That's a damn good question..........because I'm of the opinion that the IP cannot draw any fuel.........it requires the pressure from the lift pump to function.

In reality, I don't see how the engine runs at all with any air or space at the top of the secondary as the outlet to the IP is at the very top.

The secondary has a constant bleed to the return lines, and any air (unless the air leak is huge) would move out through this bypass.

How many miles on the car? Seems that lift pump failure is pretty rare....

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

165K.

Really can't be anything else at this point.

What makes you think that the IP can't draw in fuel? It expels fuel and therefore creates a vacuum. Is there a check valve? if not then sure it can't pull in fuel, but I'm guessing there is a check valve present. I'm betting on lift pump.

__________________
For Sale: 1982 MB 300TD
1995 Chevrolet Suburban 6.5TD

Sold: 1980 IH Scout Traveler- Nissan SD33T Diesel

I had a somewhat similar, but more frequent problem. The cause was air getting into the system at the lift pump. Replacing the lift pump crush washer fixed it.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

How did you diagnose that?

Wouldn't the symptoms be identical to a bad pump.........insufficient flow?

This issue could be exactly the same.........if the pump cannot move fluid.

I'll toss my hat into this discussion as I am having an almost opposite problem. I just returned from an 800 mile trip and the following started about halfway through.

After running at > 65 mph for an extended time when I come to a stop the engine will often die and when restarted have no power (floored and it will perhaps get to 5 mph). But after 10 minutes or so then all of the power returns. At highway speeds it has no problem. It is only when I slow down.

When I keep the speed below 65 mph on the highway the problem does not occur. My first thought was fuel tank vent but problem continued with the cap removed. No smoke in any conditions. No abnormal fuel usage. Actually the economy was the best yet due to driving so slow.

Fuel filters are fairly new and I don't think they are a source as the vehicle runs just fine on the highway and when accelerating when the power returns. The same line of thinking applies to air leaks.

The usual generic issues such as IP timing and valve adjustment have all been recently done.

Unless someone has run across a similar situation or has a better idea I plan on trying a high speed run with a marine fuel tank strapped in the engine compartment supplying fuel.

My take on this scenario is a definitive air leak. Here is why:

In my situation, the engine would idle perfectly and run at very low power levels without a problem. I could not get sufficient fuel to the engine. This points to the pump as an air leak would quickly stall the engine at low rpms.

In your case, you've got plenty of fuel...........it's just diluted with air...........and at high rpm's, you don't notice any issues. At low rpm's the air is now a significant portion of the injection charge and the charge is not sufficient to allow the engine to accelerate. Once you slowly get the engine up to higher rpm's, it appears to be fine again........although the air leak remains constant.

Your test with underhood fuel will rule out the lines, but there could be a leak between the pump and the IP.

> Your test with underhood fuel will rule out the lines, but there could be a leak between the pump and the IP.

Good point. How would one test this?

Not easily. Ideally, you'd rig up a vacuum pump and perform tests on the lines. But with the attached banjo fittings, this becomes problematical.

I'm betting on the air leak. Will a new lift pump fix it? Probably, but so might a $1.00 washer or O-ring.

__________________
1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

Roy came over and we figured it out. The pump was sucking air. We took the pump off to inspect it and we couldn't find the crush washer.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

Were you able to see the air in the line from the pump to the secondary?

It's difficult to diagnose air visually due to the very dark lines after 20 years.

Many folks struggle with air...........be good to know how to find it if it's not visible. The Mityvac would do it, but it's not easy to attach to the banjo lines.

Interesting......

If it turns out to be rust in the lift pump, you know what that means about the tank.....

With a 616 / 617, a quick and dirty troubleshooting technique is, pump the hell out of it with the primer pump until it buzzes in pain. If it runs well for a couple minutes after that, good chance it's the lift pump. (Works with the new style primer pump. Wouldn't bet on the old style). Of course, on a 603, that option is not available.

__________________
Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE

Assuming you're refering to the lift pump mounted on the IP, isn't the entire system beyond the lift pump's outlet under pressure during normal operation? If so any leak on that side of the system would be fuel exiting the system rather than air getting into the system.