losing power (fuel pressure?) on corners - PeachParts Mercedes-Benz Forum

240dddd · #1 11-11-2014, 05:31 PM

My '85 300D loses power on corners, i.e. It behaves as if it is losing fuel pressure.

If I am low on diesel it will actually die coming out of a longer, hard corner, e.g. a long hairpin freeway offramp. I then have to pump up the fuel pressure with the primer pump to get going again.

If I have a relatively full tank it still shudders and drops rpms in some corners, and it seems worst if I am going uphill and make a turn to the left.

I regularly change out the filters, but I have never changed out the in-tank screen filter. I plan to do this next to see if it shows improvement with a new in-tank filter but was wondering if anyone has other suggestions on things I should check?

thanks

Stretch · #2 11-12-2014, 03:24 AM

Tank screen sounds like a good start.

Then I'd consider the fuel pump (if nothing is found in the screen).

barry12345 · #3 11-12-2014, 06:22 PM

Air leak in my past experience on a volkswagon diesel. Only manifested itself on sharp left turns. Engine died out as corner was completed. Other than that car ran fine.

Initially you might look for a small bubble stream coming through the primary filter as well as the constant bubble.

It was a cracked water separator under the rear axel but it did not leak fuel at all. Took me a bit of thinking to look for something like this. Your case should be easier as your car has a primer pump you use to restore fuel and that eliminates any odd ball vacuum issue to the shutoff as the issue.

240dddd · #4 12-04-2014, 02:09 PM

I replaced the screen a few weeks back and didn't find any noticeable problems with the old screen, i.e. it looked old but was still serviceable. No clogging that I could tell.

The problem still continues, primarily on left turns, but as the fuel gets lower (3-4 gallons remaining) in the tank it seems to get worse and is noticeable on both right and left turns. The power loss doesn't really kick in until a few seconds after the end of a turn (unless i'm on a 270 degree turn like some freeway on/off ramps)

I'll go through the fuel lines and see if I can find any noticeable cracks or weak points that might be letting air into the system.

Partscarver · #5 12-04-2014, 03:18 PM

New fuel lines to the primary (clear) filter?
Sounds like air is getting sucked in.
Startron is a good tank/screen cleaner

winmutt · #6 12-04-2014, 04:33 PM

Simple solution, your hoses are backwards....

uberwasser · #7 12-04-2014, 04:55 PM

Ah, interesting point but seems pretty likely.

The return lines do not sit at the bottom of the tank like the feed does, so when cornering they become exposed to air. Worse as the tank empties.

It would be pretty simple to accidentally reverse the lines under the hood during a fuel line change.

winmutt · #8 12-04-2014, 05:20 PM

Quote:

Originally Posted by uberwasser

Ah, interesting point but seems pretty likely.

The return lines do not sit at the bottom of the tank like the feed does, so when cornering they become exposed to air. Worse as the tank empties.

It would be pretty simple to accidentally reverse the lines under the hood during a fuel line change.

Yup, its easy for it to happen and is usually a stranded quick fix for when your tank screen is clogged.

mybenz123 · #9 12-04-2014, 08:06 PM

Could also be a cracked/ripped fuel hose.

Something simular happend to one of our vehicles at
work(retired bluebird bus).
All was well when the road was flat but once the
road went uphill it started to shake, stutter and stall.
One of the fuel hoses from the tank to the fuel pump
was cracked and when going uphill it would leak
and suck in air.

Louis.

Diesel911 · #10 12-04-2014, 09:23 PM

If you shop the Internet you can get a Generic Fuel Supply/Lift Pump rebuild Kit for about $17 each. Costs more if you want a Bosch one.

The Mrecedes Valve Kit for the Fuel Supply/Lift Pump lacks the little O-ring to make it a really viable repair.

Also it is possible for old Fuel Lines to suck Air and not leak Fuel if the cracks are small enough. You have something like 14.5 psi of Atmospheric Pressure around the Hoses and Diesel has some viscosity to it at least more so than Air so sometimes no leaks.

I am not sure if the Fuel Pressure Relief/Overflow Valve can have the same symptoms or not.

Also a lower than spec Idle speed can cause stalling.

240dddd · #11 01-17-2015, 09:26 PM

I'm still trying to sort this one out...

Upon inspection the 'cigar' hose was leaking at the fitting. Replaced but no noticeable improvement.

Replaced the soft hose from the hard line to the pre-filter. The hose was very old and the end connected to the hard line was wet. No improvement to stalling issue in left hand corners.

Found a few of the return lines on the injectors were weeping a bit at the connections. Replaced, no improvement.

I'm planning to replace hand pump next to see if that does anything.

I've also determined that even when the tank is completely full it loses power on left hand turns.

Can anyone tell me how to tell the difference between the two fuel hard lines on the engine compartment end? I'd like to verify that they are not reversed but I'm actually not sure which is which and haven't been able to find that info by searching.

Thanks.

whunter · #12 01-17-2015, 10:45 PM

Quote:

Originally Posted by 240dddd

I'm still trying to sort this one out...

Upon inspection the 'cigar' hose was leaking at the fitting. Replaced but no noticeable improvement.

Replaced the soft hose from the hard line to the pre-filter. The hose was very old and the end connected to the hard line was wet. No improvement to stalling issue in left hand corners.

Found a few of the return lines on the injectors were weeping a bit at the connections. Replaced, no improvement.

I'm planning to replace hand pump next to see if that does anything.

I've also determined that even when the tank is completely full it loses power on left hand turns.

Can anyone tell me how to tell the difference between the two fuel hard lines on the engine compartment end? I'd like to verify that they are not reversed but I'm actually not sure which is which and haven't been able to find that info by searching.

Thanks.

For diagnostic testing, simply reverse (swap) the hoses and test drive it.
If the issue is gone = be happy.

.

Mxfrank · #13 01-18-2015, 12:13 AM

Long shot, but how are your engine mounts? I'm thinking that the engine may be leaning over and pinching a pipe or hose.

vstech · #14 01-18-2015, 12:42 AM

If the problem persists with a full tank, reversed lines are not the problem, and reversing them can make you think it's not fixed because the leaking line will now be on the return... But below 1/2 a tank you will run out of fuel...

240dddd · #15 01-20-2015, 05:29 PM

I replaced the hand pump last night as well as the fuel lines that connect to the lift pump. A previous owner had removed the clear plastic lines and replaced with 8mm hose, however it seemed like the hose they used was a little too big and they had just tightened down the hose clamps as far as they would go. The lift pump was definitely wet so either the hand pump or the hoses were weeping (or possibly worse).

On my drive to work this morning the car ran noticeably better, particularly it seemed to idle better and possibly accelerate a little better. That said, it is still dropping rpm's in corners, particularly left but I've noticed hard right corners have a negative effect as well.

I'm leaning toward replacing the entire lift pump at this point.