Hey people. I joined the forum to try and help my friend out here. I don't personally own a Blazer, but my friend does.. He drives a 1991 S10 Blazer, with the 4.3L V6 engine. I'm trying to help diagnose a vacuum leak, but his engine is a big different than my 383ci LT1 in my Caprice.

Here's what's going on. It was having a bad idle when it warmed up, so much it would stall or run very rough. It also was rough on starting too. It had bad rapid shaking on the vacuum readings, and there was a whistling sound that had me concerned, because if it was my engine, it'd be a vacuum leak.

So we tore the top end off, and he got a new edlebrock intake manifold, along with new gaskets and RTV black sealant on the front and back on the walls.

While doing this however, we found the distributor to be quite difficult to turn. So I had him bring it to the auto store to get a new one because that is not good on the camshaft. The pickup coil was also bad.

We get it all together, and while it is idling great, and seemingly running great (minus a MAP error on the ECM), the vacuum readings are still completely crap.

Here's a video of the readings: Vacuum Pressure Problem - YouTube

I'm out of ideas. He says he has gone over the rockers multiple times, and I believe him, but assuming it is done right, what else could it be? There's no leak on any vacuum hose. That has been tested.

I'm seriously thinking about a possible valve issue though, perhaps with the rockers.



PS: While having the motor apart, I did find it odd that the top of the valves were a glossy black like we found in the valves, that this fiber optic camera showed: http://img.photobucket.com/albums/v3...n/IMG_0021.jpg

It's not wet though it's dry.

But anyway, while it could be a bad MAP causing the code, I'm guessing that if the vacuum readings are this whiled, the MAP may be honestly telling the truth. I can't remember specifically what the MAP complained about though in the OBD codes.

If you need me to elaborate on anything, let me know. I'm getting ready to eat dinner so I might have skipped on a few details by accident.

EDIT: Oh, btw! compression was tested, he has good compression on all 6 cylinders, all well within spec.
EDIT2: This motor is a rebuilt motor from Autozone btw. It has less than 5k miles on it. If we have to touch the bottom end at all, I'm going to have him return it to Autozone because that is warrantied, unless this is something we can fix.

 

You say you've ruled out vac leaks, they only way to do that for sure is disconnect and plug all the accessory vacuum ports.
You also need to disconnect the IAC harness so the ECM wont be able to maintain rpm.

EGR dirty?

 

Actually, he just called me, he hooked up to a different vacuum line and it has a steady reading. So it actually may be ok after all. Could have possibly been the wrong vacuum line.

Granted, before we switched manifolds, we did try all vacuum lines and all had shakes, so this did improve.

I think the MAP sensor is just malfunctioning at this point.

But anymore recommendations are welcome.

 

Yeah... the only vac reading you should take is the intake plenum, away from the throttle plate. Been a long time since I looked at TBI, but IIRC, it had a 'velocity' port like a carb.

Double check that.. my previous comments still may be valid.