Low compression

Tiny
MIXERMAN158
  • MEMBER
  • 2002 FORD FOCUS
  • 2.0L
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 126,000 MILES
I have 120 compression in cylinders 1and 4 but only 65 in cylinders 2 and 3 any ideas on where to start
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Wednesday, April 5th, 2017 AT 3:48 PM

7 Replies

Tiny
CARADIODOC
  • EXPERT
Compression test only tells you the compression is low, but not why. For that you need to perform a cylinder leakage test. You may be able to find the tester at an auto parts store that rents or borrows tools. It involves placing the cylinder at top dead center on the compression stroke, then you force in compressed air through the tester, into the spark plug hole. It will show the percentage of leakage, and you can listen for the results of the leakage.

In this case, when two adjacent cylinders are affected, a leak between them in the cylinder head gasket is the best suspect. You would hear the air from the tester escaping from the adjacent cylinder, and leakage would increase, when that second spark plug was removed.
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Wednesday, April 5th, 2017 AT 4:53 PM
Tiny
MIXERMAN158
  • MEMBER
The vehicle will start sometimes and run for about 15 min but then it won't start again until the next day but it sometimes needs starting fluid to start
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Thursday, April 6th, 2017 AT 8:32 AM
Tiny
CARADIODOC
  • EXPERT
That's not a compression-related problem. The first thing you have to do is determine if you're missing fuel pressure, spark, or more commonly, both, when the no-start occurs.
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Thursday, April 6th, 2017 AT 7:42 PM
Tiny
MIXERMAN158
  • MEMBER
Has spark and fuel pressure
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Friday, April 7th, 2017 AT 1:54 PM
Tiny
CARADIODOC
  • EXPERT
When you use starting fluid to get the engine started, does it stay running, or quit within a few seconds? Are you measuring the fuel pressure with a gauge, or just observing a squirt from a test port?
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Friday, April 7th, 2017 AT 3:53 PM
Tiny
MIXERMAN158
  • MEMBER
It will run for about 15 minutes before stalling and failing to start again
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Friday, April 7th, 2017 AT 5:25 PM
Tiny
CARADIODOC
  • EXPERT
The description of what's happening isn't clear. If starting fluid is required to get the engine to run, then it continues to run for 15 minutes, the best suspect is fuel pressure is bleeding off while the stopped engine is sitting, and / or the pressure is not high enough due to weak pump output.

If there is no way to get the engine restarted right after it stalls, that is the time you must check for spark and fuel pressure. Some engines will not run when the fuel pressure is low by only five or ten pounds.

You also need to read and record the diagnostic fault codes. It is real common for a crankshaft position sensor or a camshaft position sensor to fail by becoming heat-sensitive, then they will work again after cooling down for about an hour. On most engines you will lose spark, injector pulses, and the fuel pump, but there are some that only lose the injector pulses. That means you could have spark and fuel pressure, but the engine would only fire with starting fluid.
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Friday, April 7th, 2017 AT 5:41 PM

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