Low Turbo Boost - Page 2 - PeachParts Mercedes-Benz Forum

oldsouth · #16 06-06-2003, 01:20 PM

If you have disconnected the EGR, that is why you have no boost. The computer cuts boost if the EGR is not working. Only way around is to replace the wastegate with a pressure operated one and plumb to the intake manifold with a manual boost pressure controller inline between the two. That is the best setup anyway. Quicker boost and easy to adjust.

Tasma · #17 06-06-2003, 01:29 PM

I did not disconnect the EGR. How would I know if someone did?Is there a favorite place people do this. Any other likely causes?

There sure is a mess of hoses. Having a schematic of how it all works would make it much easier.

I wonder what the dealer would sold me if I took it in. They wanted $1036 just for the rebuilt turbo not counting labor. Yikes

Tasma · #18 06-06-2003, 03:54 PM

There is vacuum coming up to the system of 2 electrically operated valves and vacuum going down to the valve that is mounted down close to the headlight, but the vacuum does not leave there to go to the wastegate. Not knowing how this is supposed to work is making it difficult. Does anyone have a drawing in PDF format from a service manual that could be sent via e-mail? It really feels good to know that it’s not the turbo or the pump!

Could the over boost sensor be bad and if so where is it located?

CJ · #19 06-07-2003, 12:27 AM

Oldsouth,

I made a bloack off plate for my EGR and I have the same problem. All of my hoses are connected. I wonder if making a block off plate effects the boost? I did notice that I only get boost on cool nights or early mornig when it is cool. the car then reall takes off and CAN and DOES plant you in the seat.

oldsouth · #20 06-07-2003, 01:50 AM

I don't know exactly how the setup is on other models, but on my 1995 S-350; and probably most other late model turbocharged, computerized diesels; it works like this:

The turbo is controlled by the wastegate. On these cars the wastegate is vacuum operated and controlled by the computer via the valves behind the passenger side headlights. This setup also controls the EGR valve. There is also an airflow sensor in the air cleaner assembly and an overboost protection circuit that also comes into play. If the computer senses the EGR is not working it says hey something is wrong and automatically cuts the boost via the vacuum controllers. You ask how it knows since there is no sensor downstream of the EGR. It knows by the airflow meter. Less EGR gases = more airflow than normal. Bingo - no boost. There is two ways around this:
1- I have heard you can use some special voltage diode in the wires going from the airflow sensor to fool the computer into thinking the airflow is right when it isn't. This is way over my head and I did not try it.
2- Take the vacuum wastegate off the turbo and replace it with a pressure operated one. Plumb it to the intake and completely do away with the vacuum transducers. (Plug the lines) It is also much easier to adjust the boost with an inline boost pressure controller than trying to adjust the wastegate. (It is hard to get to on my car) Install it between the turbo and intake on your new hose. I have found that the Dawes device works great for me.
http://www.dawesdevices.com/boost.html
It is also good to have a boost pressure gauge in the car to keep tabs on the boost since you are disconnecting the overboost protection circuit. I went with this one - very small an will mount anywhere. http://www.speedshop.org/store/Scripts/prodView.asp?idproduct=20

Why go to all this trouble to disconnect the EGR. Have you ever looked in your intake manifold. Enough said. EGR was only put on diesels to cut NOX emissions. It does so by lowering the combustion temperature by circulating exhaust gases that have already been burned into the engine. (Less powerfull explosion) More EGR = more soot. It's a fine line. The advantages of no EGR is no soot in the intake to clog it up, less soot out the pipe, more efficient use of the fuel (more powerfull explosion). The disadvantages are increased engine operating temperature usually only in hot climates, and increased NOX emissions.
End of dissertation

CJ · #21 06-07-2003, 02:06 PM

I'll have to check it out!

