Diagnosing cylinder misfire

-
Penstarpurist

Penstarpurist

Well-Known Member
Joined
Oct 28, 2016
Messages
1,313
Reaction score
630
Location
Salem
So we had got the new engine fired up, broke in and was running and idling good. Recently it’s started running rough and appears to be not running consistently on two of the cylinders. Cylinders 5 and 7. The plugs aren’t showing any burn on them. Has good compression, ample fuel, shows spark through the inline spark tester. Initially I thought maybe lost the cam lobes, pulled the valve covers in both sides and cycled the engine with the coil wire unplugged and all the rockers have equal travel up and down. What makes it seemingly odd is that it was running flawless before. Runs the same way with a 600cfm carb avs and with a brand new 800cfm avs (came free with the edelbrock kit). Timing is still the same as when we initially fired it up. Checked and double checked the cap to plug firing order, I am baffled and needing some direction on what to try next.
 
Compression test. Sounds like a head gasket problem between 5 & 7. Is it using any coolant? Report back with your findings.
 
We did a complete compression test and all the cylinders tested within 11 pounds of each other with the lowest being 110 and the highest 121. Cylinder 5 is 111 cylinder 7 is 118 pounds. No noticeable coolant loose.
Garrett Ellison said:
Compression test. Sounds like a head gasket problem between 5 & 7. Is it using any coolant? Report back with your findings.
Click to expand...
 
2F7C715E-C98F-46FF-989D-72ACCC05EAE6.jpeg
 
What engine is it along with intake and headers? Have you checked for an intake vacuum leak at five and seven?
 
Infrared temp gun on the header tubes. That'll tell real quick whether they're firing.
 
It’s the original 318 block bored .040 over. With the complete edelbrock upper end kit(aluminum heads, performer rpm air gap intake, came with the matched cam, lifters etc. stick hp exhaust manifolds with dual exhaust.
 
RustyRatRod said:
Infrared temp gun on the header tubes. That'll tell real quick whether they're firing.
Click to expand...
I’ll pick up an infrared heat gun tomorrow and give that a try if you think it would work with stock style hp exhaust manifolds. I would think it would.
 
Another quick test if you do not have an infrared tester is to squirt water onto the first header bend. Should turn to steam if firing.
Swap 5/7 spark plugs with 2/4.

Points or elec ign?
 
Bewy said:
Another quick test if you do not have an infrared tester is to squirt water onto the first header bend. Should turn to steam if firing.
Swap 5/7 spark plugs with 2/4.


Points or elec ign?
Click to expand...
Points distributor I’ll swap them out and see what that shows.
 
Penstarpurist said:
I’ll pick up an infrared heat gun tomorrow and give that a try if you think it would work with stock style hp exhaust manifolds. I would think it would.
Click to expand...
Absolutely it will. Just shoot the manifolds close to each exhaust port on the head.
 
It may be worth getting a cell phone bore scope / probe camera and looking down the intake. I know someone who amazingly was not myself who lost a shop towel down the intake and had a similar issue.
 
Check your points gap. Gap may be closing up on affected cyls due to worn dist cam lobe or worn shaft/bush.
 
Bewy said:
Check your points gap. Gap may be closing up on affected cyls due to worn dist cam lobe or worn shaft/bush.
Click to expand...
Brand new points distributor complete out of the box. But I will definitely check gap and look down the runners to see if there’s any obstructions.
 
Penstarpurist said:
Brand new points distributor complete out of the box. But I will definitely check gap and look down the runners to see if there’s any obstructions.
Click to expand...
That is meaningless. New doesn't mean good, OR adjusted right.
 
Is it just me, or does a compression test with an average of 116 seem a bit low for a modified 318? Since the compressions are all so close, I am leaning towards a probable ignition or a possible fuel problem.
 
harrisonm said:
Is it just me, or does a compression test with an average of 116 seem a bit low for a modified 318? Since the compressions are all so close, I am leaning towards a probable ignition or a possible fuel problem.
Click to expand...
Initially when we tried to start it, it had an ignition problem with it. That’s why we put in the new distributor and replaced the msd coil with an own one. I was thinking too it would have been a higher compression rate as well.
 
Retarded cam position will give low compression readings. Point is that the #s are uniformly low, so I doubt that this is related to the misfire.
 
I have a misfire off idle as well. In my case I believe my OEM CAP distributer (273 2bbl, OEM points dist) is advancing too much if I have the advance set to greater than 0 deg. (OEM spec it 5 ATDC) the today timming at 2000 rpm is around 50. ( I had a timming tape on it for a few hundred revolutions)

Also the dwell gets really small when the vacuume advance is fully advanced.

I would test again with dist at 0 and no vacuume advance and check the dwell through idle to 2000 rpm. If the misfire goes away that might give you a direction.
 
Wondering what those 2 plugs look like.
Wet with fuel or water?
Dry but black?
Maybe a cylinder leak test is in order to narrow down the issue.
Obviously, hearing shop air back feeding from the plug hole into the radiator, carburetor or exhaust with the cylinder at TDC would be a problem.
 
5 and 7 are next to each other firing order. Duly check the distributor lobes/and if The cam plate shaft bent or not. This is the pefrect example of when to use a cylendar kill meter or a oscilliscope for. Take a plug and put it on 5 and 7 wire ground it and see if you have a definate spark or not to verify it.
 
Last edited:
intake leaking sucking in air , I ve had the problem before on a 6 cylinder chevy motor. spray brake cleaner along the intake .and engine rpm should increase
 
Did you set the preload on the lifters. Sounds to me like the lifters pumped up and hold the valves open with oil pressure when running. Sometimes you won't pick that up on a compression test. Stamped steel rockers are noted for this unless you shim the shafts correctly. Just a thought. A vacuum gauge may picked that up watch the gauge as it warms up.

Also check the left side muffler . I have see varmints move in when the car is sitting.
 
You must log in or register to reply here.
-

Back
Top