New Mystery on my 124 91 300D 2.5
 

Today running on the hwy at about 65mph my engine started missing slightly and intermittently,when i got in to town and stopped for a red light it was very rough at idle.Once i got into a parking lot and raised the hood With the engine running the little clear fuel filter was just full of foam and bubbles.This car is new to me and this is the first tank of fuel,the gauge was at 1/4 so i figured the gauge was incorrect and i was out of fuel,however it only took 14gal to fill it right to the lip of the fill pipe so i guess the gauge is right.
Does anybody have an idea of whats going on here? Is it likely that the lower part of the fuel filter is so plugged to cause such a problem when the fuel gets down to the last few gallons?
I cant do much about it now as the tank is full but i sure would appreciate any advice you might have.Car runs fine once i put fuel in it,actually runs too fine as today i had a manifold pressure gauge on it for the first time and it can easily exceed 22psi but thats a story for another post when i get time to really get into it. :confused: TIA Don

at 22psi you'll blow something up.



Sounds like an air leak in the system. Replace all rubber hosing, and then start with the lift pump (tricky rebuild, use viton orings)


It's also good to replace the filters if it's a new-to-you car.

Sounds like air entering somewhere, and/or the tank strainer might be occluded? On a new to you car, definitely replace all the filters. And fluids, including brake fluid.

22psi? I'd question the accuracy of the gauge. Most stock engines can't make more than 15-18psi peak. Spec is 12psi, +/- a couple of pounds. Extra boost doesn't make power, it just overheats the intake air charge and increases EGT's... not good.

:eek:

Especially on this engine with its computer controlled wastegate. There's no wastegate hose to break which is the most common failure resulting in high boost on the older diesels. If the computer isn't receiving a good boost signal from the intake manifold at the pressure sensor, it won't just keep the wastegate closed trying to see more boost. It will throw an error code and cut boost. The transducer also does not fail open; when they go they fail closed (again, no boost). Unless somebody has piped direct vacuum into the wastegate actuator, I'd check the gauge.

The previous owners indie had told him that for $200 he could "get the turbo working".Now that i have had a chance to look at it i find that he had connected a vacuum line directly to the wastegate actuator AND connected the manifold pressure line directly to the aldl!!! The aldl has not been tampered with.I only put the pressure guage on to get readings under driving conditions as my son has a 93 2.5 that he feels is low on power and next week we will be checking it over.When i first got to the hwy where i could step on it and i didnt really floor it ,it definetly went over 20psi for an instant.I immediatly let off and drove it more carefully the rest of the trip and did not exceed 15psi again.The Gauge of course could be wrong,but its is fairly new 0-30psi industrial gauge from Grainger so i have no reason to distrust it. The really scary part of this is having been driving it without knowing all this and the other day running it WOT up an access ramp onto the interstate.I wonder what the manifold pressure was then and how the headgasket survived.I really intended to start another thread for this subject but since it got started here might as well keep it here.Will be a while before i can check out the EDS and see what was the problem that caused him to do this and decide if i will repair the original system or just eliminate it and go to a pressure operated actuator for simplicity.All suggestions and opinions are welcome. TIA Don

correction; more psi will get you more power - you'll have to come up with more fuel is all.

Sure, but more PSI alone gets you nothing.

I can't believe a "mechanic" would actually force a wastegate open at all times. That's crazy. I'm still surprised it hit 22psi max. I haven't tried (temporarily!) bypassing the wastegate on my 2.5T, to see what the peak pressure is. On my 3.0's it wouldn't go past 17-18. Regardless, it should be fixed properly. It could be relatively simple, like a flaky OVP relay or something...

