Rough idle, cylinder 4 may not be firing

[streetspirit7]

I've got a 97 240 with a KA24de engine. The car has a rough idle. With the car running I pulled each plug wire individually and noticed a drop in idle on all but #4. I did this same test with the MAF unplugged and noticed a drop on all four cylinders. I did a compression test and had between 170-180 on all four cylinders. Any guess as to what my problem might be?

[Q451990]

Well, if you have good compression it's either spark or fuel. I would put a spare spark plug into the #4 plug wire, ground the bottom of the plug to the valve cover, and look for spark at idle. If you have good spark, I would check the #4 injector resistance with an ohm meter... it should read 12-14 ohms. If it's way high or open, you have a bad injector. It's also possible for an injector to be electrically good, but plugged up - so if it ohms OK and you have spark, you might swap that injector with the one next to it (using new o-rings and bleeding the fuel pressure off first) to see if the problem follows the injector. You can read up on all of this in your factory service manual at www.nicoclub.com/FSM/

Good luck!

Heath