Engine idles and runs fine, but has a vacuum leak type sound on startup. It can also be heard when throttle is opened. Any ideas?

 

pop the hood, listen close n see if you can isolate it.

 

Likely normal airflow noise.

 

NO, it is NOT a normal airflow sound! It sounds like a massive vacuum leak, but the engine runs perfectly. It sounds like it is coming from the throttle body.

 

AS mentioned above ,it;s normal for the 50mm to make some noise at start-up.Kind of like a big 4 barrel carb opening up,sucking air and mixing fuel.If you ever noticed when you cruse at hiway speeds the throttle bodie kind of whistles when its happy at RPM and speed.Quirks of 5.9 motors.They grow on you>>>>>>>>>>>>>>>>>

 

mine makes the same sound at idle. I think its the iac because my rpms flucuate at idle and I have a bad o2 sensor. a good way to check for a vacuum leak is carb or brake cleaner. if your engine reacts to it then you have a vacuum leak.

 

Just a thought, a better way to check for vacuum leaks is to get a scanner that reads live data, esp. fuel trims. These modern engines that are computer controlled fuel injection, can get a cylinder that would normally fail with a carb n normal ignition. you might get a response with spraying something on the intake, vacuum lines or throttle body if the leaks big enough, but the computer can usually correct pretty rapidly for smaller leaks. go for fuel trims. It's more accurate, because while the hissing noise might be the IAC, the IAC might be compensating for a rich signal from the fuel trim instruction of the ECM, possibly caused by an O2 sensor thats stuck in a low or open mode...AND...that could be caused by a leaking fuel injector...n so on n so forth. Step by step, verifying fuel trims is your best bet.

 

+1 on IAC. Mine does it on occasion. Indicative of another vac leak somewhere. IAC goes nuts trying to stabilize the idle

 

Yeah +1 on being normal sound. Get a video and post it here if you don't think it's normal and we'll have a listen. I've got the same year and motor as you and get the same sound. It's especially amplified when it's a cold start in cold weather. It's just the throttle body moving a good amount of air through. But if you're that concerned, check all the vacuum lines as mentioned and replace if needed.

 

I recently had to remove the air cleaner housing to replace the temp sensor. Apparently when I did that, the round gasket that goes between the throttle body and the air cleaner housing came off. Replaced the gasket and no more vacuum sound.