Engine Run-on after stopping from a cold start.

budwich

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Jun 16, 2013
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Recently, I just purchase a 2008 TB (in line 6, 4.2) that sometimes exhibits engine run-on when shutting down from a cold start up (ie. morning start, pull truck out of garage, then shut off -> sometimes result in a second or two of run-on). Engine has 135kkm, don't know any history other than it appears to be well kept. Is this a common issue? Is it perhaps related to the Supplemental AIS system? I thought that in general with most of today's systems (computer and other things) that run-on was a thing of the past. Gas mileage appears to be in-line with "published" guidelines (ie. 14 /20 m). Possibly needs a cleaning of the throttle body / intake?? Thanks for any guidance in this area.
 

Sparky

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Dec 4, 2011
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That's what I'd start with, maybe run some injector cleaner through it also in case you have a dirty injector that is weeping a little fuel.
 

CaptainXL

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Dec 4, 2011
2,445
Yeah, definitely sounds like a leaking injector. No other way for fuel to get into engine. With KOEO see if pressure holds by using a fuel pressure guage connected to the rail. If pressure bleeds down then it is injector (s).
 

gmcman

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Dec 12, 2011
4,656
It's getting fuel and spark somewhere, possibly a lot of carbon build up as well.
 

The_Roadie

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Nov 19, 2011
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Seafoam may help the carbon buildup. Some folks swear by it, others shun it. Not sure there's a consensus.

Rare, but I think possible - a flaky ignition switch might let part of the PCM stay running, but the fuel pump relay might be going off as expected, so the engine thinks it should be running but it gets starved of fuel? Just thinking out of the box, since I've never heard of that set of symptoms in many years of reading about problems.
 

budwich

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Jun 16, 2013
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I would think it has something to do with "rich start up" followed by possibly an air leak thereafter but as suggested, I probably need some "electricity" for it to continue those couples of "seconds". I would think that a "cylinder carbon buildup" is less likely since this is on a "cold startup" (more than 8 hours stopped) and the carbon wouldn't have time to heat up in the short "start, back up, stop". Good suggestions all around though. I have to get a fuel pressure gage and also try the cleaning. Go from there. I would think all "engine electrical" shuts down pretty quick with "key off" with the new systems but maybe a slow / sticky relay somewhere holding things up momentarily so I guess I need to poke around in those areas also.

thanks
Allan
back to GM from ford... :smile:
 

budwich

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can't find anything related to "computerized cranking", a feature which this truck has, in my service reference manual but was wondering if that "sub-system" has some "hooks" into a couple of areas (ie. holding up various relays) that might be involved... maybe? More of question on the feature operation / interactions than a "that's the problem" declaration. I would think not other than holding up the starter but who knows?
 

gmcman

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Dec 12, 2011
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GM top engine cleaner, seafoam, or a BG44K treatment is what I would try after the fuel leak down test. Have you ever replaced the plugs? Can you pull a couple and note their condition?
 

budwich

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Jun 16, 2013
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I plan on pulling a couple of plugs this week.... they should give me some indication of what's happening "inside".
 

Berzerk

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Apr 7, 2013
6
bubwich,
Does the engine idle smooth as always during this "ignition off run-on" or does it idle rough with clanky noises?
If it is smooth it's what gmcman and roadie are saying, it's getting fuel and spark from somewhere and the first point of interest is the ignition switch.
 

budwich

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Jun 16, 2013
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I would say that it is more "clunky" than "smooth run-on", more closer to the "traditional dieseling" but not quite that severe where you might get alot of engine / body shake as it is relatively short in duration but just get an the audible noise / sputter.
 

budwich

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Jun 16, 2013
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It was worse this morning.... probably cause now I am looking for it.... :smile: I pulled a couple of plugs, they look clean BUT there gaps appear to be on the narrow side at around 40ish as opposed to 43. Tough to crack as they appear to be original and don't appear to have any or little anti-seize associated with them. Hopefully that will be my focus. The throttle body looks clean but I will also give it some attention. Hopefully, this will reduce or eliminate the run on.

Oops... maybe. I see some references to 43 while I see others at 40... ???? which is it? It appears that the 41-103's come pregapped at 43 while my service reference indicates 40. I can't see anything in my GM owners manual or an engine sticker anywhere... guess that was a cost reduction and "more $$ for GM service centers".... :-(
 

budwich

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Jun 16, 2013
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Probably not (video), its not predictable. On the oil side, only had the vehicle for 1 month, levels look OK. I am putting the cleaned throttle body back on and see if that helps. New plugs in the future but that still doesn't answer the "40 gap from GM service manual" versus the "come with 43" for the 41-103's? maybe too narrow of a gap doesn't give a good "pop" on the plugs.
 

