Mazda KL Misfire

We've been trying to track down a misfire on a 1996 Mazda MX-6 with a KL engine (V6). All easily accessible vacuum hoses have been replaced (we will do the last 2 tomorrow), and vacuum leaks around gaskets have been exhaustively searched for. The misfire occurs only when pulling with full throttle from below 2000 rpms. Generally a 3rd/4th gear pull from about 35mph.

There are four o2 sensors, one of which has been replaced with a new bosch wideband sensor. All of the sensors are reading sort of wonky (mostly reading rich). The replaced sensor reads zero'd out most of the time, although tonight it would peg upto 1.245V and stay there for a bit, then go back to zero'd out (with occasional bouts of reading within spec).

It throws o2 sensor codes somewhat regularly (for all of them), but it's hard to believe all the o2 sensors are bad (so we've been thinking it's got to be a vacuum leak, but cannot find it). It will also rarely throw a multiple cylinder misfire code, and then also very rarely, an EGR boost sensor code.

We've been fiddling with this car for a week now, with plenty of KL forum searches and are getting frustrated. Any help would be greatly appreciated. Thank you in advance.

***Edit: Also the timing has been set. TPS has been adjusted.

put a set of plugs in it. these cars go through plugs in about 35k miles, in my experience -- my '96 PGT is on its third set in the 80k miles i've had it. the third set went in about 2k miles ago. i like bosch platinums, but any high quality plug that is properly gapped will do you just fine.

also check the molded plastic spark plug end of the wires, where they stick down into the recesses in the valve covers. when the valve cover gaskets leak into those spark plug wells, it hastens the deterioration of the wires. i had a bad misfire that i traced to a crack one of those.

We've given this car a thorough one-over (tend to do servicing in spurts). I guess i should have mentioned it has had a full tune-up, clutch, exhaust manifolds and downpipe, timing belt, ect. The plug wires are the only thing that has not been replaced recently, but they were done within probably 20k miles. The plugs are NGK's and were done ~500 miles ago. Plug wires look fine, but I'll look at the boots one more time.

I keep going back to the O2 sensors, since I can not get a normal reading on my sensor 1's on both banks. What does it mean for these sensors to be reading rich and what sort of things would cause this condition? It especially bothers me that the new bosch sensor is zero'd out most of the time. This problem was present before any work was done on the car, so I don't think we bumped anything while the transmission was out. Any help would be much appreciated. Thanks.

The boost sensor code can be due to vacuum lines that are routed incorrectly. I had that code pop up on my probe before because the lines in that area were just hooked up wrong. Car still ran fine.

My wife's 1993 MX-6 (since sold) developed an intermittent misfire, usually when you were accelerating into traffic. I tried plugs, wires, timing, and O2 sensors before finally just taking it to a mechanic for diagnosis. The problem was a bad distributor. Apparently it wasn't the first they had seen.

There is also a vacuum line attached under the intake that could be loose but not easily visible.