Testing the engine compression is probably one of the most overlooked diagnostic tests when trying to find the source of a hard to find rough idle condition or a misfire trouble code.
In this tutorial I'm going to explain how to do the compression test and more importantly, how to interpret its results and find out if you have a dead cylinder on your hands.
This tutorial applies to both the 1993-1995 SOHC and 1996-2002 OHV 4 cylinder 2.5L Dodge Dakota engines.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1993-2002 2.5L Dodge Dakota) (at: autotecnico-online.com).
Symptoms Of Low Or No Cylinder Compression
It's been my experience, over the years that I have been working on cars, that the majority of engine compression problems can be divided into two distinct categories.
In the first category, you have the type of compression problems that provoke a rough idle condition or a misfire condition.
In the other category, compression problems will make the engine crank but not start.
Here are some of the symptoms you may see when your 2.5L Dodge Dakota has engine compression problems:
- Engine cranks but does not start (0 compression in all cylinders).
- Blue smoke coming out of the tailpipe.
- Rough idle (engine misfires).
- Bad gas mileage.
- Engine ‘misses’ at idle but ‘miss’ disappears as you accelerate.
- Check engine light is illuminated with a MAP sensor trouble code (even tho' the MAP sensor is good).
With this info under our belts, let's head down to the next subheading and get testing.
TEST 1: Dry Compression Test
The engine compression test is not that hard to do, but there are some precautions you need to take before you start.
The most important precaution being that you need to remove the spark plugs with a cold engine. So if the engine in your 2.5L Dodge Dakota has been running for any amount of time, you need to let it cool down completely.
Removing the spark plugs from a hot engine can damage to the spark plug threads in the spark plug holes.
The other precaution I recommend you take, is to label all of the spark plug wires so that you won't lose their firing order.
OK, these are the test steps:
Disable the ignition system by disconnecting the distributor and ignition coil from their electrical connectors. This will prevent the ignition coil from sparking during the test.
Remove the spark plugs.
When removing the spark plugs, be careful not to drop any of them on the floor, or you run the risk of having the spark plugs porcelain insulator crack and then you'll have a misfire on your hands.
If the engine does not start, don't worry about it being warmed up.
Thread the engine compression gauge into the spark plug hole for the #1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
Have your helper crank the engine till the needle on the compression gauge stops climbing.
Now, record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper. Release the pressure on the gauge and repeat this step one more time.
Repeat this test step on the remaining 3 cylinders.
Let's take a look at what your test results mean:
CASE 1: You got 0 compression on all 4 cylinders. This test result is usually due to one of the following conditions:
- Timing chain (or timing belt on SOHC engines) problem.
- Blown head gasket.
- Blown engine.
Any compression value below 100 PSI (even if it doesn't go as low as 0 PSI) means internal mechanical engine trouble.
CASE 2: You got uneven compression values between engine cylinders. To a certain extent, having an uneven compression values might be normal.
The next step now is to find out if your compression test results are within normal operating parameters. For this info go to: Interpreting The Compression Test Results.