Miss At Idle - The 1947 - Present Chevrolet & GMC Truck Message Board Network

mdwilf · 07-07-2010, 09:38 AM

I have an odd miss at idle - it runs fine when the engine is cold or cool, but once it warms up it has a miss that sounds more like "thunka thunka" than just a dead spot. No knocking, etc. It almost sounds like a plug wire crossed, but I've checked them several times. I've also made the idle richer with no change. The longer it runs, the worse it gets, until you can feel the truck shake a little at idle. Runs great going down the road.

Stock 350, 70,000 miles, HEI, new cap, plugs, wires, rotor, 10 gauge wire from fuse box to dizzy, timing set to low 30s at full advance.

Thanks

joes50pu · 07-07-2010, 10:19 AM

Check for a vacuum leak around the intake manifold where it bolts to the head. Spraying with starter fluid.

Lee H · 07-07-2010, 10:51 AM

Sounds like a bad plug wire, maybe a burnt boot. When warm and missing, I would start removing wires one at a time. As you remove each wire the engine will miss more, those wires are alright. You've found the bad one when the miss does not change when the wire is removed. Replace that wire and all should be fine. I would check the plug in that hole to make sure it is not fouled. Also make sure wires 5 & 7 are not touching each other, keep them separated from cap to plug.

Wrenchbender Ret · 07-07-2010, 03:32 PM

Have you run a comp. test. Sounds like a week valve to me. It's possible one is adjusted too tight.

*Fitz* · 07-07-2010, 04:13 PM

You did not say if you're running an automatic choke, but if you are I'll bet the farm on a vacuum leak. When the engine is cold the choke is closed providing a richer mixture at idle. When the choke opens the mixture should lean out to the correct stochiometric air/fuel ratio but if there is a vacuum leak the mixture leans out way too much when the choke opens and you get a very rough idle.

71tahoe · 07-07-2010, 08:25 PM

I'm guessing it could also be a head gasket leak.

wzuke · 07-09-2010, 01:23 AM

Had the same problem - turned out it was a leaking brake vac. booster. Plug the line and see if the idle improves.

IndyRich · 07-09-2010, 02:34 AM

I had a similar problem...except mine would also miss out with WOT .....I changed the plugs and I was good

07-07-2010, 09:38 AM	#1
mdwilf Registered User Join Date: Feb 2009 Location: Jefferson City, MO Posts: 83	Miss At Idle I have an odd miss at idle - it runs fine when the engine is cold or cool, but once it warms up it has a miss that sounds more like "thunka thunka" than just a dead spot. No knocking, etc. It almost sounds like a plug wire crossed, but I've checked them several times. I've also made the idle richer with no change. The longer it runs, the worse it gets, until you can feel the truck shake a little at idle. Runs great going down the road. Stock 350, 70,000 miles, HEI, new cap, plugs, wires, rotor, 10 gauge wire from fuse box to dizzy, timing set to low 30s at full advance. Thanks __________________ What I do in my spare time: http://rosebudsgarage.shutterfly.com Last edited by mdwilf; 07-07-2010 at 09:40 AM.

07-07-2010, 10:19 AM	#2
joes50pu Registered User Join Date: Jan 2010 Location: yuba city ca Posts: 31	Re: Miss At Idle Check for a vacuum leak around the intake manifold where it bolts to the head. Spraying with starter fluid.

07-07-2010, 10:51 AM	#3
Lee H Registered User Join Date: Jun 2010 Location: Upland Ca Posts: 4,147	Re: Miss At Idle Sounds like a bad plug wire, maybe a burnt boot. When warm and missing, I would start removing wires one at a time. As you remove each wire the engine will miss more, those wires are alright. You've found the bad one when the miss does not change when the wire is removed. Replace that wire and all should be fine. I would check the plug in that hole to make sure it is not fouled. Also make sure wires 5 & 7 are not touching each other, keep them separated from cap to plug. __________________ 1972 C10 SWB, Air, PS, PB, 350/350THM. Second owner. 1965 Corvette roadster, 44K miles, 327/365 SHP, 4 speed, side exhaust, knockoffs, teak, second owner (bought in 1970), Have ALL numbers matching components. My frame off restoration thread: http://67-72chevytrucks.com/vboard/s...d.php?t=556703

07-07-2010, 03:32 PM	#4
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,229	Re: Miss At Idle Have you run a comp. test. Sounds like a week valve to me. It's possible one is adjusted too tight.

07-07-2010, 04:13 PM	#5
Fitz Account Suspended Join Date: Jan 2007 Location: Tucson Posts: 2,183	Re: Miss At Idle You did not say if you're running an automatic choke, but if you are I'll bet the farm on a vacuum leak. When the engine is cold the choke is closed providing a richer mixture at idle. When the choke opens the mixture should lean out to the correct stochiometric air/fuel ratio but if there is a vacuum leak the mixture leans out way too much when the choke opens and you get a very rough idle.

07-07-2010, 08:25 PM	#6
71tahoe Registered User Join Date: May 2006 Location: Jonesboro, AR Posts: 916	Re: Miss At Idle I'm guessing it could also be a head gasket leak.

07-09-2010, 01:23 AM	#7
wzuke Registered User Join Date: Nov 2007 Location: Marion, IL Posts: 112	Re: Miss At Idle Had the same problem - turned out it was a leaking brake vac. booster. Plug the line and see if the idle improves.

07-09-2010, 02:34 AM	#8
IndyRich Registered User Join Date: Feb 2009 Location: Indianapolis, IN Posts: 372	Re: Miss At Idle I had a similar problem...except mine would also miss out with WOT .....I changed the plugs and I was good

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