MOTOR DECOMPRESSES plzhelp - Northern California Ford Owners

This is topic MOTOR DECOMPRESSES plzhelp in forum Tech Talk at Northern California Ford Owners .

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Posted by LXman25thANNV. (Member # 5359) on :

After installing a edelbrock lower and uper intake the motor will make a "decompressing sound" like its relieving pressure after i turn the car off. It runs perfectly and I am pretty sure there is no vacuum leak. What could be wrong???? [Confused]

Posted by JohnB (Member # 969) on :

You have a manifold leak; it is either at the head surface or upper to lower intake surface. Sounds like air blowing off, correct? maybe a slight whistle to it?

Posted by LXman25thANNV. (Member # 5359) on :

yes,but it only does it right after i turn it off. could it be anything else? like maybe the pvc valve? or vavles that arent working correctly? it doesnt leak while its running-its almost like there is pressure building up in then moteor [Confused]

Posted by BlackNGold (Member # 655) on :

Does this happen even when the car is warm?....

Posted by LXman25thANNV. (Member # 5359) on :

It only does it right after i turn the motor off. it could run for a only a sec or an hour and it will still do it. Could pressure be building up when the motor runs because of a bad pcv valve or something else??? does the manifold just run at a hegher pressure? I used the stock pcv valve even though the hole was alot bigger..good'ol duck tape [Roll Eyes]

Posted by AJBlackGT (Member # 3936) on :

Thats how a motor makes power, it buils pressure in the cylinders. You have pressure leaking from the cylinders somewhere it sounds like

Posted by LXman25thANNV. (Member # 5359) on :

................its not the cylinders-im not a total morron....but thanx for the idea [Smile]

Posted by Fostang (Member # 3752) on :

If it aint broke don't fix it.

[ February 19, 2005, 11:19 AM: Message edited by: Fostang ]

Posted by AJBlackGT (Member # 3936) on :

Yeah but the pressure left in the cylinder will leak back past an open intake valve when you stop the motor and it will make that sound...

Posted by LXman25thANNV. (Member # 5359) on :

"if it aint broke dont fix it"........ [BS flag]

what would that mean?-if it is leaking back out a valve? and why only after i install the manifold? [Confused]

Posted by CDT (Member # 5004) on :

try disconnecting any vacuum lines going to the climate controls..

Posted by LXman25thANNV. (Member # 5359) on :

really? why?

Posted by Gt88 (Member # 4361) on :

Could be that your new "Chicks dig me" manifold is letting you hear a sound that the O.E. setup didnt- let's take stock:

1: How is your car running after the upgrade?

2: If #1 is 'BETTER!' or equivalent, maybe you might just let it be?

A car with a manifold leak will let you know without a doubt- and, as you know, the piston that is moving on a compression stroke AFTER you turn off the key still has built compression, and that pressure has to go somewhere.

I think you found out just how much noise suppresion the stock intake tract has built into it.

Either way, just remember that the Honda guys pay big bucks to get the 'PSHSHSHSH' sound that you got for free, so cheer up!! [Big Grin]

Posted by Gt88 (Member # 4361) on :

Oh, and to answer your question to CDT, he's trying to help you eliminate the vacuum bleedoff at the vacuum reservior to the A/C unit- check valves go bad, and a reservoir takes it's sweet time bleeding off 25 inches of stored vacuum!!

Posted by sic5pointSLOW (Member # 4347) on :

i have a trickflow upper and lower and after installing it the car whistles and makes a air sound when u turn it off. my vac is at 22-20 and the motor runs fine. i e-mailed trick flow and bbk (i put a 70mm tb) and they both told me that its sometimes normal for this to happen. i had a friend with the same setup and his had the same sound but a lil louder.

Posted by LXman25thANNV. (Member # 5359) on :

It probably is normal then, i was just curios if any one else had that prob. thx for all the info. [dance]

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