Boost reading

[BSKI]

Ahh March is upon us and here in the northeast my yearly ritual of ramping up my insurance and getting the 930 rolling usually depends on some sense of a warming trend along with a good soaking rain storm to clear off the crap on the roads (which this weekend surely came thru). Tomorrow is my day. That is what brings me to my question for this post. In the off season I replaced the turbo with a K27-7200 along with a Kokeln Intercooler and c2 BOV valve setup that the intercooler comes with. After completing the install I did manage to get one decent day in January to go for a lap around the block. After warmup I got into it one or twice to get a taste of my money well spent. Although short bursts I was happy with the input I got. One thing I did notice was my boost readings (on the stock guage) were spiking right up to 1 bar. Previously with my stock turbo I was consistantly at .8 (note I did not touch the waste gate spring). Any thoughts?

[Speedy Squirrel]

Check that the wastegate control hose is connected to the intercooler.

The wastegate has been known to get sticky (the valve stem sticks in the guide) when a car has been sitting around for awhile. Drive it long enough to get warmed up (oil up to temp).

Is your overboost switch ok? You might want to make sure. It should cut off the ignition at 1.1 to 1.4 bar.

[BSKI]

Definately not 100 percent sure if the overboost switch is good where I never got over .8 prior to changing turbo's. I will definately double check that the wastegate control hose is hooked up. At this time the car is still under cover. I will be getting her out tomorrow and will check the hose. I am pretty sure it is hooked up. I did need to change the hose to lengthen it during the install of the new IC . If the new hose were to be a little (used fuel injection line 3/8 dia.) larger than the stock line do you think that would effect anything? That being noted I will over the install as well as get the car out for a while to get a good spring strech prior to driving it like I stole it. Any way of testing the overboost sensor?

[JFairman]

You put on a bigger turbo, it's gonna make more boost.
The origonal boost gauge is not accurate at 1bar so you need to put an aftermarket mechanical boost gauge that goes to 1.5 or preferably 2bar on it or permanantly in the car to know how much boost your making.

The overboost sender disconnects the white wire to ground around 1.3bar depending on how old and tired it is, and the overboost circuit with yellow relay in back doesn't do anything to the ignition, it disconnects the fuel pump relay grounds shutting off the fuel pumps.

The factory revlimiter relay under the driver seat shuts off the ignition around 7200rpm.

[mark houghton]

Originally Posted by BSKI

Any way of testing the overboost sensor?

The only real way to test the OB sensor switch is to take it off the car, hook a hose up to the threaded end, and apply controlled air pressure to it. With a VOM (ohm meter) hooked up to the spade and body, slowly increase the air pressure until you get an open circuit. Somewhere between 1.2 to 1.4 bar.

That switch working is only half of the equation, though, as Jim has pointed out. The switch in turn interacts with the yellow overboost relay to shut the pumps off. If the relay is faulty, then of course the whole protection system is null and void. But the good news is that if the relay is malfunctioning you'll generally know it....as in the car won't run, boost or no boost.

Were you running the stock 3LDZ turbo before ungrading to the 7200? As already pointed out, you will build more boost and - even more importantly - much sooner than the stock turbo. Thus, you may see momentary spikes beyond your WG spring setting, but generally you shouldn't see sustained or increasing boost at that higher level once the WG has opened.

[Speedy Squirrel]

Originally Posted by BSKI

Any way of testing the overboost sensor?

The switch is removed and a continuity tester is connected to the output spade connector and the threads. It should be a closed circuit. You then have to apply air pressure to the switch by rigging up some kind of connection to a regulated air supply. Be sure to start with the regulator set to 10 psi or less. As you increase the air pressure above 16 psi or so the switch will open and you will get an open circuit. If it has not opened by 20.3 psi it is defective and should be replaced.

I was incorrect in stating that the switch cuts of the ignition. The switch actually removes the ground to the charge air pressure relay, which cuts off the fuel pump relays and turns off the fuel pumps.

[Speedy Squirrel]

Sorry for repeating you Mark, we must be typing at the same time.

+1 on not seeing higher boost. The wastegate is still in control, but there is some data here to indicate that a K27HF can creep up to about 1 bar. If you are using the 5327 988 7200 version that came with the C2T's (930.123.013.04), you should stay well below 1 bar. Search Cobalt in this forum. He put up some good data on this.

[mark houghton]

Originally Posted by Speedy Squirrel

Sorry for repeating you Mark, we must be typing at the same time.
+1 on not seeing higher boost. The wastegate is still in control, but there is some data here to indicate that a K27HF can creep up to about 1 bar. If you are using the 5327 988 7200 version that came with the C2T's (930.123.013.04), you should stay well below 1 bar. Search Cobalt in this forum. He put up some good data on this.

No problemo....seems to happen to all of us once in awhile! It's kinda like waiting 'till the last second to swoop down on an eBay bid...you don't know who else is out there doing the same thing.