"deiseling" after turning the key to "off" - The 1947 - Present Chevrolet & GMC Truck Message Board Network

sactoC10 · 10-04-2004, 10:46 PM

Anyone else experience this? What did you do about it?

glock35ipsc · 10-04-2004, 10:53 PM

How many miles are on the motor, and when was the last tune-up (plugs, timing checked/set)? Several thing can cause this. In fact, the last one I worked on for a friend had high mileage, and had sever deposits on the top of the pistons and the valves. The build-up can get soooooo hot that "pre-ignition" can occur. That is, the deposits are actullay igniting the mixture before the plug fires. Same can happen after you shut the key off, the deposits are so hot they keep "igniting" just enough to turn the motor over, fuel flowing, running somewhat...........

But, it can also be something as simple as checking the timing.

68w/sbc406 · 10-04-2004, 10:55 PM

i tore the straight 6 out and put a V8 in

Longhorn Man · 10-04-2004, 11:12 PM

If a tune up doesn't fix anything, and the timing is right, and the idle is set properly, then a higher fuel grade may be needed.

**passthebuck** · 10-05-2004, 02:08 AM

my L6 does it when its above 15C & a 195 thermostat is installed for the winter. I change to a 180 or a 160 & it stops.

68LSS1 · 10-05-2004, 02:59 AM

I agree with most everything previously said. Another possible solution if everything checks out good is to run a colder heat range of plug.

Southpa · 10-05-2004, 10:24 AM

The usual culprit is idle set too high.

sactoC10 · 10-05-2004, 12:15 PM

Not certain when there may have been a rebuild, but suspect that there was since the rest of the truck was kept up so well. May go with the plug change, but in the meantime...

This has recently started to happen occasionally since I got the exhaust leak taken care of, but does not happen all the time. Also, since moving the fuel lines away from some heat sources and a vapor lock situation that was probably taking place. It has felt as if the idle may be a bit too high.

BUT, this is my first Q-jet and with all the nighmares I hear about them, I don't want to go tweaking with the wrong screw(s) and tweak the wrong thing(s). I'll check this out though, and if it does not work, I'll probably go with the tune-up/plugs next... maybe another octane fuel at the next fill up.

I'll keep ya posted, thanks.

poff · 10-05-2004, 12:40 PM

A lot of times the throttle plate is too far open at idle. When this happens the mixture screws have to be set rich so the engine will idle. Q-jets are very easy to tune, do not be discouraged. The factory service manual has easy carb tuning instructions.

jimfulco · 10-05-2004, 11:08 PM

How's your vacuum advance hooked up? If it's hooked to ported vacuum, try it on manifold vacuum, which will likely increase the idle speed, then turn the idle speed down on the carb. Too much throttle opening is a common cause of dieseling.

**passthebuck** · 10-06-2004, 03:34 AM

i know i said this before, but mabe try the thermostat - you are in cali & probally dont need a 195degree thermostat. Its the easiest & cheapiest (a 180 or 160 thermostat & a gasket) of all thats been suggested so far.

GMC AMI · 10-06-2004, 06:41 AM

Quote:

Originally Posted by sactoC10

Anyone else experience this? What did you do about it?

Timing and the idle speed first things to check!

**Stocker** · 10-06-2004, 09:35 AM

Not a fix, but if you have an auto tranny, get in the habit of shutting the engine off while still in gear.

sactoC10 · 10-06-2004, 10:30 AM

This board is simply great, thanks all. I may be able to work on this later today. Not certain about the vacuum lines either... just another thing I'm tentative about messing with!

Maybe someone can explain to me why it does not happen all the time?

10-04-2004, 10:46 PM	#1
sactoC10 71 DELUX Join Date: Sep 2003 Location: Northern CA / Sac Posts: 1,056	"deiseling" after turning the key to "off" Anyone else experience this? What did you do about it? __________________ 5/4 drop LWB wood bed - 15x8 front 235/60 - 15x10 rear 295/50 2003 FORD Escape 1998 Honda VTR1000

10-04-2004, 10:53 PM	#2
glock35ipsc Next project: 1970 K10 Join Date: Feb 2004 Location: Fort Collins, Colo 80524 Posts: 6,685	How many miles are on the motor, and when was the last tune-up (plugs, timing checked/set)? Several thing can cause this. In fact, the last one I worked on for a friend had high mileage, and had sever deposits on the top of the pistons and the valves. The build-up can get soooooo hot that "pre-ignition" can occur. That is, the deposits are actullay igniting the mixture before the plug fires. Same can happen after you shut the key off, the deposits are so hot they keep "igniting" just enough to turn the motor over, fuel flowing, running somewhat........... But, it can also be something as simple as checking the timing. __________________ *Bob* 1970 Chevy K10 LWB "Goldie" 350/TH350 *next project!!* 1981 Chevy C10 LWB 355/TH350 - My son's truck LS SWAP FORUM! Tons of LS swap info here! PLEASE CLICK HERE TO SUPPORT THE GREATEST BOARD ON THE NET! CLICK HERE FOR THE FAQ INDEX by KRUE

