Air in Fuel Lines? How to bleed? - PeachParts Mercedes-Benz Forum

#1 06-17-2005, 02:28 PM

I bought an '85 300SD a week ago, very pleased so far. The pre-filter plastic was clouded but I could tell there was muck in it, so just changed both fuel filters.

First, a person with experience told me what to do: clamp off fuel line from tank; replace pre-filter; loosen big bolt holding primary filter; spin off primary filter and replace; tighten big bolt; loosen small bolt securing fuel lines; unclamp fuel line from tank; use primer pump to fill big filter, 'til bubbles don't come out loosened fitting; tighten fitting; start car, and be prepared for rough spots 'til air is worked out of system.

I did all this, and car started right away, got rough, then stopped. Since then, it won't start. I feel reasonably confident that it's an air in fuel system problem, but I hate to just sit there and crank away, draining the battery, if there's anything else I can do or check. I've already been at it for a while.

Primer pump is leaky, that's for sure, but it was leaky before this started and I've got it well screwed-down and it seems tight. That is, it's not leaking NOW. Also, I backed off the primary filter just a bit to confirm that it's now full of fuel.

I've read the post by dpbrowne and TonyFromWestOz's helpful reply, so next step will be to start following the fuel to the IP and then to the injectors, but does anyone have any other suggestion? Many, many thanks!

boneheaddoctor · #2 06-17-2005, 02:50 PM

GO to NAPA...you can get an updated Bosche Primer pump for $20..and get it inside of 24 hours most likely...its not fixed until you do.

and nothing Upstream of that can be eliminated for certain because you know that is bad.

dieselbeagel · #3 06-17-2005, 02:59 PM

Press the gas pedal to the floor. Start cranking and do not take ur foot off the gas pedal until the car starts. Then hold the gas pedal for a couple of seconds at the floor and then slowly release the gas pedal at half depress for about 10 more secs.

The mistake u most probably made was taking ur foot of the gas pedal once the car started.

Anytime u change fuel filters, follow the above procedure because if it dies, like in this case, it takes a bit of cranking for it to fire up again.

Don't ask me how I know !

boneheaddoctor · #4 06-17-2005, 03:05 PM

Good way to toast a starter......the primer pump is cheaper.

If it was leaking while pumping.....odds are extremely high its gonna leak after he gets it started....if it hasn't been started how are you sure its not leaking....see my point.

#5 06-17-2005, 03:18 PM

Good advice on both counts. I'm heading to parts store now for new primer pump and will check results. If no solution, I'll then try pedal technique 'til I get paranoid about my starter motor.

Thanks again. Will posts success/failure report in about two hours.

boneheaddoctor · #6 06-17-2005, 03:30 PM

Quote:

Originally Posted by camcausey

Good advice on both counts. I'm heading to parts store now for new primer pump and will check results. If no solution, I'll then try pedal technique 'til I get paranoid about my starter motor.

Thanks again. Will posts success/failure report in about two hours.

Long cranking of your starter will overheat it..that will dramaticly shorten its lifespan. And trying to prime your system with the starter is a very easy way to do it.

Not being a smarty pants....but sometimes people can be pennywise but pound foolish. Would hate to find your starter goining out on a cold February morning becasue you overheated it in the summer.

#7 06-17-2005, 05:12 PM

I'm back with my new primer pump, but before I've had a chance to install it, I may see a problem...

Large fuel line that leaves the top of the primary filter and goes back, to follow the main line from the tank, is bloated and as I touch it, it oozes diesel fuel from many small cracks. It may have been holding together as long as it wasn't disturbed, but since I've been bumping it with the filter changing, it is really leaking. Could this be reason the engine won't start? Or is it far enough downstream that while a problem, it's not THE problem?

Also, any hints for removing the old primer pump? I'm having a heck of a time fitting any tools down there to get a grip on it. Does the entire lift pump assembly have to come off? Or just the in and out fuel lines?

And thanks, boneheaddoctor, re: being penny wise and pound foolish. I agree, starter motors are big, expensive things and difficult to replace, so I'll go easy on it.

Palangi · #8 06-17-2005, 05:20 PM

It is normal for that hose (fuel return hose) to look bloated. It is shaped that way to even out the fuel pulses in it. It is not normal for it to be leaking. You can temporarily replace it with a piece of regular fuel line if needed.

Disconnect the hose from the prefilter at the top of the fuel pump. The nipple where the hose went is threaded and can be removed. Once you get that out of the way you will have room to swing a wrench and unscrew the primer pump.

