Can someone explain the fast idle circuit for my '94' 315ci? After the truck has warmed up the idle drops down to around 900rpm but intermittently will run up to 1100 for a couple moments then back to 900rpm. Everything seems to run fine, no Engine lights or anything, I'm just curious.

 

Leak in Vac sys. or bad diaphragm of EGR Valve.

 

EGR, exhaust gas regulator i'm assuming? What does it look like and how does one test it?

 

or iac is bad.

idle air control.

 

check all your sensor plugs. When i changed my pass. side valve cover last year i didnt completly plug one back in so the idle tweaked out. Took forever to find.

 

Bolts into side of air intake w/ 3/4 in. metal tub attached to the bottom, a position sensor attached to the top, a vac hose connected just below that. and below that, a partially encased diaphragm can be seen. Remove the other end of the vac hose connected to the valve from its connection to the solenoid. With the EGR Valve diaphragm in view, suck on the hose until the diaphragm appears to reach its farthest point of travel and block the end of the hose w/ tongue to prevent vac loss. If diaphragm returns to normal position w/o allowing air through hose, the diaphragm is compromised. If diaphragm holds the position, its ok. then check all Vac hoses & connections If connection is loose, cut back hose to fresh rubber. the rubber mounting grommet for the PCV valve tends to shrink & harden over time allowing air to flow around valve as pressure builds. No better time to check PCV flow & function. The next of the best percentage shots as QL2 said is the IAC. I think if this problem is being caused by the IAC, disconnection will have no effect on idle, but if it is functioning it will start to stall. I'm sure he'll know. also if norm.oper.temp. is required. Results of KOEO & KOER scans and it would already be fixed by now.