Boost Pressure Controller Advice - PeachParts Mercedes-Benz Forum

oldsouth · #1 04-22-2003, 12:13 AM

I currently have the Dawes Boost pressure controller for my turbo. Over the last several months when I hook up a guage to check the pressure (at each oil change), the boost is lower than I had set it to and I have to re-adjust the controller. I am about to the end of the adjustment. Is it common for this device to get weak and change the pressure? I have thought about the electronic controller. Anybody have any experience with this type. Only thing is that they are expensive. Whatever I do I have got to get a guage inside the car. I saw the link on a post the other day about a small digital guage that I could probably install in the ashtray. I just don't know how I am going to route a wire through the firewall.

speedy300Dturbo · #2 04-22-2003, 02:00 AM

It would start leaking if the spring inside started to lose tension. I haven't heard of that happening though. Either that or you have a leak somewhere else. Check all vacuum lines from the manifold, etc.

jbaj007 · #3 04-22-2003, 02:05 AM

The Dawes device, when cranked down, is capable of ~ completely blocking boost pressure to the wastegate, thereby raising boost to the engine and ALDA . Take yours apart and clean it and check the ball seal. There is a very small (and I mean nearly microscopic) vent hole in the side of the device that you're supposed to clean with a FINE wire, also. Check banjo fitting for boost line to ALDA as the fuel enrichment works in synergy with boost in sort of a feedback like circuit.

oldsouth · #4 04-22-2003, 12:02 PM

Well, I dis some serious looking at how to get a hose or wire through the firewall and found a way. I can take the footwell panel out and get access to where the electrical wires go through the firewall to the underhood fuse box. Under the hood, I can route a wire into the fuse box through one of the nipples that is empty. If I take the top of the fuse box out that has all the fuses attached, I can poke a wire through to the inside. I found a link to a small digital guage here http://www.speedshop.org/store/Scripts/prodView.asp?idproduct=20
Where do y'all think. I would rather have an analog guage but I think this one would fit better with the interior and be small and hideable if I decided to hide it in the ashtray.

I will take the Dawes device apart and check it out.

sixto · #5 04-22-2003, 12:10 PM

Quote:

Originally posted by oldsouth
I found a link to a small digital guage here http://www.speedshop.org/store/Scripts/prodView.asp?idproduct=20

HUD is so much nicer

http://www.machv.com/headupdisboo.html

Sixto
91 300SE
87 300SDL
83 300SD

gsxr · #6 04-22-2003, 12:51 PM

OldSouth,

Did you notice any power gain with more boost? On my cars, I played around with increasing boost and saw zero power gain with higher boost pressure - the injection pump must have been at it's limit for fuel enrichment. The spec for your car is, I believe, 0.85-0.95 bar (12-14psi). What pressure are you at now?

Regards,

oldsouth · #7 04-22-2003, 01:01 PM

I run mine at around 15 psi. Before I got the controller, I was below 10 psi and I could tell a difference. Boost does nothing without fuel. I never have adjusted the ALDA and think it is foolish to go above specs with it. Especially with the 350 engine. The thing I like about a manual controller is that you build up to full boost quicker. Without it, as boost builds you are gradually opening the wastegate and bleeding off boost.

gsxr · #8 04-22-2003, 01:24 PM

Your wastegate is opening too soon, below factory spec. I would not dink around with an external boost controller, I would fix the wastegate properly! Assuming you have a Garrett turbo, you adjust the rod length from the canister to the valve. If you had a KKK there would be no rubber hose to tap in to so I'm assuming you don't have a KKK. The manual controller should not let boost build faster, you can achieve the SAME result by adjusting the wastegate directly.

I just checked specs for your car. Starting in 1992, MB lowered the boost spec to 0.75-0.85 bar, or roughly 11-12.5psi. So since yours is less than 10psi, your wastegate is opening too soon!

Tip: If your wastegate controller doesn't increase boost, or if you pinch off the hose and boost doesn't increase, there is not enough fuel to provide enough boost to actuate the wastegate. That means an ALDA adjustment is required. It is not "foolish" to adjust the ALDA, as the springs inside the IP weaken with time and the easiest cure is to adjust the ALDA *SLIGHTLY*. Of course the proper fix is to pull the IP and have it calibrated on a Bosch test bench, but most people don't do that. Mercedes even has a procedure to cure "sluggish acceleration with cold engine below 2000rpm" that specifies a 1/4 turn adjustment of the ALDA screw.

Regards,

oldsouth · #9 04-22-2003, 01:32 PM

You have to take the intake manifold off to get to the ALDA to adjust it on this engine. A LOT of trouble and then you have to put it all back on to check it.

gsxr · #10 04-22-2003, 01:47 PM

Not always true. It mostly depends on how your IP is rotated relative to the manifold. On my one car (similar engine to yours), I can remove the ALDA with the intake and injection lines in place. On my sister's car, I need to remove the rear 3 injection lines to get the ALDA off - they're in the way. Haven't tried my other car. On the one car though, it takes about 20 minutes total, in & out. Each car is different, and your 3.5L pump could be slightly different and since clearances are tight it's hard to tell without trying it. But yes, it's sure a lot easier with the manifold out of the way!!

BTW, this is one of the extremely few things I like about the old OM617 (5-cyl, iron head) engines - they sure had easy access to the ALDA!

oldsouth · #11 04-22-2003, 02:09 PM

Do you take the ALDA off to adjust it? I thought you loosened up the nut and turned the screw in to increase. Or is it out to increase. Anyway, I had the intake off a couple of months ago to replace a hard to get to glow plug and I was tempted to turn the ALDA 1/4 of a turn but reframed

. Car runs good the way it is set.

gsxr · #12 04-22-2003, 02:13 PM

You don't need to remove the ALDA to adjust it, as the adjustment is on top. However there is near zero room to do this with it in place. Much easier to remove the ALDA (it unscrews from a captive nut on the IP), adjust in-hand, and re-install. Or, pull the intake off and leave the ALDA on the IP. I find it much easier to pull the ALDA out than to yank the intake. If you don't want to tear off the top seal, you can also insert shims (proper sized washers of varying thickness) underneath - same effect.

sixto · #13 04-22-2003, 03:17 PM

The Blitz Dual Timer is an interesting device. It's primarily a turbo timer which would be very difficult to get functioning in a Diesel. On the other hand, the boost display with warning and stopwatch alone are probably worth $130. Plus it reads kPa so you won't have to convert.

Sixto
91 300SE
87 300SDL
83 300SD

oldsouth · #14 04-22-2003, 03:21 PM

Where to find??

gsxr · #15 04-22-2003, 03:37 PM

Yes, the timer function is a waste, IMO - the cooldown issue is nearly moot on MB diesels. But yes, as a boost gauge it does look kinda neat:

http://cars.ign.com/articles/384/384396p1.html

http://www.blitz-na.com/Blitz_Products.htm

Not sure if this comes with manual boost sensors, or if it will work on any turbocharged engine. It mentions a wiring harness that is specific to a select group of turbo ricer cars. The data sheet leaves a little to be desired.

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