fuel pressure regulator test

[w116john]

hi all

i have a slight problem with warm starting i am losing fuel pressure i think, and 1 of the culprits that comes up as a possible cause is the fuel pressure regulator.

on warm starts the car starts first time but idles at 500 and slowly rises to 750 this can be sluggish and take up to a minute,
it does the same thing on cold starts but that is assisted by the open aav and the csv so starts at 1200 and rises to 14-1500 so is less of a problem, on hot starts it starts bang on at 750rpm

i have tried starting in d for 10 seconds and this does not really improve it so i don't think its the accumulator, i did as was suggested on another thread remove the vac line and check for fuel and there was none, but when i put my thumb on the vac opening there was no change in the rpm should there be?

all the best thanks in advance
john

[jbrasile]

John,

Without doing a pressure test you may end up chasing a ghost. You need to check if you are loosing pressure somewhere. From what you describe you could indeed have a bad accumulator. There is a table that tells you how many psi you may loose over a certain period of time, it is available in the Service Library.

The cold start injector will only operate at an engine block temp below 15C so with a warm engine it will not aid starting.

What year is the car and VIN #? I ask because there is an injector plate sensor installed after a certain VIN # that  you could unplug and see if the car starts when the problem occurs, this sensor prevents the fuel pump from working unless the key is turned to the start position.If you disconnect it the pump will run as soon as you turn the key to position 1 pressurizing the system. On a bad accumulator situation the residual fuel pressure that's supposed to allow the engine to start immediately is not there.

Yes, there is supposed to be vacuum at the line connecting to the pressure regulator, which in the case o K-Jet is actually a pressure damper.

I am assuming you have a k-jet car. If you are running D-Jet, then look at the non-return valve at the fuel pump, it sort of plays the role of the accumulator, keeping the lines pressurized. If it is stuck or faulty, the car will have difficulty starting. The bad news is that the valve was never sold through MB and Bosch seems to have discontinued it...

Tks

Joe

[w116john]

thanks Joe

the car is an early k jet 1976 450se  vin 11603212047677

i know i could be chasing ghosts, which leads to chasing parts i may not need, but i like to have an idea of parts required before going to the mechanic.

i will have the mechanic look at it in a few weeks and do a pressure test, one other question do all fuel pumps prime before starting i cannot hear mine on my previous 116 a late 280 i could hear the fuel pump kicking in.

the car starts first time every time it can just be sluggish when warm

thanks again
john

[jbrasile]

John,

Does it start first time in the morning from dead cold? If so you do not have an accumulator problem.

If the issue occurs only when warm I would suspect vapor lock, a very common problem with early K-Jet equipped cars or perhaps a warm-up regulator that needs a little "tuning". Excess control pressure will cause difficulty starting when warm since the WUR will be fully heated and therefore operating at  its highest pressure setting.

I cannot determine if your car has the sensor on the plate plate by the VIN since EPC lists an engine # cut off not a VIN's. If the pump runs for a couple of seconds as soon as you turn the key you do not have the sensor. In your case it sounds like the pump does not run so you probably do have it. You can tell by removing the air cleaner and looking for a blue connector right at the base of the air flow sensor.

If after the pressure tests everything is in order, you most likely have the vapor lock issue I mentioned earlier. One way to improve things is to install a relay that runs the pump for a couple of seconds "priming" (as you said)  the system. I am not 100% sure but MB may have a kit that was offered at some point to correct this known problem. In 1980 a fuel cooler was added to K-Jet and that eliminated vapor lock for good, I have that set up in my 1980 450SEL that lives in LA, even on the hottest days the car starts immediately.

Tks,

Joe




Hop this helps.

Tks

Joe

[w116john]

thanks joe

the engine no is 117 986 12 003556 and i do have a plue plug and when disconnected i can hear the fuel pump in position 1 but that does not solve the problem, so we can rule out the accumulator probably rule out the fuel pump and check valve, i have a spare wur but its from a later 450 which i could put in and see if it makes any difference

you have been a great help narrowed it down a bit,  you might be able to help with 1 other curiosity, i have a magneto (i think its a magneto ) sticking out the back of my  air sensor plate body  part no 0330 101 028 and 12v writen underneath i have heard on this forum that it plays apart in warm starts mine is not connected to anything.






i cannot find any info about it any where

thanks again

john

[jbrasile]

Hmmm.... that's odd.... never seen one of those before...EPC does not show it either.

Can you take a picture from farther away so I can visualize the location of the part a little better?

Just for fun, try the other WUR and see if it makes any difference. If it does than that is most likely the problem. There is a way to tweak these WUR's as long as the ceramic resistance is intact and the rubber diaphragm in the lower chamber is ok.

Tks

Joe

[w116john]

hi joe

i think i will try the other wur , here is a pic of a 6.9 engine i found and the item can be seen sticking out just behind the fuel distributor

curious, i found the image this page  http://www.m-100.co/6point9/books/since1945/index.html

[jbrasile]

Hi John,

Wow.... I had never seen that before....

Looked everywhere in the EPC and no luck....

If anyone know what that solenoid is and what it does it would be greatly appreciated!

Tks

Joe

[w116john]

well i was just about to replace the wur and decided i would check the accumulator and see if there is fuel leaking from the back and when i pulled the pipe fuel ran out of the accumulator and continued to run out of the pipe going to the fuel pressure damper by the fuel pump.

am i right in thinking fuel should only go 1 way from the accumulator if broken back to the pump so is the pressure damper
broken?   

the acccumulator could be ok but not working properly because it was ful of fuel from the damper???

any assistance greatly appreciated.

john

[beagle2022]

For what it is worth I had bad warm/hot start issues on a 79 kjet 280sel. Changed the fuel accumulator and all fixed.

[w116john]

i took the tube from the damper to the accumulator off and fuel is pouring out, so that has filled the accumulator and there is fuel dripping out of that, i need to replace both.

i have been quoted over 300 pounds for both parts so i will have to search for some cheaper options asap

regards

[beagle2022]

I got a new OEM accumulator on ebay for $100 aussie.

[w116john]

thats where i am as well , accumulator about 100 euro and damper less than 50 on ebay finishing in a few days.

accumulator is new , damper second hand but it looks pretty good.

john

[w116john]

hi all

two questions i need help with,  should there be fuel in the line between the damper cage and the accumulator? i assumed it was a leak line and it was only to let fuel back to the tank in the case of a leak, and one more was there ever a valve on that line? when searching i have seen many different configurations and on the neimoller parts site it looks like there is a valve on that line. i think it might be be a later arrangement.

i definitely need an accumulator and if that hose under normal conditions is full of fuel IE fuel coming from the tank i may not need the damper cage, although the one i have looks as if it will disintegrate and i am probably better of replacing it

regards

[w116john]

hi

a quick update i replaced the accumulator and the fuel pump but to no avail, the accumulator was shot anyway fuel pouring out the back and i had a spare pump so it was no real issue.

the amazing thing is the car starts first time in all conditions but when warm even after 15 minutes ,  rpm will fall straight away to 500 and begin to rise slowly up to 750-800, and when cold it will start at 1200 and rise to 1450 but when hot it is close to normal 700-800 range.

there is no problem with acceleration even when the rpm's are low it revs freely

the aav is new fuel filter is about a year old and the car idles really smoothly even when the rpm's are low

all the best