Hello all! New to this forum and I’m in much need of help.

Got a 1999 Ford Ranger 4.0l v6 last year with 150k on it. Love the truck but in January it started to misfire and buck after 35mph. Took it in and they said that cylinder 3’s compression was really low. The diagnosis is a damaged valve or piston. Because of this they advised rebuilding the engine vs replacing the head because, as they explain it, a new head on an old engine would just eventually cause it to burn an excessive amount of oil and lead to more damage. (Paraphrased that explanation a bit hope I did it justice)

I’m just wondering if this is a sound diagnosis and if I should go with the route of replacing the engine. Estimates to fix it with labor would be getting close to the price of the truck. If I did need to replace it then I would do it with a friend who has much more experience with swapping engines than me to try and save money but just not sure it’s worth the risk.

Also just curious is this issue common for this type of engine?

Thank you to any willing to share some advice!

 

Welcome to the forum

I would get a 2nd opinion, doesn't read right
Replacing only the heads on any engine is common practice, so their statement is odd, it absolutely won't cause any stress on original block or pistons
Yes, replacing or rebuilding an engine is good too, but its like buying a new car because the ashtray is dirty in the old one, yes that IS BETTER than cleaning the ashtray, can't argue that, lol, just costs alot more to do it.

1999 Ranger has the 4.0l OHV engine, used in Rangers from 1990-2000
150k miles is just broke in, 400k is expected on engine

The heads could crack IF(big if) engine was overheated, above 3/4 on temp gauge for a few minutes
But this would cause white smoke(steam) from tail pipe along with misfire at ANY RPM or speed

Common issue for bogging or stumbling on these 4.0ls was a dirty MAF sensor, easy to clean
Or a cracked Coil pack, not expensive to replace

If you can get a Vacuum gauge you can do some tests on compression
If you can pull out all 6 spark plugs and have a look at the tips, you will be able to see the cylinder thats misfiring, it will be a darker or light color, it won't match the other 5

 

Oh, I forgot a simple test for burnt valve on an engine, called "the dollar bill test"

Google: exhaust valve the dollar bill test

There are a few videos so you know what to look for

Basically, the tail pipe should have air flow OUT all the time when engine is running
If an exhaust valve is "burnt"(not sealing the cylinder) then when that cylinder is on its Intake stroke(sucking in new air/fuel mix from intake manifold) it will also suck IN air from exhaust manifold
You can often see that happening using a piece of paper over the tail pipe, using money is just because its usually handy and thick enough to show the change in air flows direction, IN instead of OUT

Burnt exhaust valve should cause misfires at any RPM because cylinder doesn't get full load of air/fuel mix and has lower compression, but might be more noticeable under load at higher speeds, when you need full power from each cylinder

But you would just change the one head and have the other head cleaned and tested, you wouldn't need to rebuild the engine