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Time to clean the EGR pipe, one of the "most likely To Do" tasks on the list I made when I got the car.
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Don't assume EGR tube blockage until you have tested it. There are several other EGR faults on 104s that will bring u a code 5. Along with a Factory TSB part up-grade for EGR code. I will post a basic EGR flow test , if you are interested in diagnosing the fault before you go for a tube blockage repair.
Or go for the possible blockage and see if that solves it ..and if not, then repost you have the CE/code 5 again ant then you can do the flow test.

I'll wait for your flow test.

My "clogged pipe" assumption is based on the "most likely" scenarios I've read in various EGR threads. If that turns out to be the problem, I'd probably prefer a tube replacement over a cleanout, since both seem to be equally tedious DIY jobs.

__________________
2008 E350 4matic / Black/Anthracite

------------------------------------
Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black

I will post the procedure as soon as I find it in the Archieves .. I did a complete DIYer article on it a couple of years ago..hold on, it's comin'..

.... and , Yes, If you do find you have to get into the blockage, you do want to go with the new tube as that eliminates the problem rather than just cleaning it every time...

Is this the procedure? (From a 2004 post of yours):

Although there is a reasonable chance that the egr tube is blocked,[common 104 prob], an easy test for vac. flow is to simply apply some vac [ hand pump or vac source] to the egr while at idle. This opens the egr and will cause the engine to stumble..[ go lean]...if it does ,the tube is not blocked and other parts of the egr system are suspect...

__________________
2008 E350 4matic / Black/Anthracite

------------------------------------
Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black

Yes .. here is another

http://www.peachparts.com/shopforum/showthread.php?t=137068&highlight=egr+hand+pump

The basic concept of the test is the EGR is never called for at idle when car coolant is up to temp. So, if you manually cause the egr to open at that time , the engine will go very lean and stumble [ what you are basically doing is causing a very large intake vac. leak] If the tube is plugged , this can not happen as the air the egr is allowing to pass can not pass a blocked tube , so there will be no noticeable effect on the engine running.
NOTE**-The engine will not stall b/c engine management will try to correct a/f mix.
There are 5 parts to egr operation:
The ECU sends an electric sig to a SOV at the front of the engine [ SOV is just an electrically operated air valve.] It lets engine vac pass on to the egr diaphragm, which opens the egr actuator.
The common faults on 104s are the plastic vac supply line that is under the front engine cover cracks from heat fatique. This stops vac from ever getting to the SOV supply side. So, code 5. But if you have vac at SOV, then the SOV has to open.. That is also a probable for code 5 [ they get gummed up]
And if the SOV does get vac to the EGR , the EGR on 104s was changed via Benz TSB [ Technical Service Bulletin 518 or such] b/c the shaft did not have the needed clearence and they would Hang Up, causing a Code 5. These where changed to a new part and usually the Dealer would put a White paint dot on the new unit to let Techs know it was upgraded , b/c the part # never changed, just the shaft tolerence diameter design. [ but many got changed without being indexed, and some never got changed]
So , you can see why it is best to do a couple of test to see which one of these common faults are causing the ..well you get it ..code 5.

Anyway , a hand vac pump does the testing and you are looking mainly for rough running at idle when you open the EGR and you are also looking for a SNAP SHUT when you relaese the vac supply to EGR.
That, along with verification of vac supply [ hoses/sov/etc] are the test.
Good Luck ..not a hard thing to check out, but easily mis-diagnosed

Arthur, thanks.

Now I have a really boneheaded question, but bear with me since I am new to this procedure:

I have surmised that the EGR is the object resembling a flying saucer from a 1950's SciFi movie. To do the vacuum test, I assume I must disconnect the short orangish/pinkish (vacuum??) line connecting it to the engine, and attach the hand pump line in its place, correct?


Just out of curiosity, would this coolant temp dependency affect how quickly the CEL returns after its code is cleared?

After clearing the codes on Sunday afternoon, the CEL returned Monday morning after only the 4th drive/restart sequence of the day. At that time, I was finally was able to retrieve the "5" code, and cleared it again. This time, the CEL did not return until Tuesday morning, after the 8th or 9th drive/restart.

On Sunday, I was driving during warm daytime hours during which time coolant temp was at or above normal. On Monday, however, the day itself was cooler, and most of my driving was at night, when coolant temp was at or below normal. Only after a "hot" highway drive did the CEL illuminate on a subsequent start.

__________________
2008 E350 4matic / Black/Anthracite

------------------------------------
Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black

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Correct..
If you do not have a hand pump, many guys just attach a vac line onto the plastic line that feds the SOV...this gives you straight engine vac to work the egr by jumping the SOV circuit..If that works fine, then you know the SOV circuit is suspect [ SOV gummed up , or SOV coil open circuit, or SOV not getting sig from CU, etc]