OM603 idle misfire - PeachParts Mercedes-Benz Forum

JHZR2 · #1 02-08-2021, 08:16 PM

Hoping to get some thoughts on this. My 91 was in the shop for a HG as preventive maintenance after I saw some oil in the coolant reservoir (no coolant in the oil). It also get a new timing chain and related parts while the head was off.

Got that done, a few other seals and whatnot, all back together. Engine starts fine, runs fine, pulls with lots of power. But back down to idle, it is misfiring. Only at idle it seems. Not a cold start issue I don’t think, glow plugs are replaced and my understanding is it does this after running a while, well, under load/speed.

What could this be? Fuel system? Valve out of adjustment? Bad timing that is “close enough” to only manifest itself at idle?

Trying to diagnose from afar, but would like to get a jump on some possibilities.

Thanks!

Diseasel300 · #2 02-08-2021, 08:50 PM

Delivery valves in the IP are probably kaput. The 97x IP's for whatever reason had very stiff springs for the delivery valves compared to all the other versions. I've done 3 IP's from these cars with the valve faces hammered into a ridge. I sourced all new valves and springs for my 350SD and it ended the rough/lopey idle and intermittent nailing.

JHZR2 · #3 02-08-2021, 09:19 PM

I’d agree if this was something that happened over time. But it went in perfect, now is misfiring. So I think it’s something that happened during the repair.

Could air be trapped in one line only? Is fuel pressure at the injectors different at idle than at load?

Diseasel300 · #4 02-08-2021, 09:35 PM

If nothing changed on the fuel system, that really only leaves timing or something in the head. Did the mechanic do anything to the valves? Grind seats or replace guides?

Fuel pressure is always the same on the injector lines, it's the quantity that differs for RPM. Air in one of the hard lines should burp out on the first drive. It's possible that one of the hard lines isn't tight and can be weeping or pulling an air bubble.

ah-kay · #5 02-08-2021, 11:40 PM

Whatever you are going to do. Do a compression test first.

JHZR2 · #6 02-09-2021, 09:38 AM

Quote:

Originally Posted by ah-kay

Whatever you are going to do. Do a compression test first.

Agreed, that is job #1 he is going to do.

Quote:

Originally Posted by Diseasel300

If nothing changed on the fuel system, that really only leaves timing or something in the head. Did the mechanic do anything to the valves? Grind seats or replace guides?

Fuel pressure is always the same on the injector lines, it's the quantity that differs for RPM. Air in one of the hard lines should burp out on the first drive. It's possible that one of the hard lines isn't tight and can be weeping or pulling an air bubble.

Valve guides were done. Long story short, this head has already been successfully run/operated before this came up. Thus why I’d think it’s more likely something with the fuel system. But who knows in reality. That’s why I’m interested in the potentials. What could the causes be that would cause a hot idle misfire only...

Diseasel300 · #7 02-09-2021, 10:53 AM

Quote:

Originally Posted by JHZR2

Valve guides were done. Long story short, this head has already been successfully run/operated before this came up. Thus why I’d think it’s more likely something with the fuel system. But who knows in reality. That’s why I’m interested in the potentials. What could the causes be that would cause a hot idle misfire only...

Were they done while the head was off, or were they done at a point prior to this head gasket renewal? If they were done while the head was off, the valves need to be recut and the seats machined to match. The old valve guides wear oval shaped and allow the valves to wobble. It doesn't take much of a leak to cause a misfire at idle and due to the smoothness of these engines, it's quite noticeable when they do. A leakdown test might be more useful than a compression test in a case like this.

barry12345 · #8 02-09-2021, 03:36 PM

Let the car sit overnight. Coolant hoses relaxed next morning? Long shot but easy to test.

In my case I might do the milli volt test to identify the cylinder. It usually works well for me in many situation because I understand it. Then go from there. Fast test usually as well.

