Checking non-ignition related miss

[ab_slack]

I found this video about testing a miss when ignition issues are ruled out. I thought it was a clever idea, but there was no comment about how do you know if it is on compression stroke or not.

http://www.youtube.com/watch?v=8NtOkBBkuYU

Maybe this can help with that miss problem on our Jeep Cherokee which has been thus far elusive. On the other hand every time I think about that I remember adsm08's comment in response to someone else's Jeep problem.

Might as well cross that one off the list.

You can't fix Jeep.

In our driveway:

One 87 Bronco II, runs fine
One 99 Ranger, runs fine
One 02 Explorer, runs fine
One 94 Jeep Cherokee, major miss, no good to drive

I see a pattern here, he may have a point.

 

[kunar]

one important part he leaves out is that a lot of times the hose for a compression tester will have a check valve in the end. that valve has to be removed to use the hose to feed air into the cylinder. other than that, yeah its common practice. get that cylinder to TDC and pump in air. if you hear an air leak through the intake or the exhaust, its a good bet that valve is bad.

 

[FritzTKatt]

Is that fuel injected?

I like the idea of putting air in the cylinder. Definitely a relatively easy valve check.

 

[ab_slack]

Is that fuel injected?

I like the idea of putting air in the cylinder. Definitely a relatively easy valve check.

I don't think it makes a difference if fuel injected or not. You still have the air intake and all. And the injectors inject into the manifold rather than the cylinder.

It wouldn't isolate an injector as a problem, so that have to be an additional test, but on the other hand you could infer that if it holds pressure without are backing out either exhaust or intake that it isn't related to valves.

 

[RonD]

Putting air in the cylinders is called a Leak down test.

Generally you remove all spark plugs then put compression tester in the cylinder you want to check.
Turn crank manually until you start to feel the compression of that one cylinder.
Remove compression tester and put a stiff wire in the spark plug hole.
Turn crank until you are at TDC for that cylinder, watch the wire and use it to feel where the piston is, or if you can see into the spark plug hole, even better.
I usually mark the crank pulley with a white line at 0 deg, so I can check spark timing on the cylinder at a later time if needed, you can also add a number if you are testing several cylinders, also comes in handy for the Wet test, see below.

Once cylinder is at TDC compression stroke you inject air
For Leak Down test you inject a known PSI of air, so if your compressor is set at 100 PSI, and is stable, you hook it up to the cylinder with a pressure gauge.
If pressure reads 95 psi then you have 5 psi leaking, so 5% leakage, 5 psi is 5% of 100 psi.
If you use 50 psi to start with then just do the math, if leak down showed 45 psi that would be 10% leakage, 5 psi is 10% of 50 psi.
New engines are usually 3% so 5% wouldn't be bad.
0% leakage would mean someone has welded the rings to the cylinder walls and weld valves as well, lol.

That's the Dry test, then you added a couple of tea spoons of motor oil to the cylinder.
I like to rotate the crank two times at this point to spread out the oil, two times because its a 4 stroke engine, so my TDC line needs to go around once for the exhaust stroke and then again to get back to compression stroke.

Now the Wet test.
If I was at 95 psi Dry and then at 97 psi Wet the rings are leaking 2 psi, which isn't bad.
3 psi are valves, which also isn't bad.
All the seals in a cylinder are metal on metal, rings and valves, so will for sure leak, you are just testing how much they leak

 

[ab_slack]

That's cool Ron, thank you de-mystifying some more stuff.

 

[RonD]

Your welcome

As far as the miss goes, you can just do a compression test on all the cylinders.
With no spark plugs in and throttle propped open(put something on gas pedal)
The starter motor spinning the crank should generate about 150-180 psi at each cylinder when tested.
The variation from one engine to the next depends on battery and starter condition, the faster the spin the higher the compression, so the numbers are not that specific.
The point is to see what one engine has on each cylinder so those can be compared.

10% variation is the "rule of thumb", so the highest compression number and the lowest should be no more than 10% apart, so if highest compression is 160 psi then lowest should be no more than 16 psi lower, 160 - 16 = 144 psi

If there is a leaking valve you will see that easily with a compression test.
If compression on a cylinder is lower than 100 psi then it will not ignite, it might after warming up, but would be hit and miss.

One other thing to check while doing this, check the end of the compression meter when you remove it from each cylinder, it should be wet with fuel(don't disable fuel for test).
If it is not wet with fuel then you do have an injector issue.

Ford Injectors test at about 14 ohms, not sure on Jeeps

Injectors are controlled by Grounding, when key is turned on all injectors get 12 volts but no ground so don't open.
The computer grounds each injector to open it.

There are two kinds of injector systems, Batch fire and Sequential fire, there is also direct injection, where injector is in the head like a spark plug, but don't run into many gas engines with this, diesels usually.

Batch fire opens 2, 3, or 4 injectors at one time, relative to 4cyl, 6cyl or V8 engine
So one step up from carb, intake is kept full of fuel/air mix so when intake valve opens fuel/air is pulled in, very hard to get a mis-fire based on fuel with theses, under load maybe but idling probably not

Sequential injection is like the spark system, old spark system not waste spark, lol.
Each injector opens when it's intake valve is open, so injector problem on this system would cause miss at pretty much any RPM.