thedoctor

Hi guys,

Wonder if anyone can help or has any insights on this issue im having on my 2004 GT3:

What could be causing a slight drop in power between 7000 - 8000 rpm when full-throttle, and the "Check Engine" light flashing up? When i took the car in to my local dealer they said the diagnostics were reporting a misfire in cylinder 4 but when they drove the car it seemed fine. They replaced the spark plugs and fuel filter and said the diagnostics showed it back to normal with no error coming up.

I was very happy to hear this and the car felt 100% when i drove it home. But then, a week later, it's doing the same thing again.

The "Check Engine" light flashes when im at full throttle and approaching 8000 rom. It then goes off after a while... but the loss in power is concerning me.

Anyone experienced this before or have any explanation?

Thanks in advance.

Chris

997s07

Check engine lights that don't show much in the codes are usually emissions related. That could be one possibility.

The DareDevil

My 996 exhibited the same symptoms. Without getting into detail, power loss up top, and random misfire (flashing check engine light) was caused by a loose catalytic converter. It was rolling around inside the canister and blocking exhaust flow.

Macster

The slight loss in power arises from the misfire.

Since you have the codes and know cylinder 4 is misfiring and a misfire is really a cylinder that is under performing.

The real question is why the misfire?

Replacing the plugs is good but I would have been tempted to replace the coils too.

While I'm not anxious to just replace coils because one is there your engine that operates at those high RPMs really puts a demand on the coils.

Any fall off at the coil can result in a misfire and at the higher RPMs. Another time the engine might be prone to misfiring is under part throttle high load cruising which would be experienced -- with the engine fully up to temperature -- driving at a steady speed in a high gear that has the engine operating at fairly low RPMs. I'm not suggesting you lug the engine but at this low RPM relatively high throttle open operating condition the cylinder filing is quite efficient -- low pumping losses -- and the DME will want to really advance the timing and under these conditions any weakness in the spark plug or its coil can result in misfires.

If you do not want to throw new coils on the engine, if you do not want to even put in one new coil, you can swap coils and see if the misfire follows the coil or remains at the cylinder.

However, depending upon the miles I'd be tempted to just replace all 6 coils. Say you replace the #4 coil. Unless the coils are fairly new to begin with -- and #4 suffered from an infant death syndrome -- after a time another cylinder will likely start to misfire and you are back to replace another coil. (In fact, I suspect more than one cylinder would misfire now but the #4 misfires first and you back off and the other cylinders do not get a chance to misfire.)

A longer shot but be sure the oil is an approved oil and fresh. If the oil has too many miles on it the water content can be rather high. This results in the oil's viscosity being reduced. Among other things this causes the oil to be more prone to foaming. With more foaming the engine's defoaming pots are just not up to the task and some aerated oil gets pumped throughout the engine. If this happens what can happen is the oil is not able to sustain the pressure in the zero lash valve adjuster -- due to some entrained air in the oil -- and the cylinder misfires because the valve action isn't optimum.

This is a real problem. In some engines it is the aerated oil causing the valve action to become sub-optimium that limits RPM not valve float.

Porsche engines are less prone to this but if the oil has some miles on it, if you make a lot of short trips, live/drive in a humid area, this makes the odds the oil *could* play a role higher. I mean 8K RPMs means the engine obviously is really spinning. A huge volume of oil is being pumped through the engine and a good percentage of this oil is routed to the cams and valve hardware. This oil upon exiting the camshaft bearing spaces and from around the lifter bores gets whipped into a vapor and then is scavenged by the oil pumps which tend to further froth the oil. This scavenged oil is then routed to the defoaming pots and well just a perfect storm of conditions has the defoaming operation less than satisfactory, at least at these high RPMs.

pontifex4

7-8K RPM is a high flow condition. As Derek notes above, check the cats first before throwing parts at it. His were obstructed enough that there was a difference in the exhaust_flow that you could feel at high RPM from the left exhaust to the right one, while revving the engine in neutral. Look there first.

enthusiast

As 'Mactster' was pointing out...the dealership tech didn't say anything about the coil packs? Strange to focus only on spark plugs.

Macster

Sure, check the converters. Generally there is a flow problem it is because the converter brick is loose and in this case the thing rattles, buzzes, even knocks (based on my experience with a loose brick in one of the converters on my Boxster the thing knocked, too, at least when cold).

Of course the brick doesn't have to be loose the converter can be obstructed due to a break down in the ceramic matrix that is the brick. This fails/crumbles and the debris becomes the obstructing material.

I would think if this is bad enough to cause misfires if the exhaust had separate outlets for the cylinder banks one could detect a difference in volume of the exhaust coming out albeit with someone revving the engine though hopefully not up to 8K. I would not be willing to try to determine a converter blockage by feeling any part of the exhaust system.

But the most common explanation for misfires is the coils. Still it costs to replace the coils and yet costs nothing really to at least confirm the converters are flowing about the same amount of exhaust at higher RPMs.

So, sure check the converters.

gvtesse

I had a similar problem. Mine ended up being a couple of coil packs needing to be replaced.

DHI

Check the cats.

993GT

replace all coils and plugs and go from there, when was fuel filter last changed?
cats also make sense if above doesn't help

thedoctor

Thanks for the valuable feedback. Really appreciate it. I have a custom de-cat straight pipe on my car, so no cats. Have had it on the car for quite some time and never had an issue. Could this be somehow contributing to the problem? The right side does sound louder than the left lately so perhaps there's an issue there?

996FLT6

Did dealer pull any codes? I would try replace all plugs and coils. So when u say cel is flashing is it really flashing or continuous cel lite. Maybe maf? Throttle position sensor. Mine had catbypasses never had issue with power loss til my maf grenaded. Mike

sullivan6453

I had the same problem last fall. Had to replace my cat converters.

Macster

These engines are designed to produce a balanced flow of air into the engine and a balanced flow of exhaust out of the engine. Any change at the intake side or exhaust side can upset this balance and result in sub-par performance or abnormal behavior.

That said I'm less inclined to blame the aftermarket exhaust for the behavior you report. That's too easy.

However, that the sides of the exhaust sound different does suggest that if there is not a partial blockage -- I take it there is nothing really in the exhaust: converters, mufflers, baffles even; to become blocked? -- there's a difference -- a leak? -- that bares investigation.

996FLT6

Hopefully it's not intake manifold hoses develop a small leak. Motor has to be dropped. Rubber hoses can get old/brittle and crack. I replaced all my hoses on the intake manifold when motor was dropped when I popped 2 of the hoses going in a spin in reverse and didn't disengage clutch fast enuf. From memory there's 8 of them. Mike