Pulsing idle, almost stalls coming off gas in P/N, no CEL, new coils and VCGs

[FDR]

I recently did the coils, plugs, and gaskets. I checked each plug well 2 weeks and 200 miles after the repair with no signs of oily boots.

The idle pulses- about 4-5 second cycles where it'll be up at ~800 rpm for 2-3 seconds, drop to 600, rev up to 900, then come down to 800 again for 2-3 seconds, and repeat. If I rev it, it will rev up fine (maybe a short delay though), but when the speed drops, it'll almost stall, rev to 900, come back to 800 and continue the same idle cycle.

I've had a misfire with pulsing 40-50mph, driven it in blinking CEL rough mode, and driven it with a coil completely unplugged, but this idle is not like any of those. Last I checked, it had no pending codes. I thought maybe the idle was set a tiny bit too low, so I held the throttle a hair higher. When I let go after ~10 seconds, the engine stalled because it didn't have enough time to "catch" itself.

Am I looking at the IAC? Clogged cats? TPS? Fuel filter?

I used NGK Iridium plugs with the gap they came with, about .040-.045. I know it's a little high, but my mechanic said he'd just leave them as is since they came with the cardboard tubes.

 

[joegr]

What year is your LS and which engine?

By the way, if you have a V8, the gap should be 1mm (0.039"). Having a gap larger than that is sure to quickly kill the coils.

 

[1LoudLS]

so your "mechanic" knows what the gap should be, knows hat they are too high, but doesn't adjust them because of some cardboard tubes?

 

[FDR]

I went out with my Bluetooth obdii adapter. The pulsing idle was only ranging from 685-710. It just so happens that 700 causes the car to bounce so I guess it's just the right frequency, not a rough idle necessarily. I was really off on my estimate. After about 10 minutes from cold start, the idle dropped to about 650 and the bouncing was gone. If i brought the engine back up to 700, it'd start bouncing again. I tried getting the car to stall again ( by bringing up the rpms a hair and then letting go) but nothing. It'd drop to 550 and sound like it was stalling, but it picked up. Idunno

Plug gap: I put in the Accel coils. I'm think the stock gap for the mustang is .052-.055, so I wasn't worried about the coils.

I did the work myself and was getting free help from my mechanic here and the and using some of his tools, so I didn't want to keep bugging him. The range of gap may be off since it was a month ago and I don't remember exactly, but it's easy enough to pull them to check again next weekend since I left all the covers off

 

[FDR]

Whoops, forgot. 02 V8

 

[LS4me]

Plug gap: I put in the Accel coils. I'm think the stock gap for the mustang is .052-.055, so I wasn't worried about the coils.

Did you read the part where you have to reverse the wiring on the Accel coils????

 

[FDR]

Haha, yes, I reversed them. I've been meaning to post pictures of the clips, actually, because I wasn't able to find any when I was working on it....

The engine runs smoothly on the road and sounds alright. It's really only noticeably rough at idle. It might not even be rough (still slight pulsing and the rpms dip pretty low coming off slight throttle) if it's just a resonant frequency.

Maybe being 10 degrees warmer today made a difference, maybe the car needs to be driven more since i haven't been able to bring it in to get a transmission leak checked. I haven't driven it because Ive been away at college, but I did put 200 miles on it after the coils without problems