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Old 04-18-2013, 08:52 PM
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Originally Posted by LATECH View Post
Coolant is 191 degrees, engine is fully warmed up.
TPS looks a tad low
Idle speed is 1000
IAC is way too high, should be around 20 with idle speed at spec.
That is a good indicator of an intake leak.

Easy to check.
Pull PCV valve ,plug hole.Pull tube on other valve cover that allows fresh air in. Put a vacuum gauge in the hole and run the engine.If it builds a small amount of pressure...then youre normal. If it build vacuum...then you have an intake leak inside the valley area under the intake.
Wow, never heard of that test, but makes perfect sense. GREAT TIP - thanks.

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  #47 (permalink)  
Old 04-18-2013, 09:18 PM
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The test for checking the lower side of the intake manifold for a vacuum leak turned up negative, the crankcase built slight pressure as predicted but there was a good result from this anyway...

I did the setup as described, plugged the PCV hole, put a combo pressure/vacuum gauge in the hole in the other rocker cover and for lack of anything better to do, I capped the PCV port on the throttle body. Since the whole PCV system is a "calibrated vacuum leak", the engine ran a lot slower with the port capped. With the engine going slower, I began to test again around the base of the throttle body and along the edges of the manifold at the cylinder heads with a can of carb cleaner and the little red tube in the nozzle. I was able to produce a change in RPM that I couldn't do before with the engine running so fast. I have a buddy coming over to verify my findings but I think I'm gonna be taking the top off this thing soon and doing an intake manifold gasket.
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Old 04-23-2013, 09:37 PM
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Guys had cautioned me about using propane or carb cleaner to test for a vacuum leak simply because the stuff can easily be sucked into the intake and basically give a false positive indication as to the presence of a vacuum leak.
Being one to resort to the simplest home-grown solutions to complex problems, I came up with what my buddy dubbed the "Benjamin Moore vacuum leak tester".

The 4.3 V6 has a collar that sits atop the throttle body and the air cleaner in turn, sits on the collar. The collar also has a hose nipple on it where the fresh air hose for the crank case vent air intake attaches.

I took a gallon paint bucket ( I don't really know if it was Benjamin Moore paint or not) and cut a hole in the lid that was the same diameter as my air cleaner base. Then I placed the collar on the throttle body, and the inverted paint bucket on the collar and ran a hose off of the nipple on the collar down the left front fender and away from the engine in general. Now with the engine running and breathing through it's new snorkel, I could dispense the carb cleaner around the base of the throttle body and along the sides of the intake manifold where it meets the cylinder heads without fear of feeding the stuff into the throttle body and sure enough, I could make the engine change RPM with the carb cleaner. That was enough to make me decide to change the intake manifold gasket.

When removing the manifold, I couldn't find any evidence of a a vacuum leak but vacuum leaks are sneaky and they hide real well. I was hoping for something unmistakable but it wasn't there. After we finished the gasket change, we fired the engine and it had a great deal of trouble staying running during the first few minutes but once it was up to temperature it ran really well and the idle RPM was much more like what I would consider a normal speed. The exhaust note was also steady with no pops or surging. I plan to double check the timing and do the minimum air/base idle speed set up again soon.
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