I think I have a vacuum leak...maybe...

I can hear a sort of whine/whistle, but only when the engine is under load, it's not there if I give the engine a rev in neutral, only when the vehicle is underway and with a load.

I always thought you'd be able to hear a vacuum leak at idle or without load.

But with a supercharged engine, maybe the "extra vacuum" from the blower emphasises this?

I swapped over my throttle body a couple of days back, and i'm sure that noise wasn't there before. I fear that it's a leak at the gasket where the throttle body bolts on to the inlet elbow of the supercharger, because I had a hell of a time getting it all lined up and bolted together.

Does this sound likely? A vacuum leak at the throttle body only audible when the engine is under load i.e. on boost?

 

Sorry this won't help much but you can't always hear a leak at idle. Still worth checking fuel trims at idle, I think. And at speed under load (but if WOT then I think it just does table look up instead of using O2s and thus any trims may be fake if available).

I hope someone can be more help.

 

Thanks John, I've checked the fuel trims at idle and they're close enough to 0%

I don't remember what they were like at part throttle before but you can see for yourself here.





Just pottering along the short term gets up to around 3-6%, half throttle sees them up higher, in the teens and low 20's

Bear in mind these values are less than 100 miles after an ECU reflash...

 

great display of readings.
what are you using for the readings and what are you displaying them on ?

 

It's the Torque App on my Android phone, using a ELM327 bluetooth OBD2 adaptor.

Took the screenshots with a screenshot app.

 

The teens and more the 20s for fuel trims? Quite high but don't really see them as air/vacuum leaks if only at speed.

Isn't there lots of vacuum at idle, so a leak should show up then?

I never thought about it before but as the Eaton works harder what happens to pressure (I suppose must be partial vacuum) on its input? That's where the gasket you're worried about is, right?

 

That's basically what i'm thinking, the Eaton is a pump, if you have positive pressure (i.e. boost) on one side, then there would be a vacuum on the other, maybe not a lot, but still more than just a regular manifold vacuum like on a naturally aspirated engine.

I'm quite sure that this is why the leak is not audible at idle, only when moving...if it is a leak...

 

Couple things, guys. Think about how the supercharger works, this is where some folks go off the rails. The SC is ALWAYS pumping, always. The thing that keeps it from boosting is the bypass valve. There is little pressure unless you tromp on it because it just pumps in a circle, right back into it's own intake most of the time. When you tromp the throttle, the bypass valve CLOSES, cutting off the return to the inlet. That means the boost is forced through the intercoolers and into the engine. Now there is pressure. That is probably the source of the whistle. Your trouble could be that gasket you mentioned, but I think it's more likely one of the gaskets or tubes on the pressure side of the induction system.

Fuel trims...... Forget the short term fuel trims bouncing around; that's exactly what they are supposed to do. They are doing their job. The long term trims staying near zero are telling you things are working pretty well. Seek out my two papers on fuel trims in this forum and get a refresher.

The fact the trims are not affected much also seems to indicate a pressure leak rather than a vacuum leak. Think about it, a little air getting OUT will not really affect the exhaust oxygen much, a little air getting IN would.

Good luck!

 

Thanks Steve.

Some getting out was what I wondered but I thought the gasket was on the input rather than output. Supposing it's the latter, though, then air getting out will tend to cause extra richness and some lost power.

The trims would only show rich if the O2s can be / are reported in that situation. This may be where only IDS/SDD can show them?

 

Well, SDD can actually be one-upped by other stuff sometimes. The attached screen can illustrate a couple things.

First, with regard to the Short Term Trims, I said not to pay such close attention to them, here's why. This screen is from a logged drive after I made a purge valve repair, to confirm it was fixed. All the cells in the FT Map for both banks show LT Trim very near zero. That is what we want to see, this car is darn near perfect. Now look at the lower red box, you can see the Min and Max figures for both LT and ST trim. This car is perfect, and the ST shows having gone both way high and way low. The fact you see that tells you absolutely nothing, IN THIS CASE. They do have value at other times, though, just not right here.

Now if we were to map Cameron's car the same way, and our theory is correct, that is boost pressure getting out is causing his whistle, I would expect the FT figures in the cells in the upper red box [those are the load cells where we would expect SC boost] to trend negative. Negative because it would be metered air that never made it to the combustion chamber. The O2 sensors would detect the actual exhaust oxygen and the trims would respond accordingly. If the leak was to just one of the intercoolers, only one bank would trend negative, if it was the seal at the SC outlet, I'd expect it to affect both banks.

In the real world, how much it would affect performance is anybody's guess. Depends of the size of the leak, of course.

Cheers,