low vacuum question

[drayrunner]

I have a o2 chevy malibu with a 3100 v6 motor. the vacuum reading at idle is just over 14 lbs which i consider low i beleive it should be about 18. the motor is in good condition doesn't use any oil, just a normal motor except for the low vacumm reading, the reading is also low under cruise conditions. i have checked for leaks a couple of times in the normal way, without sucess. I,m starting to think its something else which i,m not aware of. anybody have any ideas on the subject.

 

[dcasto]

14 inches Hg you mean. First, test to make sure your gauge is calibrated. If you can not find any leaks on the vaccum system and your engine diagnostics do not indicate, I'd next do a compression test.

 

[RossABQ]

What altitude are you at? In the Rockies, you'd do well to get 14" at 5,000+ ft.

 

[drayrunner]

Your right on the 14 in. hg. I checked my guage a couple weeks ago against known accurate guage and its very close, i also have a scanner-programer for the car , my intake air counts are 9 -10 when they should be 20-25. I checked the compression,there are 5 cylinders from 154 to 158 and one at 165 lbs. I'll try anything anybody has to offer. thanks

 

[evelrod]

drayrunner.
Picking up a cheap leak down tester to check your engine would be my next move. The comp ck is not all that out of whack (indeed, it's pretty normal) for a street engine and, as mentioned, 14" number is not a reliable guide in and of itself. It's more important to ck how the needle acts than ultimate vacuum, IMO.

To quote you, "the motor is in good condition doesn't use any oil..." Man, what are you looking for? I have a philosophy about this sort of thing, IF IT AIN'T BROKE, DON'T FIX IT

Rod

 

[RossABQ]

Also, since this is a PCV-equipped car, check that you have one made specifically for this engine/car, not an AutoZone "one size fits all". There is a difference.

 

[FoMoCoMoFo]

lots of things that could cause low vacuum.. leak,late ignition timing. late valve timing, but,almost anything that would cause abnormal low vac in cruise mode would cause it to idle very poorly if at all.those numbers,comp and vac do seem a little low as if your at 7000 feet. but in the end i'm with evelrod if its running good whats the problem ?

 

[PLeon]

A vacuum gauge can tell you a lot of things about your engine - also, check for a clogged converter (with the gauge). If it's not clogged don't bother, some engines run low vacuum (like there is small load on them all the time).

 

[SMOKEY44211]

Partially obstructed catalytic converter. I believe your model has flange at each end. Unbolt it and run it. If vacume guage reading comes up you've nailed it.---Phil

 

[drayrunner]

I agree with the statement,if its not broke don't fix it. I have the corect pcv. I also installed headers and a 2.5 inch exhaust without a convertor. The fact that it has low vacuum during cruise is what makes me try to find the problem. actually the car operates in a normal way. The only other thing i can think of is a retarded cam, and i'm not sure thats a valid argument. Thanks for all the replys.

 

[FoMoCoMoFo]

why not valid ?

 

[FoMoCoMoFo]

if you have some reason to suspect a retarded cam then the symtoms are perfect. vac. a little low, comp. a little low, performance would also be off a little but maybe not much

 

[MikeHalloran]

Does that engine have a timing chaing? Maybe it's time to check it for slack.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

A retarded cam will reduce bottom end power, but increase top end. It will reduce manifold vacuum.

A stretched timing belt or chain will effectivly retard the cam

Regards

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[PLeon]

Perhaps the headers are huge (idle, cruise) and cylinders cannot exhaust easily (low gas speed, high pressure).

 

[drayrunner]

What i'm going to do is first look at the timing chain and i might advanve the cam a couple degrees. I had the low vacuum before i installed the the headers. The car doesn't have much botton end power, works better at higher rpm.

 

[patprimmer]

It has been known for engines to be assembled with miss matched timing gears and key ways, or even for a key way to be misaligned when cut.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.