#9 06-17-2005, 05:50 PM

Thanks, Palangi, that got me in and the new primer is working fine. Now, when I pump it, I see increased ooze from the cracks in that return line. I don't have any spare fuel line lying around, so it may mean another trip to the parts store to get the correct part.

Now I'm wondering if the leaks in the return line are a fatal problem? That is, would they keep the car from starting? Also, I see wetness around a couple of my braided cloth-covered return lines from the injectors, so I assume they're old and at least one is probably broken where it connects to the injector nipple. Would that also keep the car from starting?

I guess they'll get replaced anyway, as long as I'm going to buy more parts.

You all are giving me good advice, I always love it when I see progress.

#10 06-17-2005, 08:39 PM

The fuel return lines from the injectors and the cigar hose are replaced.

There's so much diesel on the engine, though, I can't tell where any new leaks might be. Time to let it all rest. Also, I notice the engine is cranking just a little bit slower, so I've got a trickle on the battery and will get back to all this in the morning.

boneheaddoctor · #11 06-17-2005, 10:10 PM

Quote:

Originally Posted by camcausey

I'm back with my new primer pump, but before I've had a chance to install it, I may see a problem...

Large fuel line that leaves the top of the primary filter and goes back, to follow the main line from the tank, is bloated and as I touch it, it oozes diesel fuel from many small cracks. It may have been holding together as long as it wasn't disturbed, but since I've been bumping it with the filter changing, it is really leaking. Could this be reason the engine won't start? Or is it far enough downstream that while a problem, it's not THE problem?

Also, any hints for removing the old primer pump? I'm having a heck of a time fitting any tools down there to get a grip on it. Does the entire lift pump assembly have to come off? Or just the in and out fuel lines?

And thanks, boneheaddoctor, re: being penny wise and pound foolish. I agree, starter motors are big, expensive things and difficult to replace, so I'll go easy on it.

The Primer pump comes off with a 7/8 open end wrench....its really metric but this also fits good. No the lift pump doesn not have to come off.

You can use a small adjustible wrench too...

I have a 7/8 wrench cut in half as I needed to do that to get my O2 sensors onto my downpip of my honda and I didn't have a set of Stubby wrenches at the time. (yes my Honda has two O2 sensors...one for each pair of cylinders)

I have not worked on a W126...worst case get a cheap 7/8 and find someone with an abrasive cutoff tool to cut in half.

Its tight on a W123 and a W116 too....

boneheaddoctor · #12 06-17-2005, 10:16 PM

Quote:

Originally Posted by camcausey

The fuel return lines from the injectors and the cigar hose are replaced.

There's so much diesel on the engine, though, I can't tell where any new leaks might be. Time to let it all rest. Also, I notice the engine is cranking just a little bit slower, so I've got a trickle on the battery and will get back to all this in the morning.

Spray it down with simple green or some other cleaner...while its cold and hose it off....never spray cold water on a hot IP......EVER....

When it dries you will have a good idea what you might have missed.

dkveuro · #13 06-17-2005, 10:26 PM

[QUOTE=boneheaddoctor]....................never spray cold water on a hot IP......EVER............[/UNQUOTE]

Just how, exactly, do you warm up the rain where you live ?

_

#14 06-18-2005, 04:30 PM

I slept on it and went back to what TonyFromWestOz wrote in another thread...wish I'd really paid attention yesterday. I loosened the fuel lines at the injectors and determined I was getting no fuel, so went back to check the input to the IP, and then the return line from the IP.

When I pumped the primer with the return line (banjo bolt) loosened, I got fuel but lots and lots of bubbles. I kept at it for a while, 'tiil the bubbles finally disappeared, then secured the bolt.

It only took a few turns after that for the engine to kick in. I kept my foot on the accelerator for a bit, expecting some coughing, but it ran smoothly, and it's going fine now.

I'm glad I got the primer pump, cigar hose, and injector return lines replaced, and grateful to all of you for your advice and help. Thanks a lot!

boneheaddoctor · #15 06-18-2005, 09:52 PM

I need to update my comment earlier which was in error...

The primer Pump is a 17 mm......not a 7/8 which is way too big. Sorry for any inconvienience. And for any people who find this in a future search.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
1983 300SD - No Fuel?	dpbrowne	Diesel Discussion	8	03-06-2005 08:08 PM
94 94 C280 with intermittent air distribution problem	ronin	Tech Help	6	04-29-2003 09:08 AM
steel fuel lines leaking- help?	Dave Larson	Diesel Discussion	1	01-03-2002 11:35 AM
fuel return lines question	TurboTurbine	Tech Help	7	12-23-2001 10:01 PM
Air bubbles in fuel line.	3jocks	Diesel Discussion	3	10-22-2001 11:54 PM