2002 996 Misfires on cylinders 2, 4, and 6

Hi everyone, Was driving to dinner last night and the check engine light comes on blinking and the car is running like crap with a lot of vibration. Limped home about 8 miles Bought a ODB reader this morning to read the codes Here are the codes that came up P0300- Misfire detected P0304- Cylinder 4 misfire detected P0302- Cylinder 2 misfire detected P0306- Cylinder 6 misfire detected Searching this website, it looks like replacing the coil packs and or plugs may fix it. The current plugs were replaced about 4 years ago. About 10,000 miles ago. Is there anything else I should be looking at? It doesn't make any sense to me that 3 cylinders would produce the same code at the same time. What am I missing??? Thanks in advance for any info John

Quotes source post: 11519685

Unlikely to be plugs. More likely to be coils. But even this is not a certainty, to have 3 decide to go bad at one time suggests something else is going on, that failure is a common failure. But if you tell me you drive the car year 'round and it has some miles on it I could more readily accept coils as being the source of the misfires. Before you proceed with the coils/plugs... Any chance of stale/bad gasoline? This is a bit of a long shot as fuel has to be pretty bad to cause misfires, but I thought I'd raise this possibility. Did you recently, within a day or two -- wash the car and either briefly start the engine to pull the car into the garage or not start the engine at all? Did the local weather change, saying going from warm/hot and dry to cooler with more humidity? Did you leave the car out overnight? I've had both of my cars misfire -- enough to turn on the CEL at cold start but not once under way -- several times from sitting out in the rain, or overnight in heavy fog, or once just during a mini-climate change going from hot and dry to cooler and damp but with no rain. Generally the advice when dealing with misfires is to erase the codes and afterwards then drive the car and see if the codes come back. However, if the engine is exhibiting untoward behavior I would not advise that. This makes me hesitate to suggest you disconnect the MAF at the wiring harness and clear the codes and road test the car. In some cases a bad MAF can result in misfires but I'm always concerned about a real rough running engine. Have you checked for any pending codes? What is possible, though thankfully not that frequent occurrence is -- and I mention this just to be thorough not be to an alarmist -- is a cylinder bank's VarioCam solenoid/actuator acts up and the DME will adjust fueling on that bank to get the O2 sensor readings it expects and at some point this fueling can lead to misfiring. However, one would expect this misfiring to be confined to one bank but it is possible that severe misfiring of one or more cylinders on the "bad" bank could result in causing misfires of one or more cylinders on the other bank. Last but not least, has the car been siting a while unused? This can present an opportunity for mice to get in the car and it is not unknown for the damage to be to the engine wiring harness and misfires can arise from this damage. To be thorough, you need to carefully inspect the car, the top of the engine, and on top of the panels that cover the bottom of the car, for any rodent sign.

Hi Macster, Thanks for the good info. The car is not year round as I live in Syracuse, ny, I have about 45k on it. The gas has been in the tank for about 3 weeks now, and is down to less than a 1/4 tank. I did wash the car about 3 hours before we left. I didn't start the engine again till we left. It was fine for the first 5 miles or so and then it started running really bad. I brought it home and it was in the garage overnight. When I started it this morning, it was still really rough and the cel was blinking. I ran the codes and haven't run it since then. I'm with you when you hesitate to try running the car when its really rough. It was really bad and I'm lucky it made it home. I don't know if Im comfortable trying to drive it again until its smooth. I didn't try to clear the codes, but I'll try that tonight and see how it runs. Funny about the rodents, but my kids found 2 baby mice in the garage over the last two weeks. Ill look at the engine compartment also to see if I can see anything. Is their something to look at that specifically controls the 2, 4 and 6 cylinders? Thanks for all the great info! John

I had the same issue, codes and experience after I washed her and stuck it into a moist garage. The garage had lots of humidity because of all the rain we were having. Towed it to my indy and had the coil packs/plugs replaced and all is good.

