: Vacuum Leak after WOT (P0101) - 2003 STS

06-11-12, 05:09 PM
Hi Everyone

Been snooping around these forums for quite some time as i have had my eye on the Sevilles! Well recently i found a nice one in New Jersey bought it and drove it back to Wisconsin, had no problems on that trip at all. The other day coming home from work i decided to give it some on the high way, i got a bit of smoke from the Exhaust then after that a bubbling hissing sound which is now just a hiss, the revs were not normal and the car felt very down on power. Quickly took the exit for home a noticed at a stop light my revs were surging slightly.

Pulled codes when i got back and saw P0101 (MAF), did some research on this site and quickly learnt that it was probably a vacuum leak.

So i listened all over the engine for a leak, sprayed carb cleaner in all the usual weak points (plenum, vacuum joints, PCV hose) i couldn't find what it was. So i decided to removed the Throttle body and give it a clean, it wasn't too bad. While doing that i stupidly shot the red straw into the intake manifold, so i had that off as well, i retrieved the straw, checked the plenum for the notorious crack mine looks quite new, and cleaned up the rats nest under the manifold!

When i started the car back up i had SES light with P0101 ( that is the only code)

Okay now its all back together and i still have the same problem as before if i rev the engine when it settles it will raise the rpm on it own then settle down to about 750rpm. (I had this happen to me before with a ford and that turned out to be a hole in the intake)

What i did notice has when i unplug the rear PCV hose (dirty one, lots of suction) and block it with my thumb the hissing noise described earlier stops, but when i rev the engine like this it still surges.

So really my question is this is there any other place that a vacuum leak could occur? I have checked everywhere!

My car is a 2003 Seville STS with 120000 Miles


06-11-12, 05:52 PM
2003 - The intake manifold overpressure valve - the mushroom black cap at the throttlebody end on the manifold, below the plenum spigot....OR a deteriorated rubber elbow in the PCV dirty air (suction) line.

Correct idle is 650 or so.

Pictures of your manifold in my albums.

06-11-12, 06:58 PM
Hi Sub thanks for the quick response, i have checked the elbows on the dirty PVC line and that all looks good although when plugged back into the valve cover there is hissing but no idea where from (is that normal?). The Mushroom looked good when i had the manifold off. I'll check it out tomorrow after work just to make sure.

Thanks for your help

06-11-12, 09:39 PM
Hissing is not normal. Might try listening with a stethoscope to locate it.

06-12-12, 09:04 AM
Do this in order to remove one possible source of a MAF code - ANY air intake duct leak after the MAF - as well as make engine fluids checks a LOT easier.

06-18-12, 10:47 AM
Just a quick update on this one:

Thought I'd service the area and replace the plenum and intake manifold gaskets even though it looked as if they didn't need replacing. The results are the same there is still a hiss and it looks like its coming from the rear valve cover which i can see is leaking oil anyway. I cleared the P0101 code a couple of days before the work and it did not return. Absolutely no codes on my last check!

While i was working on the car i decided an oil change was necessary and while underneath the car i looked around to see the condition of components, not too bad under there but like i mentioned i can see a leak on the rear valve cover and i think the transmission gasket is about gone as the pan was quite wet with ATF. I also noticed a coolant leak that seems to be coming from the black rubber hose at the bottom of the surge tank. Will sort that out in a few weeks as I'm not loosing coolant rapidly. My DIC shows the transmission fluid life at 49%, from what i have read on here it should stay at 99%, there's no evidence of a tow hitch, so i can only presume at some point it got hot, the fluid is still red but not the freshest I've seen.

Other than those few bits and the list of things that need replacing she's running pretty well!

06-18-12, 11:11 AM
If there's a vacuum hisssssss at the right (rear) valve cover, that's the PCV vacuum side - the "dirty air" suction line. The leak gets big enough and you WILL get mixture and maybe poor idle codes.

06-18-12, 12:50 PM
I've tested that hose by taking it off and blow into it will holding my thumb on the other end it holds pressure no problem, when connected the hissing sounds leak its coming from the rear of the valve cover where the oil leak is. I'll try clamping it tonight to see if it improves anything. Do you think that the valve cover leaking would not cause a vacuum leak?

06-18-12, 01:25 PM
The PCV (Positive Crankcase Ventilation system ) in our STS does not use a rattle type PCV - it uses a fixed orifice. The "dirty air" line runs from the right cam cover to the center log of the intake manifold. The clean air supply comes from the atmospheric side of the throttlebody to the left cam cover. When the engine is running the system operates at a constant vacuum, moving clean air into the left cam cover, through the crankcase, up through the right cam cover and into the intake manifold for burning - the system removes vaporized moisture and acids from the engine guts.

That line from the PCV orifice to the inside of the intake manifold operates under up to 30" of vacuum, depending on engine speed and throttle plate opening, so that line has a very direct effect on engine performance if it has even a slight leak.

You should be able to blow through the whole system and feel very little resistance. There are no "valves" in the system in our later Northstars.

06-18-12, 01:31 PM
Thanks Sub i'll take a look at this tonight.

07-10-12, 01:13 PM
Okay i think i have this fixed. But i now have another problem which I'll start a new thread for.