Sparky

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Dec 4, 2011
12,927
41-103s superceded the older plugs, so the gap very well could be a tad different. Iridium plugs are pregapped, don't try to regap them as you'll probably damage them (they crack).
 

budwich

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Jun 16, 2013
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thanks... will not do... but these are 41-103 so their gaps were either changed or "shrunk"... or are the "older vintage".
 

budwich

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Got everything put back together (ie. cleaning throttle body, checking all plugs -> all look the same visual and gapping). Seems to run "better" but the idle seems low now at about 600 rpm compared to about 625-650. Haven't found an "idle specification" anywhere for the engine... was expecting it to be in the 650-675 range maybe. I have to wait til tomorrow to see if the run-on has stopped. It currently turns off fine but it was OK after "warm starts" before so I don't know if anything has changed. Fingers are crossed. Anyone have ideas for "nominal idle rpm" for this engine. Do I have to "force" the computer to "re-learn" anything after a throttle body cleaning?
 

gmcman

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Dec 12, 2011
4,656
I wouldn't sweat the .003 difference. When you take it out next, either use 1st or 2nd (2nd if on a freeway) leave it in that gear and run it up to 6K at WOT. Then let it decelerate until about 3500 and floor the gas. Watch the rearview mirror for a cloud of smoke or puff and run it up to 6K again. Repeat a few times.

If you have some carbon build up then you will likely see something in the wake. If there is a big black cloud then you need to get the combustion chambers cleaned. This is likely causing your run-on and could either be from oil deposits or just nasty carbon build-up over time.

If this motor had 8:1 compression then I would say carbon, but being a 10:1 motor the compression sequence can heat up the mixture a lot more but not common to ignite with 87 octane.

Try the hard accel tests and report back.
 

gmcman

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Dec 12, 2011
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budwich said:
Anyone have ideas for "nominal idle rpm" for this engine. Do I have to "force" the computer to "re-learn" anything after a throttle body cleaning?

600-620 and absolutely you need a relearn.

Unhook NEG battery cable Remove harness from TB Perform cleaning Attach TB harness After 30 min of having the battery unhooked, reattach NEG terminal. Turn key to ON with all the dash lights on. DO NOT START AND DO NOT TOUCH GAS PEDAL TB will perform series of noises during this time. Leave key ON for about 5 minutes. Do not turn key off. Start engine DO NOT TOUCH GAS PEDAL let idle for 10 min.
 

budwich

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Jun 16, 2013
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thanks... my bad on disconnecting battery cable before playing with wiring... always forget. Anyways, will run thru the aftermentioned steps and see if that helps although it would appear that it is pretty close if 600-620 is "nominal".
 

gmcman

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Dec 12, 2011
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budwich said:
it would appear that it is pretty close if 600-620 is "nominal".

Takes awhile to settle down to that RPM when cold. Give it a few min or drive it.
 

budwich

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Jun 16, 2013
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well, the cleaning of the tb didn't help with the run-on situation this morning ... :-( although it does seem to run better. I changed the air filter this morning. Probably hope to try the top end cleaner. One thing I think I noticed yesterday that during shut down (of course, this during warm starts / afternoon driving) that it appears that the shutdown is "rapid" (ie. gages and such drop, "poof", to zero while perhaps during a run-on "event" they seem to "linger" which perhaps points to maybe an ignition switch issue. Don't really want to start tearing into the steering column quite yet... maybe if it gets significantly worse with continuous run-on on each shutdown.
 

gmcman

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Dec 12, 2011
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budwich said:
well, the cleaning of the tb didn't help with the run-on situation this morning ... :-( although it does seem to run better. I changed the air filter this morning. Probably hope to try the top end cleaner. One thing I think I noticed yesterday that during shut down (of course, this during warm starts / afternoon driving) that it appears that the shutdown is "rapid" (ie. gages and such drop, "poof", to zero while perhaps during a run-on "event" they seem to "linger" which perhaps points to maybe an ignition switch issue. Don't really want to start tearing into the steering column quite yet... maybe if it gets significantly worse with continuous run-on on each shutdown.

Did you drive it like you stole it yet?

The ignition switches on these are flaky, I would replace it now and see if it helps, either way if it's going out it may leave you stranded so at the very least just have one as a spare. From what you stated about the gauges I would change it.

If that doesn't cure your problem then it's likely a carbon issue/leaky injector. The TB being dirty isn't going to cause run-on but it's a necessary item to clean.
 