Tasma · #22 06-07-2003, 07:05 PM

Where do you find a pressure operated wastegate control?
I think this is the route to go, the black goo in the intake always bothered me. Less soot in the engine and the oil is a big plus.

oldsouth · #23 06-07-2003, 09:23 PM

If it is a Garrett AiResearch, get it from them. If other brand, get it from the manufactor. They should have a pressure operated one because it is the most common.

Tasma · #24 06-14-2003, 10:27 AM

I finally found all of the problems. There was more than one so it made it a bit tricky to find.

There was a small leak in the hose that connects to the vacuum modulator for the transmission. This small hose receives the vacuum from the same junction as the waste gate vacuum transducer. Thus the wastegate would not close.

The second problem was the EGR system was 70% blocked with carbon. Whenever the normal air flow isn’t correct based on mass air flow and engine rpm , the computer shuts down the turbo. It this would explain why a blocked exhaust system or EGR will cause low boost.
Since I didn’t want the EGR anyway, I made a small circuit board to control the wastegate at 10PSI without connecting it to the computer. Now I have full boost and no EGR.

Thanks a million for those that helped. This board a fantastic souce of info.

gsxr · #25 06-14-2003, 12:05 PM

WOW! Glad you fixed it, and saved a bundle by not buying a new turbo!

I understand the 2.5L engine management system much better now. Could you explain in more detail what you did to disable the EGR and retain normal boost operation? Perhaps with a photo or two if you have access to a digital camera? I'd love to know how to do this!

Best regards,

Tasma · #26 06-14-2003, 04:58 PM

I have a digital camera but I am not sure how to post pictures here.

My business is building pressure based bottle inspection equipment so I have a lot of pressure sensor experience. I bought a Motorola pressure sensor rated for 15 PSI. Then I made a little circuit board (another part of my business) that converts pressure into a current signal to send to the wastegate vacuum transducer. At .6 amps these devices send full vacuum the output port. The amount of vacuum is proportional to the current. My circuit starts sending current to the transducer is at 8 PSI boost and delivers full vacuum at 10PSI boost. The circuit is only about 2 square inches and cost about $35 for the parts.

This is probably not the simplest way to solve the problem but it works. The boost is up to 10 PSI at about 1800 RPM and stays there all the way up to 4000 RPM. The car seems like a brand new vehicle!! The passing power in high gear is great and the transmission shifts very smooth now.

coachgeo · #27 06-14-2003, 05:43 PM

Quote:

Originally posted by Tasma
I have a digital camera but I am not sure how to post pictures here......

Down below the box you type your post in is a way to post ONE picture from your computer. You just have to find it on your computer and select it. You can just do it several in several post to get all the pics you want on here.

I think that is best cause they pics will remain for archive purposes. I also use http://www.montypics.com/ but I plan to make sure I keep pics there till hell freezes over just for those who do an archive search.

when you use something like Monty's you use the [img] button from up above next to the [@] button. Advantage to this is you can put several pics into one post. You can also use this button but use the address to a picture that is allready somewhere on the net.

Tasma · #28 06-14-2003, 07:15 PM

I will give it a try. The board looks like a mess because I coated it with water proofing goo.

Tasma · #29 06-14-2003, 07:25 PM

There is a fuse on the board that will blow if the output transistor fails. Without the fuse, a short in the transistor would leave the turbo full bore and could cause damage if the overboost sensor on the manifold were to fail somehow. I think it's a pretty safe system.
The other bad failure would be if the hose were to come off the sensor. That would leave the turbo full bore also.

There are two systems that try to prevent over boost. One that shuts off the pressure to the IP and one that chokes off the intake air with what looks like a carb butterfly.

coachgeo · #30 06-14-2003, 07:34 PM

Ya done good Tasma,

And from that pic I can tell EXACTLY what you did..... uh huh..

yep... the resistor connects to the jizzmister that makes the whatcha tya ma call do that with the thingy. When supplied with 12v it causes the widget to modify the boost

GOT IT!

Wish I had your knowledge!!!!! LOOK GREAT.