I have a friend who I'm trying to help with a misfire. It's throwing misfire codes for all cylinders, and random misfire.

The strange part is that it's only at idle and decele. I don't know if he threw it in neutral, or engine braked. It's only done that a few times. It's got a slightly lumpy idle that is intermittent.

I've changed the injectors,plugs, swapped coils and ran a compression check. My compression tester is blowing out at 125psi, so I'm going to swap its seal and try again today. I don't have a fuel pressure adapter for the S so I'm blind there.The freeze frame data looks decent, although I think I need to check the fuel trims one more time and see if I can get the fuel pressure round-of-bout.

The really weird part is what happened yesterday. It would pull to about 6k and then the power would just drop to about 3k and then snap back full power. It did that for a while and then I decided to top the tank off. This fixed it instantly.

Is there some strange way the fuel pump could be going bad at idle, but still give good pressure at power? Maybe the brushes are shot, and the extra voltage is helping, or the full tank is helping it with pickup?

Just a stab in the dark. What have you guys experienced with this problem?

 

When was the last time you had the vlaves adjusted?

 

Hadn't thought about that, but I may give it a shot. Maybe one valve is throwing of the balance.

I think he had two problems which confused the issue. His gas was probably bad, and he added a few fuel treatments to the tank. I think all that junk floated to the top and was causing the miss once his fuel got low.

Now that that is sorted, I cleaned his IACV which helped with the rough idle, but now it just has a slight intermittent shake to it. Nothing scary, but not rock solid like a normal S. Feels like every half second it just doesn't fire right. It's very subtle.

The fuel trims, compression, coils, plugs, map, iacv, and injectors are good, and I'm hoping it's not going to misfire again. We'll see.

 

I'd check the connection to the TPS as well. The issue with pulling to 6k and then dropping to 3k sounds like the plug might be working loose.

 

I checked it first. The live data looked good.

 

i've been having nearly the same problem. multiple misfires only at startup and idle. I replaced the plugs, injectors, and coils. Been 2 days and hasnt come up yet but i think it will

 

13 gallons of rusty milk in the tank. A siphon fixed everything.