For about a month or so I didn't start my car (1986 300E W124). I was away on business. It was running fine when I left, when I got back it initially started fine. I went to the bank without any problems. After stopping by 7-11 I started the car and it began to miss. Both at idle and while pressing on the gas. I figured maybe the line got some air so I let it go.

Today the problem persisted. It even died out at a stoplight. It sounded like it was trying to suck some gas. After getting a haircut, I had a hard time starting the car. When it finally started, it missed for a while, then suddenly it cleard up. I made it home without any problems.

I have a very important business to attend to next Sunday. I cannot afford to get stuck. Are these signs of a plugged fuel filter and/or a failing fuel pump?

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JR
1986 Gold 300E 4Dr automatic

hmmmmmm.....

too short of time for gas to go bad......

you got me!

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2006 BMW M5 "Heidi" @ 109,000 miles
2005 MBZ C55 AMG "Lorelai" @ 165,000 miles
1991 MBZ 300E "Benzachino II" @ 165,000 miles
1990 MBZ 500SL "Shoshanna" @ 118,000 miles

(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)

did a little research...I think I'll try changing the fuel filter (might be plugged) and probably replace the fuel pump relay (malfunctioning)

The OVP has be replaced before (to correct a rough idle) so I'm familar with that problem.

This problem is more of a hesitation, lack of gas. I initially suspected the fuel pump, but my dad said the car wouldn't start if it was. The pump either works or don't work.

Can anyone give me their opinions?

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JR
1986 Gold 300E 4Dr automatic

feels like a filter problem. It is a fact that the car will start with a clogged filter! (gas pools aftershutting down) If you need constant gas pedal pressure to keep going and it bucks (or stalls) probably clogged filter. good luck Abe G

yeahm and with that age, I am sure that a $21 filter will benefit alot and still be relatively cheap insurance!

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2006 BMW M5 "Heidi" @ 109,000 miles
2005 MBZ C55 AMG "Lorelai" @ 165,000 miles
1991 MBZ 300E "Benzachino II" @ 165,000 miles
1990 MBZ 500SL "Shoshanna" @ 118,000 miles

(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)

A fuel filter is a good place to start. Needs to be replaced every 30,000 miles or so. The fuel relay is another thing to be looked at. you could remove and bridge it (terminal 30>87 I think) to see if the problem remains. Distributer cap & Rotor as well as spark plug & wires can all do the things you describe. The fuel pump can actually continue working and still not produce enough pressure, so put that too on your check list if it is older than three years. You can also remove the pump and clean it's insides (with care - diesel fuel) by powering it externally and reversing the polarity, thus having it work backwards spewing out crud.
I had to do all the above to fix my car.good luck

If this is a problem that presist at idel, then a fuel pressure test will at least rule out anything in the fuel delivery system.

1) Have correct fuel pressure while engine is running AND MISSING: No fuel delivery problem. Could be air flow sensor on down, ignition or compression.

2) Dont have correct pressure: start with filter first, its cheepest.

John Roncallo

Well , since you've done some of the items that are on the regular - to check first - list, then you are a step ahead in the elimination game already. That's good.
The fuel pump (I have one on my 126) should last much longer than three years though sometimes they don't. Cleaning mine kept it going for another three years. You might want to change the filter first, while at the same time checking the fuel pump relay, both DIY items and follow that with a fuel pressure test. Good Luck

Gidpor: Concerning cleaning of the fuel pump you mentioned, did you just do this on the bench making the pump run backwards using diesel fuel? How long did you let it run?
Thanks
Doug

I've always found that fuel pumps are a binary thing. They operate, or the do not. Never experienced partial operation condition.

I'm not saying it's not possible, just never seen it myself.

You may have developed some condensation water in the fuel, in which case it will clear itself up, or you can hasten with drygas. Found out with the 300E that water in the fuel will be evident after you park nose down but not nose up. Presents as a baffling intermittent problem (like yours).

Got my water from contaminated gas station tanks.

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1986 300E 5-Speed 240k mi.

Spoke with my mechanic over the phone and described the problem.

He said it sounded like a fuel pump problem. While changing the fuel pump, he said I might as well change the fuel filter too. If that doesn't clear it up, he wants me to bring it in.

I asked him about the fuel pump relay. He said it either works or doesn't work. Since my car still starts, he doubts it's the fuel pump relay. I've read different experience in this forum.

Comments?

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JR
1986 Gold 300E 4Dr automatic

Sounds like my 560....It got so it would only do it (not start) when hot. After a fuel filter, new fuel lines, and an accumulator, they now tell me it's the relay. Shuts down the fuel pumps when it gets hot... we'll see.

yes, I ran the pump backwards on a bench a couple times for just a few seconds or so while it was sucking (from the "wrong" side) on some diesel fuel. It immediately spewed ugly stuff out.
I am no expert , but was told that the pump was not binary in behaviour. So I just mentioned that.
The fuel pump relay too is not necessarily a yes or no relay. It can be intermittent because of temp changes. The solder points expand and shrink.
That is why it is good to either jump it or borrow a good relay for a test.

I don't know if they are the same, however the relay for my 75 450 SL was really inexpensive from Fastlane. Since I am having similar problems I also replaced the fuel filter and as a new fuel pump is over $600.00 I didn't want to just try it to be trying it. I have to be able to afford the gas also.

Same deal with relay as for pump. Tends to operate or not. May operate for a few tries, than not operate for a few tries, but I have not seen a partial situation here either.

Could be the filter or the fuel accumulator, or an induction problem, or an electrical problem.

If you do replace the fuel pump (only 1 pump for you), get a Bosch. Do not be tempted to get the 'made in Taiwan' pump that many parts stores stock. Will be loud, and will not last long. Been there.

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1986 300E 5-Speed 240k mi.