I suspect the lock up clutch is engaged when it should not be. Below 45 mph or thereabouts, the lockup clutch should be disengaged. This was a common problem for GM in that era and is easily fixed by replacing the sensor that controls this function.

To test, unplug the connector going to the lockup clutch to see if the slam-bang goes away.

what rpm is your motor idling at in park? 700rpm or so?

It idles around 1800. We have had it looked at by MANY guys. The latest thing is it needs an alternator. G Son is buying that tomorrow and hope that's the problem. Lots of money invested in this car. The first guy said it needed motor mounts. One was $50, the other was almost $300. It is still jerking hard like it did in the beginning.

If the engine is turning 1800 when its put in gear, its going to slam (assuming you have an automatic tranny). That's way too fast for idle. Get that fixed. Upon startup, the engine may run at 1000 to 1200 rpm and then should fall back to 700 to 800 rpm. Wait for the return to norml idle speed before shifting into gear. A cold engine may idle faster than when warmed up.

A worn down cam shaft can cause grief and make the engine controller misbehave. (Causes vacuum to be abnormally low.)

aurora motor is 4.0 northstar. 4 cams. sure, you could have cam wear still. not very likely though. sounds like the IAC issue is not solved. either major vacuum leak or hung up throttle plate? get it to a good mechanic. sure a northstar motor is complex but the intake/throttle parts are no different than a buick 3.8?

rare but it does happen. i have both a caddy northstar and buick 3.8 motor. both use plastic/nylon intakes. but any competent mechanic can fix this. they replaced the idle speed control motor or IAC motor? and the idle is still 1300rpm? they charged you for this "repair" and let you leave with a poorly running car? i would demand a refund.

We have replaced the Idle Speed Actuator and idle positioning sensor, which did fix the idling issue, it is now idling at about 700-800 rpm as you mentioned, for about 1 hour it ran fine, shifted in and out of gear without any troubles. but then started slamming into gear (and not shifting out of 2nd) Service engine soon light has been on this whole time. Haven't checked intake's yet, but had the transmission scanned and the mechanic that scanned it says the transmission is fine. a scan did come up that the alternator was pushing around 16.8volts rather than the average 12.2 that it should be pushing.

The theory at this point is that the alternator pushing too many volts will send the computer into 'haywire'. Haven't placed the new alt. in yet but will let you know how it goes.

i keep going back to the comment about slamming into gear. in park, the motor is at 700rpm? foot on brake, shift into drive. idle does not rev up while you do this? if revs seem normal when you shift into gear, and it still slams, than it sounds like a tranny issue. more so now that it won't shift out of first gear.

yes in park motor is at 700 rpm's...with foot on brake, shift into drive, idle does not change much, maybe drops by 100, yes still slams. problem is...when the car shifts (because it does when it wants too) Everything runs perfect...doesnt slam into gear...and shifts through while accelerating like its supposed too...

When we had the tranny tested at a specialist (transmission shop) they said all the solenoids were working fine...at the time it got tested it was not shifting...

The mechanic then said he reset the computer and the car was back to normal...for about 10 miles...by the time we got it home it started slamming again...he said it was a power problem...which is why we had the alternator rebuilt since previous scans showed it was pushing too high volts...

We just keep going through the motions and spending $, $, $ trying to figure out what the problem is...

If your familiar with aurora we had to disconnect the radiator to get to the alt. to replace it...also noticed idler pulley has play in it...so that is why we havent put the rebuilt alt. in yet...have to literally disconnect the front motor mount and shift the engine around in order to replace the idler pulley..."headaches-r-us" :)

[by the way most of the work is getting done by me (ruthann's grandson) sending the car to shops to be tested then doing the work myself as its a much cheaper route]

appreciate your help :)

the alt drops out the bottom on my seville. there is a small access hatch you unbolt. probably the same for aurora. too late now. i figure car is not running now since alt in not installed? does tranny bang into gear for reverse? or only forward? can you watch motor and notice engine movement when going into gear? you did motor mounts. that would steady motor. are there torque straps on aurora? "dogbones" is what i call them? how amount trans mount? is that ok?

Eureka!!!! We have found it! After a LOT of time and a LOT of money and a LOT of anger the car seems to be running OK now. For about a week. After the rebuilt alternator was installed it started working like a normal car. It cost several hundred dollars to diagnose a $50 problem. One of you said "get a good mechanic" another said to get the "slamming issue" fixed. That's all we wanted in the first place. How do you find a good mech? My husband used to know men that would repair cars from the home and we always had good luck with that. But all those old guys have died off or can't see anymore, so I have to use comm. garages now. What a bummer. They don't always know everything, but they let on that they do. Some of the garages that I called just flat out refused to look at it.

Anyway, thank you a lot for responding and helping out. We all appreciate it very much. Ruth Ann (and grandson)

How do you find a good mechanic?

A lot of good it will do to tell you, you wouldn't support him and his business with all of your service work anyway.