Help needed: 1990 350SDL quiet on cold strart, but loud engine noise after - PeachParts Mercedes-Benz Forum

dino67 · #1 10-05-2015, 06:40 PM

Sorry for the long message but I want to give as much info as possible, in hopes someone might be able to give some help

I'm a relatively new owner of a 1990 350SDL with 210k miles. I'm also new to owning a diesel car. The car is in excellent overall condition and I have service records showing that it was well taken care of since new. It starts and shuts off immediately as it should, uses no oil, does not smoke, has no blowby, seems to have good power and all vaccum functions seem to work properly.

My problem is that there is a quite loud engine noise at idle, and low rpm, from the top of the engine that I think, from reading the posts here, is injector nailing. It does not seem to be coming from any particular injector.

Here's the rub. The odd thing to me is that when starting a totally cold engine, the engine is very quiet for the first short period of time before the loud clattering begins. The colder the weather, the longer the engine is quiet after starting. Last night the temperature here got down into the 40s F. When I started the car this morning the engine was very quiet idling for the first 45-50 seconds then the loud noise started and stayed loud at idle the whole 15 mile trip I made today.

I tried the diesel purge trick of running the engine on Liqui-Moly and the engine quiets down but the noise at idle returns once the Moly is gone. The noise also seems to go away once the engine reaches about 1100rpm or so when I rev the engine while sitting out of gear. There is no detectable engine nailing (if thats what it is) during normal operation unless rpms fall down to around 1100rpm and below,.

What could be causing this? The noise is quite loud and I'm worried about driving the vehicle until I fix the issue. Any advice on correcting this would be greatly appreciated.

Thank very much

babymog · #2 10-05-2015, 08:06 PM

Your car has glowplugs which heat the pre-chamber in the cylinder for cold starts, typical of small diesels.

After you start the engine there is what we here call "afterglow", which is a timer based on engine (coolant) temperature that keeps running the glowplugs after starting (longer when colder). With the glowplugs on, my car is quiet on a cold morning, but you can certainly see the white smoke start and hear the engine start to rattle if I leave it idling as the glowplugs cycle off in cold weather.

Once the glowplugs turn off, the engine should be warm enough to run without them. Still, the engine is a bit cold, at low rpm and low engine loads the engine nails a bit.

If it is excessive when warm (and uniform across all cylinders) it could indicate a cold-running engine (should run around 85*C), advanced injection timing, or just dirty-leaky injector nozzles.

Are you running good pump diesel? You might try a better grade of diesel, or adding a cetane improver (I prefer redline, some like other brands, some prefer none) and see how that helps (much like your dieselpurge).

If the engine quiets down as soon as you add some power, it is likely normal enough and just being a diesel. Unless you are told that this is abnormal then you might just want to drive it for a while and get a feel for the car, the fuel mileage, and run a little fresh fuel through it to see if it works itself out.

People also say that (good quality) biodiesel will quiet the engine, another option.

Diesel911 · #3 10-05-2015, 11:22 PM

You said it is a 1990 350SDL with 210k miles. My own opinion is that the Injection Nozzles wear out somewhere around 125-15oK and the injector opening/pop pressures become low and unbalanced between cylinders.

Then fuel injection Pump timing becomes late/retarded and that can cause nailing (I also found out if the timing is too advanced it will also nail at idle).

Low compression can also cause nailing when the Engine is cold.

Timing Chain Stretch retards the fuel injection pump timing as well as the valve timing. Valve timing can effect compression.

You might want to post another thread and ask wich is the most likely cause of Nailing on a 1990 350SDL with 210k miles because if you turn the above list over to a Mechanic (license to go fishing) it is going to cost a bit$

As a stop gap you might add a diesel fuel addative with a cetane booster and that sometimes reduces the nailing as it changes the rate that the fuel burns.

dino67 · #4 10-13-2015, 06:53 PM

Thanks to all for the comments but they have brought up some additional questions. The engine running cold was suggested as a reason for the noise. Is this a common issue with these engines? Is there someway to correct a cold-running engine?

