‘88 300TE: Rough idle, low idle speed

Occurs mainly after it’s warmed up. Also intermittently bogs down when accelerating from a stop until about 2000 rpm. Its starts up fine warm or cold. The following parts are new: copper plugs, plug wires, cap and rotor, O2 sensor. The following I checked and found to be O.K.: OVP fuse, coolant sensor, coil, crank sensor, EZL resistor, EHA, altitude sensor. It has no codes stored but it did have codes for the EHA and altitude sensor which I cleared and they never came back. The last thing I tried was disconnecting the wire to the idle speed air valve. It seemed to smooth out the idle to an acceptable level even with A/C on and I haven’t been able to get it to bog down on a test drive. Is there a way to test the valve? I’m tempted to leave it disconnected, would it cause any problems? Any other areas to check such as EGR or injectors?

[Michael]

There's an idle control valve in there-replace it. Been there, done it on my '88 TE last year-ish (@ around 160k mi)

Just replaced the idle control valve. No change. Still idles low and rough, still bogs down starting from a stop, and still these problems go away if the wire to the valve is disconnected. I also found out that I can't just leave the wire disconnected since the car won't idle when cold. I guess I could take it on and off everytime I go to drive it, but I'm not up to third world tuning tricks yet.

I think I may have fixed the problem. I tried to adjust the idle mixture screw a few times to see what it would do, and now finally, the car accelerates smoothly from a stop. It still idles a bit low and a little rough but I can live with that, especially when many posts say that this is more or less the norm. Can't wait for a test drive in the morning from cold.

Forgot to mention that if anyone wants a new (used 10 minutes) idle contol valve, let me know. They are $151 at MercedesShop, I'll sell it for $110 including USA shipping.

[jay123]

Exactly where is this idle mixture screw that you've mentioned? Are you referring to the EHA adjustment under the air cleaner?

[Mick J]

There should be a hole in the air cleaner so you can adjust without removing it.

BTW no one has mentioned that all vacuum lines should be checked for leaks.

[dtanesq]

Have your injectors been replaced with the new brass-colored ones?

I have a post in the forum somewhere from awhile ago where I reproduced my mechanic's list of a number of the possible sources of rough idle on these engines. In my 300E, the culprit turned out to be the injectors and, believe it or not, the solution, while it seemed weird, worked to perfection. I had the original design injectors in my engine and it was explained to me that the original design did not permit a good spray pattern with modern gasolines formulated according to rules adopted after my '86 was built. I was advised to run a tank of 87 octane unleaded followed by a bottle of Techron and see what happened. The 87 octane, I was told, burns hotter, which catches up all the fuel in the cylinder, regardless of how it was sprayed in, and means that there is less unburned fuel left for secondary little explosions. The Techron assists in cleaning the injectors. It worked and I was amazed. Before y'all start spamming me, my engine didn't ping or run hot, either.

I think one long-term fix is to replace the original injectors with the new-design ones. My TE has the newer injectors and idles like a brand new Singer sewing machine - one can barely hear it turn over. In fact, when I first got the car, my girlfriend drove it and a couple of times shut it off and restarted it because she didn't think it was running upon initial start-up. When I took the car to my mechanic for pre-purchase inspection one of the techs had a TE in the service bay next to my TE (2-bay area of the garage). Both cars were idling at the same time and yet it was no noisier in there than the air conditioning in my office building.

Good luck. The rough idle issue can be frustrating and annoying.