2007 Chevrolet Impala • 154,428 miles

I did the LIM gasket and the upper plenum gasket and I changed tge plugs, now its got a top end on cal 3, and the car has low power
Compression good
Oil pressure good
Spark/plugs good
Injectors in working order
Vacuum is good but at idle it sits at about 19-20 inhg and it fluctuates constantly 3-4"hg

Low power
About 22mpg avg, was at about 25-27mpg before this happend
It has a bad hesitation on acceleration
Rpms usually sit at 750-800 at idle but now it will drop to about 550 when brake is applied in drive and will go back to. Normal when foot is off brake(intermittent)
Valve clatter very pronounced on cold start ups
Doesn't want to hold revs when it hesitates, it gets to about 2 grand on accel and will just drop. Off and make tge car hesitate
Rpms will jump 200-400 when I let my foot of the gas at cruising speeds 25-45 does it all speeds but more noticable at 25-45
I will occasionaly have power randomly but after the car gets driven for like 20-30 min it hesitates and has low power

I do have power if I make the car downshift, but it still doesn't have the power. It used to.

Iv done
New intake gaskets
Plugs/wires ac delco
Air filter
Seafoamed it (helped a. Little on power/mpg)
Oil is clean
Coil good
Maf/map good
Can't find any intake leaks
I feel like ive been running in cirlces, but I'm like 99.99% positive its a valve train issue, any ideas on how I can be 100% sure what it is before o tear into it again

March 3, 2012.

The give away is the fluctuating vacuum. The noise is a collapsed lifter and the valve is not opening enough. You most likely will have to open it up again and replace the bad lifter. It just wont pump up.


Would it be safe to try a quart of marvel mystery oil on an oil change to see if I can get it pumping up? And can a single Lifter be replaced? Can I it be replaced without removing the intake manifold?

What would you try first before blowing 500 bucks digging into it. Again!

Mar 4, 2012.
You can try the additive but I doubt it will work.
Yes, you can replace 1 but I recommend replacing all.
No, the intake must come off again.


Dang when I pull the lifters do I have to remove the guide? And what's a recommended procedure for prep of new lifters and a break-in precedure? Or point me out a place to find this info

Mar 5, 2012.
Sorry for a lot of questions, but is it damaging to drive it like this for long?

Mar 5, 2012.