mhorn

My 1991 300E with 105k miles will catch quickly when started, engine hot or cold. However, as soon as the engine catches and I release the key the motor stalls out. If I give the car gas while it's stumbling it will rev up and then idle perfectly. If I hold the key in the start position for a second or two after the enigne initially fires, it doesn't stall out. This happens with the enigne hot or cold. I can hear the fuel pump whizz for a second or two in the back as soon as the key is turned to the "on" position. My 1992 190E 2.6 would leave the fuel pump whizzing as long as the key was in the "on" position. The fuel pump on the 300E only comes on for a couple of seconds before starting. The car idles at about 1000 rpm when cold until the motor warms up, then idles at about 600-700 when warm. I have replaced the plugs and checked the fuse in the OVP. How do you remove the OVP? I tried to pull it out but it looks like it is attached by something other than the plug. Hard to get to in there behind the battery. Also what is the MAS box with the weird knob on top next to the OVP on the other side of the battery? Other than the inital stumbling after starting the car is perfect. No warning lights or anything come on. Car runs like a dream. Has anybody ever sucessfully fixed this problem?

Sambsamb

Pump fuel relay checked/replaced?

mhorn

I don't know if this car has a fuel pump relay, I looked the part up on the benzbin site and it lloks like their are two kinds. I don't see the black plastic box looking relay in the compartment behind the battery, their is the MAS box with the weird knob on top.
See link
http://mercedes.thebenzbin.com/?year...Field22222.y=8

mine has the MAS box at the bottom, I don't see the part pictured at the top of that page. I did remove the OVP and the car ran poorly without it and the ABS light came on. I tried to take the OVP apart to see if all the connections were properly soldered but it wouldn't come out of the metal housing. Any thoughts?

124-Fan

I would replace the fuel filter. When old or clogged, the pump is longer to achieve the correct pressure in the system, initially. And the 2-3 second sound of the fuel pump is normal.

mhorn

I replaced the fuel filter last night, not a fun job. If anyone is thinking of this as a DIY, I would have gladly paid a repair shop to do this. Gas in the face and axle grease in your hair is not fun, had to wash my hair with GoJo to get that stuff out. New fuel filter doesn't seem to have fixed the problem. I was reading in Stu Ritter's W124 Owner's bible that this hard starting, slow acceleration problem is many times caused by low pressure in the EHA and fuel distributor. He recommends raising the pressure from 4 bar to 4.5 bar to accomidate for wear and leaks. Any one done this? Supposedly it takes a special gauge to do this.

bamberger_1

My long lasting experience with W124 inline 6 is that the rough idle or hard starting are normally very simple things to fix. The only bad part is that it takes quite some time to find the cause. There are so many reeasons which systematically need to br followed one b one. AND DONT START WITH MOST COMPLICATED ONCE. like fuel filter swap ;.(.
The sequence I would suggest is:
1. check the spark plugs and cables
2. change air filter
3. clean the intake assembly
4. clean and grease the accelerator linkage next to the intake, especially check the function of the micro switches there
5. check the function and values of the temperatur sensors on top of the engine
6. check the function of the cold start valve. It shall not leak any gas into the intake at normal temperatures, other wise it is changing the mixture which may lead to rough idle or stall
7. check the sparks distributor in front of the valve drive for worn contacts, oily or wet housing...
8. check for any vacuum leaks and in the vacuum systems
9. ckeck for extra air may enter the intake. Crack in the rubber housing of the intake, cracks in the rubber hoses, also toward the power break canister
10.check the injector seals, by spraying alcohol or brake clean onto the injetor and see if the idle speed changes
11. Check the function and spray pattern of the injectors.
If the OVP goes bad this has way more significant impacts to the function of the engine, you will notice.
The different function of the fúel pumps in the 190 and 300 is most probably that the 190 only has one fuel pump whereas the 300 has 2 and the controlling of the pumps operation may be different. I have a `86 260E and a `90 260E and the fuel pumps are only engaging for some seconds when the key is turned, the same as you describe.
This how I would go step by step before investing in more expensive things. If you have done all of this, as a DIY you pretty much reached the limits.

mhorn

Thanks bamberger, I've pretty much done everyhting on your check list with the exception of greasing the throttle linkage and checking the spray pattern of the injectors. I'm not really sure how to check the injector spray pattern. If I can't fix it my self I may just drive it as is. I don't think it will cause any harm to the engine.

zahkunst

you know what's strange about my car is that it has a tendency to want to start up too quickly when cold. In this case when you release the key it stalls or you have to hold it longer and it bucks a little....however if you turn the key to the position past 2 right before you would push to crank it up and hold it there for about 2 sec, then it starts pretty much without incident.

mhorn

zahkunst, what you've described sounds like you have to wait a couple of seconds for the fuel pump to prime before starting. I've tried doing that, waiting with the iginiton on for a second or two before starting and it doesn't seem to improve consistantly. I think I will just live with the annoyance, I don't think the car is being harmed by this. If anyone has adjusted the pressure on the lower fuel chamber with sucess please let me know. Thanks, Matt.

Gman91932

The MAs is for air conditioning! mine did not work until I bought one from a wrecking yard. Ever since the air is as cool as a cucumber!

zahkunst

eh le suck...oh well. yeah man its proven effective 9/10 starts now that way...I figure i won't even play with pressure unless this method for me stops working.

mhorn

What does the adjuster knob on top of the MAS do? Awfully tempted to mess with it even though I know I shouldn't.

Strider93

To quote from the owners manual:

Cold Engine: Turn key .... Do not depress accelerator. Release key only when the engine is firing REGULARLY.

Hot Engine: Turn key .... Do not depress accelerator. If the engine has not fired after approx. FOUR seconds, depress accelerator and continue cranking until engine is firing regularly. At very high coolant temperatures the engine starting time can be shortened if the accelerator is depressed slowly at the beginning of the starting process.

What I think most people THINKS is a starting problem is caused simply by not holding the key in the start position long enough. I had the same problem when I first got my car a couple of years ago. Always fired right up when cold, but when hot would fire, then die. I learned that if I held the key to start just a couple of seconds longer it would start perfectly every time. Four seconds seems like forever when you are cranking the motor over. Its just the nature of these motors I think, having more mass to start moving and gaining inertia during a startup compared to an I-4 or V-6. I wouldn't go messing with the fuel pressure setting unless I knew for sure that was the problem first.