Engine Miss - I could use some help - The 1947 - Present Chevrolet & GMC Truck Message Board Network

jasons · 01-19-2011, 09:54 PM

I've been trying to track down an engine miss and am running out of ideas and tired of throwing money at it. I'm hoping for a little guidance.

It's your standard 350 with electronic ignition. It was running fine then one day when I went to leave work for lunch it had a miss. It stays the same if the engine is cold or warmed up.

I've replaced the gasket between the carburetor and intake which was leaking, new cap and rotor and new plugs. The plugs were only a few months old. #4 had black carbon deposits, #6 had some rust colored coating on it and the others looked normal. I have not replaced the wires yet, they were replaced last summer with the plugs and like I said, I'm tired of throwing money at it and not fixing the problem. I have checked them over and can't see anything visually wrong with them.

I did a compression check and all cylinders were at 115-125 psi.

It does have a cracked exhaust manifold on the LH side that is a recent development. Could this cause a miss?

Any ideas?

Edit: I forgot to mention that the miss is present at an idle and when under load.

vectorit · 01-19-2011, 10:22 PM

Those two 4 and 6 plugs, would have me concerned. Is the crack in the exhaust manifold on the same side as the 4&6 holes? If so, then the crack could be causing a problem. Is your exhaust popping on deceleration, such as crackle n' pop? If so, you could have an exhaust leak that could be causing 4&6 to foul up like you describe.

Could also be as simple as a defective plug wire, and I would again focus on 4&6.

Just a thought.

60ratrod · 01-19-2011, 10:57 PM

your compression numbers look a little low too. for most healthy stock motors the norm is about 150psi. your compression has me a little worried

hayhauler71 · 01-19-2011, 11:44 PM

Take a spray bottle of water and spray the wires at night when the truck is running and look for a spark. This is a free test

jasons · 01-20-2011, 09:15 AM

Quote:

Originally Posted by vectorit

Those two 4 and 6 plugs, would have me concerned. Is the crack in the exhaust manifold on the same side as the 4&6 holes? If so, then the crack could be causing a problem. Is your exhaust popping on deceleration, such as crackle n' pop? If so, you could have an exhaust leak that could be causing 4&6 to foul up like you describe.

Could also be as simple as a defective plug wire, and I would again focus on 4&6.

Just a thought.

The cracked manifold is on the opposite side. There is no pipping, just a small leak that can only be heard with the hood up.

jasons · 01-20-2011, 09:16 AM

Quote:

Originally Posted by hayhauler71

Take a spray bottle of water and spray the wires at night when the truck is running and look for a spark. This is a free test

I've tried this also as part of the visual check on the wires, didn't see any arcing.

jasons · 01-20-2011, 09:38 AM

Quote:

Originally Posted by 60ratrod

your compression numbers look a little low too. for most healthy stock motors the norm is about 150psi. your compression has me a little worried

After a little research I discovered the throttle should have been wide open. I did my test with the throttle closed. I'll have to do it again. I would think the numbers will be higher, but the important thing is they were all about the same.

cdowns · 01-20-2011, 10:30 AM

have you changed your fuel and air filters recently?

jasons · 01-20-2011, 10:34 AM

Quote:

Originally Posted by cdowns

have you changed your fuel and air filters recently?

Air filter is new, fuel filter is a clear one and there appears to be plenty of fuel flowing through it.

Willys47 · 01-20-2011, 10:41 AM

I have heard of a fuelmiss befor is this just a idle thing or is it through the rpm range? you may just need to retune the carbs maybe the idle mixture ?

jasons · 01-20-2011, 10:52 AM

[QUOTE=Willys47;4425315]I have heard of a fuelmiss befor is this just a idle thing or is it through the rpm range? you may just need to retune the carbs maybe the idle mixture ?[/QUOT

It is through the rpm range, and is present under all driving conditions.

Wrenchbender Ret · 01-20-2011, 11:17 AM

Pull the plug wires one at a time with insulated pliers. The weak cyl. will not effect the RPM like the good ones do. Pull the valve cover on the suspected cyl. & check the height of the spring. A recessed valve seat sometimes will not show up in a comp. check. Turn the engine over & watch if any valves do not open near as much as the others. That would signify a worn camshaft. If you isolate it to a particular cyl. you can check that plug wire with a ohmeter. How about the dist. cap. Often you can not see a carbon track.

