I'm being told she's down....cylinder number 4

[seanagins]

1997 ford ranger xlt with a 4.0 v6 2wd. Recently started getting a hesitation. I immediately made an appointment with my local firestone. I dropped it off. 4 hours later they called me and said that cylinder 4 is low compression and that is why I am having a hesitation. Where should I go from here? Thoughts? They said they hook their computer up to the truck and that's what told them number cylinder 4 was low on compression....

 

[Uncle Gump]

I've never seen a scanner that would tell you a cylinder had low compression. Now it my tell you cylinder number 4 misfire... but no way it could say it was low on compression.

You would have to run a compression test to determine compression.

 

[seanagins]

I've never seen a scanner that would tell you a cylinder had low compression. Now it my tell you cylinder number 4 misfire... but no way it could say it was low on compression.

You would have to run a compression test to determine compression.

Yea, my 1997 ford ranger is capable of virtual compression readings....like, it's a 2018 or something..

 

[RonD]

Well it is not just possible but common practice to do relative compression tests on OBD2 engines with a laptop

Google: laptop compression test video

There are two tests, one is the cranking test, no fuel added, other is running test
The crank sensor and cam sensor can tell "you" how much the crank/cam slowdown when cranking, on each cylinders compression stroke, if it slows down less then lower compression in that cylinder
Running test is done by "power added" when cylinder fires, this is how computer IDs and sets misfire codes
But there is an extra step, the computer cuts injector signal, like a power balance test, and watch the relative speed that the crank/cam slows down which is compression resistance
We are used to gross analog seconds and minutes, the engine computer and sensors gets down in the milliseconds, so timing delays is easy for it to do

These are all based on "average number test" so its like a regular compression test, the actual number doesn't matter, well within reason
If you have an un-calibrated compression tester, it might show 130psi when actual number is 160psi
But if you test 6 cylinders and 4 are 125-135psi and 2 are 100psi then you know there is a problem in the 2 low ones, so the actual numbers are not relevant except to show 2 are different


But a compression test doesn't tell you WHY a cylinder may have low compression
Generally if its not blowing white smoke on start up(head gasket) then its a burnt exhaust valve
If you want another non-intrusive test you can do the burnt valve dollar bill test
Google: burnt valve dollar bill test

You can also use a lighter next to tail pipe to see if the flame is SUCKED IN to tail pipe when #4 comes around


In any case I would change the spark plug in #4, new one or just swap it with #5
And give it a good look, if its blackish then oil is getting in, valve guide seal

 

[seanagins]

Well it is not just possible but common practice to do relative compression tests on OBD2 engines with a laptop

Google: laptop compression test video

There are two tests, one is the cranking test, no fuel added, other is running test
The crank sensor and cam sensor can tell "you" how much the crank/cam slowdown when cranking, on each cylinders compression stroke, if it slows down less then lower compression in that cylinder
Running test is done by "power added" when cylinder fires, this is how computer IDs and sets misfire codes
But there is an extra step, the computer cuts injector signal, like a power balance test, and watch the relative speed that the crank/cam slows down which is compression resistance
We are used to gross analog seconds and minutes, the engine computer and sensors gets down in the milliseconds, so timing delays is easy for it to do

These are all based on "average number test" so its like a regular compression test, the actual number doesn't matter, well within reason
If you have an un-calibrated compression tester, it might show 130psi when actual number is 160psi
But if you test 6 cylinders and 4 are 125-135psi and 2 are 100psi then you know there is a problem in the 2 low ones, so the actual numbers are not relevant except to show 2 are different


But a compression test doesn't tell you WHY a cylinder may have low compression
Generally if its not blowing white smoke on start up(head gasket) then its a burnt exhaust valve
If you want another non-intrusive test you can do the burnt valve dollar bill test
Google: burnt valve dollar bill test

You can also use a lighter next to tail pipe to see if the flame is SUCKED IN to tail pipe when #4 comes around


In any case I would change the spark plug in #4, new one or just swap it with #5
And give it a good look, if its blackish then oil is getting in, valve guide seal

Is cranking test and running test referred to as relative compression check or balance compression check?

 

[Uncle Gump]

You know I feel asleep last night thinking about this. Hell even 20 years ago (or more) the test was done with scopes like the SUN interrogator. Ron right as usual... it's a comparison thing with cranking speed/starter current/maybe even KV volts produced buy the ignition coil. Then the machine would spit out a likely diagnosis for what it seen. We would always go into an actual compression test to verify.

I had been running my garage sale in 93 degree temps with people trying to talk me down from a dollar to 50 cents all day. I probably wasn't in the best condition to answer your question.

Edit... was more like 30 years ago.

I believe the relative compression check would be cranking and running would be the balance compression.

I still believe these are just a starting point... if you really want to know whats going on... as Ron said... you will have to get your hands dirty.

 

[seanagins]

You know I feel asleep last night thinking about this. Hell even 20 years ago (or more) the test was done with scopes like the SUN interrogator. Ron right as usual... it's a comparison thing with cranking speed/starter current/maybe even KV volts produced buy the ignition coil. Then the machine would spit out a likely diagnosis for what it seen. We would always go into an actual compression test to verify.

I had been running my garage sale in 93 degree temps with people trying to talk me down from a dollar to 50 cents all day. I probably wasn't in the best condition to answer your question.

Edit... was more like 30 years ago.

I believe the relative compression check would be cranking and running would be the balance compression.

I still believe these are just a starting point... if you really want to know whats going on... as Ron said... you will have to get your hands dirty.

I have in my head that I want to perform a compression check and cylinder leak down test. What are the best ways going about these tests?

 

[seanagins]

I want to perform a cylinder leak down test on my 4.0 liter OHV. What is the best way of going about to perform said cylinder leak down test? How do I go about finding TDC for each cylinder for compression stroke?

 

[Uncle Gump]

I would start with a compression test.

Pull all the plugs... disable spark and fuel... connect a battery charger... block the throttle full open... crank the engine over until compression stops building in each cylinder.

This will make the test results equal across all the cylinders.

Once you have those results... then you start looking for why it's low on a given cylinder(s).

 

[RonD]

Compression test will give you what you need to decide if you want to pull the heads and service them, so you only need to do either a compression test or a leak down test


For a leak down test you need an air compressor with a fixed regulator you can set at a specific PSI, like 50psi or 100psi
Then you hook the compressor up to the spark plug hole with a pressure gauge on that hose, yes cylinder at TDC with both valves closed
Say you are using 100psi, if gauge showed 85psi then you have 15% leakage
If 50psi and gauge showed 43psi then 15% leakage(7psi x 2)

New engines usually have 5-8% leakage
100k miles 10+%
Metal rings on metal cylinder walls and metal valves on metal seats are NOT air tight, they are tight enough to hold HIGH pressure temporarily, like when a cylinder fires and pressure exceeds 700psi which pushes the piston down adding power to the crank

With a leak down test you can listen at the tail pipe and intake manifold to determine if exhaust or intake valve is leaking, you WILL hear escaping air

With compression test you add a teaspoon of oil, AFTER testing compression, then retest compression
Compression number will always go up with oil added, but if it doesn't go up passed the average of the other cylinders then you have leaking valves, the oil only seals rings better

So either test can tell you if Rings or Valves are the reason a cylinder has low numbers
In engines made after 1970 the ring material is good enough to last passed 400k miles
99% of the time low compression is a valve issue or head gasket

 

[Broosedamoose]

If you have low compression it will be from one of three things; either the valves, the head gasket or the rings. Regardless of which one it is you are going to have to pull the heads!