Stuck
Thread Starter
1st Gear
Joined: Apr 2025
Posts: 1
Likes: 0
Stuck
I bent a valve a while back replaced the valve and fixed everything got the car to run good but at 3500 rmp cylinder 3 constantly misfires, replaced spark plugs, ran perfect, blew a head gasket, replaced that, now it’s back to constant misfires at 3500 rpm and super rough idle, it does not chop any more at idle, doesn’t even sound cammed, I’ve been told potentially the alternator, any advice would be amazing
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,431
Likes: 3,981
From: Cape Coral, Florida
A single cylinder misfire because of a bad alternator ??…WHO told you that ??…did you check for a broken valve spring ??…other than changing plugs and wires the DIY mechanic will have a tough time diagnosing a misfire…you need advanced test equipment like a lab scope to nail it down to one part…this is what can cause your misfire…
Plug
Wire
Coil Pack
Coil Pack wiring
Fuel Injector
Fuel Injector wiring
Fuel Injector wiring- bad ECM/PCM
Valve Spring broken
Burned Valve
Cam Lobe worn
Low Compression- Rings
Plug
Wire
Coil Pack
Coil Pack wiring
Fuel Injector
Fuel Injector wiring
Fuel Injector wiring- bad ECM/PCM
Valve Spring broken
Burned Valve
Cam Lobe worn
Low Compression- Rings
Safety Car
Joined: Nov 2013
Posts: 3,554
Likes: 1,097
From: Ex DPRK, now just N of Medford, OR
C5 Diag is spot on, as always, and I hate to be harsh, but...
Most people, and maybe most 'mechanics' think the only job skill required of us is the ability to change parts.
It takes time to develop skills in diagnosis, and requires quite a bit of knowledge. Talented technicians don't grow on trees.
Maybe the hardest part of DIY auto repair is knowing when to say 'when'; recognizing that you're in over your head.
DIY is great, and I encouraged all my customers to participate, but only to the point where they were confident of success.
Asking for diagnostic advice without some basic skills in that area will lead you to replace a lot of perfectly good parts.
Like alternators....
Be careful accepting advice if your skill level isn't adequate to properly ascertain its worth!
Good luck with the misfire.
Broken or weak valve spring was my first thought, but there's no substitute for a systematic diagnostic approach.
Most people, and maybe most 'mechanics' think the only job skill required of us is the ability to change parts.
It takes time to develop skills in diagnosis, and requires quite a bit of knowledge. Talented technicians don't grow on trees.
Maybe the hardest part of DIY auto repair is knowing when to say 'when'; recognizing that you're in over your head.
DIY is great, and I encouraged all my customers to participate, but only to the point where they were confident of success.
Asking for diagnostic advice without some basic skills in that area will lead you to replace a lot of perfectly good parts.
Like alternators....
Be careful accepting advice if your skill level isn't adequate to properly ascertain its worth!
Good luck with the misfire.
Broken or weak valve spring was my first thought, but there's no substitute for a systematic diagnostic approach.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,431
Likes: 3,981
From: Cape Coral, Florida
C5 Diag is spot on, as always, and I hate to be harsh, but...
Most people, and maybe most 'mechanics' think the only job skill required of us is the ability to change parts.
It takes time to develop skills in diagnosis, and requires quite a bit of knowledge. Talented technicians don't grow on trees.
Maybe the hardest part of DIY auto repair is knowing when to say 'when'; recognizing that you're in over your head.
DIY is great, and I encouraged all my customers to participate, but only to the point where they were confident of success.
Asking for diagnostic advice without some basic skills in that area will lead you to replace a lot of perfectly good parts.
Like alternators....
Be careful accepting advice if your skill level isn't adequate to properly ascertain its worth!
Good luck with the misfire.
Broken or weak valve spring was my first thought, but there's no substitute for a systematic diagnostic approach.
Most people, and maybe most 'mechanics' think the only job skill required of us is the ability to change parts.
It takes time to develop skills in diagnosis, and requires quite a bit of knowledge. Talented technicians don't grow on trees.
Maybe the hardest part of DIY auto repair is knowing when to say 'when'; recognizing that you're in over your head.
DIY is great, and I encouraged all my customers to participate, but only to the point where they were confident of success.
Asking for diagnostic advice without some basic skills in that area will lead you to replace a lot of perfectly good parts.
Like alternators....
Be careful accepting advice if your skill level isn't adequate to properly ascertain its worth!
Good luck with the misfire.
Broken or weak valve spring was my first thought, but there's no substitute for a systematic diagnostic approach.
One of my favorite sayings..."All mechanics are not diagnosticians, but all diagnosticians are mechanics" !!
Melting Slicks
Joined: Jul 2015
Posts: 2,279
Likes: 226
From: Round Rock Texas
I would think with a bent valve a cam lope got messed up. This is why you are limit to only 3500 rpm.
On a pontiac 400 engine the sold me the wrong timing chain set. That car too was limited to 2500 rpm. We are only guessing since we dont have all the details that went into previous fix. Like parts list on what was replaces, work done etc
On a pontiac 400 engine the sold me the wrong timing chain set. That car too was limited to 2500 rpm. We are only guessing since we dont have all the details that went into previous fix. Like parts list on what was replaces, work done etc
