Hi all,
1991 940SE Turbo - I am strugling to find what is causing my erratic idle. Car idle badly regrdless of engine temperature. Checked all vacuum lines, could not find any leak. Yesterday checked the compression with screw type gauge. Engine at normal operating temperature. Removed all plugs. Disconnected all injectors. Disconnected positive lead (15) from ignition coil. Disabled main fuel pump. At wide open throttle (WOT) I got the following readings:
Sparkplug # from front of the car to rear 1, 2, 3,4 compression reading 175, 160, 165, 165. Please comment if these numbers shed any light on any issue. If not, should I be contended that engine is perfect and I should be looking at air/fuel/ignition parts to the cause of erratic idle.
Thanks as always.
Ram

Spray WD 40 or carb cleaner lightly around each inlet manifold where it meets the head, if the engine speed increases,it will be an air leak passed the gasket.

Compression is not your problem---those numbers are well within the range of normal (gauges vary quite a bit between different units, so there is not an absolute spec on this) and less than 10% difference between the highest and the lowest, which says to me that your pistons, cylinders, rings, valves, head gasket etc., are all in good shape, and therefore your problem is either fuel or ignition.

By "idling badly" do you mean that the idle speed is not constant or do you mean that there is roughness and vibration at idle. If it's the latter, a possibility worth checking is that the motor mounts have sagged and the oil pan is touching the crossmember. To check for this, go underneath the car and see if you can fit your fingers between the pan and the rubber cushion on the crossmember--there should be about 1/2" of clearance if the mounts are good. This check should be done either with the car on the ground or securely supported with ramps or jack stands on a solid level surface, and with the engine off and cold. You will probably get messy too, as grease and oil tend to collect in that area, so dress appropriately.

Not to be simplistic but have you followed the lead of the FAQ's in this regard? Throttle body clean? IAC? These would be your most likely culprits.

Hi
"Not to be simplistic but have you followed the lead of the FAQ's in this regard? Throttle body clean? IAC?"

Yes, cleaned and adjusted twice per 'FAQ' without any success.
Thanks

The variation in compression that you cite is most certainly NOT the cause of a significantly uneven idle.

The most straightforward way to proceed in diagnosis would be to put the car on an oscilloscope. Then you could see the firing of each plug and see if there is a misfire condition, which it sounds like there is. You would also see whether it is only one cylinder or a random misfiring condition happening on different cylinders.

This would help a lot in locating the problem. If the running is that uneven, there has got to be some some visible misfiring going on.