The car is a 2000SSEi with 105K on it, it is having bucking/cutting out issues under boost pretty badly.

A bit of background, this car has had the LIM done by me (mechanically inclined) and after I did that it ran good for months.

Lately it has a problem that seems to be worsening, and need some input on what I should be troubleshooting. I finally did get an SES light and pulled codes P0102 and P0171. I cleaned the MAF and even though it seemed to run better the SES light would not clear even though I did so through the scanner.

I went ahead and bought a new MAF and after a few cycles of running and shutting off the light went out. It'* been smogged and passed with flying colors.

Anyhoo, here is what is going on. General driving staying out of the boost the engine will run normally and show no signs of problems. Under boost, as soon as you hit past that 0 on the guage the engine will start cutting out and bucking around. the harder you push it the worse it gets. The guage will show full 7psi of boost even though the engine is choking. And to add it will be at -10 at idle in gear and the gas mileage is normal.

Also, when the engine is colder and the outside air is colder it will not show signs of this issue until the throttle is near or at full throttle and will accelerate normally up until then. When the engine is warm, and outside temps have increased you can barely get into boost and it gets real bad. Even more so when the ac is on, I tried to pass a truck on my way to work tonight and had quite a bit of trouble just getting speed up, until I backed off and stayed out of the boost. It just wouldn't go.

I'm kind of stumped, help

I would start by looking for a vacuum leak. Check the LIM gaskets, the injector O rings, the supercharger gasket, anything you can find.

You might want to take less than 2 minutes and check the contact between the spark plug wires and the coils. Super easy to do- I had same problem and discovered corrosion on the coil contact hidden by the plug wire boot.

Under boost there is positive pressure and therefore no vacuum issue should be possible.

Sounds like you are getting knock retardation.

When was the last plug change/tune up and with what model and name brand of plugs?

Thank you guys,

The plugs were changed with the LIM gasket about 10K ago along with the wires. The plugs are NGK R55'* I believe. I also checked the connections to the plugs and coils and are very clean no corrosion.

I need to add something here that slipped my mind yesterday, not too long ago on the freeway the tach went to 0, the engine started surging when it did this but cleared up and started running normally soon after. At the same time, the bucking under boost went away from what I can tell but the engine would detonate real bad at anything over 3-4 lbs of boost, I stayed out of it so I did not damage the engine. Also it will randomly stall for no reason, not very often though maybe once a week or so. This kind of points me to a crank sensor but I did not know those going bad would cause problems like I am experiencing under boost.

Did not last long though, the next day the tach started working again and the engine went back to having the same problem as before.

Thanks for the help so far

Start looking over the ICM plug as well as the ignition wiring harness. I had a GP here the other week that lost the tach and was running bad. The tach wire broke.