Purchased a 996 - Multiple Problems. DuraMetric Questions

Hi, Picked up a 99 996 that has multiple issues. I knew going in about the rough running problem but am having a hard time tracking down the problem. I am getting a misfire#2 and #3 cylinder on durametric. I have changed the plugs and coil packs. Still same problem. Seems that when I clear the codes it runs ok for a few seconds then throws the codes again. Car has 186K miles and has NOT been loved most of its life. I am getting voltage and a DC signal to the coil packs. They are firing, as the manifold to each cylinder is getting hot. Any thoughts? Thanks in advance!

Cracked head?

I don't Think so I have good compression numbers, surprisingly enough.

You have replaced the plugs and coils so a bad one is unlikely. What you need to be sure the connection at the wiring harness is good. The coils could be misfiring due to bad connections at the harness. The harness could be suspect too. The fact the manifolds get hot is of little value. Good compression -- are you sure of the correctness of the compression test? -- is good news. My fear would be there is one or two burned valves. What are the overrev counter numbers? Any recent over revs in the higher ranges?

Have you ruled out wiring to the coils?

Thanks for the input. I am getting zero over-rev count on durametric. I guess I could be wrong on the compression test. 155 +/- a few PSI. Strange that the car will run ok when I clear the codes. Just a few seconds but very strange. I have not OHM out the harness yet to the ECU. That will be the next weekend project. Although I do not get a ground short when checking DC signal to ground at the plug.

When you changed out the coils did you make sure that the connectors that plug into the coils "clicked" on to the coil? Its easy for the connectors to be on the coil but not really have good contact. You can confirm a good connection by pulling back the rubber connector cover at the coil and inspecting the connection. Also, how is the car running? Does it seem like it is running "rough" ? I ran across one unlucky 996 cab owner that had a worn spot in the wiring harness that runs from the DME to the coils. It may have been a cab specific problem though.

Yes the car for sure runs rough. It might be a wiring issue, I removed the coils a few times checking for connection.. Pretty hard on #2,3,4,5 to be positive but think so. Will be checking wiring harness this upcoming weekend. Thanks for the help!

are you getting a specific code? Did you rule out the simple stuff like MAF?

As geetee says - the MAF. Clean it. Takes a torx 20 screwdriver bit, MAF or carb cleaner and 15 minutes. Or just unplug it and see how it runs in default mode. Make sure your grounds are good- battery to chassis, engine to chassis. Clean both sides of the connection thoroughly and reattach. Just because you only have codes for two cylinders doesn't mean the cause is located at those two cylinders. I feel your pain, just went through this with my wifes WRX. Three trips to the dealer and he gave up. I went to the WRX website and got the list of 11 things that cause misfire codes. Turned out to be the battery chassis ground.

You have a number of things to check. About the MAF. A dirty MAF is possible, I guess, since you do not know the car all that well. While my experience is a car with an engine filtration system that has not been compromised -- by compromised I mean generally with the addition of an after market air filter and air box -- does not dirty its MAF at all (even after over 100K miles of use) you admit the car has not been loved, so all bets are off. There has been at least one case I know of that a bad MAF was the cause of engine misfires and they were all confined to one bank. So cleaning the MAF, or at least disconnecting it from the harness and clearing the codes -- to reset the learned adaptations -- and seeing if the misfires come back is probably worth the effort. Also, I'm always concerned about when one buys a new (used) car especially one that has not been loved rodent damage to the harness is always a possibility. If you find the MAF is not to blame, if you find the coils are securely connected to both their plug and the wiring harness, you must carefully inspect the wiring harness. Last but not least, what is the age of the gasoline in the tank? There have been a few cases of engine misbehavior that has been due to (very) stale gasoline. If the car was not loved chances are the previous owner may not have used the best gasoline, may not have used a premium grade. So keep that in mind.

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Interesting and wise suggestion. Ethanol blended gas also doesn't have the same shelf life as 100% gas, so if the car has sat for an extended period of time separation could also be an issue. Has the fuel filter been replaced? If the car has been seriously neglected and has an original filter at 186k, that could definitely cause rough running.

186k miles!?

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Just a clarification : all gasoline is a blend of several different hydrocarbon distillates and many include ethanol. The exact blend depends on the grade and the area of the country it is sold, but there is no such thing as 100% gas.

When you clean the MAF make sure you use Mass Air Flow Sensor Cleaner.

