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First things first, do a complete compression check, then you can rule it out and start chasing the ghosts, I've seen it time and again, guys chucking parts, replacing everything and in 10 minutes I find a dead cylinder. This may well not be your issue but it costs next to nothing to do, write all your numbers down and it'll be easier to find the % difference. If all cyls are healthy then let the chasing begin.
 

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Discussion Starter · #23 ·
Thanks for all the replies guys! I check compression in the near future then start working my way through the grounds and wiring of the sniper itself and try to swap to another HEI unit, though I did swap to all new internals.

With the fan - what might be causing it to draw excessive current? I was under the impression that would be from a bad ground but perhaps the motor itself is "sticky"? FWIW, its a newish Spal fan (about 2 years old) that i bought new to compliment the custom radiator.

- TyF -
 

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I have a 66 nova that I acquired from my father in-law 20 years ago, its been subject of a full rebuild including new wiring, trans, engine, paint, interior, paint, etc. Unfortunately it's always had a nagging misfire that I've been unable to chase down.

I suspect there's a lingering electrical issue or perhaps a wiped cam - the engine is Chevy Performance 350 H.O. (with a flat tappet cam, I've been running Royal Purple oil with HPS oil) complete crate motor with GM HEI, I've swapped out the wires (the first set got a little too close to one of the header tubes) with MSD, currently running Holley Sniper EFI. The car has been completely re-wired with an American Auto-wire kit, right now it seems to have a consistent mis-fire on cylinders 1, 7 and 8 - when I hit these cylinders with the infrared thermometer they register around 200 degrees, all other cylinders are around 500 degrees as measured at the header. I've also noticed that the voltage drops significantly when the electric fans starts running (14v down to approx. 10v), to the point I thought it was affecting spark - so far I've found no loose wires, damage, etc. that would point to the fan as the culprit. The engine runs the same regardless if the fan is running or not and idles fine but bogs heavily as soon as you touch the throttle. AFRs as measured by the Sniper display are erratic. I've tried the following:
  • Tested all plug wires for continuity/resistance - all are within spec
  • rebuilt the distributor
  • replaced spark plugs
  • cleaned and checked all grounds - currently have a heavy ground wire from battery to body, then firewall to subframe (in the engine bay - it has a TCI pro touring front clip) and subframe to core support. I also have the braided copper ground straps, one from the firewall to the head on the driver's side and one from the firewall to the block on the passenger side
  • tested the battery - seems fine, its about 2 years old
  • tested the alternator, it puts out about 14v and appears to be functioning correctly
I'm planning to pull the valve covers this weekend and see if the valve train operates correctly for 1, 7 and 8. This project is getting very frustrating - what else should I check?
VERIFY good motor with dry/wet cranking ,,,, then static cyl leakdown checks.If OK [all within 20% of ea other] Then ck cam for wear with dial indicator/Caliper
Most aftermkt EFI systems demand ALL 12v+ and All 12v - connections directly to batt for interference free op.
You need 140 amp alt [like 95 caprice cop car CS144] and 4-6G power cable to batt. These have wider pivot spread, but brackets can be modded for fit. Then you have enuf 'juice'
Big money spent for Roller cam/rockers solves many problems @ once. Power & longevity are 2 of them. the others will be apparent afterward as you wonder 'Why didn't I do this Sooner ???'
 

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Pull all 8 spark plugs, put them in order and post a pic of that. Have someone hit the starter while you block the spark plug hole with your finger, you will instantly know if you have a mechanical issue. Then replace the entire HEI unit with another one, with the power lead wired directly to the battery. If all of this checks ok, you need to eliminate your fuel system problems.
 

