Engine Run on (only after about 25+ minutes)

[72K5CSTDarkOlive]

Hey guys, first time posting in LS section and was wondering if anybody has suggestions to what I am currently experiencing. First I'll start with describing the issue and then provide background information regarding my application. My engine will stay running for about 2-3 seconds after I turn ignition off. However, it doesn't do it every time, only after the engine has been running for 25+minutes. I can get the engine temp up to 185-195 in about 10 minutes or so and shut the ignition off and the engine turns off immediately. I've tested this idling in my driveway as well as taking short trips under 20 minutes. Every time, beyond let's say 25 minutes the engine will experience run on for 2-3 seconds.

I originally thought I might have an electrical issue but if that were true, wouldn't I experience this every time?

As for my application, here goes: 72 K5 Blazer (no smog) 6.0LS, Stock stroke, ported cathedral heads, CAM is around 225/235, 113LS, and I believe 550-575 Lift. Nothing crazy. I'm running stock 42LB LS3 injectors AC Delco (part # 12576341), Holley single plane retro style EFI intake and Holley fuel rails. Holley Universal Throttle body 4150 style which has the connections for IAC, TPS, and MAT. and Summit brand 14x4" air filter. I have a new Boyd welding gas tank with Aeromotive 340lph in-tank pump, plumbed with -8AN hoses and fittings and a return style regulator by Aeromotive. Fuel pressure is 55-59. Fuel from tank goes through Holley fuel filter than enters through the fuel rail on passenger side and then crosses over to driver side fuel rail and after that it goes to my fuel regulator (one side is plugged) with the return hose hooked up to bottom of regulator back to tank. Alternator is by Power Master 1 wire 165 AMP alternator. I also have -10AN hoses coming off the back of each valve cover to a catch can in the front of engine bay on core support for vapor fumes. I had the K5 tuned on a chassis dyno ( The run on happened prior and after tuning so no change there) The idle is set around 850 RPM

ECU: Holley Terminator Max X with trans controller. The tuner did a great job and the engine runs great, smooth and healthy. I'm not burning any oil either, oil has stayed consistent levels. I've logged about 700 miles on this engine.

Also, might add this to the equation as might be part of the problem. My gas mileage is about 9.3. I tested my MPG on my last fill up and this is combined city and highway and I made sure not to do any WOT. just normal driving bc I wanted to see what my MPG was. I was told that I should be getting around 14-16 MPG. I have a 4l80e and it goes into lockup when it is supposed to. Maybe the summit air filter is suffocating it causing the engine to compensate for more fuel? But even if that was part of MPG being low, that wouldn't probably totally solve it bc that would be about a 40-50% increase in MPG, unless, I guess it really might be suffering from breathing.

Any ideas?

Thanks for taking the time to read my post and I appreciate any help in advanced. If there is any more info you need, let me know and I'll get back with that.

Mike

[JMac11]

Electric Fan running when you shut it off? Had an SS Monte we had t put a resistor in the line as the electric fans created just enough juice to try to keep the distributor firing

[Wgesnerjr]

Are you using a relay in the starter circuit? If so, it may be sticking closed after key is turned off. Swap with another relay or pull it when it's running on. If problem goes away, might be your issue.

[kglowacky]

I just had this happen on my start up. run perfectly and then the pcm would trip the relay to the fuel pump and off went the motor. I discovered the trigger wire from the pcm to the relay on the relay end got pushed down when the relay was installed to when it was making very little contact and after awhile it would shutoff the fuel pump. pushed the wire on tight and haven't had the problem anymore. Worth trying I guess.