I've been reading about adjusting the IP timing, thinking I might investigate that. But I can't seem to find anything about where the actual "adjuster" itself is if there is one? I see what looks like some type of screw adjuster at the back of the IP. Is that it? Or is the procedure just to loosen the pump and move the whole pump to adjust?

Thanks for any additional help.

jay_bob · #5 10-14-2015, 11:34 AM

Try a couple simple things first:

Cold running engine - you should get up to 80 degrees C +/- 5 to 10 degrees within a couple minutes of driving at most any normal temperature outside the dead of winter. Double check your gauge - IR thermometer or carefully remove the cap just after shut down and check the coolant temperature with a chef's thermometer. 80 degrees C is approximately 175 degrees F.
If you are not reaching temperature you need a new thermostat. The thermostat often fails stuck open, and so the engine can't gain proper temperature at the warmup phase of operation.

Also check the clear (well they are supposed to be clear) lines between the injection pump and fuel filter. If you see bubbles then you are getting air in the system. If your hoses are dark brown and brittle I would replace them on principle. The low pressure side of the system operates under vacuum with respect to the atmosphere, so fuel does not leak it sucks air.

Maxbumpo · #6 10-16-2015, 11:11 AM

dino67, start simple/cheap/easy before getting expensive/complicated.

I'm pretty sure your car has the "after-glow" feature, which energizes the glow plugs immediately after the engine starts, and keeps them on for a short period of time according to engine temperature (the colder the engine, the longer they stay on). The noise when quiet (while the after-glow is working) is what the engine should sound like normally once at full operating temperature.

As already stated by others, at 210k miles the injectors are likely tired and ready for service. When the glow plugs are on (about 1200 deg F), the injected fuel is burning nicely. But if the injectors are tired and not spraying a good pattern, the fuel is not igniting cleanly hence the "nailing" sound that you describe.

If you have the right tools and some experience working on cars etc, removing and replacing the injectors is not too hard, but you will need a torque wrench and the factory service manual. The intake manifold needs to come off, which means there are two gaskets that need to be replaced, and you will also need new fuel return hose and new heat shields to install under the "new" injectors. While you have the intake manifold off, this would be a great time to replace the glow plugs (if one or more fails in the next 100k miles, the intake manifold may need to come off to replace it). Only install Bosch glow plugs.

If you can let the car sit for a week or so, you can remove the injectors, mail them to Greazzer (list member) and he will clean / test / set the opening pressures, and then mail them back to you.

If you can't afford the down-time, buy some Bosch rebuilt injectors or get a set from a junkyard, install the new or send the old to Greazzer (or send the new the Greazzer for test / set), and then install a set that are tested and the pressures properly set. The injectors need to be balanced (opening pressures set to within a 5 bar range) to get a smooth idle.

Maxbumpo · #7 10-16-2015, 11:16 AM

Injection pump timing: This is NOT a service that needs to be performed regularly. The Bosch injection pumps can last the life of the engine and never need to be timed / adjusted. The most common cause for the pump to be out of time is due to timing chain stretch, and in that case the proper repair is replacing the timing chain.

If you suspect that a previous mechanic has adjusted the pump timing, then you may want to check it, but I don't think that is causing your current issue.

dino67 · #8 10-20-2015, 07:59 PM

I've been reading everyone's comments closely, thanks.

I checked my coolant temp and found it to be way low, so I've ordered a new thermostat (nobody around here had one in stock) and I hope it arrives in time for me to put it in this weekend. Right now new/rebuilt injectors are not in the budget; maybe in the Spring.

I'm noticing one issue in the rear-most injector. It looks like there might be some minor seepage between the upper and lower halves of the injector. Is this a major issue? Can I remove the injector and tighten the two parts to stop it or try to tighten while the injector is still on the car? Are the two pieces threaded together and are they "normal" threaded, not reverse threaded?

