350SD low idle problem... drops to 250rpm at idle, A/C shuts off, engine overheats... - PeachParts Mercedes-Benz Forum

Blevinsax · #1 08-09-2013, 07:20 PM

Hello all - long time no see...

For the last few months my 350SD has been idling too low - typically around 250rpm. It forces me to put it in idle with one foot on the accelerator and one on the brake when I have to stop at stoplights in order to keep it at normal idle speed. The low idle also causes one of the fans to stop and the engine will run hot. With the A/C on it starts blowing hot air and the engine really starts to overheat.

A non-diesel mechanic friend looked at it and when he jiggled a few wires/vacuum hoses under the intake manifold it temporarily corrected the problem. He used a zip-tie to pull the cables and hoses up together and the problem was resolved... again temporarily. The low idle is now back and jiggling the wires/hoses doesn't have any effect.

Otherwise the car runs fine - drives fine, no problems.

What could the source of this problem possibly be? I really need to know so I can just tell a mechanic what to do - otherwise they'll charge me thousands of dollars to 'figure it out' themselves and do a bunch of unnecessary stuff. Unfortunately I cannot seem to find a mechanic in Las Vegas who actually knows anything about working on these cars (and won't charge a fortune to do so) so if you know of a mechanic in Las Vegas who ACTUALLY knows what they're doing and cares to do a good job, please forward their information to me.

NJ300sdl · #2 08-09-2013, 09:12 PM

Increase the base idle using the adjustment screw at the rear of the pump if you can not find a broken wire or loose plug. Tach working? May have a bad OVP (overvoltage protection relay).

Blevinsax · #3 08-10-2013, 10:36 AM

Yep, the tach is working fine. The engine is certainly idling too low - it shuts off going around turns from time to time.

Can you help me find the adjustment screw at the base of the pump? When I find it, which way do I turn it to increase the idle? Are there other potential issues/side-effects from simply increasing the idle speed, especially if there is some other issue at play?

Where is the OVP (overvoltage protection relay) you mentioned located? It could be a broken wire/loos plug but I can't say I know well enough - I know quite a bit about my car, but I'm definitely not a mechanic.

Air&Road · #4 08-10-2013, 11:48 AM

Is there coolant in oil or water in coolant? Have you done a compression test?

Blevinsax · #5 08-10-2013, 12:24 PM

Quote:

Originally Posted by Air&Road

Is there coolant in oil or water in coolant? Have you done a compression test?

Nope, no coolant in oil or vice versa. Haven't done a compression test, but the car seems no different than ever, with the exception of the low idle.

At stoplights this condition forces me to put it into neutral and keep my toe on the accelerator to keep it at a proper idle speed, otherwise it idles so low that a fan shuts off and the engine heats up, and if the A/C is on the compressor will shut off and start blowing hot air (non-cooled air.)

Also - the low idle isn't something that came on gradually; it happened all of a sudden. This makes me think it was something that broke or failed all at once.

compu_85 · #6 08-10-2013, 01:29 PM

If you jiggle the 2 pin plug on the end of the injection pump does the idle change? That's the solenoid that the computer uses to maintain idle speed.

-J

Blevinsax · #7 08-10-2013, 03:33 PM

Quote:

Originally Posted by compu_85

If you jiggle the 2 pin plug on the end of the injection pump does the idle change? That's the solenoid that the computer uses to maintain idle speed.

-J

BINGO! That whole plug is broken in half (the passenger side is gone) and one of the wires is free-floating. When I touch the free wire to the left side of the plug the idle returns to normal.

What is the name of that plug/solenoid? Please tell me so I can instruct my mechanic what to fix!! (Aren't mechanics supposed to know this stuff better than me, a saxophone-playing financial advisor??)