E320 CDI Rough Idling on Diesel - PeachParts Mercedes-Benz Forum

na5kar · #1 07-12-2017, 09:59 PM

Hello all. Thanks in advance for any guidance. I've got an interesting story. I have a 2006 E320 CDI with 217,000 miles. A couple years ago, I successfully converted the car to run on Vegetable Oil. It was an elegant conversion and I now have close to 50,000 miles on veggie. It is my wife's daily driver. She drives about 100 miles round trip to work 5 days a week. That's why I did the conversion. We start the car on diesel and switch to veggie after the oil heats up and gets thin enough to spray through the injectors. We switch back to diesel prior to shutting down, and purge the fuel line. I take very good care of the car and go overboard on the maintenance.
Here is the issue I'm having; Recently, I noticed that after switching back to diesel, and stopping in the garage before shutting down, the car idles super rough. I also have a 2001 Ford F-250 (converted to veggie too), and the best way I can describe the E320 rough idle sensation is to compare it to when my F-250 veggie pump blew a fuse. There was no 'lift pump' fuel pressure, and the truck felt like it was a gas V8 engine running on 5 cylinders. So, my guess is that I have a fuel pressure problem in the CDI. I recently changed the fuel filter, and I occasionally put a bottle of Seafoam in the diesel fuel tank when filling it up. Now here is the helpful diagnostic benefit of running two separate fuels....when I idle while switched to veggie, the car is smooth as silk. For reference purpose, the switching junction is just before the diesel fuel filter near the intake. When on diesel, fuel passes through the fuel filter. When on veggie, fuel bypasses the diesel filter and heads to the high pressure pump. There is a separate veggie fuel filter in the trunk near the veggie fuel pump.

So, I am wondering it I have a lift pump in the diesel fuel tank (like the gas W211 cars I see on Youtube), or if there might be another thing to look at. Any help is very appreciated.

engatwork · #2 07-13-2017, 06:13 AM

There should be a pump in the tank.

Murkybenz · #3 07-13-2017, 11:58 AM

Mercedes-Benz W211 Fuel Pump Replacement (2003-2009) E320 | Pelican Parts DIY Maintenance Article

GregMN · #4 07-13-2017, 04:46 PM

I do not know anything about your VO system, so I will just let you know what I have learned from my experience with a 642. The theories should apply to the 648 as well.

Read it and see if you think any of it applies to your situation.
The injector test is easy and cheap.

_________________________________________
CDI Purge Cycle

Short purge cycles on a high pressure common rail system will leave VO in the rail.
Running on cold VO (less then 160 degrees) can cause injector malfunction and poor spray patterns.
This leads to unburnt VO in the cylinder that can turn to carbon and impact the piston rings in their grooves.
Impacted rings allow fuel past the cylinder piston to the crankcase.
That can cause washing down the oil on the cylinder walls and then wear on the unlubricated walls.
Fuel in the crankcase will dilute the oil. VO in the crank case oil... I can only assume is worse than diesel in the crankcase.
Water injection can help remove the carbon and free the rings, but it cannot repair damage to the cylinder walls or engine bearing surfaces.

On an older diesel, say a 603, the unused fuel is returned from the IP and the injectors at about the same rate and a single purge of the combined returned fuel system can purge the system of VO in preparation for the next cold start.
On a high pressure common rail system, the fuel is returned from the high pressure pump and the common rail at very different rates.
Fuel in the common rail is purged via the fuel rail pressure relief valve, and the injectors return fuel lines.
The pump purges relatively quickly in about 12 - 15 seconds.
The common rail purges via the pressure relief valve, and the injectors return fuel lines and takes about 40 seconds.
The fuel enters the rail in the middle. The pressure relief valve is on one end.
The fuel on the opposite end of the rail from the pressure relief valve can only exit via the injectors, either into the combustion chamber, or via the returned fuel line.
It takes a lot longer to purge the common rail. It takes about 40 seconds and then some run time.
If you set up a common purge for the pump and the rail, you will be returning 1/4 to 1/2 gallon of diesel to the VO tank each 40 second purge. You would not be saving much fuel doing that all the time.
So, for a common rail system there should be 2 separate purges, one for the high pressure pump and one for the rail.
I believe that there will always be a small amount of VO at the far end of the rail beyond the last injector after the purge cycle.

If a common purge is used for the pump and the rail and it is too short, you will purge the pump, but not the rail. The next cold start, you will be injecting cold VO. The high pressure injectors will not like that in cold weather. The one farthest back on the rail will fail first. It will fail by having the bleed circuit stick open under the lower pressure of the starting sequence. That will bleed off the pressure from the rail to below the firing point of the injectors (about 3000psi).

