PeachParts Mercedes-Benz Forum - 84 300D rough idle. Have done everything. Need help.

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New clear plastic lines will more pliable and less likely to crack from engine vibration.
Performance should be about the same, if the old ones were sealing properly.
Be sure to replace all the crush washers for the banjo bolts when you replace the clear lines.

Remove and check the boost sense lines for soot and hidden cracks.
Replace the two small crush washers for the intake manifold banjo bolt when you have it out to check for soot blockage.
Check that the switchover valve on the firewall is operating correctly (boost goes to ALDA when solenoid off, and boost vents to atmosphere when solenoid engaged.).
Slight boost leaks on this ALDA circuit can reduce performance by partially limiting fuel rack travel under boost conditions.

Additionally, the rubber seals in the ALDA can fail to seal adequately.
Test the ALDA seals (with engine off) by applying vacuum from a Mityvac vacuum gauge to the ALDA input line.
If the ALDA cannot hold a vacuum, then it will need to be re-sealed/replaced.

Quote:

Originally Posted by weneversleep (Post 4169239)

Thank you again to everyone for the continued help. I have an update, and this time it's good news!

@Alec300SD suggested that the fuel hoses on the intake side of the pump might be degraded, causing air incursion. Well, the tech took those rubber lines off, and they were falling apart inside. Maybe there was air incursion, certainly there was a choking-off of full fuel pressure.

Replaced those lines as well as the rubber line for return, and the majority of the shaking is now gone. The engine still shakes a bit at idle, but nowhere near what it did before. And, most importantly, sitting in the driver's seat, there is no shaking felt.

The car has always been a bit sluggish as well. I took advice from another thread and replaced the throttle linkage bushing assembly (at the firewall). (unfortunately, for an 84, this part is not available new anymore, but I took one off of another car as a test, and ordered the bushing insert from the seller on eBay to rebuild the bad one) Now, the throttle response is as it should be, engine is getting full power, and life is good!

I think the next steps are to replace the clear plastic fuel lines (which have turned brown); those have been ordered, and maybe that will make things even better. And to now drive the heck out of the car; I'm sure there was quite a bit of carbon buildup in the intake and cylinders resulting from the mis-calibrated pump that needs to be blown out.

Anyway, thank you to everyone again for all of the input, and I will continue to update this thread as I drive the car!

Out of curiosity where are you getting the replacement clear lines?

Quote:

Originally Posted by runnersigh (Post 4169341)

Out of curiosity where are you getting the replacement clear lines?

I don't know. The shop ordered them. Probably not OEM; they're just clear plastic lines.