01-19-2011, 09:54 PM	#1
jasons not enough hours in the day Join Date: Jan 2009 Location: Axtell, NE Posts: 550	Engine Miss - I could use some help I've been trying to track down an engine miss and am running out of ideas and tired of throwing money at it. I'm hoping for a little guidance. It's your standard 350 with electronic ignition. It was running fine then one day when I went to leave work for lunch it had a miss. It stays the same if the engine is cold or warmed up. I've replaced the gasket between the carburetor and intake which was leaking, new cap and rotor and new plugs. The plugs were only a few months old. #4 had black carbon deposits, #6 had some rust colored coating on it and the others looked normal. I have not replaced the wires yet, they were replaced last summer with the plugs and like I said, I'm tired of throwing money at it and not fixing the problem. I have checked them over and can't see anything visually wrong with them. I did a compression check and all cylinders were at 115-125 psi. It does have a cracked exhaust manifold on the LH side that is a recent development. Could this cause a miss? Any ideas? Edit: I forgot to mention that the miss is present at an idle and when under load. Last edited by jasons; 01-20-2011 at 10:36 AM.

01-19-2011, 10:22 PM	#2
vectorit What? Join Date: Jun 2007 Location: Southern California Posts: 7,617	Re: Engine Miss - I could use some help Those two 4 and 6 plugs, would have me concerned. Is the crack in the exhaust manifold on the same side as the 4&6 holes? If so, then the crack could be causing a problem. Is your exhaust popping on deceleration, such as crackle n' pop? If so, you could have an exhaust leak that could be causing 4&6 to foul up like you describe. Could also be as simple as a defective plug wire, and I would again focus on 4&6. Just a thought. __________________ Chris 1968 K20 Suburban 1972 K10 LWB PU

01-19-2011, 10:57 PM	#3
60ratrod yes, i do Join Date: Jun 2007 Location: Spokane, wa Posts: 2,734	Re: Engine Miss - I could use some help your compression numbers look a little low too. for most healthy stock motors the norm is about 150psi. your compression has me a little worried __________________ ERROR 404....... SNAPPY COMEBACK NOT FOUND _______________________________________________________________________________________________ -60 Chevy C10, dead 350/good th350 combo, sitting on a 76 c10 blazer frame, built from 11 different vehicles "the abomination" -07 gmt900 silverado 1500 lt xcab z71, granite blue -81 suzuki gs650g "shelah"- current project -81 kz1000m1 csr "sarge" -next project (just beautification) -07 kawasaki vulcan classic vn900 Last edited by 60ratrod; 01-19-2011 at 10:57 PM.

01-20-2011, 10:30 AM	#8
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: Engine Miss - I could use some help have you changed your fuel and air filters recently? __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

01-20-2011, 10:41 AM	#10
Willys47 Registered User Join Date: Dec 2007 Location: Nanaimo B.C. Posts: 604	Re: Engine Miss - I could use some help I have heard of a fuelmiss befor is this just a idle thing or is it through the rpm range? you may just need to retune the carbs maybe the idle mixture ? __________________ I was hooked on these trucks since 1982 when I drove my budys fathers 1968 Custum GMC 396 automatic dana 60 rear with a power lock diff what a ride more fun then we will we can ever tell!!

01-19-2011, 11:44 PM	#4
hayhauler71 Registered User Join Date: Aug 2009 Location: St. Paul MN. Posts: 1,996	Re: Engine Miss - I could use some help Take a spray bottle of water and spray the wires at night when the truck is running and look for a spark. This is a free test __________________ Fuzzy

01-20-2011, 10:52 AM	#11
jasons not enough hours in the day Join Date: Jan 2009 Location: Axtell, NE Posts: 550	Re: Engine Miss - I could use some help [QUOTE=Willys47;4425315]I have heard of a fuelmiss befor is this just a idle thing or is it through the rpm range? you may just need to retune the carbs maybe the idle mixture ?[/QUOT It is through the rpm range, and is present under all driving conditions.

01-20-2011, 11:17 AM	#12
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,229	Re: Engine Miss - I could use some help Pull the plug wires one at a time with insulated pliers. The weak cyl. will not effect the RPM like the good ones do. Pull the valve cover on the suspected cyl. & check the height of the spring. A recessed valve seat sometimes will not show up in a comp. check. Turn the engine over & watch if any valves do not open near as much as the others. That would signify a worn camshaft. If you isolate it to a particular cyl. you can check that plug wire with a ohmeter. How about the dist. cap. Often you can not see a carbon track.



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