Plug readings
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Le Mans Master
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From: New York New York
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Plug readings
I have a missfire on my 455 Olds that I need some input and suggestions.
It appears cylinders 5 and 6 are very weak firing at idle and off idle.
I have checked and double checked all the obvious.
No vacuum leaks, sprayed around the manifold, carb. and pinched all vacuum lines with no change.
I used an osciliscope to check all of the ignition and and had the distributor on a dist. machine and no problems with either.
I took a compression test today and they all read 180-190.
Since it is cylinders 5 and 6, the only common thing I can find is that they are next to one another on the distributor cap. I removed the cap and using a bright light, I could not find a crack or carbon track.
Here are some pictures of the plugs.
Should I go a colder heat range?
Thanks, Joe
These are 5 and 6
These are 1 and 2 which are firing OK
It appears cylinders 5 and 6 are very weak firing at idle and off idle.
I have checked and double checked all the obvious.
No vacuum leaks, sprayed around the manifold, carb. and pinched all vacuum lines with no change.
I used an osciliscope to check all of the ignition and and had the distributor on a dist. machine and no problems with either.
I took a compression test today and they all read 180-190.
Since it is cylinders 5 and 6, the only common thing I can find is that they are next to one another on the distributor cap. I removed the cap and using a bright light, I could not find a crack or carbon track.
Here are some pictures of the plugs.
Should I go a colder heat range?
Thanks, Joe
These are 5 and 6
These are 1 and 2 which are firing OK
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Joined: Feb 2003
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From: St. Joseph Mo
Don't change the heatrange of the plugs as they look very good.
As stated, plug wires may be the problem or carbon tracking in the dist cap.
A long shot:
On most 180* intake manifolds the power brake port is over the 5 & 6 intake runner junction. If the PB booster has an air leak those two cylinders will be effected the most. A leaky master cylinder will let the engine draw brake fluid into the intake manifold but you would notice the loss of fluid.
Modulator lines from the auto trans also obtain vacum from the same port.
As stated, plug wires may be the problem or carbon tracking in the dist cap.
A long shot:
On most 180* intake manifolds the power brake port is over the 5 & 6 intake runner junction. If the PB booster has an air leak those two cylinders will be effected the most. A leaky master cylinder will let the engine draw brake fluid into the intake manifold but you would notice the loss of fluid.
Modulator lines from the auto trans also obtain vacum from the same port.
Thread Starter
Le Mans Master
Joined: Apr 2000
Posts: 5,098
Likes: 1,351
From: New York New York
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"On most 180* intake manifolds the power brake port is over the 5 & 6 intake runner junction."
Thanks for the input but that only occurs on cylinders 5 & 8 or 6 & 7 because they are on the same runner.
The wires and all secondary ignition is OK.
When I put the engine on the osciliscope there was no excessive voltage spikes and every cylinder was firing evenly.
Thanks for the input but that only occurs on cylinders 5 & 8 or 6 & 7 because they are on the same runner.
The wires and all secondary ignition is OK.
When I put the engine on the osciliscope there was no excessive voltage spikes and every cylinder was firing evenly.
