View Single Post
Old 01-16-2013, 09:38 AM   #3 (permalink)
Master EcoModder
razor02097's Avatar
Join Date: Jan 2013
Location: ohio
Posts: 306

Tetanus - '95 Geo Tracker 4WD Base
90 day: 29.43 mpg (US)

300 - '82 Suzuki GS300 L
Last 3: 60.78 mpg (US)

Jeep - '98 Jeep XJ Cherokee Limited
90 day: 12.82 mpg (US)
Thanks: 28
Thanked 50 Times in 37 Posts
Rent, borrow, beg or buy a diagnostic vacuum gauge. Hook the gauge to a manifold port directly on the intake manifold and start the engine. Allow the engine to warm up and stabilize. The reading should be over 20" at a warm idle. If the needle isn't steady or the reading is low it could indicate a problem.

Here is a resource for you about reading a vacuum gauge.
Tips on Reading Gauges: Vacuum Gauges

If the vacuum gauge indicates a problem or you wish to further check the health of the engine the next step is to acquire a compression gauge and get an actual reading on all of the cylinders.

Hook up the gauge by removing all of your spark plugs . Install the gauge to each cylinder and hold the throttle wide open while cranking the engine over. You should be able to listen for a lumping as the cylinder with the gauge pushes air in to the gauge. Wait for 5 of those audio indications then stop cranking and check the gauge.

Write down the compression number for each cylinder and get a calculator out. Each cylinder should not be more than 10% away from the highest number. Low numbers for all cylinders typically indicate a worn out engine or incorrectly timed camshaft. If you have low numbers repeat the test on all cylinders but add a tablespoon of oil in each cylinder before installing the gauge. If the numbers rise a lot you most likely have worn out piston rings. If there isn't a significant rise in compression your issue is likely valve train related or blown head gasket.

Evaluating the health of an engine using tools you can use at home isn't very expensive but it will take research to understand what the numbers mean. The rule of thumb is even if the compression is lower... if the compression readings are within 10% of each other and the engine runs okay it means the engine is just tired and the only option is rebuild or replace.

As far as raising compression. Take this how you will. Surfacing the head will reduce the size of the combustion chamber which will increase the compression ratio. Using pistons with a different profile can also increase compression. Neither way is a fix for worn out piston rings. Once an engine is torn down to the point the piston leaves the cylinder the minimum is piston ring replacement and honing the cylinder. Expect the new rings to not last though. Cylinders over time will wear out and require them to be bored and honed. Though not by much boring a cylinder doesn't necessarily mean you are going for increased power it is to square up the cylinder again to make a good surface for the piston to travel in.

Project Avalon: E bike build
  Reply With Quote
The Following 2 Users Say Thank You to razor02097 For This Useful Post:
ECONORAM (01-16-2013), mcrews (01-16-2013)