[highrisk]

I installed the Holley Snipper 1 EFI along with the HyperSpark Distributor back in December 2019, Christmas present to myself. Once I got the program figured out it has ran great, way better than the custom built Quadrajet I originally had on it. Then one day last summer when I was heading to a local car show I noticed the RPM occasionally would hang for a few seconds around 1000 to 1500 RPM then drop back to normal. I never hand a chance to drive it again until last week. Now the car will not idle. It constantly surges up to 1000 to 1500 RPM then back to idle, then up again, and then maybe stall. I also noticed the idle AFR dropped from 14.5 to 11.5 and the IAC would vary from 25 to 65 instead of the recommended 2 to 10. After doing some research I found out that what happened last year and this year could be one of three things. The unit is severely out of adjustment, a major vacuum leak, or the IAC valve has failed or is stuck. Unless Gremlins sneaked in one night and messed with the car I doubt it got out of adjustment on its own. There are no vacuum leaks, everything is new and attached where it is supposed to be. That leaves the IAC valve. I ordered one from Summit and it should be here tomorrow. I also found out that the Gen 1 Sniper has a high failure rate with the IAC valve. Once I get a chance to install it I will let everyone know if that solved the problem. If it doesn't I will have to spend some time on the Sniper Forum and figure this out.

Hopefully this might help someone else if they ever have the same issue.

Dale

[SRGN]

You are on the right track. Vacuum leaks can be an issue when they aren't obvious, everything from a stuck PCV to a failing brake booster can throw it off. Have even seen leaks from incorrect valve cover bolts protruding into the intake ports and causing a leak. What are your MAP readings at idle and cruise? Hook up a vacuum gauge and see if the numbers match. There's a lot of charts to correlate inches of vacuum to kPa. You can also try setting it to open loop so there's no correction if you have a failing O2 sensor, it runs off the base fuel map when you do that. It's difficult to diagnose over the internet, so knowing what isn't an issue is going to help narrow things down.

[highrisk]

Originally Posted by SRGN

You are on the right track. Vacuum leaks can be an issue when they aren't obvious, everything from a stuck PCV to a failing brake booster can throw it off. Have even seen leaks from incorrect valve cover bolts protruding into the intake ports and causing a leak. What are your MAP readings at idle and cruise? Hook up a vacuum gauge and see if the numbers match. There's a lot of charts to correlate inches of vacuum to kPa. You can also try setting it to open loop so there's no correction if you have a failing O2 sensor, it runs off the base fuel map when you do that. It's difficult to diagnose over the internet, so knowing what isn't an issue is going to help narrow things down.

Sorry for the delay, for some reason I am not getting notified that someone has posted on a thread I am subscribed to. It ended up being the IAC Valve. Once I replaced it and drove the car to go through a new learning process it settled down and is driving normal again. Didn't think it could be a vacuum leak, everything is new. The car only has 1900 miles since the restoration was completed. But then again, anything can happen.

Thanks for the help,
Dale