I would think it would depend on how bad your leak is. I suppose if it was severe enough it would cause air to come in, but I would not be suprised if a small exhaust leak at the exhaust manifold would be able to keep out static pressure (fresh air coming in) on top of that, there is a positive pressure differeintial on the exhaust, especially before the turbo that would prevent fresh air from blowing in. So i would imagine that unless the leak was fairly severe you would be ok. If it does throw it off some, it would show you running leaner than you actually are, indicating a rich tune, which should be safer than a lean tune. Personally i would spend the money getting the head bolt repaired, than on a wideband. For another frame of refererance, some dyno operators and emmision control testing facilites, just shove a wideband down a tailpipe to get a/f readings (although a little laggy) the reason this works is the positive pressure forcing any fresh air out of the pipes.
Exhaust gasses, especially upstream of the turbo, will pulsate between positive and negative pressures. If you have a leak at all, you are introducing fresh air into the exhaust. This is a reality you won't be able to escape. It's probably exacerbated off-boost, but I wouldn't be surprised if it happens under load as well.
You don't want to play that "are my readings a result of this failure" game. Fix the leak, then use the wideband. Do not try to tune in its present condition!
Why would you accept crappy mileage and an inch of carbon buildup in your combustion chamber because you didn't want to fix a leak? Do it right!
Generally negative pressures are only present at no load, and even then they are marginally negative. Also the mr2 uses a twin-scroll turbocharger which negates a lot of the negative pressures. But like i said, unless the leak is significant, it should not be a problem. Most gen2 mr2s have exhaust manifold leaks, and many of those have been tuned fine. RAGARDLESS i still recommend the repair of the stud.
Yes of course it should be fixed first, which i guess makes our arguement a mute point. An exhaust leak can reduce gas mileage and reduce the life of your o2 sensor, including the expensive wideband you may want to put in there, so get it right first, and then go from there. I find that shortcuts lead to more cost down the road, if not right away!
well there is no way i can afford to fix it...i will have to drop my damn engine to get the stud out, and im at school in another state for the next year..plus this isnt my daily driver. i maybe drive it once a week +/-.
so ill prolly maybe put a couple thousand miles on it this year. im not to worried about it.
after i move back home, i gonna drop it, get all my _____ cleaned up and upgrade upgrade upgrade...ill do it then.
and honestly if i screw my head up, o well. just time to upgrade