Engine pulsating at low RPMs?
Thread Starter
Melting Slicks
Joined: Jun 2013
Posts: 3,065
Likes: 39
Engine pulsating at low RPMs?
Hello, I have a 1985 Corvette with a Miniram, 58mm throttle body, flat tappet zz409 camshaft, AFR 195 heads, Hedman Longtube 1 5/8" headers, 1.6 roller rockers, true dual Dynomax exhaust, with a bunch of other smaller modifications.
Anyway, it feels like I have a subtle pulsation under 3k RPMs. I also have a subtle idle surge and I think that's because of a small vacuum leak.
If I got WOT it doesn't feel like it is pulsating, if I bring it above 3k RPMs, it doesn't feel like it is pulsating. It feels really strong at WOT, doesn't feel like it is running on 7 cylinders.
Ignition wires were replace 6 months ago, and within the passed 2 months I've replaced the plugs, distributor, coil, ICM, pick up coil, cap, rotor, and have good compression.
All cylinders pass a power balance test.
It feels like a misfire but I don't know if it is.
Could being under a 1/4 tank of gas cause this?
Could a vacuum leak cause this?
Can too much timing cause this?
Is it the camshaft causing this?
Spark knock or anything causing this?
Thanks.
Anyway, it feels like I have a subtle pulsation under 3k RPMs. I also have a subtle idle surge and I think that's because of a small vacuum leak.
If I got WOT it doesn't feel like it is pulsating, if I bring it above 3k RPMs, it doesn't feel like it is pulsating. It feels really strong at WOT, doesn't feel like it is running on 7 cylinders.
Ignition wires were replace 6 months ago, and within the passed 2 months I've replaced the plugs, distributor, coil, ICM, pick up coil, cap, rotor, and have good compression.
All cylinders pass a power balance test.
It feels like a misfire but I don't know if it is.
Could being under a 1/4 tank of gas cause this?
Could a vacuum leak cause this?
Can too much timing cause this?
Is it the camshaft causing this?
Spark knock or anything causing this?
Thanks.
Melting Slicks
Joined: Feb 2005
Posts: 2,592
Likes: 13
From: USA
My '85 had an upgraded cam installed by the previous owner before I bought it in 2002. It often surges at idle when hot and I always thought it was "loping". Never stalled or caused any other issues.
Le Mans Master
Joined: Mar 2004
Posts: 5,107
Likes: 171
From: Dublin CA
I would suggest that as it is at idle you pull one injector plug at a time to see if there is a noticeable difference. If there is a difference by pulling the plug....you should be good on the cylinder. If there is no difference. You probably found found the cylinder in question.
If you find a cylinder that is bad with this test, then check to see if your getting the injector to fire. I believe that a noid light is used to test this.
If you find a cylinder that is bad with this test, then check to see if your getting the injector to fire. I believe that a noid light is used to test this.
Drifting
Joined: May 2013
Posts: 1,736
Likes: 31
From: Reno Nevada
Not enough timing, so try 30 to 34 degrees at idle, and upwards of 2000 as it comes off throttle and vacuum drops. This added timing will help smooth things out.
I found the snipping tool, but can't get it to the screen, or I'd post up a table for you.
I found the snipping tool, but can't get it to the screen, or I'd post up a table for you.
