To start, i replaced my plenum gasket and plate with the hughes kit and folled directions and specs to a t. Nothing has really changed. That being said, im back to checking for vacuum. Also replaced my pcv valve because it was letting oil though into the intake.


Regarding the intake vacuum, I checked it two ways (at least I think). First I got a vacuum gauge and connected it to the port on the passenger side of the manifold. Read about 13 psi at idle. So lower than normal, but I thought the cam might cause that.

Then, I went and connected my hp tuner and started scanning (including manifold vacuum psi) at idle. The average psi then read at about 5 psi. Jumped up to 8 at 2k rpms, but no higher. So I'd think that's consistent with a vacuum issue for sure.

Also, my timing advance was jumping from 7° to 22° just at idle. My rpms were also fluctuating from about 550 to 725 while idling.

I can get more data, but that being said, any ideas on the heart of the issue? Is it purely going to be an advance timing issue that's throwing everything off, or is it still some vacuum leak i can't find for the life of me?

 

I think you still have a vacuum leak somewhere. Unless you have a REALLY lumpy cam in there, you should see 20" of vacuum at idle. (stock cam.)

 

Nope, got a pretty mild comp cam. In fact so mild it was able to run with stock valve springs. (I changed em anyways, but still). but I do agree, I just can't find the dang leak haha

 

After some research, and testing with the gauge and tuner, i think ive hopefully diagnosed the issue. I believe i may have set the valve lash too tight. I used a torque wrench to install the rockers, and looking back its possible I torqued them too tight... it explains every single issue I'm having, and would explain why I can't hear or find a typical vacuum leak, but would have a lack of vacuum. I'll check on it tomorrow night and see if that's the case and update later once i find out.

Rookie mistake if so haha

 

Non-adjustable valve train, so, I don't think over-torquing the bolts really matters.

 

Do a compression check. 7.5:1 = about 145–150 psi, 8:1= about 150–155psi 8.5:1=about 160 -175psi, 9:1=about 180–190psi and 10:1= about 190–210 psi. Cylinders should have about the same reading. I'm still leaning towards a vacuum leak. You have oil throughout the plenum, how many miles have put on the engine?

 

I'll do a compression check, but it should be just fine. In answer to your question, I rebuilt the engine less than 1k miles ago. And with comp cam, EQ heads, new pistons, etc. Basically every internal component is new. So it should be making more than stock hp, and have a good bit of power. And I don't have oil in the plenum, literally redid that 2 days ago with the hughes kit. Not to mention doing it before that during the rebuild. So yeah, has to be vacuum. At this point, I've thrown a p0455 code. So I'm about to where I'm gonna take a day, remove every vacuum hose, pull the dash forward and inspect those, and put every one back on. I'm pretty sure it's not an internal engine issue unless somehow one of my pistons has low compression.... and it'd better not. Otherwise the only thing would be a vscuum leak, or massive timing issue, which I don't believe is the case (though possible I suppose)

 

Ran a compression test on 5 cylinders. ( cylinders 1234 and 5). All of them cam back reading about 95 psi..... so at least it's consistent, but wow. I didn't bore out the engine, I made doubly sure I got the correct piston size, and I have the correct piston rings, all installed the right direction, with the ring gaps all 90-180°from each other.

Given that i have a comp cam, albeit a mild one, could this be the cause of my vacuum issue?

Also, while using my scanner/tuner, I was reading a few variables whole the engine ran. For one, the long-term fuel trim bank 1 never went above +3, and would drop down to -21. Also, the reading on the timing advance varies from 6, all the way to 29.
lastly, the knock retard is reading at 120° and 7.5°.

I'm new to some of these things, so any explanations would help, along with ways to fix or change those things, possible causes, etc. All of this together is throwing me for a loop, especially the compression issue. I did install EQ heads, but didn't change the valve stem seals on them. Is it possible those are bad? Any other possibilities??

I can submit photos of the readings on the tuner if needed

 

Knock retard??? 2nd gens didn't have a knock sensor..... one of the reasons for the Death Flash....

How many miles on the engine? If it is fresh, then your rings haven't seated in yet. You may also want to check you cam spec sheet, and see if it was ground 4* advanced.... did you install an adjustable timing set? Or a stock one, with no advance dialed in?

 

Yeah not sure, it was picking up something that the tuner registered as knock retard, I'm just assuming it was spark retard (which I believe I have).

And yeah, it is fresh. About 1k miles on the rebuild.
Given that it's a comp cam, I'd assume that it is ground 4*advanced. The spec sheet doesn't specify that, but at the same time, some of the cam specs are Greek to me... and no, I did not install an adjustable timing set. And yes, I did install a stock replacement with no advance😬. Which I guess could cause low compression, due to the valves being open at the wrong time maybe? If needed, Is there a way to advance the timing short of taking of the cover and doing it manually?