Run-on, Dieseling Help - The 1947 - Present Chevrolet & GMC Truck Message Board Network

70GMC396 · 05-05-2006, 01:16 AM

Not a mechanic so bear with me - trying to get the my father-in-laws 70 396 up and running. Hadn't been turned over for 3 years. Drained all the gas and put in 87. Had "fresh" fluids, but 3 years old. Got it going after priming several times and ran perfectly, not a single hickup - knock on wood. Then it happened...

just kept running on. WOULD NOT STOP ON ITS OWN.

So did usual dropping it in D. The interesting thing was that I tried to cut all fuel and air and it would still go on...minutes at a time before finally backing out the exhuast and occasional hiss up through carb. That was the last time I'll ever hold a towel on the throttle plate. Didn't look like the plate was being held open either. Found leaks on both exhaust manifolds. Could that be enough? It does burn some oil until the engine expands - normal I guess for an old BB? Should I be concerned about valves/guides? Could it be all the California crap allowing air to get in as well? Doesn't have EGR, but does have everything else - 3 systems at least. I checked for leaks, but everything looked OK.

Water temp dead on normal - not even hot on the guage after 30 minutes on and off trying to find problem. Went to the middle and stayed there.

It had a complete tune up by his regular good mechanic 500 miles and 3 years ago. New condesor, points, plugs, timing, etc.

Apparently, it has always done this, but only intermittantly.

My check list will include:
Higher octane.
Lucas fuel additive to hopefully get some deposits out - better product suggestions (Marvel/STP just old wives tales?)
New plugs - maybe colder
Double check timing
Check idle
Vacuum leaks
Remove emissions crap if possible (special distributor setup?)
Check points/condenser/cap, usual stuff

Where would you guys start if exhaust and timing don't fix it?
Thanks!

Frizzle Fry · 05-05-2006, 02:04 AM

Residual power to the dizzy. When you shut it off the voltage in at the dist should drop to zero. If it remains high enough it'll keep on truckin. Possibly the alternator wired wrong, or the wrong dist power source if so.

70GMC396 · 05-05-2006, 02:13 AM

Excellent suggestion - will check.
Thanks - damn, great stepside!!!!!!!!!!

dwaite72lnghrn · 05-05-2006, 05:49 AM

I would run some cleaner through the carb from the fuel pump side. Sounds like the float is stuck allowing fuel into the engine. I would also check the mechanical advance in the distributor. It sounds like it rusted in advance and you have to much, Put a timing light on it and see what it looks like.

boundstaffpress · 05-05-2006, 12:24 PM

You could have a kink in the throttle linkage. It doesn't take much to get some oxygen into the cylinders. There will be hot spots in the cylinders that will ignite the fuel like a glow plug. Hence the name dieseling. Check the timing like was said above, then try to get the carb to shut some more.

my .02

**PICKMUP** · 05-05-2006, 12:47 PM

Anti-dieseling solenoid

Shut it off in drive, while it is running. If it is already dieseling, it won't stop it to put it in drive.

Set the idle as low as possible. The carb butterflies need to be closed for it to stop. Try a test: set the carb so butterflies are closed, start it, run it, shut it off and it will not diesel...it just won't idle, but that will show where the problem is. You may need that solenoid.

It is running backwards when dieseling, by itself. There is no power to the dist, so timing with the key off should not make any difference.

mrein3 · 05-05-2006, 12:54 PM

Holy need for a can of Sea Foam Batman!

Seriously, read this post:
http://67-72chevytrucks.com/vboard/s...d.php?t=198309

A can of sea foam (1/3 in the tank, 1/3 in the crankcase, 1/3 in the carb) may clean up all your problems.

70GMC396 · 05-05-2006, 02:20 PM

thanks for the advice guys - will get to it this weekend, hopefully. Heard a lot about sea foam.

shelby987 · 05-05-2006, 03:28 PM

mrein3, you beat me to it....I was gonna come and save the day!
it stopped my dieseling problem (I was doing the same thing, pop the clutch in first, and it would stop)....never dieseled for more than 20 secs. though, so you might have additional issues....but I was very pleased!

grayw0lf · 05-05-2006, 03:34 PM

I agree on triple checking the carb. I replaced the POS Q-Jet w/ a newer Q-Jet & the problem was resolved 100%.

Do you have an extra carb sitting around?

