More boost woes (92 300D) -- driving me BATTY! - PeachParts Mercedes-Benz Forum

cscmc1 · #1 04-19-2008, 09:24 PM

OK... this boost issue is driving me batty. I will read codes again tomorrow in the daylight, but can anyone suggest why boost would be fine (peaking at 15 or so, and stabilizing as 12lb or so) around town, but as soon as I get on the highway it all but cuts out (2 or 3 pounds at best). It seems to happen at around 45mph or so... it just peters out.

I am just about ready to can the whole vacuum system and go with a wastegate actuator of the type GSXR has added (bypassing the EGR in the process).

Any thoughts? Thanks all!

sixto · #2 04-19-2008, 10:15 PM

Put a vacuum gauge on the wastegate line to see if the ECU is cutting boost or your turbo is running out of breath. If you get the same vacuum signal above 45mph as you get at 12-15psi around town, I'd say your turbo ain't cutting it or you need a new air filter.

Sixto
87 300D

cscmc1 · #3 04-19-2008, 10:23 PM

Thanks, Sixto -- I am guessing the ECU is sending a signal; once the boost cuts out, it seems to be gone for the rest of the run (even if I head back into town). However, if I stop and restart in town, it's fine again. I'll see if I can't round up enough line to run the mightyvac line in to monitor the wastegate vacuum.

Thanks again; I appreciate the help.

sixto · #4 04-19-2008, 10:28 PM

That behavior suggests the ECU senses a fault (duh!). Have you done anything to the EGR?

My 93 SD had a similar problem. I didn't check for an electrical signal to the wastegate amp. There was no vacuum to the wastegate despite adequate vacuum to the amp. I stopped the investigation and went with a boost controlled actuator.

Sixto
87 300D

cscmc1 · #5 04-19-2008, 11:05 PM

I haven't fiddled with the EGR, but these symptoms have been occurring since I bought the car a couple of weeks ago. Thanks again for the suggestions -- keep 'em coming!

bobodaclown · #6 04-19-2008, 11:22 PM

Isn't the boost supposed to cut out? It's purpose is for acceleration only. Once you're up to speed there is no boost required and its normal. You're is doing the same thing as mine.

As a matter of fact when I get back on the pedal the boost goes back up...

kinda goes like this (from memory)
about 2200 boost kicks in. car accelerates a bit quick quicker when boost kicks in accelerate to about 60mph. boost drops to about 2 PSI. Step on the accelerator to get to 70 boost climbs until foot comes off accelerator.

I'm gona be gone for a bit. But look forward to reading more.

Glowplug · #7 04-19-2008, 11:31 PM

Ya i was thinking the same thing . Boost is supposed to blow off when your not hard accelerating and or on the highway unless you get into it . Right?

I have no idea, this is my first turbo car .

cscmc1 · #8 04-19-2008, 11:37 PM

That's just it... even if I stomp it on the highway, it will kick down but won't build boost. It's got to be a fault. At this point, I couldn't pass anyone safely on a 2-lane road if I needed to.

2.5Turbo · #9 04-20-2008, 12:58 AM

Check out your vacuum transducers for the EGR and flapper valve in the intake. If one is intermittent or bad, the computer will notice that the intake airflow isn't right and will cut the boost. It usually takes a few minutes of driving for the computer to figure it out. I don't remember if it stores any codes for this or if it just shows the code when it knows something is wrong (when there's no boost). An easy test for the transducers is to swap a suspect one with a known working one. The easiest place to do this is on the wastegate circuit, where you can tell immediately by the butt-o-meter if you're getting boost or not. Swap the EGR transducer and the flapper valve transducer with the wastegate transducer and see if you get boost. All three are the same part number, so they're all interchangeable.

I went through the same problems a year ago...it does get frustrating trying to track things down, but if you try to think in the primitive logic of the EDS computer, you'll find that it's not too difficult to track down bad components.

Boost should be available at all times. While cruising at a steady speed on the highway, the boost will be pretty low. Step on it a little and the boost will build to the teens and level off as the wastegate is opened.

Edit: I noticed that your car is a 92...you don't have a transducer for the EGR, just a couple switchover valves. The one on the flapper valve is the same as the one on the wastegate.

cscmc1 · #10 04-20-2008, 01:11 AM

Boy, this system is confusing! I have an actuator AND a switchover valve for the EGR (aft of the air filter box), and another actuator (the larger unit with the black, round plastic top, in front of and just below the air filter box) and 2 switchover valves (near the radiator) for the "Boost Pressure Control" (wastegate?) and pressure control flap. I tried swapping the latter 2 switchover valves with each other a few days ago; no help.

cscmc1 · #11 04-20-2008, 03:32 PM

Codes, for what it's worth:

5
15

EDIT: *ignore* codes 3 and 9; I just remembered that I failed to hook the electrical connector the top of the airbox back up just before I took codes. I had it off to facilitate swapping the two vacuum transducers. I'll go clear those last two codes and take it for a spin with those transducers swapped.

cscmc1 · #12 04-20-2008, 04:56 PM

So far so good. I jaunt to the highway and back shows all's well (for the moment). I'll keep the thread updated, but perhaps swapping the transducers (or cleaning their electrical connections while they were out) has helped. My fingers are crossed, anyway!

cscmc1 · #13 04-20-2008, 09:40 PM

Jinxed myself... no boost is back. Aaaaaargh!

BigBen · #14 04-20-2008, 11:06 PM

Quote:

Originally Posted by cscmc1

Jinxed myself... no boost is back. Aaaaaargh!

I have tried several times to fix a very similar issue with mine. With no luck so far.

I have a pressure type wastegate actuator on the bench now. After replacing quite a few known faulty parts (1 transducer, EGR valve that leaked, leaking flap actuator), I realized that the flap in the Mass Air Flow sensor gives a discontinous signal when I play with it by hand. So I would need to replace that part which I have not even asked for the price.

The very last thing I will try before I go gsxr route is to lock the MAF sensor flap in the WOT, max RPM, fully opened position with a metal screw. No idea what EDS will think.

If that ain't work, I'll be in for a major job of pulling the turbo from the hole, rusty studs to play with, lots of fun ahead. It's hard to believe what I'd do to get this car working good. One simply has to try full time vacuum to the wastegate actuator to confirm the great power this motor has hidden behind that mysterious EDS prison!

Good luck anyway!

Dana B. · #15 04-20-2008, 11:52 PM

I've had various boost problems over the years (which makes the 2.5T a joyless ^%$#%&$# dog). Not being a confidant DIYer and having no patience with power loss, I had my indy dial in the solution (s).

One fix was to UNPLUG and DISABLE the boost over-protection valve (sensor), which fixed more than a year of frustrating on-and-off boost (while some might say that's as foolhardy as going to a cathouse without a raincoat, I'm confidant -- and more importantly, so is my mechanic -- that MB overdesigned its turbo overprotection system and that there is little danger in popping the turbo under max boost).

When the turbo boost problem resurfaced recently, it turned out the problem was a minute, hard-to-detect vac leak.

My 2.5T now pulls with all 121 horses and instant get-up-and-go. In short: it's an utter JOY to drive.

Good luck with your turbo issues; once they're sorted you'll treasure your w124.