1997 oldsmobile 88 3.8L v-6 started knocking idles smoothly though. Seems to be coming from under intake manifold

The EGR valve opens when the engine is warming up, which allows exhaust to enter the intake manifold. This is essentially "extra air" coming into the engine. The engine has an "idle speed control" function, which is either a solenoid or small stepper motor connected to a valve that allows a controlled amount of air to "sneak" by the throttle plate in the throttle body. Fundamentally, it is the amount of air entering a fuel injected engine that controls engine RPM. A defective idle speed control ("ISC) motor or solenoid can cause the problem you stated. (I do not know which type is used in your BMW.) Often, a check engine light will illuminate of the engine computer cannot adequately regualte engine idle speed.

Have your idling speed adjusted. If this will not sole the problem, below are reasons why your engine stalls:
1. Defective ignition components
2. Faulty emission components
3. Incorrectly gapped spark plugs
4, Intake manifold or vacuum hoses leaks.
5. Incorrectly adjusted valve clearance.
6. Defective fuel pump
7. Clogged fuel filter or fuel lines
8. Obstruction in the injectors or carburetor
9. Water and other impurities in the fuel

Well, let me just say that you have to remove the intake manifold (the one with 6 metal pipes) to remove the knock sensor. It is not easy to remove the intake manifold on Nissans.

Check wiring harness to fuel injectors where it runs through intake manifold - these chafe through and cause injectors to stay on or off. Also check engine grounds. 2 at back of head near firewall and one at drivers side engine mount (almost impossible to see)

Rough idle is a very common problem on 4 cylinders, but I believe the 98 Regency has a 6 cylinder in it, right?

Sometimes the coil packs get iffy, and sometimes the EGR Valve gets a leak in it causing a rough idle.

When the accelerator pedal is depressed, it can change things so that the engine seems to run smoothly.

However, I would look for codes to see if anything intermittent is occurring in the engine that may need attention.

The OBD-I scanner can help to diagnose the problem, if anything is out of specifications on the engine in question.

The owner will have to find a technician that has such a scanner.

The knock sensor is located under the intake manifold.The starter is also located under the intake manifold.Hope this helps.Good luck.

could be a cracked exhaust manifold or manifold gasket.

very familiar with this. it is down under the intake manifolds in the valley of the engine. it is a single wire connector. you must remove the manifolds to replace this item. parts will run you about $100 and take about 3-6 hours depending on your compentance. parts needed are 1. knock sensor 2. intake(upper and lower plenum gasket)set 3. coolant. make sure you drain your radiator prior to doing this as to not allow a bunch of coolant to drain into the cylinders and cause hydrolocking. if this happens you must remove the spark plugs and rotate the engine to spray out the liquid out of the cylinders then reinstall.

Under the intake manifold. Make sure you get the harness for the knock sensors. and clean out all of the rust and junk you're going to find donw in the holes before and after you replace the sensors.

It's located deep in the valley! Under the "LOWER intake manifold". copy and paste this link in your browser, or try clicking on it. It may be helpful to you.

http://x.nissanhelp.com/forums/pathfinder/7890-3-3l-knock-sensor-relocation.html

I didn't find this link until after I had already disassembled my truck. This is the hardest job that I've taken on thus far, hope I can put it all back together properly. I changed the gaskets on the valve covers since I had everything off.
To answer your question: It's under your lower intake manifold, so it's a hard job!