BesideTheBox

Issue resolved - see post #26

Rennlisters,

I got a check engine light (CEL) a week ago. '97 C4S with 77K miles on it. I drive it year round, typically 4 times per week. Last week the car was idling poorly and lagged until the RPMs got above 2K - approximately. Next day, I had the code read at AutoZone. The sales associate asked me if my car was a Boxster or a 911 . One code came back - P0306 - misfire on cyclinder 6.

I did a lot of searching on Rennlist for this code. So I have a number of items to check.

1. Ignition coils - assume original
2. Distributor caps & rotors - replaced at 73.5K miles
3. Plug wires - assume original, 2 wires (II-6 and II-2) measured to be 2.78K and 2.99K ohms, misting around wires & caps did not produce any arcing
4. Plugs - replaced at 60K -and- 68K miles
5. Fuel injectors - assume original
6. Fan belt and alternator belt - with engine warm, these belts measured 15 kg and 10 kg, respectively. Not where they should be.

What else should I be checking?

I should mention that I did the test where one or the other distributor caps is disconnected to run the engine on a single set of plugs. This made no discernable difference in how the engine ran.

Engine does run better after it is warmed up. After a 50 mile spirited drive, the engine felt stronger on acceleration and the idle was smoother, but still a bit off with occasional light popping from the exhaust.

Car has 77K miles on it now. The CEL and the associated running issues cropped up the day after I replaced my motor mounts during which the car sat for 4 days - longer than usual. The timing of the misfire is suspicous. Perhaps a coincidence. Is it possible that I bumped something that would cause this? Perhaps jacked the engine up to far when finagling the right motor mount out around the AC compressor or whacked an O2 sensor?

Thank you for your suggestions and questions.

BesideTheBox

MiataR

That's a good list to start with. The one case with similar symptoms I am directly aware of was resolved by replacing the fuel injectors.

pp000830

The code is a misfire in cylinder 6 – lets see it misfires xx number of times in XXXXXXXXXX number of ignitions. I would just clear the code and see if it comes back. It may not. Then if it returns in short order just replace the one plug and then clear the code, if it come back then replace the one wire to that plug and be done with it.

LimeyBoy

^^ Agree. I had an obvious misfire, turned engine off, started again with no misfire - read misfire code, reset it and several thousand miles later not had another.

BesideTheBox

Pulled three lower plugs on the passenger side. II-6 was easier to unscrew than I would have expected.

Then I found that the plugs are 14FR - 6LDU with 2 electrodes. Are these acceptable for the twin plug 3.6? The P-car DIY says the "14F 6 LDU" is the correct plug for a turbo. Not sure if the 'R' in the plug type makes a difference.

BTW, the P0306 did go away for one day, but it has come back and stayed.

Plugs have about 9K miles on them. The ceramic on II-6 is discolored. How do these plugs look to you guys?

IainM

The fact $6 is discolored is interesting. I wonder if the insulation was cracked and the spark was up in the shaft, that would affect combustion. I'd replace #6 if not all, with Porsche parts.

JB 911

Did you put the same plugs back in? Or different ones? Sorry if I missed the answer.

I'd tend to replace ONLY # 6 out of curiosity if THAT was the problem.

I do remember reading somewhere that the fan belts are very sensitive and that some aftermarket are slightly different than oem and in some situation cause a misfire code. Are yours oem or aftermarket?

BesideTheBox

I don't have a new set of plugs yet, so I am putting these three back in for now. I might spend some time pulling I-6 (the difficult one), but I'm not sure if I have the best tools for that. Going to give it a shot.

When I get new plugs, I will first replace II-6 and I-6 to see if it is the source of the P0306 CEL code.

Funny thing about the belts and misfire posts. If I was reading them right, I thought it just threw the code, but did not have the associated poor running. My car is definitely running poorly right now. My alternator belt is not as tight as it should be, but I need to get myself a 24mm open end wrench or crowfoot before I work on that. My AC belt is definitely Porsche, I cannot tell if the alt and fan belts are Porsche or not.

I looked for, but did not see any cracking of the ceramic.

I will eventually replace them all, but I will first replace both plugs at 6 to see if the engine runs better and the code goes away.

nine9six

FWIW, when you looked at the ceramic on the plugs, did you use any magnification? I seen cracked ceramics that I could not detect with the naked eye. Now I look at them through a jewelers loop.

Steve Weiner-Rennsport Systems

77K on the original plug wires???

