Just started out of the blue a few weeks ago (I hadn't done anything to the car). Skips a beat now and then when idling and really misses when accelerating. Not noticeable when just cruising along.

 

Car has always had a slightly tapping valve for as along as I've had it (a couple years). And I may be just imagining things, but that tapping does seem to be a little louder, now. 

 

Replaced the plugs and fuel filter - no improvement. Next, I'm thinking I should replace the plug wires, air filter and PCV valve. But I get the strange feeling that won't help, either. 

 

Possible vacuum leak? How would I check for that? Fuel pump going bad? Would those symptoms match that? What other cheap things should I check and/or replace?

 

Any thoughts appreciated. Thanks.

does it only do it cold or hot or both? Is there a CEL? If there is, check that first.

To check for vacuum leaks start the car and spray the vaccum lines and intake manifold gaskets with carb cleaner. You should notice a slight change in idle speed if there's a leak and it sucks in the cleaner. Usually vaccum leak symptoms will decrease as engine speed goes up though.

Make sure your throttle is clean and if not, spray with carb cleaner with engine running above idle (around 2 or 3k rpm). Outside of the plugs, you can go to a local lube place and have them perform a fuel injector cleaner service. Sometimes that's all they need. The one that's sucked up into the manifold, not the gas tank additive.

HI,

A full tune-up rules out a  lot.

 

Remember - NGK/OEM Subaru  parts are recommended (required?)...not necessarily from the dealer, but don't buy the the local auto parts' bargain stuff.

 

If you've let the valve tap for 'a couple of years', worst case is a burnt valve.

 

CEL/Check engine light ever come on? If you've had one in the past, get it checked again for the current issue(s).

skipped timing can let a car run but with low power

 

could be the knock sensor. cheap ebay units seem to work well for people so, you're not loosing much to swap in a new one if I'm wrong.

Edited February 10, 20179 yr by 1 Lucky Texan

If it's the knock sensor, there should be a diagnostic trouble code for it specifically. Definitely check your obd. Even if there is no CEL on currently, an intermittent issue could store a history or pending code without turning the light on.

It does it both cold and hot. And there's no CEL. But I'll check for codes, anyway. Thanks, everybody!

Well, that was weird. Went out to check for codes. Plugged the scanner in, started the car and it showed one code P0303 Pending (Cylinder 3 misfire?). But the strange thing was, while checking the code, the CEL came on (even though it hadn't been on before)! Cleared the code, which also turned off the CEL. Let the car idle for a while and the CEL didn't come back on. While it was idling, I was paying attention to the misses. Engine would run smooth for a few seconds, then miss. Smooth again for a while, miss a couple times. And so on and on. It wasn't like a steady miss every couple seconds. Very intermittent.  

The CEL will usually come on when you're plugged in to the OBD. Swap the number three wire to a different cylinder. Don't forget to swap it on the coil too. Then check to see if the misfire moves to that cylinder.

Ah, OK. I never noticed that before (CEL coming on when plugged into the OBD). I'll swap the wire and report back. Thanks!

No problem. Just keep in mind that the pending code might not come right back. You may have to drive it a bit or it may have a two trip logic (honestly I don't know if it does) so if it's still missing but no code, drive for a few minutes, shut the car off for a minute or so, then start it back up and drive for another few minutes and check again.

If the misfire doesn't move, check the plug first and if the plug is good it's most likely the coil pack.

Any mechanical failure will usually be constant, not intermittent. Difficult to say for sure when diagnosing via forum. Lol

Ran out of time to play with it, today. But will do all that first chance I get. Thanks, again. 

You can move the wire from one cylinder to another to see if the code follows the cylinder.  Then you know it was a wire.  It could be coil going bad.  Those are easy to swap and not very expensive.

Thanks, 3Pin! That's the plan, first chance I get. I'm just hoping it's not that tapping valve and I go through all these other motions for nothing. If it was the valve, would there be any other symptoms beside the intermittent miss? The car's not spewing smoke or anything... 

my gut tells me it's the wires. If changing the wires doesn't move the misfire, then it's most likely the coil. Wires should still be replaced if it is. From the way the misfire is acting as you described it, it's very unlikely that it's the valves (mechanical issue) causing it. A bad valve is a bad valve.

Also worth mentioning, sometimes on these cars the wires can be left improperly seated or walk themselves out as it's kinda hard to get the leverage needed on them sometimes. Make sure they "click" onto the plug and if you're not sure, pull it and do it again. This could possibly be the "louder" noise you're hearing.

Yeah, I noticed that about the plug wires, when I was changing the plugs. It's not real easy to make sure they're firmly seated. I'll definitely double-check that, too. Thanks!

And sure enough, a slightly loose plug wire at the coil pack appears to have been the problem. Pushed it in completely and the car's back to running without missing (albeit still clicking). Oh well, I'm sure it didn't hurt to replace the plugs and fuel filter. I'll probably still do the plugs wires, too. Thanks again for all the great input!

Awesome. Any time I diagnose, I always start with the simplest thing first. More often than not you save a lot of time, money and gray hairs.

Awesome!!!

Funny addendum to the story: After pushing the plug wire more firmly into the coil pack, the car was definitely running better. But it was still missing now and then. So this morning, I decided to investigate further. I pulled the suspect plug wire from the coil pack - and that end of it just crumbled! Half of the metal terminal and about one inch of wire was completely toasted from arcing for so long.

 

Naturally, I decided to do this BEFORE I'd actually bought any new plug wires. And naturally I couldn't find ANY of the old plug wires (from other cars) that I'd saved for just such an emergency. So I had to chop off that burnt part and luckily there was enough left of the metal terminal to crimp the wire down and still have a makeshift connection. Amazingly, that was good enough to get me to the parts store (for new wires).

 

Hey, what can I say. I'm not a mechanic. I just pretend to be one when I have no other choice.   

McGyver would be proud!