colder. It also only does this AFTER running at operating temp and speed for 10 minutes or so... even at -20F it idles fine enough on startup and doesn't start dying until it hits running temp.

I did try replacing the IAC valve but the cheap one from Amazon was the wrong part- it was basically too "long". So, at idle, it ran perfectly smooth at around 800 RPM or so, but any throttle at all would have it racing because the valve was, basically, too long so it wouldn't close the idle circuit/port. I did clean (as best I could) the old IAC and reinstall it, but it runs the same so cleaning didn't seem to help. I haven't as yet ordered an OEM IAC to replace it, I'm not entirely sure that's the problem and it's easier to throw parts when they're $13 vs $100+.

A vacuum leak doesn't really make sense- it seems like that would be present/worse when cold and seal up as the engine warms up.

Any thoughts on other stuff to check? When the weather is warmer I am planning to replace plugs for sure, maybe wires also. But not particularly in the mood to do so in MN in February.

Thanks for reading, if you made it this far. And thanks for any insight as well!

 

[Fless]

Any exhaust leaks between the heads and the post cats? Also don't rule out a vacuum leak until it's thoroughly tested and verified leak-free.

When the engine is at operating temp, can you capture the short- and long-term fuel trims both when it's idling properly (above 15°F), and also those trims when ambient is colder?

Also use a good scanner to see which cylinders are misfiring. Any reason for not fixing the common knock sensor issue?

 

[Scottydoggs]

your lean codes are because you got a vac leak, never seen anything else cause them codes. the intake manifold could also be leaking, not just a plastic vac line that broke.

you can spray flammable brake cleaner at the vac lines and tb gasket, manifold gasket injectors too, if the idle jumps up while spraying a area, you found your vac leak.

 

[rockola1971]

Gen 3 engines are notorious for developing intake manifold leaks that wreak havoc on idle and cause stalling conditions. Unmetered air is getting into the intake and the fuel injection control is all over the place. Likely time for plugs, wires, fuel filter too and then check for ignition coil problems if misfires persist.

 

[greg716]

Any exhaust leaks between the heads and the post cats? Also don't rule out a vacuum leak until it's thoroughly tested and verified leak-free.

When the engine is at operating temp, can you capture the short- and long-term fuel trims both when it's idling properly (above 15°F), and also those trims when ambient is colder?

Also use a good scanner to see which cylinders are misfiring. Any reason for not fixing the common knock sensor issue?

There are no exhaust leaks. After your response and the others, I will definitely be checking for vacuum leaks and will report back.

My scanner isn't good enough to capture fuel trims. The Tahoe never has an individual cylinder misfire code, only the multiple random code and that only happens occasionally, typically under hard acceleration.

If by fixing the knock sensor issue- you mean replacing it then the reason is not wanting to pull the intake manifold. If you mean wiring the output of the bank 1 (good) sensor to both wires that run back to the ECM to trick the computer, then the reason is that I only recently learned about it and it's too cold up here right now to mess with something that isn't an immediate issue.

 

[greg716]

your lean codes are because you got a vac leak, never seen anything else cause them codes. the intake manifold could also be leaking, not just a plastic vac line that broke.

you can spray flammable brake cleaner at the vac lines and tb gasket, manifold gasket injectors too, if the idle jumps up while spraying a area, you found

 

[greg716]

Thanks for the responses

 

[greg716]

your lean codes are because you got a vac leak, never seen anything else cause them codes. the intake manifold could also be leaking, not just a plastic vac line that broke.

you can spray flammable brake cleaner at the vac lines and tb gasket, manifold gasket injectors too, if the idle jumps up while spraying a area, you found your vac leak.

I had an avalanche once that did the opposite- ran like total garbage until it warmed up and the manifold leak closed itself up when things warmed up. That's why this one seems so weird to me. Anyway, I'll try the brake cleaner test and let you guys know what I find.

 

[hagar]

Like said above, most likely a vacuum leak, check the pcv hose at the rear drivers side of the valve cover, it is common to crack and leak back there and is hard to see if you don't look for it.
It makes sense it's a leak since it gets worse when it's cold out, more air gets in when it's colder due to increase air density. There is a maximum range on the idle air control, if the leak is bad enough, the iac will continue to close more and more as the engine runs to keep the idle down, all the way until the point where the iac is fully closed, then it has no ability to prevent a stall on throttle transitions or increases in loads like from the heater or ac. The iac needs to be in a range to let enough air in when the throttle closes. If the valve is closed from a leak, it cant respond open fast enough to save a stall. Once you turn the truck off, then start it again, the iac resets, and the whole process starts over again.
When you first start the truck cold, what do the rpms go to right after fire up?

 

[houstontaylor]

Maybe use a good scanner to check the intake air temperature sensor against the actual outside air temperature.