OEM or factory belts?

Magdaddy

maybe way off here...but you mentioned messing with the engine mounts...verify that the distributor vent hose is still attached at BOTH ends. When I was checking tightness of the newly installed Wevo's I accidently pulled the vent hose off at the airbox end. I didn't realize it till after driving thru a rainstorm, then parking the car. Started rough, ran like crap around idle, would sometime rev just fine.

Moral of the story, little moisture in the dizzy cap...don't know where #6 sits considering the cap orientation, but could have pooled the moisture there.

Just a simple look is all it'll take to rule this out...

For the belts, don't forget you'll need a triple square bit also, to hold the end shaft.

Also, if you go after all the plugs, it'll be handy to have a 3", and 1", 3/8" drive extension. All the DIY's mention a 3/8 nuckel, but I had the above mentioned extensions, and a swiveling head ratchet...worked fine. A set of long metric hex socket will come in handy for the heat shield bolts also. I put the extensions together in succession, to get in the tight places...worked fine. My problems were getting the heatshield out on the pass side, and the tranny vent tube was a bitch to put back together also.

Best of luck

BesideTheBox

Yep, I knocked off that little vent tube when I was doing the mounts as well, but I got it replaced.

I do have the triple square.

Thank you for the recs on the extension sizes. Just pulling and replacing I-6 boot was a nightmare.

I bet the plug wires are original. I measured them. I imagine that this doesn't rule them out entirely, but none of them appeared completely broke.

Upper coil wire: 1.069
Lower coil wire: 1.063

...Upper......Lower
1. 2.928......2.972
2. 3.015......2.99
3. 3.053......2.948
4. 3.155......3.169
5. 2.901......2.778
6. 2.906......2.78

The fan belt is worn by the sensor, so I have no idea if Porsche OEM or not.
The alternator belt says Roulunds on it.
New belts are on order today.

BesideTheBox

I decided yesterday to start the car and just let it warm up for 5 minutes. The ambient temperature here in MN in my garage was about 37F (3C).

I recorded the engine audio with my phone and loaded it into a program called 'Audacity', that graphs the audio signal over time.

The first image is a high level view of the first 60 seconds.
Engine start at 3 seconds. Driver door closing at 21 seconds.
You can see the engine starts 'popping' at 34 seconds into the clip and continues until 55 seconds.
The pops are reflected as the blue spikes away from the central lines of the right (higher) and left (lower) channels.
After that the idle settles down.

Normally, I do not let my car warm up by idling. I start the engine, wait a few seconds and then drive off.

4 hours later, I took the car out for a drive. I did not allow the engine to warm up and started as I normally do.
As I was going down the road from my house, the bucking started and the CEL came on.
The rough running stopped after 30 seconds maybe.



Now for the real technical. The 'pops' come on 100ms boundaries.
Can someone confirm that the computer is making adjustments at these intervals?



So I think the root cause of my P0306 is due to a temperature sensor.
How many of these are there?
Where are they and how can I test them and verify that they are connected, etcetera?

Thank you for all of your help.
I am moving ahead with ordering and replacing maintenance items, but I think this may be the root cause of the P0306.
I will know for sure after I can locate and test the temperature sensor(s).

geolab

The temperature sensor is working as should. (but this does not include erratic behavior from some parts)
If it pops with warm engine, regardless of size of pop, temp sensor has no role on warm engine. So Temp sensor not the cause.
There is only one temp sensor, on cylinder three, as you know

Your spark plug no. 6 is shot from the looks of it.
As the error designates no. 6 , and the picture of the plug, I would go from there.
I mean from plug to distributor caps - rotors
The plug might be the culprit but maybe the culprit rendered the plug like this.

What bothers me is the sound chart.
Reminds me of the old ferrari carburator tuning with a hose stuck on carburator inlet, and to the tuner's ear. They used to tune all the carbs to the same pitch sound

In the sound chart, every one full second, there is about 11-13 iterations, regardless of the strength.
The crankshaft does around 13 full turns in one second.
So there is a pop for every crank revolution ?
One of the few things that is measured every rotation is the flywheel sensor (impulse sender?)

anyway, lets see what the plug change would do.
I do not know if you have this doc.

bruce7

When I changed spark plugs in my car someone had also installed these same
plugs. If staying with Beru, the correct plug is the 14FR-6 DTU with 3 prongs. I think this
must be a common mistake from other posts I've seen.

-bruce