99 Mustang 3.8L misfire at idle, and snap throttle on all cylinders


Misfire at idle 3.8L 99 Mustang can hear a puff every so often sitting at idle.

So I have had this 99 mustang for a while and I decided to try and chase this sound in the exhaust at idle. I have true dual exhaust side pipes with rear 02 eliminators and no cats. At idle I can hear this puff happen every once in a while. I ended up lab scoping the secondary ignition bank 2 since I heard it less on this side. I noticed that ALL cylinders are misfiring. I pulled a plug, and I noticed they were burned pretty bad past 1.0 gap. I replaced all the spark plugs and gaped them to .50 and am still having the misfire on all cylinders. I swapped the coil pack with another mustang I have and it seems to have gotten worse, the spark seems very strong from the coil pack. I am not really good at reading the secondary wave forms, but I can definitely tell there is an issue with all cylinders. I ohmed out the coil pack, and it tested between .9-1.5 ohms at the connector. The waveform looks to me like it could be a fuel issue, I check fuel pressure and KOEO I have 25psi, and running I have 35psi.I have cleaned the MAF, and it did not seem dirty. Short term fuel trim was ranging between -2, and 3 Long term was 4.7, and 6.3 I want some advice on what I should do now before I start spending money on parts. Huge thanks for your time!

Cylinder 3 misfire

Cylinder 3 throttle snap

Cylinder 4 misfire

Cylinder 4 throttle snap

Cylinder 5

Cylinder 6

Comments (2)
No. 1-2

I checked for a blown head gasket, and it seemed fine. I smoked the intake, and there was 2 leaks. One from the EGR gasket, and one on the intake manifold. Huge thanks for your help Scotty!



often a blowing head gasket does that, test for that first. Also check for intake manifold vacuum leaks