Can air enter the fuel delivery system? - PeachParts Mercedes-Benz Forum

123 300D driver · #1 09-10-2003, 12:59 PM

I have a puzzling problem with my 1979 w123 diesel. After the engine starts, it misses randomly for about 15 seconds and then the missing disappears. Meanwhile, no exhaust smoke is visible meaning a bad glow plug. All parallel glow plugs are working fine. The missing problem does not occur after resarting a hot engine. But it reappears after not running for a couple of hours or so, when the block is still warm to the touch.

It sure seems like there are air bubbles being injected, causing the missing problem. But there is no fuel escaping anyhwere, so where could any air be entering?

Puzzled

Gary

ned2683 · #2 09-10-2003, 08:16 PM

i have had air enter the purge pump thing. what happened was it will miss and eventually my car would stall. it went away after i wrapped it with pipe tape - i still need to replace it.

psfred · #3 09-10-2003, 09:07 PM

Air will enter through a bad hand pump (the red handled screw down types usually) and through bad suction side hoses, the ones where the clear filter is. Check for the presence of diesel fuel on these hoses -- if they are wet, they are leaking, and you will get air sucked in when the pump pulls fuel up from the tank. Old style fabric covered ones are the worst, since the rubber can go to Perdition and you cannot see it through the fabric.

A bad hand pump will spill fuel when you attempt to use it. Replace with the new style "thumb" pump with internal spring. Doesn't screw down and never leaks.

Clogged fuel filters will also cause trouble. Change with the engine hot, it cranks better.

Peter

Wasuchi · #4 09-10-2003, 10:48 PM

Can an injector leak down into the prechamber/cylinder area, let the fuel out basically? I have the same symptoms after sitting for over 18 hours and that was my guess. I don't have any leaks and it goes away after a few seconds or a blip of the accelerator pedal. I have the new primer pump also.

psfred · #5 09-11-2003, 07:38 PM

Yes, injectors can leak down, making for hard starts. So can the plungers in the IP, (you will have diesel fuel in the oil) and delivery valve holder seals can leak, causing knock and poor performance at low rpm.

Peter

123 300D driver · #6 09-16-2003, 06:27 PM

psfred,

I changed up the primer pump with a new "old style" pump and it made no difference to the missing problem. I guess that leaves the possibilty of the supply line from the tank allowing air in somehow. Curiously, no fuel is leaking anywhere, no fuel smell or driveability problems after running the engine for 15 sec or so.

This missing problem is not really a big deal, but I am just curious and determined to find out the source.

Gary

madsen · #7 09-16-2003, 08:05 PM

you may also need to try the glow plug relay
i had the same problem on my 300tdt until i changed the relay to the new style that stays on after you start the car. the brief miss at start up went away when i put it in .
just to check i put the old one back miss came back .. you might want to wait until it gets a little colder to see if the miss lasts a little longer when cold that would then tell you that it was cold related. just my thoughts

JOHN M

psfred · #8 09-16-2003, 08:09 PM

Gary:

Check the hoses from the fuel supply line to the in-line filter and IP lift pump. On the W123, you can get mild suction back to the tank with the engine off, so fuel won't run out, it runs back to the tank. The hoses will be damp with fuel, and show cracks when you flex them.

You could also have a tired IP -- the fuel tends to leak down from the plungers while sitting. This will give you fuel in the oil, though.....

I'd also check the injectors if the problem isn't the fuel lines. They can leak, and it will take a while for them to re-pressurize on cold startup.

Peter

P.E.Haiges · #9 10-14-2003, 07:15 PM

300D driver,

Have you adjusted the valves recently?

Have you done a compression test lately? You may have a cylinder that is low on compression and won't fire until it gets warm.