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Pull all 8 spark plugs, put them in order and post a pic of that. Have someone hit the starter while you block the spark plug hole with your finger, you will instantly know if you have a mechanical issue. Then replace the entire HEI unit with another one, with the power lead wired directly to the battery. If all of this checks ok, you need to eliminate your fuel system problems.
I'm not sure what you mean by blocking the hole and turning the engine over, you really need a reliable gauge to determine actual compression, you will never know the difference between 70 psi and 140 psi doing that. A reliable compression gauge is cheap, write the #'s down and figure out the difference between lowest and highest cyl pressures. If you have a bunch of cyls in the 140 to 150 range and one that is 70 you found the bad cylinder. Now squirt some oil in that hole and do the test again, if its still 70 its valve related, if it comes up its piston/ring related. I personally do compression and cyl leak-down check on any vehicle I suspect has a mechanical issue.
 

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Make sure you have full power to the HEI. I had a miss that turned out to be PO’s terrible wiring job that spliced into the original points resistor wire. The low voltage was killing it.
While I agree with you he states he has 3 cold cylinders, 1,7, and 8, he verified it with an infrared gun, if that were me the next quick thing I would do is just put my in-line spark tester between the plug and plug wire and run the vehicle, if it has bright blue spark I'd move onto compression testing, you could even compare the spark quality to a known good cylinder. In-line spark testers are cheap and beyond simple, in fact I use the same tester on vehicles with COP (coil over plug) ignitions.
 

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Put your finger over the spark plug hole and crank the motor. If the engine has enough compression to blow past your finger, and it will unless there a problem, it will run on that cylinder. Post a pick of the plugs, people on here can tell at a glance if it has an ignition issue. These are easy tests anyone can do with no tools/experience.
 

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Put your finger over the spark plug hole and crank the motor. If the engine has enough compression to blow past your finger, and it will unless there a problem, it will run on that cylinder. Post a pick of the plugs, people on here can tell at a glance if it has an ignition issue. These are easy tests anyone can do with no tools/experience.
I respectfully disagree, I have had engines with 60 psi and will still blow your finger away from the hole, there are times in this trade where you need to invest a few dollars to make the proper diagnosis, backyard diagnosing a misfire will cost you far more in the long (and short run). Every mechanic should have a compression tester as well as a spark tester at the very least. I tell my students diagnose diagnose diagnose before you make an assumption on a failed part, and we all know what the definition of "assume" actually is. I have had cars towed to my shop with new coils, fuel injectors, even a fuel pump and I find a cyl with a burnt valve facing. I ask the customer why and they claim all the cylinders blew compression with the plugs pulled....I say what does that even mean? Show me the numbers.
 

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I have a 66 nova that I acquired from my father in-law 20 years ago, its been subject of a full rebuild including new wiring, trans, engine, paint, interior, paint, etc. Unfortunately it's always had a nagging misfire that I've been unable to chase down.

I suspect there's a lingering electrical issue or perhaps a wiped cam - the engine is Chevy Performance 350 H.O. (with a flat tappet cam, I've been running Royal Purple oil with HPS oil) complete crate motor with GM HEI, I've swapped out the wires (the first set got a little too close to one of the header tubes) with MSD, currently running Holley Sniper EFI. The car has been completely re-wired with an American Auto-wire kit, right now it seems to have a consistent mis-fire on cylinders 1, 7 and 8 - when I hit these cylinders with the infrared thermometer they register around 200 degrees, all other cylinders are around 500 degrees as measured at the header. I've also noticed that the voltage drops significantly when the electric fans starts running (14v down to approx. 10v), to the point I thought it was affecting spark - so far I've found no loose wires, damage, etc. that would point to the fan as the culprit. The engine runs the same regardless if the fan is running or not and idles fine but bogs heavily as soon as you touch the throttle. AFRs as measured by the Sniper display are erratic. I've tried the following:
  • Tested all plug wires for continuity/resistance - all are within spec
  • rebuilt the distributor
  • replaced spark plugs
  • cleaned and checked all grounds - currently have a heavy ground wire from battery to body, then firewall to subframe (in the engine bay - it has a TCI pro touring front clip) and subframe to core support. I also have the braided copper ground straps, one from the firewall to the head on the driver's side and one from the firewall to the block on the passenger side
  • tested the battery - seems fine, its about 2 years old
  • tested the alternator, it puts out about 14v and appears to be functioning correctly
I'm planning to pull the valve covers this weekend and see if the valve train operates correctly for 1, 7 and 8. This project is getting very frustrating - what else should I check?
A broken valve spring can give you a miss, take a close look.
 

