Looking for input on simplest and reliable way to convert 5/16 hose to 3/8 hose - PeachParts Mercedes-Benz Forum

Texasgeezer · #1 01-30-2018, 09:41 PM

My 95 E300D seems to have 8mm or 5/16 hose for the fuel in the engine compartment. I want to use 3/8 hose because my new metal large prefilter has 3/8 connections. When I looked at new plastic prefilters from Mercedes they seemed to have a very small opening for fuel to flow through.

From my reading the 606 has a much higher fuel flow than the older diesels and smaller openings can create bubbles in the fuel due to suction pressures.

What is the simplest most reliable way to make my 5/16 metal lines take a 3/8 rubber hose?

Some have suggested using a tool to create a double flare. Some have soldered brass ferrules onto the metal lines.

I could use a 5/16 to 3/8 brass barbed fitting but want to keep hose clamps to the minimum. Plus that barbed fitting is not easy to find locally.

Diseasel300 · #2 01-30-2018, 09:53 PM

Wheel reinvention. The hard lines and all the fittings all the way back to the tank are 8mm. Replacing the rubber with 3/8 will do squat. Stick with 8mm or 5/16 and make life easier for yourself. The lift pump on the 603 is the same internally to the 606. 8mm is considered marginal by Bosch, but it works fine on these cars since the orifice in the return side of the IP is so small that they can never develop full flow.

Mxfrank · #3 01-30-2018, 11:04 PM

+1 I find it hard to believe that Mercedes would have designed that problem into their systems. If the plumbing is prone to cavitation, the hard lines are as likely a place as the rubber transitions. And if leaks are the problem, replace your hoses and tighten the clamps.

If this is really a concern, come up with an electric pump that sits under the car close to the fuel tank. This would address either problem: cavitation would be reduced because pressure would be higher at the pump inlet, and air leaks would be reduced because the entire run would be pressurized. And the added benefit is that you would have no priming problem when you change filters.

Texasgeezer · #4 01-31-2018, 07:06 AM

I understand the above,

I want to eliminate the three o-rings related to the pre-filter built-in to the housing. One from the fuel heater, one at the top of the pre-filter and the one at the bottom of the pre-filter.

The plastic pre-filters for 8mm fuel line have a very small opening, smaller than 8mm. That is the flow restriction I'm concerned about. I found a larger capacity pre-filter with 3/8" connections that I want to put in-line before the lift pump. I don't need a fuel heater in Texas and I don't want the o-rings.

I find it difficult to believe Mercedes has done a lot of things that weren't really good engineering. Why have ' marginal ", it's fairly easy not to have that. Why have bio-degradable wiring, especially when there was already a history of failure for that type wire.

I have nothing but good memories of my 78 CD. I'm trying to reduce the future problems with my 95 E300D. Not that much to do, just takes a little time now. Things like removing old grease, lubing slides, using 'better' when available.

I want to have a car I look forward to having and not be concerned about stupid little repeating failures.

My question was on how to convert from 5/16" to 3/8" with minimal clamps and other failure points.

I do regret buying the E300D because of the wiring issues, weak o-rings, evaporator location and a few other armadillos. I didn't do my research because of my previous love affair with the 78cd. I would have sold it fairly quickly but it is low mileage and exceptionally clean. Rear seat had never been used. Front seats covered with sheep skin put on when the car was new. Always garaged. Now I want to make it reliable.

Best car in my history of cars was the 95-99 Olds Aurora. Rock solid smooth good performance up to 200,000 miles then overheating. Just buy one, put on Michelin tires and use Mobil 1, no worries. I owned five of them, better than the four BMW's before that.

JB3 · #5 01-31-2018, 10:38 AM

get a different metal prefilter with 5/16 openings and use the hoses present.

you are creating more leak opportunities with zero benefit otherwise, all because of 3 o rings that are generally a non issue.

funola · #6 01-31-2018, 04:34 PM

The easy and cheap Macgyver way is 1/4 i.d. thin wall PVC tubing, soak it in boiling water to soften it so you can stretch it over the 5/16 barb to build it up to 3/8. Use 2 spring clamps (do not use worm clamps) since the PVC will have different durometer depending on temperature. Spring clamps will self adjust, worm clamps can't.

Texasgeezer · #7 02-01-2018, 04:43 PM

Had to look up durometer. Hopefully what I've done will be long lasting. Luckily the lift pump feed connection was a very close fit inside the new 3/8 Gates Barricade MPI fuel injection hose. I used two hose clamps to get a tight secure fit.

