Now that a rebuilt turbo has been installed, the engine surges when idling. Until the turbo went out, this engine idled normally at about 600 RPM and started easily even when cold. Now, sitting at a stop light, the surge feels like two additional cylinders have come on line. Have never felt this while driving at speed-just at idle. Also, I hear a rattle at just above idle. Start from cold is rougher than it was before. However, at speed on freeway, engine is completely normal.

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87 300D 265Kmi
Factory rebuilt crate 603.96x engine at 200K

When it's surging, unplug the 2-pin connector at the ELR, the round red plastic thing on the tail end of the IP. That will determine whether the IP is surging on its own or responding to an EDS signal.

Rough start from cold is probably a lazy glow plug.

Sixto
87 300D

The rattle was the chain tensioner.

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87 300D 265Kmi
Factory rebuilt crate 603.96x engine at 200K

Was it the original tensioner or had it been replaced?

[edit] Oops. I just saw that the engine was rebuilt 5K miles ago. Odd that a factory tensioner would quit so soon. I'd take that up with the rebuilder.

Sixto
87 300D

Car now has close to 270Kmi on it. And it was the belt tensioner. It could have been reused from the old engine so I cannot say how many miles on that part.

Hi Sixto. I have this surging problem with my newly acquired 87 300D. It starts with 1 spin ( cold or hot ) and then idles well. Every now and then it will rev up for no reasons for 1-2 seconds then back to normal again. The car is no-op so I have not driven it on the road yet so not sure how it behaves it high rev.

Can you explain what is ELR and EDS in layman terms? How do I check them? Thanks

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

EDS is Electronic Diesel System. It's the control unit that manages idle speed and emission control systems. Once you disable EGR (exhaust gas recirculation) and ARV (air recirculation valve), it only manages idle speed. I don't remember what ELR stands for but it's the round actuator at the tail end of the IP that keeps idle speed where you set it.

There's a trimming pot next to the brake master cylinder you can use to test the ELR actuator. Take note of the current setting. Pull out the knob and turn it to a different setting then push in the knob. Idle speed should go up if you selected a higher number and down if you selected a lower number. If you can make such an adjustment, we can assume the ELR actuator is working.

I don't know how to test the broader EDS without an MB dealership's equipment. SDS (Star Diagnostic System) and OBD2 (US standard on-board diagnostics, second generation) didn't exist in 1987. I suspect there's an intermittent anomalous signal to the EDS that causes it to send the maximum signal or count to the ELR actuator. Why, in my case, that only happens as I come to a stop is beyond me. One day I'll disconnect the engine speed sensor to see if that sends EDS into open loop and high idle. Somehow I doubt it.

Sixto
87 300D