Possibly the fuel system is getting air into it when the engine is hot as well. Yet not when it is cooler.

JHZR2 · #9 09-19-2022, 09:26 PM

Quote:

Originally Posted by ah-kay

Whatever you are going to do. Do a compression test first.

So, this situation is still haunting me. I don’t have my car back.

Apparently the misfire the first time was due to a bad valve seal/seat (?). He did checks, pulled the head, had the machine shop check it, they found a slight leak, supposedly fixed it. So I was told. Head back on, car still has a misfire. Cylinder #2. For a low mile 603 and the misfire NOT in cylinder #1, and the fact that it ran perfectly before I brought it in (just a seeping HG letting oil go into coolant), we are pretty confident it’s NOT any sort of rod bending situation.

But what is it? He is going to do a leak down test hopefully this week, and get back to me. It’s very frustrating.

I’ve been told that this round, injectors have been swapped. Only cylinder 2 shows no change when fuel line is cracked. I’m told it runs smooth and strong around, it just shakes badly at idle.

I’m not so convinced that it isn’t either a fuel issue (though again, it went in running perfect, so it wouldn’t be some valve or something that is worn out), or a head issue. I guess it could still be an issue with the workmanship on the head, though one would hope that was resolved.

He said leak down test, filling with air and seeing what is lost. He also mentioned something about filling someplace (around the valves?) with liquid to look for leaks.

I just want to be as informed and prepared as possible to make smart decisions and get my car fixed.

Supposedly it runs smooth and well, just idle - if I were to take the car myself, and drive it with a known misfire issue, say, 20-30 miles at slow highway speeds, would this do harm?

Is there a chance something timing related could be off? If a cam jumped one tooth could it be just enough to make a misfire on a single cylinder but not affect running?

What else should I be asking/doing at this point?

If the leak down shows that cylinder 2 is bad, and it’s not the prechamber or injector, then what? Should I assume something happened to the ring or cylinder wall? Or still a head issue?

Very discouraged, and I know it costs the shop owner money to redo work all the time.

dieselbenz1 · #10 09-20-2022, 05:52 AM

Smoke or no smoke?

JHZR2 · #11 09-20-2022, 09:07 AM

Quote:

Originally Posted by dieselbenz1

Smoke or no smoke?

He didn’t say there was smoke. He said that it runs smooth under load and shakes a lot at idle. If I can get down there on Friday perhaps I’ll try to idle it a while after the leak down test if possible.

ah-kay · #12 09-20-2022, 01:45 PM

Quote:

Originally Posted by JHZR2

He didn’t say there was smoke. He said that it runs smooth under load and shakes a lot at idle. If I can get down there on Friday perhaps I’ll try to idle it a while after the leak down test if possible.

Shake a lot at idle?

This is what I normally try.

1) replace the ovp relay to eliminate electric issues.

2) loosen one injector top nut at a time. If it get worse then that cylinder/injector probably is ok. If there is no change then that injector, cylinder, fuel delivery may be suspect.

3) compression test?

This is my experience, no hard and fast solutions.

JHZR2 · #13 09-25-2022, 03:02 PM

Quote:

Originally Posted by ah-kay

Shake a lot at idle?

This is what I normally try.

1) replace the ovp relay to eliminate electric issues.

2) loosen one injector top nut at a time. If it get worse then that cylinder/injector probably is ok. If there is no change then that injector, cylinder, fuel delivery may be suspect.

3) compression test?

This is my experience, no hard and fast solutions.

I’m not sure that the ovp can cause issues on this engine. If so the idle speed and edc functions would be broke. But good point.

He limited it to cylinder #2 based upon cracking injector lines in the standard test like you show as #2. Compression and leak down test was going to happen late Friday, I’ll know something maybe Weds.

We did see a lot of fuel at injector #2. I have to wonder if it loosened up when he pulled it or something.

Also got to see the original HG. Not sure where the oil would have crossed over to the coolant.