wondering about idling speed

[Andy Berry]

i have a 1992 740 with I-4 engine. what should be the proper idle rpm? the engine fires right up on the first try, and it drives beautifully, but when sitting and idling, the sound of the engine cranks up and drops off. i havent noticed any change in the tachometer when this happens, but i wonder if it idles too low. it likes to sit at 500 rpm but im thinking it should be 700 or 800 rpm. any ideas?

[billofdurham]

Idle speed should be 775rpm on the non-turbo engine and 750rpm on the turbo engine.

Usually the idle speed drops as the throttle body becomes fouled by contaminants from the crankcase ventilation system.These are droplets of oil which condense on the throttle body leaving a varnish like deposit.

Bill.

[Andy Berry]

good tip, i checked my hose between the air filter and the throttle body. it has some damage and someone in the past wrapped it with duct tape. then the glue melted, that would be the source of that gunky deposit you mentioned. im going to get a new hose for it, but even after that, can i do something to clean out the throttle body?

[billofdurham]

To clean the throttle body:

Disconnect the wiring harness from the throttle switch.
Disconnect the three vacuum lines. Two have spring hose clips which must be compressed to loosen and pull off.
Disconnect the throttle actuator rod. The plastic ends which have the ball and socket attachment on the link rod could break as you remove them from an old or very cold engine. Be very gentle with them and warm them with a hair dryer if it is cold.
Unscrew the three nuts holding the throttle body on to the intake manifold.
Remove the throttle body from the intake manifold. The paper gasket may lift off without problems but if you have to scrape it off use a plastic or wood scraper so you don't to damage the aluminum facing. You can reuse the gasket if it is in good shape but it is better to replace it.
Clean the entire throat and the throttle plate paying particular attention to the edges and pivot points to make sure it can close completely. Use an old tooth brush and spray fuel injection (not carburettor) cleaner. Don't spray the cleaner on the black plastic throttle position switch. Remove the base idle black adjusting knob if so equipped and blow out that passageway and blow out another bypassing passageway that you will see in the throat. If you don't have compressed air, take this base idle boss off and pour solvent in to soak. The internal passage is quite small and engine oil easily plugs it. Clean out the tiny vacuum holes in the fittings that lead to the vacuum tubes which may be completely plugged. If you don't have an air compressor to blow out the passages, you can use one of the cans of air used for blowing dust out of computers.
Optional - clean the outside of the throttle body so it will look nice.
Assemble in reverse of disassembly. The holding nuts go on tight enough to prevent air leaks at the gasket, but don't overdo the torque on the aluminum manifold.
Lubricate the ball-and-socket ends to free them up for next time.

Bill.