70GMC396 · 05-05-2006, 04:05 PM

Nope, wish I did. It's actually in good visual shape, no real old varnish or leaks around gaskets - never been rebuilt though - so floats probably are jacked. Sure it is overdue. Just trying to get the thing running so I can get it home without engine damage.

70GMC396 · 05-08-2006, 02:19 PM

Thanks for all the suggestions. Fixed it this weekend. Here is what did it:

1) Timing was way off 12 degrees advance above base of 4B. Reset to 3B.
2) Put in 91 premium - ouch.
3) Sea Foam - 1/3 in tank, 1/3 in oil - held off in carb/vacuum then
4) Sea Foam DEEP in carb

After this it dieseled a little, but much 1/2 better.

5) Sea Foam 1/3 in manifold vacuum - rear port line - took awhile to distribute as most went in to rear 2-4 cyls.

6) Now runs perfect - NO DIESELING : )

Thanks

05-05-2006, 01:16 AM	#1
70GMC396 Registered User Join Date: Apr 2006 Location: 93420 Posts: 98	Run-on, Dieseling Help Not a mechanic so bear with me - trying to get the my father-in-laws 70 396 up and running. Hadn't been turned over for 3 years. Drained all the gas and put in 87. Had "fresh" fluids, but 3 years old. Got it going after priming several times and ran perfectly, not a single hickup - knock on wood. Then it happened... just kept running on. WOULD NOT STOP ON ITS OWN. So did usual dropping it in D. The interesting thing was that I tried to cut all fuel and air and it would still go on...minutes at a time before finally backing out the exhuast and occasional hiss up through carb. That was the last time I'll ever hold a towel on the throttle plate. Didn't look like the plate was being held open either. Found leaks on both exhaust manifolds. Could that be enough? It does burn some oil until the engine expands - normal I guess for an old BB? Should I be concerned about valves/guides? Could it be all the California crap allowing air to get in as well? Doesn't have EGR, but does have everything else - 3 systems at least. I checked for leaks, but everything looked OK. Water temp dead on normal - not even hot on the guage after 30 minutes on and off trying to find problem. Went to the middle and stayed there. It had a complete tune up by his regular good mechanic 500 miles and 3 years ago. New condesor, points, plugs, timing, etc. Apparently, it has always done this, but only intermittantly. My check list will include: Higher octane. Lucas fuel additive to hopefully get some deposits out - better product suggestions (Marvel/STP just old wives tales?) New plugs - maybe colder Double check timing Check idle Vacuum leaks Remove emissions crap if possible (special distributor setup?) Check points/condenser/cap, usual stuff Where would you guys start if exhaust and timing don't fix it? Thanks!

05-05-2006, 02:04 AM	#2
Frizzle Fry Registered User Join Date: Aug 2005 Location: Hillsboro Oregon Posts: 6,449	Re: Run-on, Dieseling Help Residual power to the dizzy. When you shut it off the voltage in at the dist should drop to zero. If it remains high enough it'll keep on truckin. Possibly the alternator wired wrong, or the wrong dist power source if so.

05-05-2006, 02:13 AM	#3
70GMC396 Registered User Join Date: Apr 2006 Location: 93420 Posts: 98	Re: Run-on, Dieseling Help Excellent suggestion - will check. Thanks - damn, great stepside!!!!!!!!!!

05-05-2006, 05:49 AM	#4
dwaite72lnghrn Registered User Join Date: Jun 2002 Location: central pa Posts: 1,525	Re: Run-on, Dieseling Help I would run some cleaner through the carb from the fuel pump side. Sounds like the float is stuck allowing fuel into the engine. I would also check the mechanical advance in the distributor. It sounds like it rusted in advance and you have to much, Put a timing light on it and see what it looks like. __________________ 72 longhorn c30 502BB/th400/fact air/4.11rear/ custom camper 72 bug(the better half's) under reconstruction 2009 HHR aqua blue 71 GMC k20 350/sm465 2009 Chevy hd3500 6.6 Victory Red crew cab dually Dave & Jeanne

05-05-2006, 12:24 PM	#5
boundstaffpress Jamie Hyneman is my twin Join Date: Nov 2005 Location: La Junta, Colorado Posts: 705	Re: Run-on, Dieseling Help You could have a kink in the throttle linkage. It doesn't take much to get some oxygen into the cylinders. There will be hot spots in the cylinders that will ignite the fuel like a glow plug. Hence the name dieseling. Check the timing like was said above, then try to get the carb to shut some more. my .02 __________________ Justin SOLD 1967 K10 Fleetside, 283, Front and Rear Posi SOLD Color Wiring Diagrams Now in High Resolution V8 and Cab and Cab and Chassis

