Tomasphil

I have a 2004 996 C2, 320 hp, with 90k miles. About once a year the engine starts missing. By the time I get it to 9Elf Motorsports in Santa Rosa CA, it stops missing & is working ok. Like yesterday, I started it up, having driven it 4 days ago, when it was running fine. Yesterday, as soon as it starts, it starts missing, the "Check engine" light comes on, the speedometer msg "Drive to shop" comes on. I drove it about 5 miles, it's missing the whole time, like only 3 or 4 cylinders are firing, a real rough engine, poor acceleration, feel like I'm in an old VW! This morning I start it up, and it's not missing, drives normally. Any ideas what might be causing this? I've been using hi-test from CostCo for 6 months that has 10% ethenol, but have had no problems before. A year ago when it did this, 9Elf said maybe it was bad gasoline. Any ideas? I'm due for the 90k service. Thank you.

Imo000

Scan it. The code will give you an idea.

C4CRNA

Is it during damp or wet weather?
Car stored outside or in garage?

vange_c4s

codes please. coil pack maybe. use good gas.

street rod

Faulty spark plug?

DBJoe996

You cannot do this diagnoses without the codes. When it throws the CEL then the codes are stored. You can either buy an OBDII reader (I got one for about $56 at Autozone), get the Durametric and read the codes (I have that too), go to the nearest Autozone and they will read the codes for you for free, or, go to your mechanic or dealer and get the codes. But without those codes, it is anybody's best guess for an intermittent problem. The codes will lead you directly to the problem.

jrgordonsenior

Agree with BDJoe above. But just for the hell of it my guess is the MAF sensor. Next time it starts missing unplug the MAF and see it the miss goes away.....

Macster

Would be very beneficial to know the error codes.

One of my cars -- the Boxster -- sat in a covered parking space and when the weather turned damp (in fall about this time) misfired for the first time in over 250K miles. Original coils too.

Two times another car -- the Turbo -- has misfired. Once after sitting outside all night in a rain storm earlier this year and the 2nd time after sitting out a Berkeley hotel parking lot overnight when a heavy marine layer moved in.

Get the codes read.

Post for comment.

I would advise you to switch away from Costco gasoline (even though like other gasoline brands it probably *does not have* the full 10% ethanol the pump sticker says -- word is the ethanol content is around 7 to 8% the gasoline refiners electing to be conservative) and instead switch to in no particular order Shell (uh, my favorite so maybe there is an order here...), Chevron or 76 or Philips, one of the major gasoline name brands, as long as it is a top tier gasoline.

Buy from a busy station too. Now it just so happens where I live a very nearby Shell station is the cheapest of 3 other nearby name brand stations and busy so the gas is fresh and so my choice of which brand of gasoline to buy is easy.

Also, when you switch gasoline consider using a bottle or two of Techron as per directions. After the last tank is down around 1/4 level change the oil/filter.

But post those codes.

And just to be thorough and safe: Keep an ear tuned for any engine noises of the scary kind. While I do not suspect anything serious if you hear anything shut off the engine right now and that's all she wrote. No limping the sucker home. No starts for your buddies. Get the car flat bedded to a qualified shop for diagnosis.

Sincerely,

Macster.

sheatley

I am resurrecting this old post which unfortunately went unsolved. Hopefully the OP is ok. My car is doing the same. Today was the second time. First time was after the car was sitting for 2 weeks.Lasted 10minutes. Gone after turning off then restarting. Today, months later it did the same but for 20min. After a heavy rain. I almost never have my car in the rain. Running better but still has a little miss under load up a hill at rpms under 3200.
Codes
P0300 multiple cylinder misfires
P0301 1 misfire cylinder1
P0305 cylinder 5 misfire

Any thoughts? I was going to let it dry out, then clear the codes and restart to see if I am down to zero codes or just one. Feels like I have just 1 cylinder down at this point. No scary sounds like grinding or marbles in a coffee can.

B-ran

As I just had this happen to me a few months ago, and it freaked the crap out of me. It was after a few days of rain and the car was parked outside at the time. I went to fire her up and it sounded like a dying Subaru (i.e. A few cylinders short of a flat 6). After changing my pants I went back to it and scan the codes. Same ones that you're getting except different cylinders. I pulled the offending coil packs off, didn't notice any cracks but decided to order six new ones and new plugs. Patched her all up and no problems since in rainy weather. Hope this helps

Schnell Gelb

Sheatley,
For the work involved in removing and correctly inspecting the coils - if you don't know how old they are and teh are the old design thin ones- just get a whole set of coils and plugs. Be careful with part numbers because of revisions. I recommend and use RND because of free shipping and no tax & lowest price. Plus I like their street address.
BTW, clean the MAF anyway - who knows but it is easy. After that ,let's see what codes remain. In my case -NONE !Nice 1/4 horse btw.
After you've done this - look at the coils.Most may look fine. Then examine them under bright light and a lens.I bet you'll find hairline cracks in the plastic on the sides of the coils. Those will 'suck' moisture in by capillarity. My plugs also looked great but the PO had fitted the wrong ones ! It ran remarkably well with the wrong plugs !

sheatley

Thanks guys I thought I would be crawling under there, but been there before and not too bad. I have a 2003 so the plugs need to go as well . I have done the plugs but not the coils. Do they replace right at the wire, or do they have to be replaced all they back closer to their origin at the electronic ignition?

808Bill

I stopped using Costco gas a few years ago because of rough running issues that went away.

B-ran

The coil packs connect right at the coil. Super easy. Personally when I changed my coils, plugs, and plug tubes, I found it worth the fifteen minutes to take the mufflers out for the extra work room. I know many others don't, but my monkey hands appreciated the extra space. Also don't be a goof like me and accidentally put one of the old coil packs back in (I'd cleaned up real good to look for cracks and accidentally grabbed it instead of one of the new ones) and at WOT had a misfire that I didn't notice for a few days. Freaked me out a bit when the CEL started flashing. And like Schnell Gelb said (and I think pretty much do whatever he says) clean the MAF as well. Super easy thing to do.

sheatley

Looks like I will be a busy boy this next week. Thanks everyone.