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Discussion Starter · #1 ·
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?
 

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Sounds like a cam issue to me... However, are you sure it's not a complete misfire causing this? Pull each of the suspected plug wires ( one at a time...) at the plug and stick a new plug in, ground it to the block and run the motor through the RPMs and see if there's a spark... That can help eliminate the ignition system to a point, though not perfectly accurate...
 

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Discussion Starter · #8 ·
Good call Andy - I did replace the hei module and tested the coil - all ok.

I have not tested compression, I did pull the valve covers on the odd side, no obvious signs of damage and all valves open as expected.

this is my second sniper, the first was replaced under warranty, though it was outside the warranty period. Holley never said what the issue was with that one… I’ve also swapped out the O2 sensor to confirm both read the same, presumably accurate.

thanks for the suggestions all!

Ty
 

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I'll throw this out there. I had a misfire that was driving me nuts. Started finding burned plug wires at the header. Repeat Header. Turns out that the Headers I have are too close to the Plug Cap and was burning thru, so I ended up with some of those Shorty Racing plugs, which solved the problem. It's only one or 2 that the plug is directly under the exhaust port in this manner if I remember right.
 

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Discussion Starter · #14 ·
I'll look more into the voltage drop, if I can get the car to run long enough for the fan to kick on. It really stumbles and hesitates to run, smells exceptionally rich too...

it had a few weeks where it ran great about a year ago but hasn't been the same since. I don't recall swapping any parts at the same time but it did run good breifly. Since I've built it, the car hasn't consistently run great ever. I pulled the valve cover and visually inspected the offending cylinders, they appear normal, I didn't measure anything though... How would I go about doing that?

The distributor is the factory GM HEI unit, my first thought was a faulty/open ground, confirmed the ground wire is solid and properly attached. I assume grounding is completed from the hold down clamp to the intake, correct?

The headers are TCI tubular units designed to work with their front clip, I had an issue with burnt plug wires early on but have since cleaned that up with some better wire routing and fiberglass insulating boots on the two plugs near the headers.
 

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You would use a dial indicator on the valve retainer to measure total lift. You could also try to cut open the oil filter. If it wiped out cam lobes there would be metal in the filter. If careful you can use a tin snips to cut it open, be careful lots of sharp edges!
 

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TyF

curious if your kit is a TBI or MPI set up? Your injectors can get stuck, especially if it sits and has ethanol gas. Which the gas can be an issue too depending on how much it actually has been driven. I am really leaning into this being digital vs mechanical issue. My Edelbrock pro flo 4 was acting up yesterday, I had to reset all the learned data as it went bad somewhere. I am also not sure how old of a generation your sniper is, mine is 3 years old and has bluetooth to tune or scan.

ultimately guessing as I have gaps in what you have for a setup.
 

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Either way:
Insure proper charging voltage at the alternator.
Check and recheck (and double up on) all your grounds... and there should be several if running aftermarket EFI.
Go over your EFI wiring directions.
Also
Check compression since it sounds like you haven't yet.
Rule out cheap or shitty ignition components. Borrow a buddies HEI distributor if you need to. Mine was only happy with a Dales Unified Ignition distributor.


I ditched the Sniper efi I had. One of the biggest wastes of money I've made on a car. My Sniper had a bad IAC and TPS out of the box.
The "Sniper" branded Bosch O2 sensor also failed within a month.
I returned everything and put a $300 Holley on that engine and it ran perfectly.
 

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TyF

curious if your kit is a TBI or MPI set up? Your injectors can get stuck, especially if it sits and has ethanol gas. Which the gas can be an issue too depending on how much it actually has been driven. I am really leaning into this being digital vs mechanical issue. My Edelbrock pro flo 4 was acting up yesterday, I had to reset all the learned data as it went bad somewhere. I am also not sure how old of a generation your sniper is, mine is 3 years old and has bluetooth to tune or scan.

ultimately guessing as I have gaps in what you have for a setup.
I agree the issue seeems electrial not mechanial.
 
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