Best way to find vacuum leaks?

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Jul 7, 2007
West Chester, Pa
I know that to find vacuum leaks in the intake portion I can spray some carb cleaner and see if the idle goes up, but what about areas on the block, such as oil dip stick, distributor or around the valve cover and side cover? Thanks for any input. Jeff
The distributor does not get vacuum unless there is an advance or diaphragm on the side. The lines going to the top of the distributor cap are vents. The valve cover and side cover would leak oil if you have a leak. There is not a vacuum in the crankcase like the manifold. Look at the vacuum diagram on your sticker on the fender. What are you reading on a vacuum gauge hooked to the manifold? Why do you think you have a vacuum leak?
If you are getting 18HG at idle, you DONT have a vac leak. If it is idling rough, it is due to a different issue.


The best way to find a vacuum leak is to KNOW you have a vacuum leak. If you read any of my 60 threads you will know the hell I went thru with vacuum leaks. But KNOWING there was a leak is 99% of the battle. From there its just a matter of tracing all the culprits down.

But 18" at idle is NOT a vacuum leak. Typical vacuum leak will drop you right down to around 10.
Exhaust leak and dizzy

OK, so no vacuum leak. Well that's one thing I can cross off the list. :)

I cleared all the hard lines and checked to make sure they are going to the right spot.

I am going to check the O2 sensor today for function and a leak as recommended by you and the 3FE group.

I do hear a ticking sound in the engine compartment, on the driver side near the manifold, or maybe down by the O2 sensor, tough to tell, which I will dive into today. The PO put a custom exhaust on with only one O2 sensor so who knows. I was going to purchase one of the Remflex gaskets (REMFLEX Exhaust Gaskets: Home Page) if it's around the manifold, I heard they are pretty good.

Question about the distributor. I tested it according to the FSM and got some results that I'm not sure about. It does not say whether to do the test hot or cold. When the truck is hot I get 194 ohms on the N and G pickups. When cold I get 178 ohms on both pickups. The FSM states its good between 140 and 180 ohms. So it is good when cold and bad when hot. :confused:

thanks for any input
The PO put a custom exhaust on with only one O2 sensor so who knows.

This could be one if your issues. The 3FE need input from both oxy sensors to work properly.
O2 sensors bad

Well after running threw the FSM flow chart, and a little help from Steve C, I determined that the O2 sensors are bad, even thought they have less then 5k on them, albeit a few years old. I'm sure this will turn into a nightmare because of the PO custom exhaust.
Does the 3Fe have a egr valve? does it at times plug with carbon? If the EGR valve is partially plugged with carbon/vanish then its possible that it could be partially stuck open. this will cause a rough Idle. I do not know how ever how this would affect the vacume readings when stuck.
This could be one if your issues. The 3FE need input from both oxy sensors to work properly.

unless both O2 sensor wires are routed into one Sensor ;)

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