:sultan:

I agree, "mechanic"is used to describe alot of people who dont deserve the title.I worked with mechanics all my life,some good some not so good.Today i spent alot of time verifying Pressure and Vacuum lines are connected properly and with the exception of having bypassed the wastegate control transducer and the overload switchover valve things are properly connected.I tested the switch for the switchover valve and determined that it does not complete the circuit until about 20 psi,well above the 1.3atm spec but since the valve was bypassed anyway that didnt matter but will have to be replaced when i get the rest of this straightened out.tomorow i will test the 3 transducers and determine which way i will correct the problem.I am leaning toward just going with a pressure type actuator and eliminate the many possible faults with all these vacuum lines and devices.Simplicity is good!All sugestions and opinions are welcome. TIA Don

checked the 3 transducers today,when i apply vacuum to the topmost nipple with no voltage applied will not hold any vacuum,however when i connect 12v to the terminals they do hold with no leakage.The 3rd one is down under the air cleaner box and not easy to get to the wire connector but it holds vacuum without voltage applied.Does that mean that 2 of the 3 are bad or 1 is bad.Or could it be that i am not testing them properly.The intake flap actuator leaks down, the egr holds and can hear it drop when the vacuum is released and the wastegate actuator holds as well. All sugestions and opinions are welcome. Looking forward to you comments. TIA Don

Here's some reading and procedures for ya
 

EGR - Vacuum Transducer Leak Test

Date: August 1999

Order No.: P-SI-MBUSA-14.20/13

Supersedes:

Group: 14

SUBJECT:
ALL DIESEL MODELS
EGR VACUUM TRANSDUCER LEAK TEST

Following are procedures for an EGR vacuum transducer in-car characteristic curve test, internal leak test, and external leak test.

In-car characteristic curve test

The Mercedes-Benz Special Tool vacuum gauge (see below) is used to monitor the vacuum signal of the EGR vacuum transducer while operational in the vehicle. The leak test is performed at 25%, 50%, and 75% duty cycle (DC) using the Hand-Held Tester (HHT). Duty cycle is activated by way of the HHT 'activation' menu (the second screen of # 3 "EGR valve").

1. Check the vacuum pump for proper output. See appropriate diagnostic material for specifications.

2. Make sure the vacuum gauge needle reads "0".

3. Connect the vacuum gauge hose to the EGR vacuum transducer OUT cover port.

4. With the engine idling, activate the duty cycle using the HHT and observe the vacuum gauge readings.

5. The vacuum gauge should read steady, with no oscillation, within the below applicable tolerance ranges.

6. The vacuum reading should drop to its nominal value again once disconnecting the HHT.

7. If the vacuum reading is not within its applicable tolerance range, replace the vacuum transducer.


Tolerance Ranges

Note : The HHT can only access the duty cycle of the EGR vacuum transducer. To test the pressure flap/waste gate transducer, the electrical plug must be disconnected from the vacuum transducer and connected to the pressure flap/waste gate transducer. Make sure the electrical plug is connected back to its original position upon completion and all DTC's are erased after the test.

Internal leak test

1. Connect the vacuum gauge hose to the EGR vacuum transducer ATM cover port.

2. Leave the VAC and OUT cover ports open.

3. Pump up the vacuum until the gauge reads approximately 450 mbar (13").

4. If the vacuum gauge reading drops from 450 mbar to 400 mbar in less than 2 seconds, replace the EGR vacuum transducer.


External leak test

1. Cap-off the OUT and ATM cover ports.

2. Connect the vacuum gauge hose to the VAC cover port.

3. Pump up the vacuum until the gauge reads approximately 600 mbar (18").

4. If the vacuum gauge reading drops from 600 mbar to 500 mbar in less than 15 seconds, replace the EGR vacuum transducer.

http://www.peachparts.com/shopforum/showthread.php?t=140425&highlight=602

http://www.peachparts.com/shopforum/showthread.php?t=166429


http://www.peachparts.com/shopforum/showthread.php?t=152040&highlight=y31%2F5

Thanks very much for the help and information.I will be away from home for about 2 weeks so willl not get back to work on my 91 but i will be in NC helping my son with his 93.we will be doing his timing chain and then tackling a similar lack of performance on his car.I will revive this thread when i get back to my car as i really dislike threads that dont follow thru to the conclusion whatever it may be. Thanks Don