air bubbles in the inline fuel filter - 79 300SD - PeachParts Mercedes-Benz Forum

bmonk · #1 01-11-2010, 11:14 PM

So when I first bought this car a year ago, the first odd thing I noticed was that it had no inline fuel filter, only the screw on. Regrettably I did not install one until about two months ago. I should mention that I've been having some extremely rough cold starts for the past 3 months, too which i reacted by using some diesel addative with cetane booster, changing the glow plugs, adding the inline fuel filter, an oil change, and a new battery. However if the temp gets close to or below freezing, its still a ***** to start.

So now that I have the inline pre-filter installed, I'm able to see air bubbles rushing in. I know a pocket of air in the filter is expected even after bleeding with the hand pump(I have the new version btw), but I'm seeing a constant rush of bubbles while the engine runs.

So if I understand correctly, fuel is coming from my tank to the inline filter, through that to the fuel pump, and then straight to the injectors, or is it going through the screw on filter before reaching the injectors. It looks to me like the screw on filter is only filtering overflow diesel heading back to the tank via return lines. Am I wrong about that?

Either way, I just want to know if anyone thinks this constant flow of visible air could cause these rough starts in cold weather, and lately, hard idling. And has anyone ever dealt with this. I have searched the forums and it seems most "air in the fuel filter" stories don't talk about seeing an actual flow of bubbles.

kerry · #2 01-11-2010, 11:26 PM

Yes, you are mistaken. The fuel goes thru the secondary filter before going into the injection pump.
It's possible that air in the system could be contributing to hard starts. You could test it by feeding fuel from a soda bottle to the inline filter, bypassing the fuel system upstream and see if you get the same bubbles and hard starts.
Have you tested the glowplug system in addition to putting in new plugs?

bmonk · #3 01-11-2010, 11:46 PM

Well about the secondary filter, I'm glad I was mistaken, since that would mean the lack of the pre filter was giving the inj. pump dirty fuel. Creating a false fuel tank is a great idea. I just got a couple cans of LM diesel purge, so I have to do that anyway.

I did use a multimeter a a while back to test my relay, and all the connections at the plugs, but since then the relay has done some weird things. Like a few months ago, my glow plug indicator light was blinking, and after giving the relay a little smack it clicked on. It did this a few times, but has not happened in a while. It would be a good idea to test the system again.

kerry · #4 01-11-2010, 11:48 PM

A flashing glow plug light after the engine starts on a 79SD is an indicator of at least one bad plug.

bmonk · #5 01-11-2010, 11:55 PM

Aw this was flashing before starting. I would turn the key to engage the glow plugs and it would blink for a few seconds and go dark. But a smack on the relay fixed it each time. When I did finally change my glow plugs a month ago, 2 were basically broken. Must have been Bosch....

Ether · #6 01-12-2010, 08:05 AM

Have you adjusted the valves lately?

Despite having changed out the glow plugs recently you may still want to take a look at them. Here's a good procedure to follow:

http://dieselgiant.com/glowplugrepair.htm

bmonk · #7 01-12-2010, 06:01 PM

I have not adjusted the valves yet, and it certainly needs it. I don't have the spare funds for the special wrenches that everyone seems to say are needed. All the pairs I see range from $80 to $100 which is just too much for such a simple and surely cheap tool. Am I wrong to think I need to spend that much? The dieselgiant glow plug repair is what I followed to check the system 6 months ago, but its time to check again.

On a separate note, its time to change the spin on fuel filter, should I do that before doing a purge to get the best results, then change it again, or do the purge with the old filter, then change it. It seems like if the filter is already really dirty, it might be a good idea to do it before and after the purge...

Also, does the fact that when I remove my fuel cap I get no pressure release mean my tank screen is clean? Or I suppose it might mean my cap seal is bad, cause it is hard to believe the tank screen is clean.

kerry · #8 01-12-2010, 06:38 PM

It's easy to change the spin on filter when purging because you can simply fill it with purge when you put it back on.
It is possible to adjust the valves with standard wrenches or make your own. there are some pictures on here somewhere of homemade ones.
Screen is probably clean. To test if a dirty screen could be causing your air bubbles by increasing the suction in the input line, simply switch the input and return fuel lines and see if your bubbles go away.

toomany MBZ · #9 01-12-2010, 08:22 PM

^ Yeah, on the valve wrenches.
I do a purge first, then install new filters.
If you don't hear anything when you open the fuel cap, especially on a low tank that means the vent is not obstructed, nothing to do with the screen. If you leak fuel out of a closed cap during a left turn, replace the seal. If you're not getting fuel to the engine, check the screen, especially on a low tank.

bmonk · #10 01-12-2010, 08:45 PM

Just tested the glow plugs and all the plugs seem fine. However I'm getting a slightly lower voltage at the relay and plugs than I am at the battery. With nothing on I get about 12.6 at the battery, and about 11.9 with the glow plugs active. But at the relay and plugs I'm getting about 10.8 while active. However my multimeter was showing a neg voltage while touching nothing, so I had to add to these finding to be correct.

Also, there is a part directly lined up with the steering wheel that looks like a vacuum switchover valve or something with a couple vacuum lines and electrical connecting coming from a small metal box screwed into the firewall. I will take a picture of it, but when i engaged the glow plug system, this box kept clicking, and when the relay turned off after about 40 seconds, it began clicking more often, until I turned the keys to the off position. Just wondering what part this is, and is it operating as intended.

It looks like this part, except the #3 connector is not on mine, and mine has a small white plastic cylinder on the face.

http://www.*********.com/images/full/W01331617610OES.JPG

toomany MBZ · #11 01-12-2010, 08:52 PM

That's the switchover valve, I just replied to different thread with pics. "What purpose does the switchover valve serve". The test procedure indicates a clicking sound when it's engaged, to see if it's working. But I don't think it should click as often as you report.

miamirex · #12 01-12-2010, 09:35 PM

Hello, I have driven these diesels for years and many times seen bublles in my inline filter. Is this really a problem? PS, change the spin on filter after the purge.

toomany MBZ · #13 01-13-2010, 11:06 AM

^ No, a bit of air is normal.

Ether · #14 01-13-2010, 11:15 AM

10.8 volts at the glow plugs when the relay is engaged seems about right. You should also expect to see lower voltage at the plugs than at the battery if they are working and drawing current. Seems like pretty typical numbers for a working system.

bmonk · #15 01-13-2010, 06:51 PM

Well I'm going to re-check those numbers today, since my meter was being weird. Turned out to have a bit of moisture in the casing, which after opening and drying overnight, it now reads accurately.

I do have champion plugs in there right now.... I bought a $100 battery the same day, and cheap plugs sounded nice.... I regret that now. I know champion and autolite have horrible failure rates, but has anyone shown them to simply not work as well as say, beru and bosch? I generally try to stay away from bosch these days since they don't seem to make anything in germany anymore. I'd love to give the new wellmans a try. I believe they make a plug that's compatible with my 617.

So I bled the air out of my fuel lines using the hand pump while shaking the crap out of the filter to get the air through the screen and out the spin on vent. There was a very small, normal size bubble in the filter when done. Drove about 5 miles, and saw that more than half the filter was filled with air. So there is a definite leak. I am reluctant to replace the hoses with anything but viton, if it comes to that.