Confirmation on Skipping

[Bronto]

Hello,

I have a 1991 Ranger 4L, and I am having a few problems with it. She blows blue smoke when idling, not all the time, but some. There is no noticeable smoke when being driven but it does skip every once in awhile, especially after taking off from a stop. I ran my diag computer on it and it says low pressure in Cylinder 3.

I am thinking it is a cracked ring or seal. Has anyone else had this same issue, or would know what it could be?

 

[RonD]

You should do a compression test on that bank.
A diag computer can't tell actual compression, it is basing its conclusion on a good fuel/air mix, and if you got blue smoke you don't have that, so each cylinder will not be firing at full power.

Blue smoke is oil burning and yes it can come from older rings, but also old valve guide seals.

If compression on #3 is within 15psi of cylinders #1 and #2 then its the valve guide seals, compression on the 4.0l should run about 160-175psi.

The valve guide seals are the most likely problem from your description.
These seals are under the valve covers, inside the valve springs at the top of each valve stem.
When engine is idling the vacuum in the intake is the highest, this causes oil vapor in the valve covers to be sucked in passed worn intake valve seals, the valve stems are in the intake passage.
So after idling for a bit, at a stop light, when you step on the gas the oil in the intake passage is sucked into the cylinders and burned causing blue smoke for a bit, also less power and misfiring because oil don't burn like gas does, lol.

Bad rings would tend to always blow blue smoke.
I have a '94 4.0l with 300k, rings are still fine.

If you have a compressor you can do a leak down test to see the condition of the rings.

You can replace the valve seals without removing the heads, just the valve covers.

While this won't fix the problem it can make it burn less oil.
Replace PCV valve, and clean it's hose, and then clean, and check for cracks, the Vent hose, usually this comes from the valve cover or oil fill tube and runs to the air cleaner or air plenum.
Both of these pull air from the valve cover area and crankcase, this lower its internal pressure, especially at idle, if there is less pressure then less oil vapor will be pulled into the cylinders.

 

[Bronto]

Thanks for all the info! Hopefully I can save some money by not doing a complete valve job.

Would this also explain why it would stall after it has been driven for a little while? I had it out yesterday and when I had it parked, it sputtered and stalled on me, and was rough running until I started out on the road again, and even then skipped a few times at first.

 

[RonD]

When an engine starts to burn oil with the gas the spark plugs don't last as long, it is called "oil fouled".

The oil doesn't burn completely and coats the spark plugs, this can cause misfiring at lower RPMs, well more noticeable at lower RPM.

If changing the spark plugs doesn't help it could be the IAC(idle air control) valve.
You can test this valve by unplugging it while engine is warmed up and idling, if idle doesn't change when IAC is unplugged then it is not working, it can be cleaned and retested.

Redoing the valve guide seals with heads on isn't complicated, but it is time consuming.
You can only do one cylinder at a time, but both valve seals on that cylinder.

You need a valve spring compressor and a few feet of small diameter rope.
The valve spring compressor can be rented, but they are not that expensive.
The small diameter rope is to put down the spark plug hole on the cylinder you are working on, the rope coils up in the cylinder, then you rotate the crank to have the piston compress the rope in the top of the cylinder, this holds the two valves in place when you remove the springs.
You can also use compressed air, although I like the rope better.

One heads up, anytime you remove a valve cover, or intake for that matter, get some rags to stuff in ALL exposed holes, this does two things, it prevents random debris from getting into the engine, and the more important one, it prevents that last nut or bolt from falling into the oil pan at 8pm when you can't replace it until the next day and then spend the next few months wondering if engine will blow up because that nut or bolt didn't fall all the way thru, lol.