W109 dying at idle when hot - PeachParts Mercedes-Benz Forum

kishg · #1 08-12-2022, 10:09 PM

Hey everyone,

i'm having some issues with my 4.5, after i have been driving the car for a some what extended period of time 30-45 minutes+ the car will shut off as soon as it hits idle.

the motor can heat up fine in the first 20 minutes (according to the temp gauge) or so and i will have no issues. after the extended period of time she's still not in the red zone but just starts shutting at red lights. all the traits are very similar to vapor lock on warm start issue, but i have never had this happen while driving.

weird thing is, if i put it in neutral at the light she won't shut off. as soon as i put it back in drive if i don't give it gas she shuts right off.

any thoughts?

thanks

alabbasi · #2 08-12-2022, 10:14 PM

If the engine is under load when it dies, try unscrewing the idle air adjustment screw to see if you can allow it to idle in gear.

Before doing that. I'd check the timing with a timing light (preferably one that has a rev counter built into it) to make sure that it's set up correctly.

Graham · #3 08-15-2022, 07:18 PM

Is the 4.5L 109 an M117 Djet?

If so, this is a common problem. Sometimes caused by mixture being too lean in hot conditions.
- Check the fuel pressure. Book number is about 29 psig. You could increase this a bit to say 32psig. This will give you a slightly richer mixture and reduce vaporization in the injectors that reduces their capacity. This should help with the cut out when hot.
- You could also try insulating the injector fuel rails. This too will help stop vaporization in the injectors.
- Also as Alabassi said, it could just be that the idle speed is set too low. Once the car is well warmed up, adjust the idle screw so that engine idles at about 750-800rpms hot. You may then find that when engine is cold it will surge up and down in rpms. If so, try and find the sweet spot.

kishg · #4 08-15-2022, 11:28 PM

Yes, M117 DJet. Thanks, I was able to adjust the idle screw and the warm idle is stable now but I feel like its a tad high now. I need to get an external tach as the car does not have a tach. Also will check valve adjustment.

Graham · #5 08-16-2022, 09:17 AM

My tach works, but I did buy a laser palm type so I could check rpm from engine bay. Link should be in this post

Some timing lights also read rpm and are useful when setting timing.

Tomguy · #6 08-18-2022, 07:33 PM

What is the temp when "Heated up"? Should be in the middle of the temp gauge. These older cars aren't like modern engines designed to run at 200+ F.

There's a switch that shuts off the vacuum retard at 212 F / 100 C. If timing was set too retarded, the vac retard will make it even worse, and cause it to idle way too slowly. If the switch is faulty and it's above 212, but the vac retard isn't turning off, it may be the culprit.

kishg · #7 08-18-2022, 08:43 PM

Quote:

Originally Posted by Tomguy

What is the temp when "Heated up"? Should be in the middle of the temp gauge. These older cars aren't like modern engines designed to run at 200+ F.

There's a switch that shuts off the vacuum retard at 212 F / 100 C. If timing was set too retarded, the vac retard will make it even worse, and cause it to idle way too slowly. If the switch is faulty and it's above 212, but the vac retard isn't turning off, it may be the culprit.

just at middle. never goes up even when i drive it around for a while. I will check the timing anyway.

Graham · #8 08-18-2022, 10:29 PM

On most Djets, the A/C has the same effect as the 100C sensor. If you turn the A/C on (even if not working) it will over-ride the retard that normally takes place at idle. The valve that controls this is located on a vacuum tube that runs between the throttle body and the distributor vacuum pod.

I can hear the change in idle when I switch A/C on and off. Provides an indication that the valve is functional.

auriga · #9 08-27-2022, 05:49 AM

Still remmbered the day that is 95C outside, I drove my car, then park at a parking lot. My car got a vapor lock, first time ever i see on a FI car.
It took me 13 turns of the key to get the vapor out, so the engine will fire.
I remembered for a period of time, my engine will only fire on some cylinder, guess the injector is vapor locked not the fuel rail.

My car was retarded at 8atdc that time, and it run really hot. I replaced with a aluminum radiator from ebay(exact fit), and advanced the timing to 8btdc. Now even in Tx, my car never overheat.My needle stay at 176 all day, no matter what I do, it wont move over ~185.

I used to afraid of highway traffic, or long red light. now no more!