Would you consider this a rough idle? (video) - PeachParts Mercedes-Benz Forum

Shern · #1 03-25-2019, 08:08 PM

https://www.youtube.com/watch?v=rXYqZTPV_r4&feature=youtu.be

I'd prefer it without the oscillation, but also have no frame of reference.

BWhitmore · #2 03-25-2019, 08:18 PM

Looks and sounds pretty normal to me.

pawoSD · #3 03-25-2019, 08:23 PM

Same here.....are the enging mounts and shocks in good shape? Sounds like a healthy 616 idle to me....

Shern · #4 03-25-2019, 08:26 PM

I appreciate the weigh-in. Shocks are new stabilus, mounts are new lemforder.
Greazzer injectors/OFV spring,
no air/fuel leaks,
rebuilt lift pump,
polished IP delivery valves and replaced seals and springs,
Set IP timing at 25 degrees
Valves adjusted 5k ago...

Think I'm about out of things to do.

Diseasel300 · #5 03-25-2019, 08:44 PM

Sounds ok, but that sideways shaking seems excessive. I'd imagine you're feeling that in the vehicle or you wouldn't be asking about it here?

One thing you failed to mention here: What's your compression numbers like?

Shern · #6 03-25-2019, 09:16 PM

You’ve got the gist of it.

At this point, the need for that step seems fairly conclusive. From what I’ve read, if there’s an imbalance, short of a rebuild, or maybe a head gasket if I’m lucky, there’s nothing for it.

Say I discover my numbers are within tolerance (10%?), what then?

Quote:

Originally Posted by Diseasel300

Sounds ok, but that sideways shaking seems excessive. I'd imagine you're feeling that in the vehicle or you wouldn't be asking about it here?

One thing you failed to mention here: What's your compression numbers like?

Father Of Giants · #7 03-25-2019, 09:41 PM

Sounds dead on. Mounts are probably shot

Shern · #8 03-25-2019, 09:49 PM

See post #4, friend.

Quote:

Originally Posted by Father Of Giants

Sounds dead on. Mounts are probably shot

Diseasel300 · #9 03-25-2019, 09:49 PM

A compression test can help you determine if you have an imbalance that needs to be looked into. If you have 2 adjacent cylinders with low compression, you may have a breached head gasket between those two cylinders. You're looking for relative compression between all 4 holes. You want to see a minimum of variance between cylinders and between each cylinder and your average compression value. The cause of low compression can be anything from worn valves/guides/seats to worn piston rings to blown head gasket.

If you find a weak cylinder, you can do a leakdown test, or just pump some compressed air in the cylinder and listen for a leak. Leaks past the valves can usually be heard in the intake manifold or exhaust pipe. Leaks past the rings can show up as pressure in the crankcase and can sometimes be heard at the valve cover.

FWIW, my SDL's engine had 1 cylinder (#4) that was ~30PSI lower in compression than the average across the rest of the cylinders. Car ran fine, did upper 20s fuel economy, and had tolerable oil consumption, but had that shaky idle that was usually worse when hot and the A/C running in Drive. Wasn't bad enough that I felt the need to rebuild the engine so I just lived with it.

Shern · #10 03-25-2019, 10:17 PM

Thanks man. This is essentialy where I’m at. Fuel economy, oil consumption, cold starts, power, all reasonable. I imagine I’ll pick up the harbor freight tester this weekend and enter via the glowplug holes.

Point of operation, while I have you: I’d prefer to leave the hardlines connected if possible. From what I’ve read, after removing all glowplugs, the move is to wire the stop leaver to prevent the engine from starting? Or is it actually better to disconnect them at the injectors?

Re: hot or cold testing (apologies for inciting a riot) did you find one more conclusive in determining imbalance? Figured I’d test on a hot engine, running it one last time after removing the glowplug wires (but obviously before removing the glowplugs).

Diseasel300 · #11 03-25-2019, 10:28 PM

If you wire the stop lever down, you don't need to worry about the injectors. In my case, I went in through the injector holes since the 60x engines are a PITA to get to the glow plugs. Made it a non-issue.

