Carbon Buildup on turbo, Car running poorly.
 

While I was headed home earlier I got into boost in 2nd gear, and was quickly greeted with a huge stumble; the car fell on its face. I lost most of my power, and had to nurse the car home (it would enter just enough boost for the BOV to be audible). It would barely idle at stop lights, and it frequently died. It was running very rich.

Upon further inspection, my downpipe has a 3"-4" crack at the elbow where it meets the turbo. It is also tremendously caked with carbon, along with the turbine exhaust housing. I had taken the downpipe off expecting to find oil, since ive had some in the charge pipes, and figured the turbo seals had just bit the dust.

so heres a rundown of the symptoms:

• No power
• missing boost
• stumbles/dies while idling
• running very rich
• 3"-4" crack in downpipe elbow
• massive carbon caked on downpipe/exhaust turbine

Could just the crack in the downpipe be causing these symptoms? Is it the stock O2 sensor reading a lean condition and trying to compensate? Or should I look elsewhere?

Here is a cat picture for your trouble:

https://www.miataturbo.net/attachmen...ine=1360121949

A crack would introduce fresh air into the exhaust stream, which makes the o2 sensor think it's running lean, therefore falsely enrichening the AFR. This would cause excess carbon, which you're seeing, and would be most noticeable at idle where the mix of fresh air would probably be greatest (least backpressure).

Fix the crack, and see what happens from there.

What ECU? Other info? What SJ said might be right, or it might not. Depends.

With an MS, the amount of closed loop adjustment should be limited (I only allow 5% on mine) and would NOT cause these symptoms if the limits are tight.

Resolved the problem today. I had the downpipe welded back up, and upon entering the vehicle to try to start it, I noticed my boost gauge line was knocked off the tee for my megasquirt's map signal.

Im assuming the downpipe has always been leaking, but the rich condition exacerbated the smoking issue. I hooked the vacuum line back up, zip tied it down, and she runs like a champ.

Thanks for the help gentlemen.