I think onewhippedpuppy was referring to 100% gasoline as gasoline sans any ethanol component. Might mention that even premium octane gasoline (with or without ethanol) has a shorter shelve life than regular gasoline.

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Correct. Around here they are marketed as either an ethanol blend or "100% gas", meaning no ethanol content.

Not to belabor the point but I'm pretty sure all gasoline is required by federal law to contain some "renewable fuel", i.e. ethanol. It replaces MTBE which has been banned . Google says not all states require it to be labeled as such but I can't vouch for that as I haven't researched it. Ethanol is a scam...

if you think about it, if you clear the codes and car runs normally, but then the cel is thrown and runs rough, its something the ECU is doing, which makes me think sensor. If you disconnect the maf, does it run differently? I remember that being diagnostic on BMWs Out of my league here, but just a thought

Thank you all for the idea's! I have not messed with the MAF yet but on the top of the list this weekend, also wiring harness inspection with ground checks. Yes 186k miles. Surprised it even starts. Purchased as a project and that is exactly what I got. Have new gas in the tank and fuel filter changed. I also think it's a sensor problem at least that's what I hope. Please keep the ideas coming. I will take some screen captures of the dura metric readings. Thanks in advance.

If its a sensor problem, it could be the grounds not the actual sensors.

I'd replace your primary o2 sensors. If bad they will cause you're car to misfire. Or you may have two faulty inj. Swap around some injectors to see if the misfire changes. Also check your timing reading on that bank with you car running with a durametric to see if your cam timing solenoid is stuck

You need to hook up your Durametric and get some baseline numbers to start working with. Not only will the Durametic give you the error codes, but with the realtime feedback you can observe what your timing is on Bank 1 and Bank 2, O2 sensors, MAF, temperatures and so on. Also, you can activate your Bank 1 and Bank 2 timing solenoids to check if they are working. I think you are on the right track with the plugs and coils, but, if you have replaced those with new parts then you are going to have to dig deeper. Run some Techron in your gas to possibly eliminate clogged fuel injectors and carbon buildup on the valves.

I am getting a 1.0 reading when i check the O2 sensors on Durametric. Does not seem like they are reading anything. Possibly a ground short. Didnt think to much of it until a few of you mentioned it. I am not sure the procedure or baseline info for the bank1&2 timing solenoids. Can anyone give me a cliff note on that. I am in the process of running techron through the system now. What should my MAF readings be? Thanks you again!!

Hi, Went home for lunch and took a few shots of the car running at idle. The last is with the MAF disconnected, which actually made the car even out but still has a miss. I am not sure of the reading and what the guide lines should be exactly. The O2 sensors seem to be a bit wonky. start reading then back to 1.0.. Thanks!

Sorry forgot the O2 readings

Was the idling rev normal (i.e., ~680 rpm) when the above was taken? The normal MAF reading at idle fully warmed up and no accessories should be 15-17kg/hr. 35kg/hr sounds really high. Any mods on the intake? Also, the cam deviations and TRAs are all 0.0...look quite suspicious. Have you just recently reset the DME? The normal idle precat voltage should swing between 0.2v to 0.7v about once a second.

DME probably reset as I can't keep the battery hooked up. Interior lights will not shut off. Another project. I changed the oil this afternoon, which I should have done from the start I know. Anyways the oil smelled horrible??? No signs of coolant but pretty sure some fuel had made its way into the sump. Another head scratcher. Going to order a couple o2 sensors as they seem not to work at all and keep at it. Thanks all!

If there's fuel it's prob from a bad AOS. If it were me I would just replace the o2 sensors, maf, and AOS as they're known to fail. Specially with those miles. When I bought my damaged 996 I literally replaced just about everything that is known to need replacing at some point.

If you search AOS here it will tell you how to know if it's failing. If my memory serves me right when you remove the oil cap place your hand over the hole. If it's sucking on your hand it's bad. But it could be the other way around lol that's why I suggest the search button.

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I believe it is state specific. Though the bastards are doing their best to not only increase the content but make blends mandatory, despite the negative impact on many older vehicles and small motors. Unfortunately the farm lobby here in KS removed the requirement that ethanol blends be identified as such, so unless otherwise labelled our gas is assumed to have some ethanol content. Some stations locally still sell non-ethanol blended fuels, particularly in 91 octane. They get lots of my business, especially for my boat and yard equipment.