I want to first start off by saying this is my first post and my first Yota. 91 Pickup 3.0 4x4 MT. I have recently brought the truck back to life from much help from this forum so I would like to thank all of you for sooo much information. I do have an odd one though I would like to get some opinions on.

I just got a valve job done about a month ago (burnt and bent valve) which also included the obvious head gasket. (Side note, they said they couldnt find any of the valve pucks so the valves were not adjusted but confirmed that cyl 5 and 6 were out of spec.) Anywho, the truck is still not running optimal and spent the week looking into vacuum leaks, spark plugs, electrical etc etc etc and in the process noticed a slight hiss on the passenger side. Rechecked vacuum lines again for leaks with carb cleaner and propane (shop also did smoke test) and found nothing. I finally got a hose and tried to isolate the exact spot the hissing was coming from and seems to be coming from the exhaust manifold/cylinder head area (If i put the hose on the manifold right before the spark plug is the loudest spot). Checked drivers side and it is less noticeable but there also.

As anyone ever had/heard any experience where manifold leak is acting like a vacuum leak. From what I have read so far, a manifold leak sounds different or at least haven't been able to find anything about them "Hissing" the way mine are. Is it a matter of just tightening the manifold further or possibly something more serious ie warped heads not sealing around the edge (Compression is good/no coolant consumption)?? Any Ill effects of going over recommended torque specs if not completely sealed?

 

Probably a dumb answer but I had a similar issue and the sound was coming from the little solenoid valve filter on top of engine. Little black round filter. I had the vacuum tubes going to ac idle screw backwards. I though for sure it was an exhaust leak but it wasn't.

 

Are you referring to the EGR modulator?? Def not that, just replaced that along with EGR and EGR VSV/Solenoid and ran through all the vacuum lines with diagrams today and all checks out. Thanks for the reply though.
Side note: Could a clogged cat play a part? Im putting parts in when I can and I'm 90% sure its original, sitting at 150K..

 

I did the whole hose to the ear to pinpoint exactly where the hiss was coming from and its most definitely coming from right where the manifold heat shield goes into the head.

 

Not the egr modulator but the vsv. Mine was the white one that goes to ac idle screw. Sounds like you already checked it. Those little filters make a hissing noise when tubes are backwards