[Eldyfig]

Does anyone want to confirm my suspicion of a leaking intake or give me a hint to a different solution?

On my 94 Eldy, about a week ago, after I started the engine cold, I got a high pitched whistle from under the hood.:wtf: I went to look and couldn't find the source. The whistling stopped after about 10 seconds. I had recently used my cruise control a few days earlier for the first time in a couple of years, so I figured it may have had something to do with that. Well, the sound went away, so I went on my way.:elvis1: Checked codes while driving...none.

Around about the same time a week ago, the engine started experiencing a rough idle every once in a while. I wouldn't say a surge, but more like a slight drop then a kick back up. Let's say it is idling in drive at 650. It drops to just about 600 then kicks back up to about 750 and then settles back out at 650. Not real bad, but you can feel the roughness. Now, this doesn't happen all the time, doesn't matter if the engine is cold or hot, a/c compressor on or off. Acceleration is not effected. I went into PS04 PCM Override and did an injector disable test. I have done this before to diagnose a bad coil pack. The change in exhaust note was the same for each cylinder that was disabled. No luck finding anything there. Then again, the idle wasn't rough at that time. Checked codes...none.:cynic:

Now back to the whistling...just the last couple of days I have noticed the whistling again. After driving for about 8-10 miles above 65 mph on the interstate, I will run into some slower traffic before I exit the expressway to get to work. Well, when I let off the gas the whistling starts. As soon as I touch the accelerator, even a little, the whistling stops. Today, it was whistling as I parked the car at work. I got under the hood to check it out but it was too loud to pinpoint the source.:dammit: I pressed on the throttle lever and it went away. When I let go of the throttle, it came back. I continued to look around and found nothing conspicuous. The whistling disappeared after 15-20 seconds.

When I go home in the morning, I think I will go into PCM Data while driving and watch

PD02 - Manifold Absolute Pressure Sensor in kPa
or
PD03 - Barometric Pressure in kPa

to see if there is a change in pressure while it whistles, if it whistles.

So, any confirmations of an intake leak or any new ideas?

 

[Ranger]

Definately sounds like a vacuum leak. Idling or deceleration will give you max vacuum (whistle). When you throttle up, vacuum decreases (whistle stops). Use a stethescope or a length of hose to your ear to locate it. You can get a mechanics stethescope for $10 -$20 and will be amazed at the things you will hear and ohe well it magnifies sounds. Listen around the manifold cover seal and all the vacuum hoses.

 

[Eldyfig]

I have a stethoscope. I plan on using it tomorrow. I haven't had time to pull it out so far. That and the fact that the whistling hasn't happened at home since the first time it happened.

 

[Krashed989]

I don't think you're going to find anything with the PD03 because that is the outside air pressure.

With the PD02 on the other hand, it shouldn't go over 48. Even if you try to make it go over that number.

Good luck!

 

[Eldyfig]

That is why I posted both data codes. I was unsure of which to go with. What is this, a PCM that tells weather conditions?!

I am not near my manual at the moment. Is 48 the normal number for PD02? If the intake is leaking, I would think it might change. Doesn't the air pressure change as the throttle changes? This I do not know and am curious to see.

 

[Krashed989]

At idle (700rpms) it should be about 35, then if you hold it steady at any rpm higher than that, it should be lower. When the rpms are changing it will raise but should never go above 48. I can't get mine to go over 44. I know I have no vacuum leaks.

It senses the outside air pressure so that it can calculate the density of oxygen more easily so that it can be more accurate with the fuel mixture.

 

[Eldyfig]

Well here is what I had while driving home from work. Never did hear the whistle though.

Barometric pressure outside was 102.
Manifold pressure before starting engine was 102.

This makes sense to me while the engine is off. After all, the manifold isn't completely sealed so it equalizes with the outside air pressure probably through the throttle body and air intake piping.

Started the car:
Cold idle in park - 37
Cold idle in gear - 44

While driving home, the pressure raised as throttle opened. The highest I got it was about 80 when I was going on the on ramp to the expressway, about 3000rpm. The pressure reading moved in accordance with the throttle. When I let off the accelerator, the number lowered.

Got home and parked:
Warm idle in park - 35
Warm idle in gear - 40

I realize I have a sporadic vac leak somewhere, but these numbers seem like what it should do. I believe your numbers are from checking while parked maybe?

At idle (700rpms) it should be about 35, then if you hold it steady at any rpm higher than that, it should be lower.

In park, warm idle - I did have 35. Gave it gas and the number lowered. Just like you mentioned.

But, when I put it in gear and gave it gas (kept foot on brake because I was parked) the number raised.

Give your car a drive while watching that number if you are curious. I'd like to know myself. I really think it is doing what it is suppose to. I checked a couple of other things while parked at home. At one point, the idle did get rough. I checked the actual rpm in the digital display and it fell off then jumped to hit 800. Of course, when the idle flickered, the manifold pressure flickered.

It senses the outside air pressure so that it can calculate the density of oxygen more easily so that it can be more accurate with the fuel mixture.

Don't you mean it senses the manifold pressure? PD03 is the outside pressure.

 

[Eldyfig]

I did the injector disable test again. This time I was able to do it while the idle was rough. Found a couple of weak drop offs. I happen to have a new GP Sorenson coil pack I keep as a back up. I exchanged it with the coil pack feeding one of the weak cylinders. It helped. Now, I have already changed out two coil packs already in the last year. I still have two originals. I went and picked up two new acdelco packs and will be installing them in the morning. That way all four will be new.

My idle problem should be fixed.

I used my stethoscope today. Of course, the engine wasn't whistling. So I went and listened to other parts of the engine. The ISC, that was interesting listening to that work. I let my daughter listen to the coil packs click, the oil slosh under the cam covers. She really liked listening to the trans. You could really here the fluid moving in there.

 

[Ranger]

Put the stethescope on the injector and you can here it cycling.

 

[Krashed989]

Eldyfig, you're right. I did do that when it wasn't in gear. Also I did what you did today and got identical results as you. So your vacuum system is fine, or was fine when you did the test.

I wonder if cruise control has anything to do with the whistle...?

 

[Eldyfig]

Well , I am prepared the next time it happens. I drive with my stethoscope around my neck. They call me the CarDoc.

No, no...but I am keeping it in the car behind my seat so I will be ready to investigate.

 

[Eldyfig]

Well, it did happen again. If you are interested I am chronicling it at this thread:

http://www.cadillacforums.com/forums/showthread.php?t=59333