240D running poorly after routine maint.

A few weeks ago, I snapped a belt. No problem... replaced all belts and back on the road. Now, my 240D is running a little rough and sucking the fuel - down almost 15 mpg on last tank.
Last night, I peeked under the hood and I found a crack in an air-check valve that is in-line running to the intake. Could this be my problem with poor mpg and sluggish driving? All else seems fine, fuel lines are intact, no evidence of leaks, and nothing else out of place. Also had valve adj. less than 4K ago.
I am going to replace it anyway tomorrow, but sure would like insight as to why this crack would offset my mileage and power this much.
Thanks,


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William
76 240D - 550K miles
78 300D - 200K+ miles

[Benzmac]

I don't know which one of the 240D's this applies to, but I have seen some with an EGR valve. At times the valve will hang open and cause problems.

Also, be sure to check for a fuel leak>>

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Benzmac:
Donnie Drummonds
1991 GMC Syclone
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

I've checked for leaks... cant find one, yet.
The valve is creamy coloured, transparent, and looks like a check-valve. It is probably 1/2 inch thick and about 1.5 inch in diameter. There is no noticable force of air in or out of it at idle or at low rev. Could it be a vacuum line with a check valve?
I believe that the crack occured when the belt broke and flopped around under the hood.
If this is an EGR, it is news to me

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William
76 240D - 550K miles
78 300D - 200K+ miles

In a diesel, any vacuam leak or leak in intake system should be fixed because it allows dirt in the engine. Not a good thing.

However, a diesel is not throttled, it takes a full gulp of air at every intake stroke. The engine is controlled by the amount of fuel injected into the cylinder. This shouldn't cause fuel mileage problems. It doesn't sound like you're describing an EGR valve.

When you lost the belt, did the engine overheat?

Good luck,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

Larry,
Yes, it started to overheat, but I shut it down before it passed 230F.
I had the car checked by my mechanic for damage, all is fine. Replaced the check-valve on Friday and the car has never run smoother. Aparently it was cracked and leaking before the belt smacked it around.
Thanks,

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William
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles

[stevebfl]

Well for all you young guys I'll straighten out some inaccuracies and maybe give some insight as to why fixing this "check valve" made such a difference.

First, no EGR for years from the date of this guy. Second, sorry Larry, that diesel does have a throttle and a pneumatic governor; which gives the reason why a manifold leak affects running on that guy and not later models (77 on - no more throttles).

I presume the check valve was the one in the line to the power booster. A hole there would alter the manifold vacuum that works on a diaphram in the back of the injection pump. This directly moves the pump rack.

It's easy to see that I have no experience with MB diesels earlier than the 123 car. I did not pay close enough attention to notice that this was a '76. However, even if I had, I would have probably replied the same way. I was very surprised to find that there was a throttle on a diesel. I'm now trying to understand why it would be there.

Thanks for the correction Steve, and I'm glad you've got it going again William.

Have a great day,
Larry

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

[stevebfl]

Larry,

The early pumps were all vac controlled mixture (pneumatic control). Even the first gas pumps were that way. I have a 58 300d ch189 that uses basically the same pump as did the Gull Wing ch198. The throttle housing has jets in it to modify the application of vacuum to the pump.

In the 621,615, and 616.916 the pumps were pneumatic controlled. They had very low manifold vacuum (I think I measured it once at about 6in at idle. It allowed a diaphram against a spring in the back of the pump to move the rack rich or lean.

Many mistakenly think that the linkage to the back of the pump controls mixture; it does NOT. Its sole purpose was a dampening action similar to the rack dampeners used at idle (on 617.95x) to prevent the ruummp...ruummp...ruummp that occured at constant speed.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Steve,
Excellent as usual in your direct approach and flawless insight.
Thanks a million for explaining this for me and others.
Hard to believe how simple, yet effective a little vacuum pressure is in proper adjustment of mixture control.... oh well, learn something new everyday!
Thanks again,

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William
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles

I'll add a detail here thats been overlooked - the pickup for the vacuum line that runs to the injection pump is NOT just a hole in the manifold - it's a calibrated venturi. If you measured vacuum directly off of the manifold, the reading is VERY different from the signal going to the IP. Also, not mentioned, but implied, its a REVERSE feedback system - closed throttle induces a higher vacuum, which REDUCES the amount of fuel being injected. Therefore a vacuum leak will give bad mileage, smoke, and, if the pump calibration is off, increased idle speed. The check valve mentioned is most likely the one that's between the intake manifold, and the vacuum pump on the front of the engine.