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Discussion Starter · #1 ·
Hello All!

I am having some issues with my fuel system and would greatly appreciate any insight and assistance that you could help me with. I have struggled to get the vehicle running properly for awhile now.

I recently had my fuel distributor and warm-up regular rebuilt -also, my fuel pump, accumulator, and injectors have been replaced. The issue that I am currently experiencing is when first starting the car cold it struggles a little to reach 1000 RPM (it starts at 500 RPM for 5 seconds then runs smooth for the initial warm-up). After idling and reaching 80C (takes about 5 minutes) the car is good to go -after driving it and when put in park it idles properly at 750 RPM (however, sometimes when shutting the car off it diesels a little bit and the fuel pump will thump several times (4) then stop).

On the other hand, if I start the vehicle and drive the vehicle before it reaches 80C it lags in acceleration and the air intake plate pops until I press full throttle several times to raise the RPM -then the vehicle runs without the air intake plate popping.

I am wondering if the fuel pump relay or safety switch near the intake could have anything to do with the issue?

I would like to run some tests on the fuel pump relay (as mentioned in the service manual) but am having a hard time finding the exact location -could you assist me with the location and the fuel pump relay part number?

I have read several other threads and someone posted a picture of the fuses and relays with an arrow pointing to the "fuel pump relay", on the other hand someone mentioned that the fuel pump relay had a red/white/black wire going to it...the relay that I found with that wire is just above the fuses under the dash (I was told that it is a lot more difficult to get to?).

Thank you for any help/insight that you might have!

All the best!
 

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Hello All!

I am having some issues with my fuel system and would greatly appreciate any insight and assistance that you could help me with. I have struggled to get the vehicle running properly for awhile now.

I recently had my fuel distributor and warm-up regular rebuilt -also, my fuel pump, accumulator, and injectors have been replaced. The issue that I am currently experiencing is when first starting the car cold it struggles a little to reach 1000 RPM (it starts at 500 RPM for 5 seconds then runs smooth for the initial warm-up). After idling and reaching 80C (takes about 5 minutes) the car is good to go -after driving it and when put in park it idles properly at 750 RPM (however, sometimes when shutting the car off it diesels a little bit and the fuel pump will thump several times (4) then stop).

On the other hand, if I start the vehicle and drive the vehicle before it reaches 80C it lags in acceleration and the air intake plate pops until I press full throttle several times to raise the RPM -then the vehicle runs without the air intake plate popping.

I am wondering if the fuel pump relay or safety switch near the intake could have anything to do with the issue?

I would like to run some tests on the fuel pump relay (as mentioned in the service manual) but am having a hard time finding the exact location -could you assist me with the location and the fuel pump relay part number?

I have read several other threads and someone posted a picture of the fuses and relays with an arrow pointing to the "fuel pump relay", on the other hand someone mentioned that the fuel pump relay had a red/white/black wire going to it...the relay that I found with that wire is just above the fuses under the dash (I was told that it is a lot more difficult to get to?).

Thank you for any help/insight that you might have!

All the best!
I've owned a 1975 450SL for sixteen years and had a problem with fuel tank contamination. The tank screen or other internal material were corroding and would move into my fuel pump. At first the contaminants to the fuel pumps were minimal first, but as it progressed I could actually see material buildup at the pump intake. I installed an inline marine grade fuel filter water separator with a cutoff valve and attached it exterior to and above the spare tire compartment. Your problem sounds very similar. Initially, I would have to replace the fuel filter every 3-4 months because it would hesitate and run like crap. Now when I start to feel any hesitation, I replace the filter.
 

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Hello All!

I am having some issues with my fuel system and would greatly appreciate any insight and assistance that you could help me with. I have struggled to get the vehicle running properly for awhile now.

I recently had my fuel distributor and warm-up regular rebuilt -also, my fuel pump, accumulator, and injectors have been replaced. The issue that I am currently experiencing is when first starting the car cold it struggles a little to reach 1000 RPM (it starts at 500 RPM for 5 seconds then runs smooth for the initial warm-up). After idling and reaching 80C (takes about 5 minutes) the car is good to go -after driving it and when put in park it idles properly at 750 RPM (however, sometimes when shutting the car off it diesels a little bit and the fuel pump will thump several times (4) then stop).

On the other hand, if I start the vehicle and drive the vehicle before it reaches 80C it lags in acceleration and the air intake plate pops until I press full throttle several times to raise the RPM -then the vehicle runs without the air intake plate popping.

I am wondering if the fuel pump relay or safety switch near the intake could have anything to do with the issue?

I would like to run some tests on the fuel pump relay (as mentioned in the service manual) but am having a hard time finding the exact location -could you assist me with the location and the fuel pump relay part number?

I have read several other threads and someone posted a picture of the fuses and relays with an arrow pointing to the "fuel pump relay", on the other hand someone mentioned that the fuel pump relay had a red/white/black wire going to it...the relay that I found with that wire is just above the fuses under the dash (I was told that it is a lot more difficult to get to?).

Thank you for any help/insight that you might have!

All the best!

Loffler,

I was given a 1979 Mercedes 450 SL and in the process of getting it to run. 1st did you ever locate the fuel pump relay on the car? For the life of me, I cannot find the thing. Going through the threads on the internets and anything mentioned about the possible location for the fuel pump relay on the car did not correspond to what I could see in the 1979 model. I am new to the car so there is a steep learning curve. Any help and response are greatly appreciated. Thank You.
 

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Discussion Starter · #5 ·
I've owned a 1975 450SL for sixteen years and had a problem with fuel tank contamination. The tank screen or other internal material were corroding and would move into my fuel pump. At first the contaminants to the fuel pumps were minimal first, but as it progressed I could actually see material buildup at the pump intake. I installed an inline marine grade fuel filter water separator with a cutoff valve and attached it exterior to and above the spare tire compartment. Your problem sounds very similar. Initially, I would have to replace the fuel filter every 3-4 months because it would hesitate and run like crap. Now when I start to feel any hesitation, I replace the filter.
Loffler,

I was given a 1979 Mercedes 450 SL and in the process of getting it to run. 1st did you ever locate the fuel pump relay on the car? For the life of me, I cannot find the thing. Going through the threads on the internets and anything mentioned about the possible location for the fuel pump relay on the car did not correspond to what I could see in the 1979 model. I am new to the car so there is a steep learning curve. Any help and response are greatly appreciated. Thank You.
I've owned a 1975 450SL for sixteen years and had a problem with fuel tank contamination. The tank screen or other internal material were corroding and would move into my fuel pump. At first the contaminants to the fuel pumps were minimal first, but as it progressed I could actually see material buildup at the pump intake. I installed an inline marine grade fuel filter water separator with a cutoff valve and attached it exterior to and above the spare tire compartment. Your problem sounds very similar. Initially, I would have to replace the fuel filter every 3-4 months because it would hesitate and run like crap. Now when I start to feel any hesitation, I replace the filter.
Thank you for the response. I replaced the fuel filter and it didn't fix the issue, unfortunately.
 

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Discussion Starter · #6 ·
Loffler,

I was given a 1979 Mercedes 450 SL and in the process of getting it to run. 1st did you ever locate the fuel pump relay on the car? For the life of me, I cannot find the thing. Going through the threads on the internets and anything mentioned about the possible location for the fuel pump relay on the car did not correspond to what I could see in the 1979 model. I am new to the car so there is a steep learning curve. Any help and response are greatly appreciated. Thank You.
There are cube like fuses on the passenger side floor, to the right. You have to remove the cover and panel around it. The relays are cheap, just replace all of them (I think there are 6?? maybe 4?? can't remember).
 
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