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22 in Hg at idle needle dead straight.....

5K views 6 replies 4 participants last post by  FST4DR 
#1 · (Edited)
22 in Hg (22 inches of Mercury at idle) and the needle is dead straight. What does that really mean? Well, as engines get older and more and more worn, they exhibit problems which are found using different tools. One of which is a vacuum gauge. Remember that air is traveling through your intake manifold to get to your multi-cylinder engine (V6-V8) creating a constant vacuum. Because this vacuum is consistent, we can use it to see how well the engine is sealing against atmospheric pressure. Taking these readings at different engine operating conditions will give an overall reading of the interior.....valve stems, valve guides, valve seats, valve faces and compression rings. This is done on a tap to the intake manifold or with a T-fitting. (Never use a vacuum canister or EGR line as they aren't accurate for this test).

Typically an engine under 50,000 miles will pull around 20-21 in Hg of vacuum at idle (V6-V8).

An engine over 100,000 miles will pull 17-19 in Hg of vacuum at idle (V6-V8)

An engine in the 150,000 miles will pull 16-18 in Hg of vacuum at idle (V6-V8)

an engine over 200,000 miles usually will pull 15-17 in Hg of vacuum at ldle (V6-V8)

Over 250,000 they are in the 15 - 16 in Hg.

We use these numbers that are provided by ASE Certified Technicians. They use these as a typical reference for customers coming into their shops.



We had the 2005 Dodge Grand Caravan SXT in the repair facility for a transfer tube coolant leak behind the back of the engine between the firewall and the exhaust manifold. While they repaired the leak, they also put a vacuum gauge on the 3.8L to see the condition of the engine since they don't get many vehicles in there with 272,000 miles on them.

To their utter surprise, the engine at idle pulled 22 in Hg and the needle was dead straight and not varying or moving at all.

The ASE Technician called me and said that not a single vehicle in their shop has pulled more than 20 in Hg in a very long time and these are 5 year old cars not 12 year old cars etc.

He asked me how often we change the oil in the van and I replied ....."every 12,000 miles with our oil and filter" and he said WHAAATTT!!??

He then told me that he knew about us in our business as he has worked on this van for it's entire life. He knows it has the original valves, pistons and rings and was only open once to replace a broken intake spring and a broken exhaust spring.

He said he now has empirical evidence that our materials are something that really truly works and as he said......"the vacuum gauge doesn't lie!!" ACES IV and QuantumBlue produce these results!:rocker:

I thought I would share this with our customers and potential customers.

Regards,
Brian
BND Automotive LLC:driving:
440-821-9040
www.bndautomotive.com
 
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#2 ·
This really determines the health of the engine. This is an old school thing to do but really helps understand internal leaking of the components. Grand Caravan just hit 277,000 miles yesterday!

ACES IV and QuantumBlue are the reasons for the health of this engine. :beerchug:

Regards,
Brian
BND Automotive LLC:driving:
440-821-9040
www.bndautomotive.com
 
#4 ·
Glad you liked it. We have had many people ask about how to do it on their cars as they are encouraged by the results from our post. This is one of the most independent and simple tools that can be used. Like a blood test if you are a diabetic. The meter simply reads reality and so does the vacuum gauge!;)

Regards,
Brian
BND Automotive LLC:driving:
440-821-9040
www.bndautomotive.com
 
#5 · (Edited)
the 513 cube stroker I had in my 68 Newport would only pull 15", but that thing had a wild 288/292 cam. The original 383 4 bbl held a rock steady 20" when it had 110,000 miles on it. I used a vacuum gauge to set the timing and adjust the carb..this tool is indispensable for an old carbureted engine.
 
#6 ·
We use a vacuum gauge for our 1969 Charger with a 383 4 bbl also. Holley Carb, turn the idle screws in all the way on both sides of the carb. Open them up 1 turn each and then try to start the car. Turn them 1/2 turn equally until the car will start. Then turn the drivers side open at 1/4 turn until you get the highest vacuum. Then do the same for the other side. I agree with you on indispensible here.

What was great was that with 272,000 miles on it, that is still pulled 22 in of mercury and the needle was dead straight. Engine has 278,269 miles on it now. :beerchug:

Regards,
Brian
BND Automotive LLC:driving:
440-821-9040
www.bndautomotive.com
 
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