05-05-2006, 12:47 PM	#6
PICKMUP "Trucks with Class" Join Date: Jul 2000 Location: OLYMPIA, WA, USA Posts: 8,125	Re: Run-on, Dieseling Help Anti-dieseling solenoid Shut it off in drive, while it is running. If it is already dieseling, it won't stop it to put it in drive. Set the idle as low as possible. The carb butterflies need to be closed for it to stop. Try a test: set the carb so butterflies are closed, start it, run it, shut it off and it will not diesel...it just won't idle, but that will show where the problem is. You may need that solenoid. It is running backwards when dieseling, by itself. There is no power to the dist, so timing with the key off should not make any difference.

05-05-2006, 12:54 PM	#7
mrein3 Registered User Join Date: Feb 2000 Location: Center City, MN, USA Posts: 3,253	Re: Run-on, Dieseling Help Holy need for a can of Sea Foam Batman! Seriously, read this post: http://67-72chevytrucks.com/vboard/s...d.php?t=198309 A can of sea foam (1/3 in the tank, 1/3 in the crankcase, 1/3 in the carb) may clean up all your problems. __________________ '70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN

05-05-2006, 02:20 PM	#8
70GMC396 Registered User Join Date: Apr 2006 Location: 93420 Posts: 98	Re: Run-on, Dieseling Help thanks for the advice guys - will get to it this weekend, hopefully. Heard a lot about sea foam.

05-05-2006, 03:28 PM	#9
shelby987 Registered User Join Date: Mar 2005 Location: Just outside Durham, NC Posts: 894	Re: Run-on, Dieseling Help mrein3, you beat me to it....I was gonna come and save the day! it stopped my dieseling problem (I was doing the same thing, pop the clutch in first, and it would stop)....never dieseled for more than 20 secs. though, so you might have additional issues....but I was very pleased! __________________ 1969 SS 350 Camaro Sold 2008 1970 3/4T K20, stock height with 33's, 250 I6, 3 on the tree, and locked front and rear 4.10's 1980 Chevy Malibu 2 dr Sold 2007 1993 Olds. Cutlass Ciera Wagon Traded 2006 2003 Saturn L200 w/5pd. D/D My list spans 5 decades with One common thread.....GM!

05-05-2006, 03:34 PM	#10
grayw0lf Registered User Join Date: Apr 2004 Location: Clanton, AL Posts: 1,253	Re: Run-on, Dieseling Help I agree on triple checking the carb. I replaced the POS Q-Jet w/ a newer Q-Jet & the problem was resolved 100%. Do you have an extra carb sitting around? __________________ 85' CUCV M1009!!! The newest addition! 6.2diesel, Th400, NP208, & only 36k miles! 70' C-10 LWB Fleetside - Looking good these days! 05' Dodge Neon 88' Winner Escape Sport 1750 - 4.3v6 94' Seadoo SP 84' Honda TRX200, bare nekkid. Just a frame & tires. Always looking for another project or any good deal!

05-05-2006, 04:05 PM	#11
70GMC396 Registered User Join Date: Apr 2006 Location: 93420 Posts: 98	Re: Run-on, Dieseling Help Nope, wish I did. It's actually in good visual shape, no real old varnish or leaks around gaskets - never been rebuilt though - so floats probably are jacked. Sure it is overdue. Just trying to get the thing running so I can get it home without engine damage.

05-08-2006, 02:19 PM	#12
70GMC396 Registered User Join Date: Apr 2006 Location: 93420 Posts: 98	Re: Run-on, Dieseling Help Thanks for all the suggestions. Fixed it this weekend. Here is what did it: 1) Timing was way off 12 degrees advance above base of 4B. Reset to 3B. 2) Put in 91 premium - ouch. 3) Sea Foam - 1/3 in tank, 1/3 in oil - held off in carb/vacuum then 4) Sea Foam DEEP in carb After this it dieseled a little, but much 1/2 better. 5) Sea Foam 1/3 in manifold vacuum - rear port line - took awhile to distribute as most went in to rear 2-4 cyls. 6) Now runs perfect - NO DIESELING : ) Thanks



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