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kamil 08-23-2004 02:49 AM

Changed Glow Plugs on dads 92 2.5 turbo and I think I screwed something up....

Well so I decided to change the GLOW PLUGS on my dads 1992 300D 2.5. It did take me a while to do since all the damn things I had to take off in order to get to the plugs. It all went pretty smooth without incident except the OVERFLOW LINES that go from injector to injector. I touched them and they disinegrated like magic. It was pretty unbelievable. I thought to myself that I will get new ones tomorrow which I have yet to do till MONDAY. So I put everything together and started the car with the new BOSCH glowplugs and it runs like crap. It shakes and runs rough. Basically it ran better until I touched it.

I'm thinking that it has something to do with the overflow lines not being there. I did not think that the lines mattered in engine performance. Today I will hit up the stealership to get the lines.

Any ideas ? Did I do something wrong ?


JimSmith 08-23-2004 03:20 AM


You most likely have only intermittent fuel getting to the injectors. Whenever fuel can come out of a place where it is not supposed to come out, remember that air can get in. Air is compressible and wreaks havoc with the injection system as it is designed to have no air in the high pressure side of the system. Put the lines back on, run it to purge the air out (I do not think your model has a hand pump) with the starter until it fires, and it should smooth out fine. Jim

oldnavy 08-23-2004 10:07 AM

Kamil about the time I'm thinking your a pretty sharp youngester (damn I wish I was a youngester again, and know what I know now) you get silly on me. Did taking those lines off and then starting the engine sounld logical? NO!!!! :(

Oh well if you learn from the mistake and no damage was done to the car, it (mistake) has had a positive affect. :D :cool:

kamil 08-23-2004 11:49 AM

the only reason i was thinking that the lines would not have any effect was from someone posting in here about it. they said that the OVERFLOW lines have nothing to do with an engine being rough....

it was in my thread about fuel injectors :)

psfred 08-23-2004 09:23 PM

Overflow lines missing will do nothing but dribble copious amounts of diesel fuel down the side of the engine, no other effect on running.

If you remove the injector lines, you have air in them. It's also possible that you don't have the wires connected correctly to the new GPs, so they don't work, this will make the engine run rough until it warms up a bit. How long did you run it? Not long, I'd bet!

You don't need to remove anything to change the glow plugs if you have a 1/4" drive ratchet set and some swivels and swivel sockets, they are all (barely) reachable.

Also check to make sure you didn't break the wires to the rack positioner solenoid on the back of the injection pump, this will cause the idle to drop to about 500 rpm and shake like crazy.

Last, if you didn't get them sealed correcly, you will have low compression on the affected cylinders, and it will run very badly. check for air leaking out around the glow plugs.

Those overflow lines can be VERY brittle -- the ones on my brother's 300SDL broke like glass when I changed them, rock hard. Do make SURE you have all of the old rubber off -- Sam's left a ring of hardened stuff on there that would simply split the new tubing right off. Make sure you don't scratch the nipples on the injectors, too!


oldnavy 08-23-2004 09:36 PM

Sounds like someone who knows what he's talking about. I did however think they could pass air to the cylinders, but I could and probably wrong as I'm not all that fimiliar with diesels.

kamil 08-23-2004 09:42 PM


thanks guys.... I solved all the problems I think. I went to the dealership and bought all new overflow lines and I installed them. I also performed the following on the car: coolant flush, oil change, air filter change. Car runs great.

thanks again ;)

psfred 08-24-2004 01:13 AM

The injectors hold 1800 psi or so all the time, how is air gonna leak in past that?

The overflow lines just collect the fuel that leaks past the nozzle holder parts as the injectors fire.


michael cole 08-24-2004 02:04 PM

:) when the injectors snap shut at the end of the injection cycle for a brief moment air can be sucked in thru the overflow lines.had this experience on my 300 sd

psfred 08-24-2004 07:48 PM

The injectors hold pressure just below opening pressure all the time via the pressure valve on the injection pump, they never go negative at all. There is no way air will travel all the way down the body into the nozzle, anyway -- the overflow lines just collect the inevetable leakage around the pintle shaft in the nozzle (there has to be clearance so it can close).

If you DO get air in there, the pressure valve is bad at the pump or you are getting air into the line at the cap nut or via the injection pumpt itself.

Normal injection volume is 50 microliters (50/1000 mL) per cycle on a Benz diesel and full fuel. To deliver this small volume (about 2 drops, more or less) from the IP to the nozzle, the line must be at near nozzle opening pressure and full of fuel, else nothing comes out.....


JimSmith 08-25-2004 09:51 AM


I think the No. 1 injector "forward" return line goes back to the filter housing, which is not at injection pressure, especially with the car shut off. With the car shut off, not having hoses on the injector return lines is just asking for trouble later, and, I believe, will allow air into what is supposed to be a solid system. Purging air on the post W123 models is even more of a drain on the starter and battery and can take a while. And, yes, a copious amount of fuel will be spilled. Makes for serious Diesel odors while driving. Jim

michael cole 08-25-2004 10:24 AM

:) there is however a reverse pressure wave created at the end of the injection of the reasons for the oneway valves at the pump

psfred 08-25-2004 09:25 PM

I repeat, there no way missing overflow lines will allow air into the injectors! If air could get "sucked" back in, all the fuel would spray out there instead of out the nozzle.

What will happen if the lines are off is the overflow from the IP will dump down the side of the engine, along with any from the pressure relief valve in the filter housing. The only place the fuel goes with the lines intact is down the return to the tank, air won't bother anything there.

There is quite a bit of "chatter" in the inejction lines, just put your hand on them while the engine is running. There are injection timing devices that use a transducer to convert the pressure pulse expansion of the line into a strobe flash. Very nice, if you can get the right one at a price you can live with. However, there is never enough negative pressure in there to pull air in unless you have a cap nut loose or something.


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