slideruleracer

I figured I'd chime in with a quick update.

So, I had to actually crack open the throttle blade adjuster to get the IAC counts down to around 60, which suggests there is no extra air getting into the engine - at least not enough to cause such a big vacuum leak.

So I pulled the intake and a valve cover off to check more details. At TDC on the overlap, I read that the intake valve should be .030-.060 more closed than the exhaust and if this is the case, then the cam/timing chain is installed correctly and not off by a tooth. Well, my intake valve spring was about .040 shorter, so that suggests that the timing chain is indeed installed correctly.

I also double checked the torque on the rocker arm nuts and they, too, appeared to be set correctly. However, one strange thing was that when I torqued the rocker arm nut to spec (with the lifter back on the base circle), the valve appears to be closing almost .1". The nut is practically free running until it hits this wall where the torque can be applied, so something in the lifter or on the rocker stud is bottoming out. That suggests that either the pushrod lengths are off or the rocker arm studs are off. But these are all stock parts, so I'm really scratching my head. I did change heads because my original heads had some corrosion issues that I was worried about having to deal with but the heads I put on have all matching numbers. They are from an earlier engine (2001 vs 2004) but according to the parts schematics, all of the valvetrain parts are the same between them, so it doesn't make sense that they would be off. I didn't do a compression test yet, so I'll do that to a couple of cylinders tonight and see what the numbers say.

So, at the moment I'm stumped and have a little more work to do.

Ryan00TJ

iTrader: (8)

When you are applying the 18 ft lbs torque, the pluger in the lifter is compressing to the correct preload. If the lifters are no longer pumped up and you torque them to spec, it can seem out of whack until oil pressure pumps them back up.

slideruleracer

Thanks Ryan, but wouldn't that only serve to compress the spring (open the valve) even further when the engine is running? I understand that I want to compress the lifter, but I would have thought that the valve spring *shouldn't* be compressing when on the base circle. That doesn't appear to be the case. I'm going to double check tonight and see if the valve spring compression is relaxing over time from oil bleeding out in which case I guess it's OK.

But, if that's the case then I'm REALLY stumped, because then I have no theories left on why MAP is so high (engine vacuum so low).
- No apparent intake leak (IAC should have been shutting down at idle)
- cam chain timing OK
- valve lash adjustment OK
- All that's left that I can think of is ignition advance, but I wouldn't have thought a few degrees of ignition advance could cause a 7" drop of vacuum at idle. Could it?

If anybody has any other theories, I'm all ears. I suppose bad rings could cause this, but everything is new and I was meticulous about ring grinding and installation. And the vacuum is steady, so all 8 cylinders would have to be equally bad, which seems unlikely...

SB

iTrader: (1)

what is the specs of this HP2 cam ?

slideruleracer

The Hp2 Cam is the Mag HO Cam. HP2 was GM's designation. The info I had (from this site, actually, courtesy of RMBuilder) is:

224º/233º @ .050” (222º/232º)
.300”/.300” Lobe
.510”/.510” Valve
114º/115.5º

SB

iTrader: (1)

Thanks. What are most people getting from this cam ? 12-14"HG ?

have you used a real vac gauge to double check your computer reading ?

slideruleracer

Concensus is ~17inHg @ idle. And yes, I did use a vacuum guage. Hovered right below the 10" mark, probably a 0.5" variation peak-to-peak.

Ryan00TJ

iTrader: (8)

With the lifter on the base circle of the cam. You torque the rocker to spec. If the valve spring starts to compress, and valve opens the lifter is fully pumped up with oil. Overtime lifter will bleed off, valve will close and you will be at correct lifter preload.

If this is not happening something is out of whack.

slideruleracer

Thanks Ryan - you're description is indeed what was happening. It turns out I was just not giving it enough time to bleed off when I checked it the first time - I guess I expected it would bleed off quickly.

So, I proceeded to run a quick leakdown test on a few cylinders and found that I was getting 25%+ leakdown past the rings. I guess that's pretty normal for a stone cold engine that hasn't been broken in yet? So, I talked with Larry at Raylar and he thinks I need to go run it and break it in before I get too wrapped up about something being wrong at this stage. Maybe the lack of ring sealing is the cause for my low vacuum.. I did gap the rings at the higher end of the range because I didn't want any risk of ring binding - maybe that's also contributing.

I'll get some hours on the motor and report back.