Mechanic replaced the water pump on my 93 300E (3.2 liter M104), and now my idle fluctuates up and down at idle, when the transmission is in any gear, and only when the engine is completely warmed up to operating temperature.

My independent MB mechanic (own shop for 19 years) has gone over everything he knows to go over.

He has a Baum CS2000 scanner, and it gave him codes for the coolant temperature sensor and the mass air sensor.

I ordered both parts from Fastlane, and neither solved the problem.

He hooked up the scanner again and then got a fault code 31, which means: fuel adaptation (lambda) control, mixture too lean (rich stop), (intake air leak, fuel injectors, fuel diaphragm pressure regulator).

He checked and there is no intake manifold air leak. He replaced the fuel diaphragm pressure regulator. Didn't help.

The weird thing is the check engine light never comes on, even though it should.

He's going to sleep on it, and try one more time tomorrow. If not, I guess I'll take it to the dealer.

Any help - Benzmac, M.B.DOC, Stevebfl?

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

I think you have a typo on the code.
Your discription would be a Flash code of 13 or a digit code of 35.
Sure about the code # ??
These codes are from the HFM-SFI module , which do not trip the check engine lamp.
Ck eng trip is only on Diagnostic Mod for Emissions on that car.

Your're right, the digit code for the scanner is actually a 35, which translates to a digit code 31 in the Mercedes manual that apparently comes with the scanner.

The car has 134,000 miles on it.

Every maintenance interval and most repairs on this car were done at the dealer.

The engine wiring harness was replaced at 107,000 miles (before anyone asks).

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

I assume the fuel pressure was checked when the regulator was changed ??

I didn't ask, so I don't know if he did or not.

Fwiw, the fuel filter, fuel pressure regulator, and fuel pumps were replaced at 127,000 miles. I was chasing another problem by a different mechanic, and the problem all along was a bad spark plug wire.

I think my car really needs a priest or a rabbi.

What's strange is - why would the rhythmic, fluctuating idle only occur after the car has warmed up? I think this is the key to the problem. The car must be completely warmed up until it starts happening.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

< What's strange is - why would the rhythmic, fluctuating idle only occur after the car has warmed up? I think this is the key to the problem. The car must be completely warmed up until it starts happening.>

That indicates fuel management after closed loop .
Another possible is a sticking egr valve, The egr is not activated til warm eng. That would prob. throw the check eng. lite , though..
As a stab in the dark [ seeing the water pump change was when the
fault was noticed] ,you may want to pop the front cover [front of valve cover-plastic] and look at the vac lines to the switch over valves
that run in front of the motor. [ egr and air pump sov's are here]
More than likely your guy has already done so, but worth verifiing
before getting into fuel management, as a vac leak here could cause this complaint.

Didn't see any mention of engine wiring harness??
Are there any codes in the EA/CC/ISC control module?

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
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Retired Moderator

The engine wiring harness was replaced by the dealer at 107,000 miles.

I'm not sure what EA/CC/ISC means, but the tech hooked up his scanner (Baum CS2000) and got digit code 35, which I described in my first two posts.

Thanks.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

I'm fighting the same problem on mine, and it surfaced after the water pump was replaced...but it happens regardless of a cold or hot engine.

I have read other related threads that pointed to a faulty shifter switch curing the problem, but I am more inclined to go with Arthur Dalton's inference of a vacuum line coming loose somewhere.

I also swapped out the air mass sensor and temp sensor, and neither did the trick...

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

I could be wrong, but I believe that changing the coolant temp sensor and mass air sensor was a waste of money. The water pump was changed, then the problem started. It simply makes no sense that either of these sensors are involved.

Think about what Arthur said...vacuum lines, etc. I believe that Benzmac mentioned the large vacuum line that runs underneath the intake manifold in another post.

I'd bet a couple of bucks that your mechanic knocked something loose when he pulled the pump or something related to this task.

Just came back from the mechanic and confirmed the following:

Fuel pressure is normal.

The coolant temerpature sensor was changed because he got a code for it on the scanner, and he assumed it burned out because I lost a lot of coolant when the water pump went out and air probably got in the system.

All of the vacum hoses checked out. He even unplugged the servo unit, and that didn't change anything.

He called the dealer for advice and they suggested the throttle actuator (damn it, it's an expensive part).

I guess I'll give Phil at Fastlane a call and order it.

If there's anyone who disagrees with this diagnosis, please speak now or forever hold your peace.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

>>He called the dealer for advice and they suggested the throttle actuator (damn it, it's an expensive part). <<

I would only like to mention to you that MB Doc asked if there were any codes in the AE/CC/ISC control module because that is where you may find a throttle code fault.
The code 35 you are addressing is from the HFM module diagnostic plug # 8.
The EA/CC is gotten from plug # 14.
More than likely, your mechanic has gone into that module also
[ prob has scanned all mods], but you may want to check that out.....
Let us know what you find.....

I would go to one more mechanic and have him again check the vacuum hose which goes to the air intake and runs near the water pump area....

I just replaced my water pump, had the exact same Idle problem...took it to Benzmac.....he found the hose I disconnected by accident while getting to one of the water pump mounting bolts.

By the way.....anyone who lives in the Atlanta area should definitely go for service to Benzmac (Donnie and Bill have a first class operation and are very good to work with)

I ordered the throttle actuator, and it arrived, but when I took it to the mechanic, he was reluctant to install it since he's not 100% sure it'll fix the problem. Once you install an item, you can't really return it, so he thought it would be best if I take it to the dealer and have them diagnose it, and then he'll do the repair.

I took it to the dealer this afternoon, and I just got a call, and the diagnosis is that there is a vacum leak at the intak manifold. They are replacing the seal at the manifold to see if that takes care of the problem. If it doesn't, then they say the manifold will have to be replaced.

They also said they got codes for the O2 sensor and the mass air sensor, but think the vacum leak at the intake manifold is causing these codes.

I'll find out in the morning if this solves the problem.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

I believe it is normal to get O2 and Air mass codes when you have a vacuum leak.......the computer tries to compensate for the vacuum leak by adjusting the idle electronically.....therefore you get codes......when the real problem is the vacuum hose got knocked off during the water pump install.

I would be real hesitant to agree to a new air intake manifold when the problem is simply they need to just plug the hose back in....

Let me know how it goes.