CaptainXL

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Dec 4, 2011
2,445
Its best to go rent or buy a fuel pressure gauge to see if the system is holding pressure. That will tell you if an injector or seal is leaking. Wouldn't waste any money trying anything else unless certain maintenance is due. Triz is right. Its either fuel, spark or computer related. I'm leaning more towards fuel. Mainly because there shouldn't be any when the key is turned off and the computer stops signalling the injectors to cycle. You may have an ignition problem as well but cylinders can't fire without fuel no matter if there is ignition or not.
 

budwich

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Jun 16, 2013
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Got a chance to pick up a pressure tester. Running pressure is around 60-63ish. Stopped, the pressure "slides" a bit to about 56-57, basically sits there. Over about 20 minutes, the pressure has slowly eased to about 40. Not sure but I would suspect that that might be "normal leak down" as I would expect a more rapid descent after a few minutes if there was a leaky injector. I haven't done the "top end engine cleaner" work yet.... maybe next on my list.
 

CaptainXL

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Dec 4, 2011
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Please fill out the rest of your profile Bud.

If the pressure drops off at all you have a leaking injector or bad fuel pressure regulator.
 

budwich

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Jun 16, 2013
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Since this is a returnless system, I assume it is unlikely that a faulty regulator (leaky) would feedback any fuel to the engine (ie. thru the vacuum port) and thus wouldn't have much / any impact on the run-on issue. Probably means that its more likely a leaky injector somewhere. I guess I will try some injector cleaner next to see if that helps.
 

CaptainXL

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Dec 4, 2011
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I think you are definitely headed in the right direction now. Throw a bottle of Chevron Techron Concentrate Plus + and a couple cans of Berryman's B12 Chemtool in the tank when it is down to 1/4 left. That should dissolve just about any deposits on the injectors. Good luck.
 

HARDTRAILZ

Moderator
Nov 18, 2011
49,665
CaptainXL said:
I think you are definitely headed in the right direction now. Throw a bottle of Chevron Techron Concentrate Plus + and a couple cans of Berryman's B12 Chemtool in the tank when it is down to 1/4 left. That should dissolve just about any deposits on the injectors. Good luck.

Seems odd that you would advise going WAY outside the makers recommended dosage for these additives and mix them. Berryman only calls for an once per gallon. Have you researched that mixing these two chemical solutions will not cause a detrimental reaction? Just like drugs, some chemicals do not mix.
 

budwich

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Jun 16, 2013
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So I haven't really resolved this issue (injector cleaner, some hard driving as suggested, but haven't tried the top end cleaner) but maybe this observation might mean something. IF I pull start up my vehicle, back it out out of the garage and then "momentarily" place it in "D" before then "P" ing it and shutting it down, it appears not to have any run on issue.... at least, I haven't had an event happen yet (5 or 6 cold start / shut down cycles... if I remember to use the "technique"). Having said that the run-on doesn't always occur in the first place so I am not sure I am seeing anything different or just wishful thinking. I thought with the cold weather the condition would happen more often / worsen but it doesn't appear to be related to temperature. Anyways, just thought I would throw the observation out there just in case.
 

Envoy_04

Member
Jul 1, 2013
749
budwich said:
So I haven't really resolved this issue (injector cleaner, some hard driving as suggested, but haven't tried the top end cleaner) but maybe this observation might mean something. IF I pull start up my vehicle, back it out out of the garage and then "momentarily" place it in "D" before then "P" ing it and shutting it down, it appears not to have any run on issue.... at least, I haven't had an event happen yet (5 or 6 cold start / shut down cycles... if I remember to use the "technique"). Having said that the run-on doesn't always occur in the first place so I am not sure I am seeing anything different or just wishful thinking. I thought with the cold weather the condition would happen more often / worsen but it doesn't appear to be related to temperature. Anyways, just thought I would throw the observation out there just in case.

How long is the vehicle running before you shut it down? I never have been a fan of cold starting an engine and then shutting it right back off, though sometimes it cannot be helped. If the engine is still on high idle when you shut it off, you can expect it not to just drop dead like it would at normal idle. It's spinning with more momentum than normal (higher RPM) and simple science says it is going to take longer to stop spinning. This is just my :twocents:, but it could be that you don't have a problem, the cold engine is simply turning at a high RPM (Probably 1000-1300, right?) when you shut it down and therefore not going to act the same way as when you turn it off warm at the normal idle of 630 RPM.
 

budwich

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Jun 16, 2013
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Probably about 20-30 seconds, basically enough time to start it, check things around in the mirrors, back slowly out of the garage and then stop. I do believe that the rpm's are below 1k.... so a high idle from cold start might be the base problem but one would think that a computer driven vehicle wouldn't have an issue as it is supposed to shut everything down with key OFF. Haven't had a vehicle with run-on since a "carburated" GM van. Of course, it is "funny" that shifting to "D" prior to "P" and then key "OFF" appears to stop the problem (maybe 2-3 more seconds). I am thinking it has more to do with "secondary air injection" system continues to run but that's just a guess.
 

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