So I have a couple of problem or concerns. I have an '85 350 Tpi. There is a VERY loud hissing/sucking noise coming from the upper plenum area so loud that it is actually annoying! I have never heard another car with this same engine be so loud. Second... high idle. I put in a new IAC motor with no cure. I taped off the front of the throttle body to see if i could stall out the engine but that didn’t even affect it. It seems to me that there is a vacuum leak somewhere. All of the vacuum ports on the throttle body and plenum are blocked off properly. I also made a EGR block off plate on the intake manifold. Could that have something to do with it? Is the high idle and noise some how related? I can’t figure it out! Please help.

 

vacuum leak. spray some carb cleaner around the hoses and intake gaskets and listen to the engine. If the rpms go up you've found your leak.

 

have you tried setting your idle...just curious?

 

Yes I have set the TPS sensor correctly. I should have mentioned that I am using the Holley Commander 950 computer. Stock is out and the laptop is in! I can watch the TPS sensor movement on the laptop screen and know that it is functioning properly. But the thing is that I can tape off the throttle body completely and the engine wont stall out, to me this means that the air is comming from somewhere else! Maybe the loud nose of air rushing past something where its not supose to?

 

It can only be a few things. Gasket leak, open vacume port or broken hose, or crack in the intake parts. All should be easy to find with the above mentioned carb cleaner trick.

 

Not the TPS but actual idle is what I am referring too. I have the same noise you do but its my IAC on a cold start and after it warms up, it goes away.

 

Im sorry i put in the wrong letters earlier. I meant to say IAC instead of TPS. I guess I should read over these things a couple of times before I submit it, or pull my head out of my a**! Im going to do a few more test and see how it turns out. Thanks for the comments.

 

not sure about thr hissing noise but I had a high idle in my Z and I checked everything including the IAC and nothing fixed it untill I changed the 02 sensor and the intake temp sensor.

 

Can the IAC pull that much air into the engine with the throttle body blocked off? I've never had any problems with mine, but I wouldn't think it could. The RPMs should have come down some when you blocked off the throttle body. This makes me think you have a massive leak somwhere. Can you locate exactly where the sound is coming from? Take a tube or pipe and put it to your ear. Then move the pipe end around the motor until you find the loudest spot. This should pinpoint the source of the leak.

 

I eliminated the IAC passage compleatly and it still idles about 1400-1500 rpms. Throttle plates are closed as far as the can be. Couldnt get the rpms to jump using carb cleaner. Could it possible be not a very good seal at the base of one, or more, of the injectors? Would the intake temp sensor really cause a problem like that? My intake temp sensor is reading about 145 degrees, that seems pretty normal to me.

 

It can cause higher idle if it reads high. Cooler air is more dense, so it will let too much in.
I got this from the autozone repair site, see if it helps any. Wherever the hissing is coming from is the problem, find it.


TESTING
See Figure 1

1. Visually check the connector, making sure it is connected properly and all of the terminals are straight, tight and free of corrosion.
2. Unplug the IAC connector and check resistance between the IAC terminals. Resistance between terminals A to B and terminals C to D should be 40-80 ohms. If resistance is not within specification, the IAC valve must be replaced.

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Fig. 1: Idle Air Control (IAC) valve circuit

1. Check resistance between the IAC terminals A to C, A to D, B to C and B to D. Resistance should be infinite. If not, the IAC valve is faulty.
2. If the resistance is within specification, check the IAC circuits back to the PCM for continuity.
3. If the valve and circuits are functional, the computer control module may be faulty.

 

I just saw a guy that had the same problem on a corvette forum. He turned something on his EGR and it stopped hissing and idled down. Try twisting it to reseat it.

 

Could you send me the link to that forum I would like to check that out.

 

http://www.goingfaster.com/spo/89formuladiary.htm

In the Oct 8th 2004 entry about 3/4 into paragraph

 

Thanks, that sounds exactly like what is going on. Except I took out me EGR and put in a delete plate.

 

Make sure the plate is sealed well. It could be leaking. Did you remove the EGR program from the computer? If not, you may want to get that done.

 

As i stated above, im running the holley commander 950 so im doing all of my tuning through the laptop. No EGR program in that computer!

 

Cool, I want one!!!!
Did the EGR plate have a leak?

 

So I think I found the problem. It looks be a missing plug, sensor, or something to do with the EGR next to the ninth injector in the lower intake manifold. My car ran out of gas tonight while testing so tomorrow when i get more petrol ill plug that hole and see if it is the cure!

 

See attached photo!

 

That's where the fitting for the PCV valve screws into the manifold. That's a huge vacuum leak!

 

Yeah for sure that a big leak! Instead of carb cleaner.....you should have been sticking your finger in all the holes.

 

Well that was A VERY big leak. With that hole pluged it idles right down like it should! With a bit more tuning I should be able to hit the road tomorrow with my first test drive in 6 months! for all your input.

 

FYI:
That looks like a '89-'92 runner because it doesn't have the EGR tube, or the ninth injector port. The red arrow is pointing to the bolt hole once used to secure the ninth injector hold down flange and would cause a huge leak as you discovered.

 

I just used that picture off the internet as an example... Mine is an '85, the hole was just behind the ninth injector on my intake.

 

is that the cold start injector hole?

 

So where did you plug the PCV hose into?