Low vacuum...

[ryan.hess]

Here's the deal... I'm reading 13" of vacuum at idle, and according to the books, when the throttle is blipped to wide open, and snapped closed, the vacuum should quickly drop to 0 (or nearly), and then jump up to ~3" higher than at idle, then come to rest at it's idle vacuum reading. When I do that, it drops to near zero (as it should), but then takes nearly a second to come back to 13". It never peaks over as it should... I was told that that's because of an exhaust restriction... but when I took off the exhaust, I got the same results. now what?

[ryan.hess]

forgot to add... It idles just fine, has no intake leaks, and compression is roughly 135 on all 6.

[mattdez]

check your brake booster line... they are known to leak.

[SappySE107]

135! Thats low, way low. Mine was reading near that on a cylinder with a head gasket leak. My cams are timed different as well which reduces the cranking compression as well.

Vaccum on the other hand is fine.

[ryan.hess]

135! Thats low, way low. Mine was reading near that on a cylinder with a head gasket leak. My cams are timed different as well which reduces the cranking compression as well.

Vaccum on the other hand is fine.

So the vacuum is alright then?? I thought 15-25" was normal? My haynes says as long as the compression is above 100 it's fine - but what is the normal range for compression?

[edit] - could my cam lobes be worn? Would that cause low compression? Wrong cam timing? So many questions!!

[SappySE107]

wrong cam timing can cause it. I don't know what stock cam timing specs would yield, but I was at 170 on 3 cylinders, 150s on 2, and 120 or 130 on the cylinder next to the blown part of the gasket.

Normal vacuum is 16 pounds for the pushrods, and I believe its damn near the same for the DOHC. 13 is close, a little low but given the compression it sounds fine.

Worn lobes could do it but I haven't seen that happen on these engines yet. You can check it out but I doubt thats the problem. It could be bad lifters too, not pumping up, and causing less lift during cranking.

[ryan.hess]

I'm pretty sure the cam timing is good... otherwise it'd be idling funny. Plus, when I changed the belt, the cams "snapped" into place, which would imply that the valves were closed at that position. I should probably squirt some oil in and test it to eliminate worn rings...

question - what does your vacuum do when the test in the first post is done?

[4spdz34]

damn 130. thats horrible. mine has stock timing and i get 195 on all except the one with the headgasket leak which is 130. but it used to be 195 just like the rest

[ryan.hess]

maybe this is why my 0-60 is 11sec...

[jmgtp]

I have no clue what my dohc is at. My 3.1 that hardly runs checked ~160 psi, dry, across all 6 :P . Since plenum would have to come off to check the dohc, thatll have to wait till I pull the heads, even then its gonna suck snaking that rubber tube through the cam carrier and getting it to thread. I guess I could do a before headwork/after headwork comparison to see if the bit of material that gets removed in the combustion chamber (to rid it of sharp edges) takes any toll on compression. I'm thinking not even enough to measure the diff but who knows. Gettin the new heads decked a hair too to clean them up so I bet if anything compression will go up.

[ryan.hess]

Okay, well, here's the deal... I have checked, double checked, and triple checked the cam timing in relation to TDC. (this is only the front, because I can't get at the rear carrier) A big question IS: the cam hold down flats - when exactly at TDC, are they supposed to be perfectly flat in relation to each other? If so, then I think all my cams are advanced by like 15* crank rotation.

[SappySE107]

They should be parallel with the top of the cam carrier. I have a small metal rectangle piece that I sit on the flat, and it is flush with the top of the carrier. They will of course be flat across each one for stock timing.

[ryan.hess]

They should be parallel with the top of the cam carrier. I have a small metal rectangle piece that I sit on the flat, and it is flush with the top of the carrier. They will of course be flat across each one for stock timing.

Okay, so for stock timing, at TDC, I can lay a small ruler (or something) across both flats to see if they're parallel?

[SappySE107]

that should work, yes. you want to test each cam seperate this way.

[ryan.hess]

Sent you a pm