Misfire Mystery

[RonD]

Yes, with MAF unplugged computer switches to Open Loop so no longer does the air/fuel calculations, and ignores O2 sensors, just runs engine on Factory tables for that engine size, same as it does when engine is cold

With engine warmed up and idling
Unplug the 2 wire connector on the IAC Valve
Idle should drop to 500-600rpms(barely running) or stall, either is GOOD it means no vacuum leaks
If idle doesn't change then there is a leak

 

[01WHTRNGR]

Ok, awesome. I’ll do that after work tomorrow

 

[01WHTRNGR]

Turns out

it is a vacuum line leak. Didn’t notice it in my previous tests because it’s under the engine cover. If anybody knows where it connects too or where the line would be it’d be greatly appreciated

 

[RonD]

Cap it for now and wait to see if a code comes up, or a system like cab vents is acting funny

 

[01WHTRNGR]

I covered it with my thumb while the truck was running, and the engine died

Nevermind, plugged it again while it was running and the truck didn’t die. So looks like that’s the plan for now

 

[RonD]

Cap it then start engine

 

[01WHTRNGR]

Cap is on and she’s running beautifully. Thanks for all the help Ron