Missing on one or two cylinders
July 8th, 2012, 12:46 PM
#1
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Location: West of Boston
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Missing on one or two cylinders
Help! I have had a 1971 Cutlass Convertible for about 10 years, but hardly drove. Probably looking to sell soon, but it runs "lumpy".
The engine is original, bone stock, and I don't believe it has ever been rebuilt. 98k miles. Two cylinders on the driver side bank don't seem to be working. I can remove the spark plug wires from them and it doesn't seem to affect the way the car runs. I have checked that the cylinders have spark (ouch!, they do), and I have checked (and rechecked) the firing order and I know the wires are in the right order. I have replaced the plugs but there is no difference. I replaced the wires when I first got the car and it didn't seem to change anything.
Do you think I burned a valve or two? Is there an easy way to diagnose the problem? Or is it something else?
Larry
71 Cutlass Convertible 350/4bbl/TH350 Saturn Gold with Black top and Interior
The engine is original, bone stock, and I don't believe it has ever been rebuilt. 98k miles. Two cylinders on the driver side bank don't seem to be working. I can remove the spark plug wires from them and it doesn't seem to affect the way the car runs. I have checked that the cylinders have spark (ouch!, they do), and I have checked (and rechecked) the firing order and I know the wires are in the right order. I have replaced the plugs but there is no difference. I replaced the wires when I first got the car and it didn't seem to change anything.
Do you think I burned a valve or two? Is there an easy way to diagnose the problem? Or is it something else?
Larry
71 Cutlass Convertible 350/4bbl/TH350 Saturn Gold with Black top and Interior
July 8th, 2012, 12:52 PM
#2
same but different
Join Date: Jun 2007
Location: Central Missouri
Posts: 2,880
It sounds like its time for a compression check. A leak down test would be helpful too, as to pointing to the valves sealing or not.
Have you had the valve cover off of the offending bank? That might shed some light on it.
Lets see what some of the other guys think.
Have you had the valve cover off of the offending bank? That might shed some light on it.
Lets see what some of the other guys think.
July 8th, 2012, 12:56 PM
#3
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Posts: 2,250
Are the 2 cylinders next to each other? It is very easy to mix up 5 and 7. If they are, see if they share a common intake runner and check for a vacuum leak. With the engine idling, slowly cover the top of the carb with a clean towel, if the engine speed increases, you are running lean. Also, the carb being out of adjustment can cause this, but not likely. If no hoses are off and the firing order is correct, I would do a compression check.
Edit; The EGR valve is on the drivers side, right? Make sure it is not stuck, that will act just like a vacuum leak.
Edit; The EGR valve is on the drivers side, right? Make sure it is not stuck, that will act just like a vacuum leak.
Last edited by captjim; July 8th, 2012 at 12:59 PM.
July 10th, 2012, 08:39 AM
#5
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Thanks for the tips. I am working long days next couple of days and going to visit my in-laws, but will report back with results soon.
My nagging suspicion is a burnt valve or two, but I am hoping it is something simpler than that.
Larry
71 Cutlass Conv 350 4bbl TH350 in Saturn Gold/Black
My nagging suspicion is a burnt valve or two, but I am hoping it is something simpler than that.
Larry
71 Cutlass Conv 350 4bbl TH350 in Saturn Gold/Black
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