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3 bad cylinders with 3 separate mechanical issues? More than likely he has a combination of burnt spark plug wires and/or fouled plugs, easy to diagnose with a pic of the plugs.
I worked for Chevy from 79 till 81 and yes, cams with several wiped lobes on the 305 and 350's were beyond common, so much so that I was the dedicated cam, valve guide guy.
 

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And three years of that was probably far more than enough...
That's an understatement!! But the experience was something I will never forget, I was a very young guy working along-side veteran guys who could do 4 cam swaps in a day, made my head spin. Later on when I was with Toyota (now I'm a veteran) I worked alongside a guy who could do full frame swaps on Tundra's and Tacoma's in under 8 hours, nice to be in your 20's again lol
 

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3 bad cylinders with 3 separate mechanical issues? More than likely he has a combination of burnt spark plug wires and/or fouled plugs, easy to diagnose with a pic of the plugs.
I agree with you. Bad wires will definitely cause a misfire. I had a friend change his transmission even because he thought it was slipping. He ended up having 4 wires not firing. He even had changed Cap, Rotor and coil. Wires had been on the car since he bought it. Never thought they could be bad.
 

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I agree with you. Bad wires will definitely cause a misfire. I had a friend change his transmission even because he thought it was slipping. He ended up having 4 wires not firing. He even had changed Cap, Rotor and coil. Wires had been on the car since he bought it. Never thought they could be bad.
There are probably 50 things (ok slight exaggeration) that will cause a misfire, from bad wire, dist cap, rotor, ignition module, valve spring, burnt valve, cam wiped,hole in piston. The reality is that it comes down to the proper way to actually diagnose it. You know you have 3 cylinders in question, before even pulling those plugs put your spark tester in line between the plug and wire to see if spark is available, if so is the spark nice and blue? If you have no spark ohm the wire, if it's open you have a bad wire, if you have in the neighborhood of 30K or so ohm's resistance the wire is (most likely) not the issue. Once you rule out the ignition move onto a compression test, if it's low squirt oil and do it again, did it come up? If so it's bottom end related if not it's valve train/cam issue. This is a good starting point, just guessing or chucking parts is what's known as "throwing darts" to dealership tech's, not an approved repair method.
 

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"throwing darts" to dealership tech's, not an approved repair method.
... not to mention terribly frustrating and expensive! We've all done it and that's why so many are trying to guide you to the correct diagnostics @TyF The "throwing darts" is shade-tree crap which only makes the mechanic money, it seldom fixes the problem and if it does eventually, you likely won't really know what was the issue in the first place.
 

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Do you know if the 350 crate motor has GM vortec heads?
If so, you need to check that you are using spark plugs that are the correct length for these heads.
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The Vortec head needs a longer reach plug (green arrow). If the incorrect spark plug is installed, the tip of the plug may be recessed into the spark plug port (red arrow)... and cause engine misfires.
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Discussion Starter · #40 ·
Thanks for all the feedback and suggestions guys - sorry for the delay, I was out of town for a few weeks.

I was able to finally check compression as follows:
  1. originally measured 148, then 120 - I wasn't sure if the gauge was reading correctly so I tested this one twice, I added some oil and it still tested at 120
  2. 142
  3. 145
  4. 140
  5. 142
  6. 140
  7. 140
  8. 145
  • I checked spark using an inline tester at all 8 plugs. All produced strong blue/purple spark
  • double checked wiring of sniper, ground and positive both go directly to the battery
  • @RifRaf this engine does have the VORTEC heads. I've been running autolite 103AP which (allegedly) are the direct replacement from the original ACDelco MR43LTS - per the owner's manual
  • I put new plugs in it the other week, went around the block once - it was running terrible, came back and parked it. Pulled the plugs again (attached) looks like my dead cylinders aren't consistent - 1/7/6 this time around....
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