Hey guys. I got a rough idle until the car is warmed up and exactly on the 80C mark. It's a 1999 E300 183K miles.

When the car starts in the morning it idles rough, like motor mounts are bad, and is a lot louder than usual. I understand these engines are common for fuel and air leaks. The delivery valve O-rings were replaced about seven or 8 months ago. I don't really know where to look. It does idle rougher in drive, and neutral is slightly better. The engine is still loud, but the vibration is a lot less in the steering wheel and passenger seat than drive.

When I do accelerate, it smoothes out immediately. I'm thinking it's an injector possible? I plan on seafoaming it in the next week to see if it remedies anything. Also, to seafoam it, you would run the seafoam into the prefilter correct? and the output is main filter?

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

You can find which cylider is the issue by lossening one Injector Hard Line Nut at a time and seeing the how the Engine reacts. It cuts off the fuel to that cylinder.

On the good cyilinders cuting off the fuel should make the shaking obviously worse.

On the bad cylinder cuting off the fuel to it will show less or no differance in the shaking.

That determins which cylinder the issue is on.

They you remove the thought to be bad injector from the bad cylinder and swap positons with an injector from a good cylinder. And, do the test again but only on the 2 cylinders.
If the problem follows the bad injector to the new posuiton you have a bad injector.

If with the good injector the problem still remains on the bad cylinder then you have an Engine issue (compression check) or something is amiss with your Fuel Injection Pump on the Element that feeds that cylinder.

This does not sound like your issue but if one Glow Plug is bad the Engine would start but that cylinder would knock and likey run rough till warmed up. Checking the Glow Plugs is easy if you have the Volt Ohm meter.

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84 300D, 82 Volvo 244Gl Diesel

EGR and resonance flaps - start reading

Fuel system seals, clamps, orings. - more reading to do.


Those should be your first priorities.


If car has a history of alt fuels - get the injectors tested. At your mileage, they could likely benefit from replacement if they were not abused (nozzle replacement on all of them on principle) and are original. But likely theyre still 'good enough' to get by without causing really noticeable issues.

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you could put the seafoam in - its unlikely to solve anything.

...but it might dis-solve something. ha! (...I'll see myself out now)

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