So i just bought a 1998 Subaru Forester ( 4 cylinder 2.5L) for dirt cheap to run around for the winter.  The passenger side front cylinder is misfiring when at idle, car runs great when I give it some gas, even a little.  I checked the compression it was a little over 100 and I changed the spark plugs.  The spark plug on the misfiring cylinder was cleaner than the rest of the spark plugs.....   I checked the plug for spark to make sure the wire wasnt bad and I did see spark and when i pull the wire off the coilpack when the car is running I see plenty of spark at the coil.    I am thinking of trying to change the injector first, just looking for any reccomendations or advice out there.  Should I replace the wires and coilpack too?  Any other ideas of what could be causing the misfire?    Oh yeah not that it matters I dont think but the misfiring cylinder is passenger side front.  What cylinder number is that?   Thanks everyone!!!

Front passenger side is number 1.

100psi is low, could mean a burned valve. Might check valve lash clearance on that cylinder.

Did you check compression on all 4 cylinders? Pull all 4 spark plugs and hold throttle wide open? (Disable ignition by unpugging the coil or the igniter on the firewall, and unplug the injectors.)

 

Do you have a misfire code?

Any other codes?

Check for battery voltage at the injector plug. Also check resistance of the injectors. All 4 should be within about 5%.

Edited December 8, 201312 yr by Fairtax4me

Thank you for you response!! I did not check the compression on all 4 cylinders, shouild I do that?   Why should I pull the plugs and hold the throttle wide open?   DId I do the compression test wrong?   I pulled the plugs but I did not hold throttle open, I just turned the car over a few times.

     

      I read somewhere you could check for a burned valve by holding a dollar bill up to the tailpipe and if it sucks the bill inward at somepoint it would mean I had a burned valve...... Is that true?  

 

    I have a number of codes coming up when I put it on the machine, I cant remember them all exactly but I know the O2 sensors were on there,   I will bring it to the mechanic shop on Monday and write down the codes I have coming up.   Im basically trying to determine if the car is worth putting some money into or maybe I just drive it as is untill inspection runs out next September.  Thanks for your help!

I will check the injectors resistance tommorrow.   Never looked for valve lash clearance,  I will do some research on that

Edited December 8, 201312 yr by kungfufunc

Throttle should be wide open for a compression test so the engine pulls in as much air as it can. Pull all 4 plugs so it can turn easily. Pulling only 1 plug causes the engine to turn slower and lowers compression reading.

Disable fuel injectors and ignition so the engine doesn't catch on fire while testing.

 

The dollar bill test does work, but its not 100% conclusive. There could be other causes but it gives you a good idea that there's a mechanical problem. The same conclusions can be drawn from a compression test. If you have low or no compression there's a mechanical problem and the cylinder head will have to be removed to check.

 

Re-check compression and check the injector resistance. Also be sure to check for battery voltage at the injector plug. If the voltage is low there could be a wiring problem on the injector.

i will recheck compression tommorrow and then check the injector resistance and voltage and ill get back to you.  thanks again

So i did not have time to work on the car today but my father switched the injector with the rear injector to see if the misfire followed the injector but it did not, after switching injectors the misfire was still on cylinder 1.   Not sure if this provides you with any more information or not,...