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Discussion Starter · #1 ·
I have a pretty rough idle as of late so I checked my vacuum and I am at 11 with a needle that barely jiggles. I am running a 92 460 bored 60 over, headers with multiport EFI and over sized valves and a milled head. My cam is:Voodoo part number61602. 219/227 @ .050" lift. I know what to expect out of a vacuum reading in a totally stock engine but with this cam what should I expect for vacuum at idle and how steady of a needle should I expect?
I have sprayed carb cleaner around the intake and the throttle body base to try to locate a leak and found I can get the engine rpms to drop around several of the bolt heads? Seems odd but I thought I would bounce it of you guys so what do you think?
 

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My brothers 61602 cam held 17" of vaccum at 900 rpm's with 20 degrees of initial timing. If engine rpm is affected by spraying starting fluid around the base of carb their is a vaccum leak. The needle is really steady on the vaccum guage with that cam.
 

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Discussion Starter · #3 ·
Well now I am really stumped. I literally flooded the intake with water to keep the gaskets and bolts underwater hoping to see a jump in vacuum but no dice. Didn't change a thing. Now I am looking for other ideas as to what could lead to the bad idle and low vacuum. Obviously the worse the engine is running the lower the vacuum so could a bad TPS or IAC cause it? )2 sensor? I don't think it is the valves cause the needle is pretty steady.
 

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Water will not have the same effect as carb cleaner.
 

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"Spray" around the engine seems to work better. We had a mystery vacuum leak for a while on our new build. Turned out it was the bushing for the shaft of the secondaries on our rebuild holley carb.

It took me two tries and maybe an hour of spraying around to get it pinpointed.

I also learned that vacuum reading will be different based on your altitude. I never considered our house to be high altitude but we were only getting 17" or vacuum with the lunati 61600. Turns out 3500 elevation really does mess with the vacuum since we get much higher #'s when I checked it near Yuma Az (close to sea level there). I'm just throwing that out there in case your at altitude too.

~Mark
 

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Discussion Starter · #7 ·
I have a pretty rough idle as of late so I checked my vacuum and I am at 11 with a needle that barely jiggles. I am running a 92 460 bored 60 over, headers with multiport EFI and over sized valves and a milled head. My cam is:Voodoo part number61602. 219/227 @ .050" lift. I know what to expect out of a vacuum reading in a totally stock engine but with this cam what should I expect for vacuum at idle and how steady of a needle should I expect?
I have sprayed carb cleaner around the intake and the throttle body base to try to locate a leak and found I can get the engine rpms to drop around several of the bolt heads? Seems odd but I thought I would bounce it of you guys so what do you think?
Well spring is coming and I still have not resolved my low vac and idle issues from last year. So I thought I would revive this old post and throw a couple more facts in to see if it helps narrow down the possibilities.

I have high pressure in my radiator once the motor warms up. If I fill the radiator up completely the pressure will force some antifreeze out thru the overflow and eventually a happy medium is found and the antifreeze is no longer forced out and I manage to drive around without overheating. You can smell a trace of antifreeze from around the engine all the time. (not the exhaust) The oil stays clean and the antifreeze does not contain any oil and yet pressure seems to be leaking into the cooling system. Both headgaskets were replaced last year. Today I pressurized the cooling system with a cooling system pressure tester and pumped it up to 7lbs (engine cold) and it held 7lbs for several minutes. Next I started the engine (cold) with the gauge at zero and it remained at zero until eventually the tstat opened and the pressure rose to a steady 10lbs and held for a couple of minutes. Then it started to rise steadily until it hit 25 lbs and held there until the fan kicked on (electric) and it dropped down to 10lbs again. Now I don't know if these systems can or are contributing to my vacuum issues but I am throwing it out to you guys for opinions. What do you think?
 

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If you've got a compression leak into the cooling system you'll get bubbles in the radiator when it's idling. If you've got bubbles do a compression check to narrow it down.
If you've got an aluminum intake manifold the bolts tend to loosen off all by themselves. You should check them a few times every year.
When you spray penetrating oil around to check for vacuum leaks the oil richens up the charge and makes the engine run better. It doesn't temporarily stop the leak or anything like that.
Also try blocking off the PCV valve and brake booster and any other vacuum lines and check the vacuum reading again.
 

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Discussion Starter · #9 ·
If you've got a compression leak into the cooling system you'll get bubbles in the radiator when it's idling. If you've got bubbles do a compression check to narrow it down.
If you've got an aluminum intake manifold the bolts tend to loosen off all by themselves. You should check them a few times every year.
When you spray penetrating oil around to check for vacuum leaks the oil richens up the charge and makes the engine run better. It doesn't temporarily stop the leak or anything like that.
Also try blocking off the PCV valve and brake booster and any other vacuum lines and check the vacuum reading again.
Thanks, I will check for bubbles. I got a feeling this is one of those leaks that doesn't show up until the block warms up cause while it is warming up the idle remains pretty steady. I checked the vacuum lines earlier and the intake but I will give the intake another try just to be sure. As far as internal block or head crack are concerned; can they turn into major vacuum leaks or at least prevent the engine from creating adequate vacuum? If so where would I look for those cracks?
 
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