I have a 2000 SSEI with 185K on it and in good shape.
Today I was driving down the highway for a good 70 miles with the cruise set to 75. Started to pull a slight grade and as throttle pulled in, engine began to stumble pretty badly. Disabled the cruise and backed off accelerator problem eased. Manually back in on accelerator and stumbling began again and then began to knock. I backed off and babied it for a few miles to ensure no smoke, leaks or temperature rise. Began to pull another slight grade and noticed that boost gauge rose abnormally even with slight increase in throttle. As boost gauge exceeded 7psi into the yellow, the engine would stumble very badly and begin to knock.

Stopped at off ramp and shut engine down and restart to see if engine computer glitch. Same problem existed.

Well, headed back home and babied it all the way. As long as boost gauge did not go positive car would run fine. Got back home let car cool to see if any obvious problem. None found. Took back out for 40 Mile hwy drive and problem did not reappear.

Any suggestions?

Thanks

Check the boost bypass valve. It should move as you move the throttle.

Thanks Dan,

The bypass valve does move when throttle is pressed. Would you see this problem if bypass was stuck or loss of vacuum? I have a GPS with engine monitor function data gathered through the OBD connector. I am able to monitor intake manifold pressure and normally the boost gauge and mainfold pressure on GPS monitor track exactly. When this problem happend the boost gauge read significantly higher than intake pressure. Example. If boost guage was in the yellow above 8psi the intake pressure was only about 1psi.

let me know if that clue helps.

Thanks. I have read many of your post and they have helped with many fixes on the old bonnie. Saved me a ton of money.

It is possible the boost gauge is reading wrong. Is it possible that you can scan for Knock Retard, KR. A stock supercharger will give you about 6 psi of boost. To go over that can happen in two ways. A smaller pulley, or a restriction causing back pressure. A clogged cat could cause that. However, I would look over the coils first. You may be having a misfire problem. Do you get a flashing SES light?

The Pulley on the supercharger is stock. I did not get any flashing SES light. When it started knocking, I expected it to thow a code but didnt. I didnt keep pushing it as to not damage the engine. I did ( at least visually and mechanically) inspect the coil packs wires and plugs. Looking for anything loose, rubbing shorted or such. The problem came on suddenly just cruzin down the highway and I haven't had any such symptoms before that would lead me to think that cat is clogged. The problem went away as quickly as it came. I'll keep pokin on it to see what I can find. if you think of anything, let me know. Thanks again

You can test the fuel pressure. Should be around 48psi. If it'* low it could start knocking because it'* lean.