Need help diagnosing..idle too high - PeachParts Mercedes-Benz Forum

robaincltnc · #1 09-20-2006, 05:16 PM

Could someone help me diagnose? The 91 300te is running great but appears to be idling too high. When I had the car checked the local MB tech said it will not hurt anything, and it probably want..but I like things to work as they should.

Idle: no air on 1600rpm
Idle: with ac on 1350rpm
Idle: in gear 1000rpm

I am not very mechanically savvy so keep that in mind with your answer. The tech had the air filter off. He checked to make sure that the throttle cable was OK (and it was). He unhooked something electrical (it looked like it was going into carb area)...when he unhooked the car idled at 800rpm. He then hooked it back and rpm's went up.

Anyone have any ideas?

Thanks!

Gilly · #2 09-20-2006, 05:23 PM

Probably disconnected the idle control valve. It must be getting power since it's "doing" something when it's disconnected. The power comes from the OVP, so we'll assume the OVP is supplying power. The engine control module provides the ground for the idle valve. Possibly this wire is shorted to ground and opening up the idle valve. Or for some reason the engine control unit THINKS the engine needs that much air, OR the idle valve itself is bad, which is what I would suspect #1. If you know what you're doing, I'd disconnect the wire connector from the idle control valve, and then the harness from the engine control module, and see if the ground wire to the computer shows a short to ground, if not I'd really suspect the idle control valve is junk.
Gilly

robaincltnc · #3 09-20-2006, 05:28 PM

Thanks Gilly..is this a part I should buy new or buy used? Is it an expensive part? I'll have it checked when I replace the axle boot and shifter bushings.

Gilly · #4 09-20-2006, 05:52 PM

I'd go new, just my opinion, hey if a "known good" one drops in your lap, what the hey, but myself I'd go new. Probably $50-75?? Not sure, you can look it up here at Mercedesshop, they desrve the business, you get lots of good info from this forum. No financial interest, just my 2 cents.

Gilly

robaincltnc · #5 09-20-2006, 10:13 PM

G..was on my way home from my brothers..we packed it full of supplies as we have a BBQ competition this weekend..that sucker can hold a lot of stuff!

Anyway..out of the blue, the car is idleing fine now..I was at a stop light and I felt it just let go and now is idleing at 750.

Still sound like the idle control valve?

Gilly · #6 09-21-2006, 04:31 AM

Probably be my primary suspect, yeah. I'm not saying that's what it "has" to be, I'd maybe wait until it fails to idle correctly again and test it as I recommended. THEN another thing is you can try tapping on the idle valve to see if it's sticky, not with a hammer, maybe a medium sized wrench, just a few taps to see if that straightens it out. Good luck with the cook-off, sounds like fun, you tow your grille with the Benz?
Grilly, er Gilly

robaincltnc · #7 09-21-2006, 07:27 AM

Grilly,
No, we tow the cooker with my brothers F250diesel. Tow the goods in my car..usually I have been using a clients van (Dale Jarrett Ford) or my Subie Outback. The subaru is just not big enough..the wagon seems to be perfect size. Thanks for the well wishing on the cookoff. My bro and I took #1 in the state in pulled pork in June with the Kansas City BBQ Association. The event this weekend is call Blues Brews and BBQ and is sanctioned by Memphis in May. ITs in Charlotte.
Have a good day and thank you again for your help!
rob

robaincltnc · #8 09-21-2006, 02:31 PM

Also..what gas mileage should I expect on this car?