Swap the coils from side to side

Assuming the codes come back after a reset, I would switch coil 6 with coil 5 since they are the most accessible. According to the engine diagrams I've seen, when standing at the rear of the car, coils 1, 2 and 3 are on the left (driver's side) with 1 closest to the rear of the car (closest to the serpentine belt). Coils 4, 5 and 6 are on the right (passenger's side) with 4 closest to the rear of the car (closest to the serpentine belt). You can visually check the state of coil 6 as well as the wires.

Mice. It's always mice.

I thought the whole mice thing was a joke until they got under the hood of my Toyota truck. Truck ran like **** , opened the hood found some chewed wires, spliced them back together.

I hate those meeces to peeces..

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Based on my experience the fuel is not old enough to be stale enough to cause misfires. I was thinking on the order of months old fuel, not weeks old. The couple of times I've had one of my engines misfire after sitting out in a rain storm all night, or out all night as a heavy fog came in, or in the case of the Boxster sitting in an open carport as the weather changed from summery (dry/warm) to wintery (damp/cool) the misfires were short lived and the engine smoothed out and ran normally within seconds after the CEL came on. Essentially it was fire up the engine� hey this thing sounds rougher than usual� CEL!� then I pay closer attention to the engine, scanning the dash for any other warnings�finding none letting the engine idle and note that it sounds better and then my heart starts beating again. Thus I had no real qualms about driving the car but I did read the codes and note them and cleared them to turn off the CEL. Coils can fail -- well, go bad -- at any time and I could believe if they were really bad it might take some time for the coils to recover -- make that dry -- after being doused from a car washing. I could even believe that the coils -- at least some of them -- could be bad enough -- cracked -- that even once dry they would not work right. If the plugs are original they could be due to be changed based on time if not miles. While I do not think the plugs are at fault, if they are due to be changed this is a good time to change them as it gives you a chance to inspect the coils. Or you could just inspect the coils leaving the plugs alone for now. Anyhow, I wouldn't have a problem replacing the plugs -- provided they were due to be changed on miles or time -- and at the same time the tech can inspect the coils and advise you if they should be replaced. If he finds them cracked that's good news. If he finds then looking ok�Well, the problem could be marginal electrical connections at the coil/wiring harness connectors. The car I gather probably receives regular washings and doesn't always get driven afterwards to thoroughly dry the engine. Even if the coils are ok this exposure to moisture over time can affect one or more coil electrical connections. The "fix" is to carefully disconnect the coil at both the plug tip and the electrical connector then reconnect being very careful and being very sure the coil is secure to the plug and its connector is secure at the wiring harness connector. If you feel up to doing this it might be worth a shot. Baby mice around means there's a mommy and daddy somewhere. This is not a smoking gun but it does tip the scales more towards rodent damage as explanation for what's going on. Off hand any common failure point for cylinders 2, 4, and 6 would also be common for other cylinders as well. What you look for is rodent sign. At the same time you have to visually and then with your hand run/trace each wire and hose looking for/feeling any signs of having been chewed. You do this with the engine off and cold. Since there are misfires, with a flashing CEL, and a rough engine, this is rich misfire, and this points to the coils. Pay attention to the coils. Look at the wiring from the coil to the wiring harness. Given there are misfires, rough running engine, the possibility of rodent damage, you might be advised to flat bed the car to a qualified shop and let a pro handle this. Discuss with him the symptoms, and get his input on how he would approach this.

Thanks for all the responses guys. I unfortunately didn't get to look at the car sunday night like I wanted too and left on a business trip for the week this morning. So I wont be able to start diagnosing tiil Saturday. I will definitely look at the wiring harness for the coils for mice damage and swap two coils and see if that makes a difference. If it's coils, I think I'll be able to change them myself. If its not the coils, off to the indy!

Same happened to my 996. After a night of rain, the moisture build up got to it and had me misfiring. However, on a hot sunny day they were fine, light went off and no misfiring. Had my coils and plugs done and the problem was fixed, that simple. With 4 years and 10,000 miles on yours, I would see no harm in swapping them out.

Thanks Venom, I ordered Plugs and coils on Monday and they should be here by the weekend. Im going to install on sunday. Ill report back what happens. John