10-04-2004, 10:55 PM	#3
68w/sbc406 Got Light Emitting Diode? Join Date: Apr 2004 Location: Newberg, OR Posts: 2,485	i tore the straight 6 out and put a V8 in __________________ 1968 1/2 ton 2wd lwb 6 lug disc and bags up front. Next c notch and rear bags '02 Chevy Silverado LT ext cab short bed 2WD 5/7 drop on 22's(the family car) 1993 Cherokee work ride/weekend wheeler

10-05-2004, 02:08 AM	#5
passthebuck Sisyphus was my mentor! Join Date: Sep 2002 Location: Carleton Place, Ontario CANADA Posts: 984	my L6 does it when its above 15C & a 195 thermostat is installed for the winter. I change to a 180 or a 160 & it stops. __________________ passthebuck #5642 -TWO 1967 GMC 910's. One with L6/3-on-the tree and the other with 355 w/435hp & a 700r4. -a 2013 Honda Civic as my "sensible" car

10-05-2004, 02:59 AM	#6
68LSS1 Registered User Join Date: Dec 2002 Location: Tampa Posts: 1,191	I agree with most everything previously said. Another possible solution if everything checks out good is to run a colder heat range of plug. __________________ '68 Short Step LS1/T56, Hydratech, Fatman Fabrications Stage III, Baer, Hot Rods to Hell, US Body, S&W, etc

10-04-2004, 11:12 PM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	If a tune up doesn't fix anything, and the timing is right, and the idle is set properly, then a higher fuel grade may be needed.

10-05-2004, 10:24 AM	#7
Southpa Registered User Join Date: May 2000 Location: Victoria, BC, Canada Posts: 2,410	The usual culprit is idle set too high. __________________ 1970 GMC 1500 Custom Original 350/TH350 Victoria, BC, Canada You can wish in one hand and crap in the other. See which one gets filled first.

10-05-2004, 12:15 PM	#8
sactoC10 71 DELUX Join Date: Sep 2003 Location: Northern CA / Sac Posts: 1,056	idle suspected Not certain when there may have been a rebuild, but suspect that there was since the rest of the truck was kept up so well. May go with the plug change, but in the meantime... This has recently started to happen occasionally since I got the exhaust leak taken care of, but does not happen all the time. Also, since moving the fuel lines away from some heat sources and a vapor lock situation that was probably taking place. It has felt as if the idle may be a bit too high. BUT, this is my first Q-jet and with all the nighmares I hear about them, I don't want to go tweaking with the wrong screw(s) and tweak the wrong thing(s). I'll check this out though, and if it does not work, I'll probably go with the tune-up/plugs next... maybe another octane fuel at the next fill up. I'll keep ya posted, thanks. __________________ 5/4 drop LWB wood bed - 15x8 front 235/60 - 15x10 rear 295/50 2003 FORD Escape 1998 Honda VTR1000

10-05-2004, 12:40 PM	#9
poff Registered User Join Date: Dec 1999 Location: san diego, ca Posts: 546	A lot of times the throttle plate is too far open at idle. When this happens the mixture screws have to be set rich so the engine will idle. Q-jets are very easy to tune, do not be discouraged. The factory service manual has easy carb tuning instructions. __________________ '70 K-20 San Diego, CA

10-05-2004, 11:08 PM	#10
jimfulco Registered User Join Date: May 2003 Location: Shreveport LA Posts: 3,170	How's your vacuum advance hooked up? If it's hooked to ported vacuum, try it on manifold vacuum, which will likely increase the idle speed, then turn the idle speed down on the carb. Too much throttle opening is a common cause of dieseling.

10-06-2004, 03:34 AM	#11
passthebuck Sisyphus was my mentor! Join Date: Sep 2002 Location: Carleton Place, Ontario CANADA Posts: 984	i know i said this before, but mabe try the thermostat - you are in cali & probally dont need a 195degree thermostat. Its the easiest & cheapiest (a 180 or 160 thermostat & a gasket) of all thats been suggested so far. __________________ passthebuck #5642 -TWO 1967 GMC 910's. One with L6/3-on-the tree and the other with 355 w/435hp & a 700r4. -a 2013 Honda Civic as my "sensible" car

10-06-2004, 09:35 AM	#13
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,694	Not a fix, but if you have an auto tranny, get in the habit of shutting the engine off while still in gear. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay

10-06-2004, 10:30 AM	#14
sactoC10 71 DELUX Join Date: Sep 2003 Location: Northern CA / Sac Posts: 1,056	This board is simply great, thanks all. I may be able to work on this later today. Not certain about the vacuum lines either... just another thing I'm tentative about messing with! Maybe someone can explain to me why it does not happen all the time? __________________ 5/4 drop LWB wood bed - 15x8 front 235/60 - 15x10 rear 295/50 2003 FORD Escape 1998 Honda VTR1000

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