rough idle when hot....any ideas?

[tdhardin]

my car is idling pretty rough when hot...
no drivability issues.
Damion
e28m5
e39 wagon sport pak

[Sheldon]

Start with the easiest thing, check your intake boot for cracks.

-Sheldon
140k

[ekdemos]

valves!

[Philo]

Hi Damion... Step one is to make sure the engine is strong. First do a compression test and if all looks good, cylinders within 15% of one another, then do a leak-down test. If the cylinder are sealed then go onto other things like un-metered air entering into the intake, dirty injectors, tune up items like cap, rotor, plugs, etc...

Like Erik..., my car had an ok idle with good compression but it suddenly started idling like total crap, especially when cold. After the car warmed up a bit the idle was ok but still not great. Driving on the freeway was perfect and had plenty of power.

After the compression test I knew what was up.., three burnt exhaust valves.

Do the test..., it's not that hard and will help you determin where to spend time chasing a rough idle.

Philo.
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[Philo]

[Philo]

[damion]

hi folks,
recently i had my mechanical guru look at that hot idle issue, we tested the ohms across the idle vale ...good, found the screw located on the throttle bodies(center area) and smoothed it out there. great results...car idles smooth in all conditions, and the slight increase in rpm (back up to around 800) seems to have help my oil pressure at idle also. I have around 200k on the engine and am holding 18 lbs at idle when at operating tmp ( driven for 1/2 hr at 90mph)
the temp(oil) is at 130 at 90mph, and will increase to about 150-160 if driving around town after being on the interstate. pics of the pressure and temp install shortly...( i know...i said that a week ago)
Damion

[barlowtc]

When was the last valve adjustment?

My E24 had the same symptoms - OK idle when cold but very rough when hot. I decided to do my first valve adjustment - used to be a lazy sort and had the shop do the yearly valve adjustment.

The intakes were all ~0.011" but the exhausts were between 0.008" and 0.009" which is way too tight. I adjusted all the valves to 0.013" (0.012 feeler gauge would fit but the 0.014" wouldn't). Now the engine idles fine cold or hot.

I found out later that the shop would cool the engine with a fan and do the valve adjustment on a warm, not stone-cold, engine - likely the source of too tight valves.

Tight valve adjustment can result in burnt valves.

HTH, Tony 1986 635CSi