Engine run-on

Gmtb03

Original poster
Member
Jun 22, 2020
4
Indiana
Hello, this is my first post. I have a 2003 trailblazer ltz 4.2 and the problem im having is almost a dieseling effect when i turn it off. It does this shake shudder thing for 2-3 seconds about 90% of the time and on rare occasions shuts down normally (instant stop). Im also having lack of power issues and low rpms then really kicks in at higher rpm.... and when i use the power windows at idle it almost stalls and idles low for a second. Not sure if this is all related or not.
Ive replaced the cat. converter, spark plugs, fuel filter, vvt solenoid, and thoroughly cleaned throttle body off the vehicle. Checked for loose intake bolts, and checked coils. No check engine light. Any help would be greatly appreciated. Thanks
 

Mike534x

Member
Apr 9, 2012
894
The low idle/power issue I'm not sure of, a wild shot in the dark could be your battery or alternator being the issue. Second *might* be your ignition switch, though things cause all sorts of weird electrical things to happen for some reason.

The dieseling symptom is something I had similarly happen with my Avalanche, it usually happens when theres a large amount of carbon build up in the cylinders. I fixed mine by purchasing 3 bottles of redline fuel system cleaner (or BG44k) and I poured 1 bottle in every fill up. When I did this, I would fill up 3 gallons short of what a bottle treats to keep the mixture more "potent". The next thing I did was seafoam the engine, I used the aerosol with the bending straw to spray it through the throttle body while having something hold the gas pedal to 3K RPM so it doesn't stall/until the can was empty. Another method used by others on here was pulling the brake booster vacuum line and having it suck up small amounts into the intake. Let it sit for about an hour-ish (do this with the engine at operating temp for best results), be prepared for a smoke show and then take it for a nice long drive to burn the remainder off. If it feels like its helping do another treatment, and run the fuel treatment stuff afterwards.
 
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Gmtb03

Original poster
Member
Jun 22, 2020
4
Indiana
I was thinking alternator too, and the battery is fairly new (year old) but i will still get it checked.
I wasn't aware of an ignition switch problem-i will look into that too.
Carbon buildup makes sense- i know the converter was clogged up pretty bad and former owner drove it that way. I will try that method you mentioned.
Thank you for your input!
 

gmcman

Member
Dec 12, 2011
4,656
You could verify your voltage at idle and rev it to about 2K and see what you have.

The BG44K works wonders and I would surely try that, may need two applications.

How long have you owned the vehicle? What is your oil change history? Synthetic or reg oil, oil change interval?

Another possible area would be a leaking/clogged fuel injector.

Since the engine diesels when you shut it off, safe bet that if you have carbon build-up, under load the knock sensors might be retarding your timing quite a bit resulting in sluggish performance.

If you have the Torque app and bluetooth OBD adapter, you could monitor the knock retard.

Without having any codes, I would definitely try 1 or 2 treatments of BG44K. Just add to a low fuel tank first when you fill up to ensure mixing.
 
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NJTB

Member
Aug 27, 2012
612
Flemington, NJ
This used to happen with carburated cars back in the day. The carbon would 'make' a hot spot that would act as a 'spark plug', and since there was no fuel shut off, as in FI, the engine would run on.
It seems there's a leaky fuel injector(s), so yes, follow the advice for the decarboning procedure.
 

Gmtb03

Original poster
Member
Jun 22, 2020
4
Indiana
I appreciate the input. I've owned it for only a month and dont know much about the history of it. I changed the oil when i first bought it. Im running valvoline synthetic blend in it.
I have recently noticed i smell gas after i shut it off and get out. It seems to be coming from the "evap box?" next to the fuel tank. I also smell gas in the vacuum line of the fuel pressure regulator. My mileage hasnt been great lately but no black smoke or other symptoms of bad FPR. Thought about testing the fuel pressure and see what it reads.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
I also smell gas in the vacuum line of the fuel pressure regulator.

That could certainly cause run-on as well as other issues you are experiencing. It's probably toast and should be replaced. Surprised it's not giving a rich running code. Start with replacing that first then look at the evap although that too could be introducing fuel into the engine after shutdown.
 
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gmcman

Member
Dec 12, 2011
4,656
You should not smell fuel through the vacuum line of the FPR. I agree, needs replaced.
 

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