P E H

123 300D driver · #10 10-15-2003, 06:19 PM

PEH, I am inclined to go with your idea about the compressioin. I noticed that the missing problem is more-related to engine temperature than on how-long-it's-been since the engine was running. On my first post I said that the missing contiued for 15 seconds after cold (+50 deg F) start-up. Now this morning @ +14 deg F, the missing problem persisted much longer than on a "warmer" morning start. I think it smokes a bit more when cold and "missing", meaning the fuel is getting there, just not igniting when cold. It misses once about every 3 or 4 power srtokes on average, so likely only 1 cylinder is the culprit for now anyway.

I'm going to skip the compression test and just go ahead and adjust the valves. It has been 25K mi since I did the last adjustment and never did check for timing chain stretch. I am going to leave this repair until spring 2004. I will only be driving the MB until the road salt season begins in few weeks. I will reply back to this thread at that time on the results.

P.E.Haiges · #11 10-16-2003, 11:56 AM

300D Driver.

I forgot to mention that there is an air bleed hole in the top of the fuel filter. This purges all the air from the system before it gets to the injection pump. And even if some does get to the injection pump, the pressure relief valve is at the top of the injection pump so it will also be purged there also. So you have to be sucking a lot of air before it causes a missfire.

Another reason I thought your misfire might be caused by low compression is that you didn't mention any knocking noises. Low compression equals no ignition , therefore no knock. Usually a bad injector that leaks will cause a knock because the fuel is injected too early.

P E H

#12 10-16-2003, 04:35 PM

A cold sensitive injector (they DO exist) will behave as you've described (test it after it's sat in a freezer overnight) as will an injector if the spray pattern is a little off so that atomization is less than entirely sufficient, or injection pressure is out of range or you have a marginal delivery valve seal (rthese all usually make noise). A glow plug with slightly higher than normal cold resistance or a misshapen tip can do it too as can some uneven carbon distribution in the prechamber. When bad enough, all of these problems will prodcue visible smoke, but early on there may not be detectable smoke. If the 10 psi valve in the return line fails open, sometimes fuel levels might drop a bit and you could suck air, but that wouldn't be confined to when the engine was cold. If the hand pump leaks fuel when you pump it - air can enter the system when it sits for a while and this will occur only after the engine sits for a while so is cold although the temp has little to do with the air. If the hand pump leaks at ALL - replace it - new ones are only around $20.

Marshall

123 300D driver · #13 10-27-2003, 02:05 PM

Well, I did the valve adustment suggested by PEH and the missing on cold startup is still there. I am going to live with the missing because it eventually goes away. Just weird because no fuel is escaping anywhere. Maybe the injector problem mentioned by Marshall Booth is the correct diagnosis. It's not severe enough to warrant removing the injectors though.

The valve adjustment certainly improved the general running of the engine. 5 valves were tight. I notice less smoke nowl and a smoother idle immediately after returning to idle following a good high speed run.

I bent some cheap 14mm wrenches for the valve adjustment, similar to leathermang's jpg, but didn't need the valve spring retainer wrench. I like the method in http://www.pindelski.com/ that ensures that you get, for example, at least a 0.100 mm gap to 0.13 mm gap (the next larger size). The chain stretch using the cam tower mark is 7, so next time I will roll in a new chain.

So thanks everyone for the help and the jpgs for the wrenches and 617 timing procedure.

#14 10-31-2003, 06:40 PM

If your miss at cold start problem WAS a tight valve and you corrected it, do you have ANY idea when the miss will go away? NOT the next few times you start the car!!! It might be better, but it WON'T be gone.

Marshall

kerry · #15 10-31-2003, 10:02 PM

I posted a thread on this same topic about a year ago. My 85 TD does the same thing. It is worse in cold weather, but also noticeable in warmer weather. I have never been able to determine the cause. I replaced all the glow plugs and it made no difference. I sometimes think that I can notice the miss begin once the glow plugs stop glowing. It runs smoothly for a number of seconds after it starts and then begins the 'hiccup' but not for very long.
I live in Denver, a mile high. I think the thinner air may contribute to the problem. I bought my 409d at sea level. I noticed no hiccups when I bought it. Now that I have it home, it does the same thing as the TD.
I've decided it is nothing to worry about.