By Frank Condos
A vacuum gauge added as an accessory to the standard set of Gauges provided by GM can provide useful information for driving as well as trouble shooting problems. First, let's review a little technical background. Vacuum is produced in the intake manifold because the throttle is closed or partially closed and the suction of the piston creates the vacuum. If the throttle is fully open there is no vacuum in the manifold since the piston intake stroke is getting all the mixture that it can hold. But enough theory, (maybe too much) how can this help us?
If you have a vacuum gauge hooked up to a vacuum port on the intake manifold at idle, the gauge should read 17-20 inches of mercury ("hg). As you open the throttle while in park and the engine speeds up the vacuum will actually increase to in the order of 20" hg, abruptly close the throttle and the vacuum may go as high as 26"hg all is well.
While driving, as the speed increases, the vacuum will drop depending how wide the throttle is opened. As the vacuum drops to below 8" the power piston in the carburetor begins to rise and enriches the fuel mixture. Mileage begins to drop. The further the throttle is opened the lower the vacuum and the more fuel is introduce. At 4" the maximum rich in the primaries is reached.
A near wide open throttle while climbing a grade the vacuum will approach zero and if the speed is high enough the air dam on the secondaries will also open proving the maximum power and poorest mileage. Depending on the grade and road speed, this may be a good time to drop down to the second or super gear.
Now some drivers watch the vacuum gauge continually, attempting to maximize mileage. (They do look down the road occasionally). Caravanning with these drivers is a real experience as they speed up and slow down. The fact remains that using a vacuum gauge while driving can aid in improving fuel mileage.
Low vacuum at idle and across the driving range can indicate a number of problems but the most likely cause is a leak. Finding the leak takes a systematic approach with all the lines around the engine. One such approach is to cap the vacuum ports one at a time at the manifold with the engine at idle. If the engine speed changes, that line has a leak. Multiple line leaks may require capping several ports or checking each line with a hand vacuum pump or the mouth. One owner complained of low vacuum and poor performance and thought it might be a misstimed camshaft. Further investigation proved a badly cracked intake manifold into the exhaust crossover.
The vacuum gauge along with an air/fuel meter can provide more useful information about the engine that we'll discuss in a subsequent Shade Tree.
By Ken Booth
Lake Havasu City, AZ
On our way to the Cable Airport rally, our co-traveler's experienced a thermostat stick that blew a rubber line going to the hot water heater (real messy). After removing the thermostat, we discov-ered it was the "R" type. Which means it had only two braces on top (straight across). "R" type is a poor choice for desert dwellers. The Stant 330 "B" style is 100% reliable. The "B" stands for balanced. The "B" thermostat has 3 braces on top like a mini pagoda. It has never failed me and keeps my engine cooler. Kragen Auto Parts was the only dealer we found in Upland that stocked the part... not Pep Boys, Napa or Auto Zone. We purchased Prestone #330-180... 180 deg model. This part comes highly recommended.
Home | Shade Tree | Schedule | Scrapbook | Marketplace-GMCs
Items for Sale | Items Wanted | Join GMC Pacific Cruisers | Links
Last Change: 10/6/2008
Suggestions/Corrections: email@example.com Copyright 2006, 2007, 2008. All rights reserved. Hot-links welcome.