My cousin has a Cobalt with the supercharged Saab engine in it, and he’s been having some problems lately. Here’s a copy and paste of the problem from another website:
Okay, so when the car is idling, the boost gauge now sits slightly above -20, whereas before it would be below -20 vacuum. It also boosts around 5-7psi unless I bring it to redline which in that case it’s lucky to go past 10-11psi.
I had our family mechanic inspect it, and he even removed and re-sealed the supercharger to intake manifold gasket which did absolutely nothing. There aren’t any exhaust leaks (not that are visible anyway), actually the OEM down pipe only has like 10,000 miles on it (was replaced under recall).
I don’t understand. It has to be a vacuum leak, but where? It sounds like its coming from the middle or passenger side of the car, almost like where the blower motor is.
The weird thing is, the car doesn’t really feel any slower, unless I’ve just gotten used to it being slower since this has been going on for over a year now. Just this whooshing air noise under load is driving me insane. The engine also sounds louder than normal, but maybe it’s just the muffler being worn out after being driven in the salt for the last 5 years. I don’t know.
Can anyone help me out? Is there a smoke test or boost leak test that can be done to figure out what is wrong? I made an appointment at the stealership as a last resort, but the last time I brought it there they didn’t have a clue as to what was going on.
Any help would be greatly appreciated. Thank you very much.
It sounds like a boost leak to me. If the blower is a part of the intake manifold, I suppose that would mean it would also be a vac leak. With the engine idling, spray brake cleaner or carb cleaner around areas you think might be leaking. If you hit a leak, you’ll hear the engine react.
I just looked around under the hood about 10 minutes ago and you can definitely hear some air hissing from the back of the motor. I can’t tell if its from the brake booster or closer to the exhaust manifold. I used a lighter and checked around for exhaust leaks but couldn’t find any. Is anybody a mechanic on here that would be willing to look at the car on a weekend?
I sprayed soapy water as well as carb & choke cleaner around all of the intake manifold areas and found no leaks. I’m at a loss here.
I don’t know of any places with a smoke machine. I’m actually driving the car for the day and you can definitely hear some kind of hissing when you hit the gas. The engine also sounds noisy but the exhaust looks completely fine.
OK, if I pump the brakes while at idle, should the car lose vacuum? The idle gets rough and when I stop pumping the brakes the car sounds like it wants to die. Also, I looked and the vacuum assist for the brakes is pulled from the blower. Could this be why boost has dropped?
yes there should be some minor fluctuation. The brake booster operates via manifold vaccum. Also a faulty booster could cause a leak and give you issues, however that would be rather unlikely IMO. I work at a dealer and have never had to replace a booster on a cobalt. I would look at the car with you, but I’m up in Plattsburgh. Honestly your best bet really is to PM 88slowmarro and throw him a couple bucks to look at the car for you rather then trying to have the collective minds here on shift diag it through the keyboard. Also CEL on or off?
The only thing done to it is the GM stage 2 kit and some box that lets you shift with the gas wide open. The car is surprisingly fun but it feels like it was built by George W. Bush lol
with the stage 2 the car should be making close to 15psi when in WOT (as long as your over 2k rpm’s) if your only half throttle then 5 to 7 psi is normal.
i would look in and around the intake system as that to me would seem to be the only place where you could have a small air leak without having a code thrown.
these motors have a ton of check points between having both a maf and a map sensor, a wot sensor in combination with a throttle position sensor. and don’t forget it’s also got a coolant temp sensor both pre and post intercooler. If any of the tables are off on that car it will throw a code so i don’t think it’s got anything to do on the front end unless it’s a small leak.
on the back side of the motor the exhaust, as long as it’s been recently replaced. “should” be ok. I do know first hand that exhaust leaks on these cars, especially when stock, are VERY common. The flex pipe will break and can cause a “wooshing” sound at wot. having a high flow cat in the car as well can also cause the same condition so it’s hard to say exactly what it might be.
if i had to guess without driving the car, your noise is coming from the exhaust side. but i think it really needs to be taken for a test drive to know for sure
We just ordered a new manifold, downpipe, and also purchased remflex gaskets for all of the flanges. Hopefully this helps lol. Those dumpy cast manifolds apparently crack and the stock downpipes are notorious for failing so its a wise investment I suppose.