Asking about Hot vs. Cold compression testing will likely turn this into another "oil thread". Everyone has an opinion. In my case, problems were present cold and were backed up with a hot test. You should be able to identify any problems either way. Compression numbers are usually slightly higher hot from expansion of engine parts and hotter air in the cylinder. The FSM gives compression numbers on an engine at operating temperature (hot). The same trends apply hot or cold, you're looking for the relative balance between the cylinders.

Shern · #12 03-25-2019, 10:34 PM

I appreciate it.
Onward, with semi-confidence.

Phillytwotank · #13 03-25-2019, 10:44 PM

Fuel quality can make a diffence in idle quality. Maybe try some cetane boost to see if that smooths out and or quiets your idle at all.

Shern · #14 03-26-2019, 03:04 AM

Agreed. I do run the grey power service in every tank.

barry12345 · #15 03-26-2019, 01:05 PM

What is the operational fuel pressure in the base of the injection pump? It does impact idle to some extent. Plus other things as well. The 616 should be at least at reasonable fuel pressure levels.

I even believe it reduces noise at highway speeds. Getting it to about 19 PSI one way or another I consider both safe and adds a little more power that is felt in the posterior.

A test that has been in my mind for sometime. If things are in decent condition when you start a 616 engined car in the morning. It should catch almost instantly.

My thought is if not the fuel has back drained to some extent. A simple test is perhaps pump the primer pump up just before trying to start after sitting overnight. If it starts a lot quicker the conclusion is obvious. It is probably back draining through poorly seating lift pump valves.

To back drain substantially there may also be another leak. There may not be enough fuel available for the injectors initially for the engine to start on. Until the elements of the injection pump have adequate fuel available to provide it.

To me there is not exactly a lean situation. Just inadequate fuel available to strongly combust provided initially.

Even the factory manual wanted dealers to check this pressure periodically. Every thirty thousand I think. I take the testing a little farther because these cars are now so old and have not had many things checked and maintained..

The importance should easily be understood. When the injection pump is or was calibrated. It must be done with a specific base fuel pressure. If the base pressure has changed with time and use. Some component of the calibration factor has been lost.

The lift pump only recharges or tops up once every two strokes of the crankshaft. If the essentially accumulator effect of the lift pump design pressure declines too much in the interval is not good. The intended pressure retention can be lost if the lift pump valves are excessively leaky or the pressure regulator has gotten tired with age.

People seem to still ignore in general this area. Even though it is both cheap and fairly easy to check. Also if you know the system it becomes much easier to find any faults that can arise. A checked and maintained system is also not as prone. To develop those issues.

The last of the 616s are now over thirty five years old. Some more than 40 years old. Things do get both worn and just in general tired. If you find no issues at least this area has been eliminated.

Many of my thoughts are based on that the 616 is the only engine I demand full power out of frequently in normal driving. I want to know the engine is producing it.

I also look for cheap and economical approaches to issues. If a 616 powered car cannot make the normal 0-60 time is to me a solid indication things need checked. If it can make it things are probably still pretty good.

One last test that is worth doing in a situation like yours. At least in my opinion. Temporarily close off the return line from the injection pump. No change means there is an issue somehow with the fuel pressure. Normally the idle should change. Here is the hard part even if it does the operational fuel pressure still needs checked.

The first test if it fails indicates the system is not even generating enough fuel pressure to open the return valve or as it is properly called the fuel pressure regulating valve. The second test you do indicates that valve is having some function but you do not know at what pressure it is actually opening at.

I hope I am being clearer than mud. That I really like the 616 engines.

You have adjusted the valves and checked that they are retaining their clearance after a week? Of average driving. All too often they have not been done over a long period of time and carbon builds up on the seats that has to be pounded off. Once again reducing the lash clearances. Requiring many adjustments until the situation stabilizes. If hot idle is worse than cold idle especially.