Hi all, I looked through this forum, and others regarding this problem but couldn't find my definitive answer, hoping someone can point me in the right direction.

A few weeks ago, my 1999 Ford Ranger went from running fairly smooth to pretty (but not REALLY) rough. No CEL, but he checked it anyway and it threw a code reading misfire on 5it idles really rough and seemed to be losing a little power. I took it to my mechanic who I trust implicitly and he checked everything that usually causes a misfire: Plugs, Injectors, Coils, Compression test, computer, electrical system, and could not find the cause. He did a little research and discovered that this vehicle sometimes blows out seal within g the valves, which requires the head to be removed and then sent out to another shop to be diagnosed. He told me it would cost a grand just to remove the head (six hour job), and then whatever it might cost to fix.

The truck has 275K miles on it and I love it to death, but I don't want to put that much more into it. Any thoughts would be greatly appreciated.

Some other info: Replaced the head gasket last December
It's my second vehicle and I don't drive it that much, but I do rely on it
I recently found a mouse in the cab

 

In a gasoline engine 3 things are needed for a cylinder to fire.
Spark, at the right time
Gasoline, in the correct mix with air
Compression

2004 to 2006 Rangers with 3.0l engines had the exhaust valve issues, your 1999 is to far away unless it has a newer engine installed.
Doesn't mean its not a compression issue, just not that compression issue

First thing I would check for is a vacuum leak
Warm up engine, over 5 minutes, then let it idle, should be about 700 RPMs
Pop the hood and unplug the 2 wire connector on the IAC Valve
Engine RPMs should drop to 500 or engine may even stall, either is GOOD, it means no vacuum leaks
If RPMs don't change then you have a vacuum leak, near #5 intake manifold

Spark, get an old fashion timing light and hook it up to #5 spark plug wire, start engine and watch timing light pulse, should be nice and steady no skips

Rent/get a NOID light, these are use to test fuel injectors, they flash when computer opens injector, hook it to #5 and test
And/Or when engine is cold, unplug the 4 wire connector on coil pack, so no spark
Crank the engine over a few times
Pull out spark plugs 5 and 6, compare the tips, BOTH should be equally wet with gasoline if #5 injector is working OK

Compression test
These can now be done with a laptop computer and the right software
Most shops can do this for you, for minimal charge
It literally takes less than 15 minutes to hook it all up and run the test 4 or 5 times

Or you can use the old fashion, and my preferred, method of removing all the spark plugs and testing each cylinder manually for compression

Your trusted mechanic did all this, but there was no fault found so....................
Well there is no so, because thats ALL there is, spark, fuel and compression, there is nothing magical about newer engines with all the sensors and the computer, they are sill just gasoline engines, EXACTLY the same as Henry Ford used in model T.

It is actually easier to trouble shoot engines now

 

Thanks! I wish I knew how to do any of these things.

 

Call around, find another shop that can do an electronic compression test AND cylinder balance test

This SHORT video shows how hard it is to do electronic compression test: www.youtube.com/watch?v=YDX83bWu3Vg

High numbers on any type of compression test is not the point of the test.
Its the difference in the numbers from one cylinder to the next

A 3.0L engine should run 165psi compression
But if all 6 cylinders tested 135psi to 140psi then all is well, problem with testing equipment but not the engine, lol.

So you need a print out or photo of the test results
Post them here

Reason you do Compression first is because compression can't be intermittent like fuel and spark can be.
You either have good compression or you don't, black and white
If it is the problem then you don't waste time and money on "non-fixes"
If it isn't the problem then you can take it off the table and KNOW fuel or spark is the issue

 

My mechanic did a compression test and said all was fine.