I've got a sbc idle problem
 

I know I know, it's the Ltx section.
Buttt...
Just bought a 96 k1500 with the sbc vortec set up.
Only 120k Miles, I picked it up cheap with a few issues, it's idling high and very sparadic (anywhere from 1200 to 3000) with no real rhyme or reason..

I assumed what would seemingly be the obvious, a vacuum leak. So I sprayed all around the intake with brake clean, sprayed vacuum lines, sprayed everything to find no change.

Pulled the IAC, a little gunky, cleaned it... No change

Brand new plugs, wires, cap and rotor.. No change

It has a pretty Nasty misfire when it idles down lower and when taking off from a stop it almost does out.. I drove it to autozone to have them check the codes and it was throwing the p0300 random miss

When I get some free time next I'm gunna scope the timing to make sure it's correct, I've also heard story's of the brake booster leaking internally and causing a vacuum issue, so I'll pull the vacuum hose and plug it to see if there's any change.. I dont know what else to look at..

Any other ideas?

I appreciate any help guys

-kyle

If you have not sprayed around the injector pod, top of the manifold behind the throttle body, try that. They are known to suck in the silicone gasket that seals the pod to the manifold.

I found a vacuum leak like that (back, bottom of the TB) on my dad's truck years ago. I thought I had checked everywhere, too.

Ill have to try that. I did pull the plug on top of the injector assy. It almost looks brand new under there, could just be well protected underneath the electrical connector, but it also makes me wonder if someone before me replaced the injector "spider" from what I've read the poppet design on the central sequential FI setup is somewhat of a design flaw and can be swapped for the more reliable multi port set up. And from the looks of it, intake gaskets were replaced (I see RTV arround the lower intake and none of the wire harness is clipped into the brackets)

I also read that it can be a real PITA to get that seal seated correctly on the upper plenum..
So I'll have to check that out

Another thing I question.. Wouldn't I be able to hear a vacuum leak? Especially one causing a 3k rpm idle?

If you can hear a vacuum leak on a roaring engine at 3K RPM, you have pretty good ears. :D

Haha, true true

So tonight Im sure I found the problem, there's an extreme amount of vacuum in the crankcase. I pulled the valve cover inlet tube out of the intake elbo and I swear I could vacuum the floor with it.. I'm pretty sure the only reasoning for vacuum in the crankcase/valve covers would be a broken gasket on the inner side of the intake

Sound right? I bought the gaskets today and will be doing the job one day next week..
Fingers crossed!

Is there a pcv valve in place and working correctly? That would be a direct link from the intake to crank case

Yes, pcv is operational. I can pull everything from the valve covers and still have enough vacuum there to suck a dollar into the oil fill

Intake gaskets. Willing to bet money on it.

Had the same problem with my 5.7 vortec in my silverado .
I was able to plug every hole in the valve covers and with no go pedal the truck would idle at 2500rpm while sucking air through the oil fill.

On another note, before doing this job. Make sure you have a new quick disconnect heater hose fitting for the intake. Those bastards never go back together and will leak like a sieve. Either that or buy just a regular coolant nipple and cut the heater hose end and just clamp it
On. Make sure it has the restricted in the nipple though.
Ultra black rtv, it will be your friend for the front and back of the intake. Make damn sure mark the distributor to the manifold before removing. If this doesn't
Go back in properly it won't run properly and will need a tuning program to check the cam offset .( I used hptuners)

Anyways, pm if you want.

Those engines commonly need intake gaskets around 100K, nothing unusual there. The heater hose connector comment is certainly correct.
Any scan tool will check the CMP off-set. If you follow the distributor installation procedure in the GM shop manual it will be dead nuts or very close.

Luckily me and a buddy just did his truck a couple weeks ago so I've got a good idea of what to expect.. The quick disconnect wasn't a problem but the dist. Was a bit of a pain

I do find it kind of unusual that from the looks of the gunk build up on the RTV in there, there's only maybe 30k on the last set

We'll I tore her down today, turns out the last guy to do this job was clueless

The intake was barely torqued down for starters.. And the passenger side gasket was sunken into the valley about a half inch, which explains the vacuum in the crank case.

Tomorrow after work it'll get pieced back together and I'll let you guys know how it runs. Fingers crossed!

remember that those intakes are only to be torqued to about 13-14 ft lbs. dont just crank it down by hand, seen a couple guys crack the end of the intake off that way..

Yeeshh, I'd be in unhappy man. Thanks for the info, I plan on looking up the tourq specs before tightening it down..

I'm fairly surprised the only problem I had tearing it down was the egr feed pipe, fittings were siezed and I had to cut the pipe. A couple compression fittings and a section of copper should do the trick

X2! They do not get torqued very tight. The bolts also break easily. We replace them where I work when doing these intakes

Do you think I should look into picking up replacements?

And when I say the last guy didn't tourque em very tight I mean I was able to break most of the bolts loose with a nut driver

If they were that loose then something definetly wasn't done properly. As for the bolts if they look ok then they are probably fine just be careful when you torque them.