Anyone's input would be appreciated, thanks.

dude99 · #9 10-20-2015, 08:25 PM

Quote:

The Bosch injection pumps can last the life of the engine and never need to be timed / adjusted

This is false, well mostly false. While yes, the pump should last the life of the vehicle, the timing will go off. Yes it will continue to run, and possibly run well, but not as well as if it was properly adjusted. This is due to wear\stretch in the chain. While it doesn't need to be a regular thing, it is something that shut die checked and adjusted if necessary. I'm a bit of a hypocrite in this regard as I've never checked\adjusted mine...

dude99 · #10 10-20-2015, 08:26 PM

Quote:

I'm noticing one issue in the rear-most injector. It looks like there might be some minor seepage between the upper and lower halves of the injector. Is this a major issue?

Odds are its actually the rubber return line on the top of the injector that has begun leaking and is running down the injector as supposed to the injector itself leaking. Change the rubber lines and see if your leak disappears.

mannys9130 · #11 10-21-2015, 01:48 AM

Do not attempt to tighten the injector halves. It won't work, and it'll cause you WAY more of a problem. Don't try tightening the injector in the engine either. Replace the rubber overflow line and nothing more.

When your new thermostat gets to you, replace the gasket, use only yellow Zerex G05 coolant, and be sure to burp the engine before you cap it and drive off.

babymog · #12 10-21-2015, 08:58 AM

This is the "injector holder", the nozzle is inside. I believe that disassembling the two halves to replace the seal will require re-balancing as this affects the shim stack height? Others will be able to verify.

If the injector holder is leaking as you have observed, it will affect nailing at idle as the slight leak will delay the injection event and thus ignition timing. If you get more nailing from one cylinder than others this is likely your problem.

Quote:

Originally Posted by dino67

I'm noticing one issue in the rear-most injector. It looks like there might be some minor seepage between the upper and lower halves of the injector. Is this a major issue? Can I remove the injector and tighten the two parts to stop it or try to tighten while the injector is still on the car? Are the two pieces threaded together and are they "normal" threaded, not reverse thread?

Maxbumpo · #13 10-21-2015, 04:51 PM

Quote:

Originally Posted by mannys9130

Do not attempt to tighten the injector halves. It won't work, and it'll cause you WAY more of a problem. Don't try tightening the injector in the engine either. Replace the rubber overflow line and nothing more.

Well, maybe. The two halves of the injector body are lapped at the joint to form a perfect seal. Leaking there is more than likely due to contamination of the joint from when it was last assembled. If you over-torque the two halves (normally threaded, left-loosey and righty-tighty) you might crack the body, and then it will always leak. I don't know the torque spec off hand, but it is not very high (something like 70 Nm). You could put a short wrench on it and just give it a little tug in the tightening direction, and see if that helps, but if you don't know the torque value and don't have a torque wrench, then just leave it alone until you have the time/money to get that fixed by a proper Bosch service shop or send it to Greazzer.

mannys9130 · #14 10-21-2015, 04:57 PM

No. Leaking at injector halves is caused by corrosion or by wear. The halves are lapped completely smooth and there is no gasket between them. Any corrosion or wear destroys the perfect surface. Tightening won't help at ALL and taking it apart to lap it smooth again will require resetting the pop pressure.

Hence my advice, replace the overflow line, send them all to Greazzer, or just leave it alone until you can.

dino67 · #15 10-25-2015, 04:49 PM

Again, thanks for everyone's input.

Per some earlier discussions on this thread, I took some time this afternoon to install the new thermostat I bought thinking that it would just be like very other car I've ever worked on and it would be a simple matter to remove the upper hose and housing to remove the thermostat..silly me. Imagine my surprise when I found that the thermostat is not there. So apparently it's under the car at the lower hose; a much bigger and messier job since I don't have a lift.

I've been unable to find any articles or videos to give me the best procedure for replacing the thermostat on a 350SDL. Does anyone have some hints or links to any videos, etc.?
The engine clattering at idle seems to be getting worse so I'm still looking for a solution.

Thanks