996 engine runs rough at 1500-1800 rpm

Sunday I was warming up my 1999 996 when I noticed that the engine begins to run rough when I slowly increase the rpm from idle. As it passes 1500 RPM it starts to run rough, like one (or more) cylinder is missing, and then smooths out around 1800 rpm. This was under no load. I think it does it a little when in gear and moving but its hard to tell. I can see the tachometer bobble a little as it goes through that range when sitting still. When not moving, you can feel it through the seat, or even if you put your hand on the body. I have noticed some times when at cruising speed it seems to miss once in a while, but its very quick and not consistent. The car has just under 80,000 mikes and except for the IMS I have no service records of engine work, coil replacement, etc. the other service records seem to show it has had its regular service and was well cared for. I have had it a bit over a year. Thoughts? Things to check?

Check engine light illuminated? Have you pulled any codes? Noticeable drop in fuel economy? Sounds like a misfire, which could be due to a number of things (spark plugs, coil packs, O2 sensor), or something as simple as a dirty MAF. If you don’t have any service records, and mileage is 80k then it could probably benefit from a tune up regardless (fuel filter, spark plugs, coils, air filter). I’d start with seeing if there are any codes and work from there.

Thanks. Do not have a code reading device. Looking at options for a code reading device. Foxwell NTS510 seems to be an option some have used and the Durametric. Looking on line now. Did the air filters and fuel filter. Been thinking about plugs and coil packs No tune up recently. I bought the car from the guy who serviced it for about the last 7-8 years. I will ask him what they have seen.

^^^^ Are you getting a CEL? You can get a cheap generic code reader almost anywhere these days... Mike

I had a rough idle on my 99 with 75k. Changed plugs and coils and resolved my issue. I didn�t experience the 1500-1800rpm issue you describe though.

Used a generic reader I borrowed and it showed no codes, but it did not have the Porsche software uploaded. No lights. Reading more about this I might just get a coil pack with new plugs and put them in. Silly to ask but does any one have an opinion (seem to be some strong ones on forums these days ) on coils or plugs to use? There are cheap ones out there on the internet, and then the OEM ones. And which plugs? I see NGK and others. Thanks for the feed back folks.

^^^^ I went with new-style OEM coils and spark plugs. Go with good parts if you're trying to trouble-shoot an issue. Mike

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Coils - pelican parts #997-602-107-02-M47 $42.50 each. you will notice the housing for replacement coils are slightly taller than original. They don�t make the shorter style any more. Since these are taller be careful when reinstalling the dust shields as they will bend if you tighten all the way. May want to add a small spacer on the bolts between the shield and the cylinder head to compensate for the additional height.

I did resolve it by installing new plugs and coils. I bought the Behru coils and bosche plugs. I had no error light or codes so could not tell it it was one coil, so did them all. The plugs looked ok, except for # 6 which was a little dark compared to the rest. The service records I have say the plugs were new 25,000 miles ago but the coils look original. No more roughness now and it feels a bit stronger than before, but it is so hard to tell. If you do this yourself and have not, this was my first time so I can offer these tips: If you do the coils and plugs, take off the mufflers. It makes it so much easier to get to these. You only have to remove 3 nuts on studs that you can get to from below and two muffler clamp bolts. You do not have to remove the rear bumper. Put it on jack stands and remove the rear tires. I had the exhaust off a year ago so the nuts were free and it took 10 minutes each to take them off and another 15 to put them back. Might take longer if they are more rusty. Loosen the muffler clamp, then the three bolts, pull the muffler away from the bracket, then slide it forward. Putting them back is the reverse, but once the nuts are on the studs loose, do the muffler clamp, then tighten the nuts on the studs. I struggled for an extra hour or so with the first side before I gave up and took the muffler off. It saved me at least an hour and a half and much swearing at German designers. I bought new tube orings and I bought a couple spare tubes just in case. I did not need the tubes but the orings were flat and I did have leaking at one tube. I used a 1" boat transom plug from Walmart to pull the tubes. Get the one that screws in with the T handle, not the one that flips up. No room to flip it up. The heat shield does need to be pushed out because the new coil is bigger and needs to be spaced out. You need longer bolts for that and spacers. The old coil allen head bolts actually are the right length and thread. I bought some new hex heads bolts from the hardware store because its easier to torque them. I bought the bolts and some spacers at ACE Hardware but you could as easily use a couple 13mm or 1/2" nuts as spacers. (putting the spacers in is a real pain as they slide off the bolt as you try to get it started when you reinstall the heat shield. I used a tiny dab of high temp gasket seal to "glue" the spacer to the back of the heat shield and they held in fine as I maneuvered it and the bolts into place.) One other thing - the grounding cable on the passenger side is sort of in the way. It is attached to the frame with a bolt on a plate that is welded to the frame and to the engine block with a bolt into a threaded hole. Remove the cable by removing the bolt at the block end. then push it aside. I sheared the welded plate way from the frame when I tried to loosen the bolt. Had to reattach it to another location.