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2004 Subaru Forester - 4th Cylinder compression

Our beloved 2004 Subaru Forester has the check engine light on. We’ve been told that the 4th cylinder has either low or no compression.
Is this fixable/worth fixing? Would you replace the engine? The car has 183K miles on it, and runs great! It is stuttering now, due to this issue, but otherwise - runs great and is in good condition!

These engine valves need to be adjusted occasionally, and if you’re lucky it’s simply a valve out of adjustment. Have someone check the valve adjustment first. Don’t drive it any farther until the valves are checked, because you could do serious damage. Have it towed to the dealer and ask them to check the valve clearances. If you’re lucky, a valve adjustment will solve the low compression problem.

if you’re not lucky, it’s something bad like a blown head gasket or a burned valve.

There is no error code that can tell you about a compression problem. What error code(s) was pulled from the computer? What was done to try to find the cause?

I’ll have to go on the assumption that you had something like a misfire code for cyl 4 (P0304). I’ll further have to assume that normal things like spark plugs and wires, and fuel injector operation were checked and an actual compression test was done. In this case, the 4th cylinder will have either low OR no compression. There’s not much to guess about if someone actually checked the compression. Assuming that someone did actually check the compression, what were all of the numbers for each cylinder? Was a wet test performed? A leakdown test will tell you if that cylinder has a problem in the bottom end (piston/rings), top end (valves) or head gasket. Where compression is being lost has a lot to do with how much “surgery” the car needs.

Jesmed is right in that the cause of the low compression needs to be diagnosed before it’s possible to guess whether it’s worth saving…

Did the shop that told you the cylinder was low give you any reason as to why? It seems strange that a shop would tell you this without suggesting (or doing) additional testing to determine cause.

If this has just started, it could be due to a tight valve, most likely an exhaust valve. If the compression is in fact low, I would check the valve lash next. If this has not gone on very long, that is it just started and it is the valve lash, you may get lucky because if the valve isn’t completely closing, it will burn in short order.

All I have from the shop that tested the check engine light is that it was code 304 and that the compression on the 4th cylinder is 60.
Should I find another shop to do a test on where the problem is? Is that expensive?

It could be a head gasket or valve lash or bad rings. Valve lash would be the best outcome. Subes seem to have head gasket problems. A leak down test could verify it. I have not heard many ring problems.

@Mbaer, a leakdown test should probably run an hour’s labor charge. If you have a somewhat average level of mechanical savvy and an air compressor it would be possible to learn enough on your own to see where things stand.

Is that something they should’ve done when they diagnosed the check engine light? Is it something one can do at home? Would it be expensive?

A leakdown test is something that should be done, but I’d say only if the shop knew that you were serious about fixing it. If relatively extensive engine work is not on your list of possibilities then it would have been a waste of their time and your money.

It will probably be as expensive as an hour’s labor. Do you have an air compressor? Have you ever changed your own spark plugs? Would you confidently do so? Have you ever turned an engine over by hand? A “real” leakdown test involves a gauge that tells you how much leakage a cylinder has and in the process you can find out where the leaking air is going. In your case, if you just want to know where the leakage is headed, it’s more or less as simple as pulling the spark plugs, putting the cylinder you want to know about on its compression stroke and blowing some air into it. That’s not a “true” leakdown test, but it would tell you where the issue is. If that all sounds too weird then you’d want to pay the hour’s labor.

Just so you know,

  • if the engine is losing compression past the pistons/rings that means total engine rebuild and you’d probably be better off getting a used engine out of a junk yard. This is the biggest bucks scenario and you might end up deciding the cost isn’t worth it.
  • if the engine is losing compression at the valves, this means the cylinder head comes off and it gets a “valve job.” This is also big bucks but not as big as the former.
  • if it is losing compression from a faulty head gasket then it’s -probably- just a head gasket replacement. It’s not cheap, but is the most affordable of all of the scenarios.

If this vehicle is in otherwise very good shape and you would like to keep it for at least several more years then I suggest you have the repair done. I assume you have kept up with the recommended service for the engine up until now. If that hasn’t been done then you need to get those things done also. I would guess just repairing the bad cylinder (most likely a bad valve) is going to run you at least $1,500 dollars. Having other work done also could run you another one thousand dollars possibly depending on what is needed. You should consider replacing the radiator also. The repairs are going to be a little costly but you should have an engine that will go many more miles and will keep you from making monthly payments to the bank for a new car. Another thing to consider is the fluids for the rest of the car. If those changes haven’t been kept up you should do that also.

If this is more than a simple valve adjustment, and you really love the car and it is otherwise in great shape, I would recommend that you look into a complete reman engine for the vehicle from one of the quality reman companies like Jasper or Proformance. It may cost you $6-7k total, but you will be getting another 100-150k miles out of it. On a cost/mile basis, that pretty good.