To determine purge cycle timing:
Install 2 separate clear sight glasses in the fuel return lines for the high pressure pump and for the rail.
Put a couple of quarts of ATF in a gallon container and run hose from it to the input side of the VO fuel filter.
Run a fully warmed up engine on VO until the fuel in the sight glasses are extremely red with ATF. At idle, this will take a long time because our engines are so dam efficient at idle, but do it. It will be worth the time.
Start your purge cycle and time how long it takes for the fuel in each sight glass to become completely clear of all red ATF.
Understand that the purge cycle will be shorter if you purge while driving, but how much shorter will vary a lot depending on engine speed and load. So to be safe I go with the time to purge at idle.
I use adjustable time delay relays to manage my 2 separate purge valves. They start when the fuel source is switched from VO to diesel.

__________________________________________________
Test your injectors for leaking fuel to the return lines.

Remove the return fuel lines from all the injectors.
Install at least 8" of clear tubing standing straight up.
Run the engine. Watch the amount of returned fuel rise in each tube. If one, or more, of the injectors is returning more fuel than the others over the same time, that injector is bad.

na5kar · #5 07-16-2017, 11:04 AM

Thanks GregMN. That's a great writeup. I will absolutely use it. I don't believe that we are having an injector or engine issue. On veggie, the engine runs great. It's when we are on Diesel that we have the rough running. I believe that I need to look at the point where the two fuels equally touch the engine and work backwards from there. The veggie side is good, but the diesel side has the problem. So, the components to look at are; the diesel fuel filter (brand new), the fuel lines, and likely the lift pump in the fuel tank (if this car has one). Thanks again for your input.

ROLLGUY · #6 07-16-2017, 11:33 AM

It is nice to see someone running WVO in their CDI. However, as much as I like veg, and have ran it in all my Diesels for many years, I can't bring myself to the point of running WVO in my CDI. I did run half a tank of B-100 (the rest D#2), and it did not like it too much. I had to give it a little pedal to get it started when cold. It also stumbled for a few seconds after start, and then smoothed out. If I used the car for long commutes/trips, it might be worth it. I only use the car around town, and occasionally drive it on a trip of 100 miles or more. I believe I have the cleanest and driest WVO that can be had, so that would not be an issue. It also seems that a two tank system would be a plumbing nightmare, and I am not prepared to deal with that. Again, it is great to hear that someone has been successful with their CDI on veg, I am happy for you. It is also interesting reading about the purge process, and how it is accomplished. There may be a chance in my future that I will run veg in a CDI, so I will be following this thread to learn as much as I can. Thanks, Rich

GregMN · #7 07-16-2017, 07:45 PM

Get an Utragauge: UltraGauge OBDII Scan tool & Information Center

You can view the rail pressure in real time. See if it changes when you switch fuel sources.

GregMN · #8 07-16-2017, 09:56 PM

Take a sample of the diesel fuel in your tank. Tap the fuel input line at the high pressure pump. Turn on the ignition. The low pressure lift pump in the tank will run for a few seconds. You may have to cycle the ignition a few times to get a meaningful amount of fuel. Find a tall, narrow, clear, glass bottle ( think of a lab beaker ), pour in the fuel and let it sit for a while and separate. Smell it, feel it, see if you detect any VO in your diesel tank. Running cold VO in the unheated diesel side of your fuel system could cause the symptoms you are having.

chronometers · #9 07-17-2017, 11:04 AM

Quote:

Originally Posted by ROLLGUY

It is nice to see someone running WVO in their CDI. However, as much as I like veg, and have ran it in all my Diesels for many years, I can't bring myself to the point of running WVO in my CDI. I did run half a tank of B-100 (the rest D#2), and it did not like it too much. I had to give it a little pedal to get it started when cold. It also stumbled for a few seconds after start, and then smoothed out. If I used the car for long commutes/trips, it might be worth it. I only use the car around town, and occasionally drive it on a trip of 100 miles or more. I believe I have the cleanest and driest WVO that can be had, so that would not be an issue. It also seems that a two tank system would be a plumbing nightmare, and I am not prepared to deal with that. Again, it is great to hear that someone has been successful with their CDI on veg, I am happy for you. It is also interesting reading about the purge process, and how it is accomplished. There may be a chance in my future that I will run veg in a CDI, so I will be following this thread to learn as much as I can. Thanks, Rich

Agree....CDIs are not as tolerant of conversions as earlier types of diesel engines and because of their closer fuel system tolerances, more suceptible to damage.