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Old 10-03-2012, 12:02 PM   #39
sailor
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Join Date: Jun 2008
Fan#: 57268
Location: Williamsville, NY
What I Drive: 2004 Pitch Black ZTS 2.3 5spd.

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Rental compression gauge will work, key is to do the test in a manner that'll give useable results.

Got a battery charger?

The best way to get good numbers that I know, is to:

1. remove all spark plugs

2. reinsert plugs into their wire & ground to engine (laying them on a metal engine part that's NOT insulated from the block works) this gives the spark somewhere safe to go when you crank the engine

3. Screw adapter/gauge into the first plug hole (which end you start with doesn't matter, but #1 is by the belt end)

4. crank over engine with gas pedal to the floor until the gauge needle stops going up, then record that number - release the pressure to remove the gauge - and go to the second cyl.

5. after doing the same procedure on the second cyl., charge up the battery before doing the third & fourth (how essential this is depends on your battery condition, a good battery could do all 4, while a poor one might need charging between each to get consistent cranking speed)

6. Now you can check the results

Exact numbers can vary between gauges, the first check is to look for variation between the cylinders - this should be within 10% for an engine in good condition. For example, if you see 140 psi on 3, and one is at 125 psi - that one is JUST out of the "normal" range and bears watching in the future. If you saw one at 75 psi, the engine may have a problem and is likely running rough - time to recheck by redoing the test to confirm your results.

As a rule of thumb, 5 to 10 psi variation falls within the normal range for a good engine, while more than that should be rechecked & watched for changes (it's good practice to check compression at each engine maintenance time to get base line numbers & note any changes).

Unusually high or low numbers across the board CAN be indicative of certain issues (engine wear or carbon build up at two extremes) but it's not time to panic when you aren't sure of the particular gauge's accuracy. There are other signs & tests we can describe, besides trying a different gauge, to determine exactly what's going on if the initial results don't look good!

Luck!
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