fuel pressure - PeachParts Mercedes-Benz Forum

Graminal95 · #1 04-01-2009, 03:28 PM

There has been a lot of discussion about the fuel pressure made by the lift pump and its effect on starting, fuel economy and even bearing failure. So I decided to install a fuel pressure gauge that I can monitor while I drive.

Initial reports show that at idle I have ~0.75bar right in the middle of the manuals 0.6-0.8bar min. The interesting thing is that I stretched my spring out last summer ~ 17k miles ago. At that time the pressure at idle was closer to 0.9bar.

If I rev. the engine up to ~3000rpm I get 1 bar above the 0.8bar min.

Now here is the interesting thing. When I go for a spirited drive the fuel pressure drops to 0.75bar at higher rpm and full throttle. I wonder if the lift pump can keep up with demand on the highly modified engines.

I'll update as times goes on.

winmutt · #2 04-01-2009, 03:51 PM

FWIW I think mine is starting to fail. Not able to keep up with demand. Do the cavitate?

Number_Cruncher · #3 04-01-2009, 04:15 PM

>>0.75bar at higher rpm and full throttle

Sounds like your pump's doing just fine.

Are you measuring downstream of the secondary filter?

Graminal95 · #4 04-01-2009, 04:44 PM

It is measured down stream of the filter.

pizzachef · #5 04-01-2009, 05:12 PM

Can you give a little more info, like what kind of gauge you installed?
I want to do the same thing to monitor the fuel filter (mainly the VO filter). I want to put the pressure sensor just after the lift pump. Did you use an electronic or mechanical gauge?

TonyFromWestOz · #6 04-01-2009, 05:46 PM

I have successfully used Turbo Boost gauges as fuel pressure gauges, although if the fittings are not tight, the fuel can travel up the hose and cause needle jitter.
To allow it to monitor the pressure of both fuels (I use separate fuel pumps and filters for my start fuel and veggie), I installed it at the IP inlet.
Dedicated fuel pressure gauges are available on eBay and should be suitable for this application.

Tony

JonL · #7 04-01-2009, 07:30 PM

I'm not usually a fan of idiot lights, but this might be a good application for one. Inexpensive pressure switches are available with a range of opening pressures. It would be very simple to wire up a light that would warn any time the fuel pressure dropped below the opening pressure. Easier installation than a gauge, and safer than running a fuel line to the dashboard or glove box.

TonyFromWestOz · #8 04-01-2009, 07:44 PM

I removed the ashtray & mounted a boost gauge and fuel pressure gauge as well as a couple of 12v sockets on a woodden panel in place of the ashtray. All visible while driving and lights up at night with the dash lights.

Doktor Bert · #9 04-01-2009, 08:49 PM

An associated thread of interest...Robert

My Results timing advanced to 28 degrees 617.952

pawoSD · #10 04-01-2009, 08:51 PM

Interesting....I am going to change my filters tomorrow!

Doktor Bert · #11 04-01-2009, 10:06 PM

I think the oiling diagram may yield some cluse to the rod bearing issues. This version is not that easy to read, but it would appear that the oil is pumped to the rear of the engine first, through the filter and to the #1 rod bearing/main journal last.

IIRC, Pascal's law dictaes that all points in a pressurized system will see the same pressure, but that doesn't take oil starvation, out of round main saddles, etc., into account.

The theory that the #1 cylinder works harder than the others is inaccurate. If all cylinders have equal compression and similar fuel delivery, all will impart the same amount of force to the crankshaft. The position of #1 at the front of the engine doesn't make it any more of less susceptible to damage by reason of it 'seeing more load' than any other cylinder.

However, if it is the last to receive oil, then a reduction in volume could create an issue. My 617.950 never drops below 30 psi...ever...Robert

winmutt · #12 04-01-2009, 10:41 PM

Your 617.950 oil pump or fuel pump never drops below 31psi?

Doktor Bert · #13 04-01-2009, 10:44 PM

Oil pressure....It is always 30 psi hot, at idle in gear and never drops below that figure. The oil jets in the block open around 15 psi, IIRC.

My Dad's 617.952 drops down to around 10-15 psi at hot idle. Since the 617 oil pump is chain driven, as opposed to gear driven, there isn't a wear issue using higher oil pressure. When I rebuilt the engine, I used a new MBZ oil pump and added a custom made, .060" stainless steel shim under the pressure regulator spring. We will be doing the same thing on Dad's 617.952.

I did this on EVERY 617 I worked on at the dealership and have many still in service with good results...Robert

Graminal95 · #14 04-02-2009, 07:04 AM

So what your saying is that the oil pressure reg. is failing at idle, allowing oil to leak out, and is causing low oil pressure at idle.

I was always under the impression that a reg. set at 8bar would be sealed until the pressure was ~ 8bar, not beginning to leak from < 1bar.

I was always under the impression that the reason for lower oil pressure at idle, was from worn bearings and a worn oil pump itself.

I guess I better pull the pan off and add a shim to my oil press. reg to up my oil pressure at idle, its only around 1.9bar.

vox_incognita · #15 04-02-2009, 09:10 AM

I don`t see how shimming the oil pressure relief valve helps raising the hot oil pressure at idle

..unless the spring is reeeeally tired or broken.