Hey guys, first time posting here 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

 

You might just have one or more hot spots in the chamber(s). It acts like a glow plug.

 

The only time ive had this issue was when my tune was way to rich. The car would diesel for a second when i shut it off.

 

Tune, and cooling system. Pay particular attention to how much the temp rises after you cruise at high speed for a long time, then pull off and idle. Like a road trip where you come off the freeway and the first thing you come to is a traffic light. That shows how much heat is built up and stored in the engine castings, how long it takes for the cooling system to move it out to the radiator, and how long it takes for the radiator/fan to dissipate it into the ambient. Instinct tells me your setup will have a temp spike at that time, which indicates a problem in one or more of those areas.

 

Only times I've ever seen this is due to flyback voltage. The person wired up the ECU wrong, powered off the fan circuit.

When you shut the key off when the fans were on, it would keep running for a while till the fans stopped.

 

Did a quick search on this subject. I believe this might be happening. One of the causes they mentioned was inefficient cooling and I did install a new radiator. Gonna give a quick burp and see if that helps

 

My AFR is reading around 13.8-14.3 at cruise, sits at about 14.1 at idle and WOT is 12.5. Although, a faulty 02 sensor could send a false reading to the ECU. It doesn't diesel after run on, it just shuts down like normal. I'm starting to lean towards hot spots in one or more of my chambers like Atsma suggested.

 

Yes, I will try this out. I dropped the K5 off at the body shop earlier today for some paint corrections. I'll have to wait until next week to report back on this. But if my temp gauge is reading normal temp levels and not experiencing sudden spikes in temp, it's still possible for heat to be trapped inside one of the combustion chambers bc the thermostat is reading from only one area of the cylinder head?

 

Spark plugs are NGK TR6

 

OK, I'm sorry in advance if this is a dumb question, but I'm confused. Is the engine just staying on or is it dieseling?

What I call "run-on" is after the key is turned off the engine runs as though the key was not turned off. Very smooth, all eight cylinders firing, etc. This is typically electrical. Most likely I would think ground-hunting. I once had a goofy ground loop between my headlamps and electric water pump. When I turned the lights on and then back off, the water pump would try to run, and the lights wouldn't go up or down, but the bulbs would come on. It would keep trying to run until the battery died. Literally just grounded in the same spot and the bolt cam loose. Separated the grounds, and my water pump was back to full power and my lights were functioning correctly

Sometimes what is called "running on" is actually "Dieseling", which is excess fuel coupled with hot spots. Typically one or two cylinders will do this, and you will get what sounds like a couple of really hard knocks as the engine randomly fires and spins over a few more times. Often times in EFI, dieseling is caused by leaking fuel injectors, and with your low mpg, that may be something to investigate. The plugs can get as hot as they want, but without fuel it can't diesel. Carbed cars were more susceptible, because if the engine kept turning and generating intake vacuum, you kept getting fuel.

 

Good thinking Darth! I was totally thinking dieseling.
An electrical fault causing continued operation of the engine did not occur to me. And maybe others...

 

That's exactly what it is doing, stays running, smooth all cylinders firing for about 2 seconds. I made a couple of post late last night with pictures of my spark plug to see if it tells a story and I also detailed my electrical scheme but it's pending approval. So after thinking about this while going through my wiring, it all the sudden hit me. My electrical fan relays are not triggered by the ignition switch, I actually have two toggle style switches that I turn on manually to give them power. The ecu with the ground trigger wires acts as the thermostat . Of course, I 'm always Turing the ignition / engine off first before I turn my switches off for my fuel pump and fans. So they actually still have power running to them since they are directly to the battery. Although, my ECU touch screen display shuts down immediately when I cycle ignition off but all of that stuff is switched from the ignition. I didn't know that since the fans still having power to them would flyback voltage to the, I assume ECu to continue firing the injectors.

Now I need to test this. Going to get it up to temp and at least 25-30 minutes of running, make sure the fans are activated, then turn the fans off and kill the Ignition to see if that solves it. The reason I did the relays the way I did (directly to battery was to take the load (amperage) off the starter during cranking bc I heard if you have to many accessories coming on all at once it could end you starters life relatively quickly.

 

sorry about the confusion, I made a couple more post last night that would have been more helpful, but still waiting approval since they had pics

 

Good deal. Thanks for the clarification. That means, you don't need to be chasing anything mechanical about the engine itself. it's all in the controls and grounds. With that explanation, you ruled out a LOT of things to not need to chase down.

If it turns out to be the fan relays, that will be interesting. sometimes controllers like to keep the fans running a minute or two to help with heat soak after turning the car off. the GM ECU does that actually.

 

I'm gonna give Holley a call and see what they say. They might have something similar thats not necessarily spelled out in their documentation. Thanks for your help. I'll make an update after I test the fans some time next week.