Hi Everyone,
I am having a hard time finding this one. My wife came home earlier today and asked me to look at the Discovery (2004). She said it was making a very high pitched sound on deceleration. I got in and took it for a ride and noticed exactly what she had been hearing. Here are the symptoms:

1. Happens on engine deceleration. I can duplicate it parked with the hood up and using the cable to the accelerator.

2. It sounds like vacuum leak not mechanical and it is effected by rpms.

3. The sound is very, very loud. Is a high pitched whistle sound.

It seems to becoming from somewhere around the ICV (Idle Control Valve). I disconnected the ICV during a time when the sound was still present at idle and the sound immediately went away. I thought I had it solved, but I increased the rpms again with the ICV disconnected and the sound reappeared, although it was less intense. I did my best to do what I could for checking for leaks on the hoses and fittings, but a full investigation will have to wait until this weekend. It's very strange as my wife said it came on all of a sudden. Any input would help. I checked a few thread here with similar symptoms but a solution was never posted.

Thanks,

Mike

 

Sounds like a vacuum hose is off.
Check for loose hoses.
Or, you have a crack in the IAC hoses.
You need to wiggle all those hoses while you are using the throttle.
Two hands - so you can manage it.

Also check that the piping is on from the throttle body back to the airbox.

 

I will check everything today. It seems louder toward the drivers side, but its hard to tell. I'll let you know what I find.

--Mike

 

spray a little wd40 around the intake it may help you locate the leak, the idle will go up when you hit the spot.

 

There is also the possibility that either you metal plate between the 2 manifolds is loose or you could have a blown head gasket shooting exhaust from between the head and the block, which if that is the case, needs to be dealt qith ASAP cause it can cut a groove into the block.

 

My brake booster hose did that cause the last owner didnt have the thread in insert piece so just jammed the hose into the back of the throttlebody house

 

I had about an hour last night to look for leaks. I let the engine run at idle and then would give it a bit of throttle by hand. The whistle returned shortly after the engine was almost to operating temp. Same symptoms as before. I wiggled and tested every line and there was no change in the sound. It still appears to be coming from on top on the engine near the IACV maybe to the back a bit. I will next be looking for leaks with carb-cleaner or WD40 as suggested.

Here is where things got interesting. After about an hour of this the whistle started to become constant and would even tale longer to go away after throttle was added. As I went to the cab to check the temps I noticed I was now throwing a Check Engine Light. I pulled the codes P1551 and P1510. I shut everything down and then wiped the codes. Started it back up and within a minute they returned. Both are IACV codes it looks like. I am wondering if the sound could be coming from the IACV maybe? I am not sure how it works internally. When I first had this issue, unplugging the IACV would make the whistling go away or at least cut it down to very light. Can these be cleaned or should I order another?

Thank You,

Mike

 

IACV is a stepper motor attached to the valve.
The stepper runs a worm screw or screw shaft which opens and closes an air valve.

Usually a whistle is a higher pressure leak of air going in to a vacuum source or outward.

OK here is how to find it..

Get a rubber hose and hold it to your ear.
With the other end - move the open end all around until you hear the
whistle strongly.

Warning - contact with a pressure or vacuum source could wreck your ear drum.

 

There was also a long ago post of partially obstructing the air intake to make the vac noise even louder.

 

Easiest way for an vac leak is to just spray starter fluid in small doses around areas methodically and wait for a jump in rpm, or if in any particular area it seems to dry instantly. use your body or the can to help block wind from the fan and squirt small doses very closly along seems and hoses at different angles, not from far back misting the engine. Also benefit, by the time you are done, youre engine will be nice and clean