idle surge
I have a GM 7.4 crate motor with about 5000 miles on it in a 1973 pickup. Edelbrock intake, stock heads, Hooker headers to dual 2.5" pipes. Edelbrock Performer Plus 2162 cam which gives me a duration of 218* and 228* at .050" lift. There is .5" of lift at the valve on both sides. This setup is pulling 12-16 in.Hg at idle, depending on where in the surge it is. I've tried running with both a "1" and a "2" cam setting, and it seems to run smoother at the two. Using a FiTech 30002, but not using the ignition timing feature yet. My intent was to dial it in while still using the vacuum advance on the distributor, then change out the ignition, so I only have one thing to worry about at a time. I'm using a GM temp sender. No EGR, no other computer controls on the engine. I have 48 psi fuel pressure at the throttle body using an inline gauge. The pressure remains constant no matter the throttle input.
Here's the issue: Warm idle, in park, it will start to surge after about 30 seconds. The surge continues getting worse until eventually it will stall out or I'll have to intervene. Putting the trans in drive will solve the problem. I've tried adjusting the IAC to no avail. It surges when the IAC goes to O, in surges when the IAC never goes below 20. It seems the computer is trying to get ahead of it, but just can't.
I have twice gone over and checked for vacuum leaks, but have found none. There is the open port on the passenger's side of the FiTech unit which I was instructed to leave open. In the newer instructions, it says to attach this port to manifold vacuum. Should I do this? The blue wire is shielded as recommended in the electrical section. The throttle body has it's own ground to the battery. TPS is at 0.5 or 0. I adjusted the secondaries as we discussed, they were slightly open, but no significant improvement after the adjustment. I can't really set the IAC due to the surging, so until I clear this up, I'm stuck at step 0.5. Should I keep looking for a vacuum leak, or could it be something else? I can take it to my buddy's house over the weekend and smoke it if need be. Also, should I go ahead and hook up the ignition end of things and do this all together, or is it easier to do one step at a time like I planned?
https://www.dropbox.com/s/0t5joc0mad4n1x5/7.44x4idle.csv?dl=0
https://www.dropbox.com/s/yyu69fv6kmp3ief/7.44x4dash.csv?dl=0
I'm attaching a couple of short logs as requested.
Putting the trans in drive increases your manifold vacuum, so it does seem to be vacuum related. I would recommend what you suggested with the smoke test. Wish I had access to that with the same unit on my wife's '71 vette with it's multitude of vacuum lines. I would also cap off the vacuum advance for now to see if that makes a difference. It's interesting that it gets worse after 30 sec, which is about the time it takes to transition to the next AFR setting.
Are you suggesting capping off the vacuum advance to the distributor or that open nipple on the FiTech, or both?
Oh, for sure capping off the fitech nipple. I don't think capping off the vacuum advance on the distributor does anything.
Once we finally got the top of the throttle body sealed off, we found there were no leaks in the system, anywhere. Once the system was pressurized to about 1psi, it would take about a minute to leak down, and even then, when you opened up any of the vacuum lines, smoke would pour out. The cellophane we used over the throttle blades would actually balloon up when the smoke was applied. We tested the engine two different times, and as surprising as it was, there are no vacuum leaks.
It has worked itself out and held a fairly steady idle twice now. If just let it go on and on for a few minutes (like a full 5 minutes of up and down) and it doesn't stall out, eventually it would hold a somewhat steady 800rpm idle. Still not smooth, though, just without the up and down. Once I put it in drive, then back in "p", though, the whole process starts again. To add to the issue, before it's up to operating temp, when the auto trans shifts from 1 to 2, the motor will almost die out. It's not keeping the rpms up during the shift. It's almost like it thinks my foot came out of the throttle because there's a momentary lack of demand on the motor.
I'm wondering if the IAC loop up and loop down settings may have something to do with the surge? They are currently set at 19 and 22. Other than that, I have no idea what could be causing this, any suggestions would be greatly appreciated.
How plugs look?
They look pretty good to me, I pulled one from each bank and they're consistent. I attached photos. Maybe a little rich? Another odd thing I noticed today: On startup when the engine is cold, it won't surge until it gets hot. Once it hits about 200*, the surging starts, and if I restart it warm, it surges right away. But from a cold start to about 200*, it's fine, The IAC counts will settle in at about 20, slowly dropping to about 8, then it gets warm enough and the up and down starts....from in the 50's to 0, it gets a worse and worse until it will almost stall, then the IAC will go way up, and start wobbling it's way back down. I've tried changing the idle settings from 750 rpm to 900 rpm, doesn't really change the surging.
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The IAC is the symptom not the cause. Usually happens when it is trying to keep running do to AFR issues.
OK, I'll look at the AFR settings and see if I can find something amiss there. Based on the plugs, it's not going to be too lean, so I guess I'm looking for a rich issue, and at a low MAP/RPM range.
Yeah, I'm perplexed about the hot surge. Watch data then. Wonder if fuel is getting hot.