I clipped the plastic line off from the fuel heater to the stock prefilter location and plugged the remaining openings with a little black permatex silicone. I made a stubby connection to the bottom of the fuel heater from 5/16 hose slipped it on and plugged the hose opening with the black silicone.

I used the Gates 5/16 hose to replace the original hardened plastic hose. The little brass in-line valve had fairly smooth connections on both sides, so that was easy to retain. Tried to search for the reason it is in the return flow path. Virtually no explanation for it's use can be found. Must be a good thing to have because different diesel engines use the same part #. Zip tied the hose to keep it away from the serpentine belt and to keep it from rattling.

Will probably zip tie the supply fuel connection ( plastic with o-ring ) to the lift pump. That should help keep it tight if the stock clips ever failed.

I'm thinking about adding a 3/8 brass ball valve to shut off the supply fuel to the lift pump. I would use this to prevent the injection pump from bleeding back to the tank if I ever have to do delivery valves again.

Also thinking about adding a brass T connection after the new ball valve to allow the easy use of the Moly purging liquid. Could also use that connection to feed new diesel fuel to the lift pump to help it prime the injection pump faster. I don't like the excessive cranking required to push all the air out.
Gravity might self prime the pump through that T connection.

Doing this eliminated 3 o-ring connections, the Gates hose is supposed to be good for bio-diesel, the pre-filter is large enough it will probably never need to be changed ( but I'm old enough that could be weeks or a few years ). The area above the injection pump lines is a little more accessible now.

Might have to fiddle with the hoses to get the washer liquid tank back in place.

I used the 3/8 hose primarily because NAPA only had a metal pre-filter in that size and the 5/16 plastic pre-filters had a tiny opening on each side.

I had previously researched an electric lift pump to pressurize the supply lines. I've done this a few times before on chevy, ford, and cat diesels. Very easy and no problems if I used Walbro pumps. I couldn't find the flow requirements for the 606 lift and injection pump. I've read the 606 injection pump requires a much higher flow than the old style diesel cast iron pumps. I don't know if this is to help reduce the temperature, better lubrication, or to help with purging of air from the top of the filter. I was also concerned about the Walbro and other low pressure pumps to keep up with the flow requirements ( since I didn't know them ). Also concerned with having an electric pump that could fail. Only reasonable place to put a larger electric pump would have been the truck area. Didn't want that due to possible diesel fumes or liquid if connections leaked. So I decided to stick with only the stock lift pump and try to make the hoses and connections more reliable than the o-ring method.

JB3 · #8 02-02-2018, 02:51 PM

you want your double hose clamps on opposite sides, so the clamping pressure is even.

That is a clean engine bay!

ykobayashi · #9 02-03-2018, 09:24 AM

Quote:

Originally Posted by JB3

That is a clean engine bay!

Yeah! I almost tipped over in my chair after I opened that. Nice work.

:

Texasgeezer · #10 02-03-2018, 08:12 PM

Thanks for the hose clamp suggestion. I wanted to use the less damaging Mercedes type hose clamps but dealer parts dept said they were not available. Found similar but not as good injection hose clamps that work but still don't form perfect circle.

The engine is so clean because the previous owner seldom used the car. I did power wash it just before I started doing work in that area.

OM617YOTA · #11 02-04-2018, 11:23 AM

Why not go somewhere else and buy a 5/16" fuel filter? This really seems like the long way to grandma's house.

Also, why remove three o rings failure points, only to add in valves and tees, which are additional failure points. Not understanding the logic there.

funola · #12 02-04-2018, 12:25 PM

Quote:

Originally Posted by Texasgeezer

Thanks for the hose clamp suggestion. I wanted to use the less damaging Mercedes type hose clamps but dealer parts dept said they were not available. Found similar but not as good injection hose clamps that work but still don't form perfect circle.

The engine is so clean because the previous owner seldom used the car. I did power wash it just before I started doing work in that area.

These are the constant tension spring clamps I mentioned earlier..

https://www.ebay.com/itm/10Pcs-6-15mm-Spring-Band-Type-Fuel-Vacuum-Hose-Silicone-Pipe-Tube-Clamp-Clip-Hot-/142271891811?var=&hash=item212010c963

I bought 10 of ea size which didn't cost much and has come in handy already.

How long are the sections of 5/16" hose you have replaced with 3/8"? If restriction and bubbles due to the smallish 5/16 hose was the problem, have you thought about replacing it with 3/8 all the way back to the tank? Maybe even going to 1/2".

ROLLGUY · #13 02-04-2018, 01:19 PM

It may be too late, but you could use half of a cigar hose from a 123. The ends are 5/16, and the hose gradually expands to about 1/2".