Help diagnosing an engine problem
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Postmaster
Joined: Jun 2004
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From: Riverside, So Cal
Help diagnosing an engine problem
My daily driver 56 has a remanufactured 351C engine. It has run great for 10k miles. Lately, it has a dead miss on cylinder #1 when cold that smooths out after the engine heats up. It used to go away entirely by 140 degrees. Now it is smoother, but not entirely gone at the 170 degree running temp (thermostat temp).
Compression tests OK on all cylinders. Plugs look good.
Any ideas? I need some fresh perspectives. I have run out of diagnostic ideas.
Thanks.
Compression tests OK on all cylinders. Plugs look good.
Any ideas? I need some fresh perspectives. I have run out of diagnostic ideas.
Thanks.
Postmaster
Joined: Apr 2004
Posts: 3,005
Likes: 2
From: Santa Clarita
You may want to check the ignition wire going to the cylinder, it may have a short and periodically touches something on the engine. Pull the wire and check to see if the end connections are also tight and making a good connection.
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Postmaster
Joined: Jun 2004
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From: Riverside, So Cal
Thanks all for the input. Please keep the ideas coming. This weekend, I'll try the suggestions.
Ed - I have spark at #1 wire/plug, although the spark duration is pretty short (according to a chassis dyno I had run in March). I'm running a Mallory Magnetic Pulse distributor. I plan on changing out the wire to see if that is it.
Norb - I'll try the cold compression test. I really didn't think that after 10k miles, I should have a sticking valve or lifter, but who knows?
Lou - No, there aren't any ticking sounds, etc.
The baffling thing to me is that the miss smoothens out when the motor warms up. I just can't think of anything that would cause that. I'm just trying to fix this without pulling the heads, etc. to track it down.
Thanks all.
Ed - I have spark at #1 wire/plug, although the spark duration is pretty short (according to a chassis dyno I had run in March). I'm running a Mallory Magnetic Pulse distributor. I plan on changing out the wire to see if that is it.
Norb - I'll try the cold compression test. I really didn't think that after 10k miles, I should have a sticking valve or lifter, but who knows?
Lou - No, there aren't any ticking sounds, etc.
The baffling thing to me is that the miss smoothens out when the motor warms up. I just can't think of anything that would cause that. I'm just trying to fix this without pulling the heads, etc. to track it down.
Thanks all.
Postmaster
Joined: Apr 2004
Posts: 3,005
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From: Santa Clarita
Randy Jack, good luck on fixing the problem. It really sounds like an ignition wire problem. Spark plug wires will act like that. When they get warm they work OK, and vice versa. Also check the other wires, it may not be the number one cylinder and could be another one or intermittent. That happened to me on several occasions. Let us know what happens. Ed
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More Turbo
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If you don't find an ignition problem, look for a vacuum leak. The most likely culprit would be the intake manifold gasket.
Lou
Lou
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Postmaster
Joined: Jun 2004
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From: Riverside, So Cal
Updated Information
Hey folks - An update. Yesterday, I pulled the plugs and checked them. None looked bad (burned, lean, too rich, cruddy, etc). Gap was good. (Using Bosch Platinum, gap at .035)
I did a cold compression test. Lowest was #1 at 140 psi, highest was 155 with the average being 149.3 psi - all within expected range.
Cleaned and regapped the plugs and refired it. The problem was unchanged - a dead miss in one cylinder, which smoothens some at temp, but doesn't go away.
I didn't do anything with the plug wires, since I didn't have another set to swap out. I'll buy those today/tomorrow and try swapping out wires one at a time to find out if I have a bad one. The current wires are taylor 8mm spiral silicone, less than 2000 miles old.
Any thoughts?
I did a cold compression test. Lowest was #1 at 140 psi, highest was 155 with the average being 149.3 psi - all within expected range.
Cleaned and regapped the plugs and refired it. The problem was unchanged - a dead miss in one cylinder, which smoothens some at temp, but doesn't go away.
I didn't do anything with the plug wires, since I didn't have another set to swap out. I'll buy those today/tomorrow and try swapping out wires one at a time to find out if I have a bad one. The current wires are taylor 8mm spiral silicone, less than 2000 miles old.
Any thoughts?
Postmaster
Joined: Apr 2004
Posts: 3,005
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From: Santa Clarita
Even if the wires have 500 miles on them, they could have a short. Sometimes, only by bending or pulling the wire it can cause a break (particularly with suppression wires). I wouldn't waste time change one at a time. Get a new set install them and fire it up. Still sounds like a wire to me. Otherwise, you'll be guessing.
Lead Driver
Joined: Apr 2002
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From: Littleton, New Hampshire
This is probably not your problem, but thought I might share this anyway since I had similar symptoms on an engine years ago. It would miss cold and then smooth out and the miss would go away when the engine was hot. I chased ignition and fuel delivery until I was blue in the face and could find nothing. It turned out to be a cracked head. The crack was so small that when the cylinder head heated up the crack would shrink just enough to allow the the cylinder to fire. I never did a compression check though and I would think that if you had something like that it would have shown up on the compression check. Keep pluggin'. I'm sure you will find it eventually. 
Vern
Vern
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Did you ever look inside the cap? How about running the engine in the dark to see if sparks are happening? My son's car just needed a plug wire restained away from an exhaust manifold. 
John
John
Postmaster
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From: Santa Clarita
You may want to change the carburator gasket (gasket between the carb and manifold. Sometimes they may not be flush or the manifold may have some warpage. Use a Fel Pro gasket. It may explain why it smooths out when it gets warm. Metal expands. I still think it's a ignition wire. Or, take it out and crank it up pass redline and see if it idles smooth. (Just kidding, don't do this!!!!!!!!!!).
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Postmaster
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From: Riverside, So Cal
Ed - I didn't mention that I also tried the unlit propane bottle idea to look for vacuum leaks. None found. Nothing caught fire. Oh yeah, the revving it over the red line idea might work!
Vern - An engine guy I know also mentioned the crack in the head idea. I really didn't want to hear that! I'm trying to rule out everything else before I have to tear my engine down. If that works out to be the problem, I assume that magnafluxing the head is how to find a crack and then it can be welded and resurfaced? Or would you just replace the head? What does one have to do to match it to the intake manifold height?
John - I haven't looked inside the distributor cap. Not sure what I should see since it's an electronic ignition (Mallory Magnetic Pulse). What would I be looking for? Also, no wires are near the exhaust headers, although they do lay on the intake.
Thanks guys. Keep those ideas coming. I'm about out of them. I will try changing out the whole plug wire harness.
Vern - An engine guy I know also mentioned the crack in the head idea. I really didn't want to hear that! I'm trying to rule out everything else before I have to tear my engine down. If that works out to be the problem, I assume that magnafluxing the head is how to find a crack and then it can be welded and resurfaced? Or would you just replace the head? What does one have to do to match it to the intake manifold height?
John - I haven't looked inside the distributor cap. Not sure what I should see since it's an electronic ignition (Mallory Magnetic Pulse). What would I be looking for? Also, no wires are near the exhaust headers, although they do lay on the intake.
Thanks guys. Keep those ideas coming. I'm about out of them. I will try changing out the whole plug wire harness.