HELP! My stock turbo hits 20PSI, isn't that bad? - PeachParts Mercedes-Benz Forum

derherr65 · #1 03-10-2007, 11:21 PM

I just did a test drive with my new boost gauge. It's plumbed in the ALDA line between the intake manifold and overboost switch. I eased down the road until second gear then floored it. Boost rose to 15PSI, and spiked to 20PSI at the shift to third. Boost rose from 15PSI to 20 PSI in third, then held 20PSI all through 4th and I quit at about 75mph.

Isn't this bad? Super high EGTs kinda bad?
I'm supposed to be hitting 14PSI max, right?
Why am I hitting 20PSI? Frozen wastegate... broken wastegate line?
My car just doesn't pull well during highway passing, at 20PSI the overboost switch is cutting out my ALDA, isnt it?

#2 03-10-2007, 11:32 PM

VERY bad.

Look for a broken wastegate line. Your overboost protection circuit (Pressure switch on the intake and overboost valve in the ALDA line) should have prevented you from passing 16psi.

You need to figure it out ASAP and drive it very gingerly until you do.

derherr65 · #3 03-10-2007, 11:40 PM

Ok, will limit boost to 10-12PSI by throttle.
How do I check:
The wastegate line? With air pressure, the turbo end should be open and the wastegate end should be closed until about 12PSI, then open, right?
The pressure switch on the intake? And what does it do exactly?
The overboost valve? It's supposed to cut off boost and let the ALDA drop to 0 PSI, right?

Thank you very much for the fast reply.

#4 03-10-2007, 11:53 PM

Quote:

Originally Posted by derherr65

The wastegate line? With air pressure, the turbo end should be open and the wastegate end should be closed until about 12PSI, then open, right?
The pressure switch on the intake? And what does it do exactly?
The overboost valve? It's supposed to cut off boost and let the ALDA drop to 0 PSI, right?

The wastegate line is just a normal hose. Check to see if it's broken, cracked, soft, oil soaked or crimped closed. It may be worth a try to get some hose from a local auto store to try and see if it solves the problem.

The pressure switch on the intake is an electrical switch that connects the overboost ground circuit at 16psi. The overboost valve on the firewall is constantly fed positive voltage from the fusebox (Check it's fuse, BTW). At 16psi the switch connects it's ground and the valve actuates. It vents the boost signal going to the ALDA which cuts fuel down to safe levels. If it's working correctly, you should noticeably feel the engine's power cycle up and down on it's own as the ALDA gives and takes away fuel.

derherr65 · #5 03-11-2007, 03:22 PM

I pulled the wastegate line. It was fuel hose. It was still in one piece but so brittle the ends broke as I removed it. I replaced the hose.

The real problem is the fact that my wastgate diaphragm is gone, I think. When I blow into it the air comes out the bottom of the valve. No pressure build up = no wastegate. Where do I get a replacement?

Also how do I test the overboost protection circuit? I want to know if the sender works, and if the overboost valve works.

#6 03-11-2007, 05:55 PM

Quote:

Originally Posted by derherr65

The real problem is the fact that my wastgate diaphragm is gone, I think. When I blow into it the air comes out the bottom of the valve. No pressure build up = no wastegate. Where do I get a replacement?

Also how do I test the overboost protection circuit? I want to know if the sender works, and if the overboost valve works.

You can check the diaphragm by removing the cap like you would if you were adjusting boost pressure. Take a very small flatblade screwdriver and work out the retaining spring from the inside lip of the wastegate lid. That will let you directly view the diaphragm to see if it's torn. If it is torn, your only options are to replace the turbo or find a used diaphragm to replace yours.

The simple way to complete the circuit with a nail into the connector that plugs onto the manifold switch and touching the manifold with it (With the key in the run position). If the rest of the system is working you will hear the valve on the firewall click when you touch the manifold. That means the pressure switch is probably bad. The cheapest route would be to get a used one (from the junkyard or another member here) because new ones cost about $50. If it does not click, check the fuses and make sure the valve on the firewall is completely plugged in.

derherr65 · #7 03-11-2007, 06:05 PM

Quote:

Originally Posted by ForcedInduction

If it is torn, your only options are to replace the turbo or find a used diaphragm to replace yours.

I was afraid of that.
Ok, will test and let you know.

derherr65 · #8 04-05-2007, 04:35 PM

Got my turbo wastegate diaphragm repaired. Thanks everyone for all the help, and especially bgkast!

Youtube videos:
cruising at 60mph - http://www.youtube.com/watch?v=OOOHYYY1LKg
0-60 in the 14s. Gonna have to work on that. - http://www.youtube.com/watch?v=mHtPQi78KNM

DIY - garrett wastegate diaphragm replacement and boost adjustment.
http://www.schumanautomotive.com/forums/showthread.php?p=30668&posted=1#post30668

DslBnz · #9 04-05-2007, 04:40 PM

Quote:

Originally Posted by derherr65

Got my turbo wastegate diaphragm repaired. Thanks everyone for all the help, and especially bgkast!

Youtube videos:
cruising at 60mph - http://www.youtube.com/watch?v=OOOHYYY1LKg
0-60 in the 14s. Gonna have to work on that. - http://www.youtube.com/watch?v=mHtPQi78KNM

DIY - garrett wastegate diaphragm replacement and boost adjustment.
http://www.schumanautomotive.com/forums/showthread.php?p=30668&posted=1#post30668

Your speedometer appears to be reading too low.

derherr65 · #10 04-05-2007, 04:48 PM

60 mph on the speedo is a GPS checked 63mph. As best we can tell the tach reads a little high. The combination of speedo and tach errors makes it look like I have the 3.69 gears. I don't, just the normal 3.07.

winmutt · #11 04-05-2007, 05:11 PM

Tachs are notorious for being inaccurate.

derherr65 · #12 04-05-2007, 05:52 PM

Are they adjustable?

#13 04-05-2007, 10:36 PM

14s 0-60 is great already.

derherr65 · #14 04-05-2007, 11:21 PM

Lol, a 14 second ET is a leisurely 1/4 mile run, not a good 0-60. My goal, for this car, is an 11.5 second 0-60. Sadly the only claim I've heard under that time required a Myna pump and a few other mods.

2.5Turbo · #15 04-06-2007, 01:27 AM

Intercooler, possibly a bigger turbo, and a Myna pump might get you down that low.

Otherwise, you'll need to get a 124 with a turbo 602 (12 sec 0-60) or a turbo 603 (10-11 second 0-60). I would think it wouldn't be too hard to get a 11.5 second 0-60 out of a stock 602.962 without having high EGT's, as it already does darn near close to 12. A stock 603 gets you under 11 in the 124.