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Hey Guys,

I have a 1999 996 3.4lt C2 Tip Cabriolet. It has been in the Porsche workshop for 3 weeks now entering the 4th week. My mechanic has tried many things without luck, so I come to you for help.

The issue is, when driving along and keeping at a specific speed, the revs will constantly surge up then down. It does not do this under accellaration but only when you sit at a speed and keep the throttle constant to stay at a speed, for example 60 km/h or 100 km/h.

We thought the 80,000 km major service would fix this but it didn't. New spark plugs did not change anything. Spark plug coils were check and no issue found.

It already had new MAF Sensor in Jan but we replaced this again to be sure and the issue remains.

Engine was checked for leaks including vacuum lines etc using smoke machine and no issues found.

Transmission was serviced in Jan, oil changed. He has also checked the oil again and cannot see any issues.

It may be the transmission but he doesn't think is so due to the above and also that this surge can occur in any gear.

We are out of ideas, and I am losing patience, if anyone has any suggestions that would be great.

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Hey Guys,

I have a 1999 996 3.4lt C2 Tip Cabriolet. It has been in the Porsche workshop for 3 weeks now entering the 4th week. My mechanic has tried many things without luck, so I come to you for help.

The issue is, when driving along and keeping at a specific speed, the revs will constantly surge up then down. It does not do this under accellaration but only when you sit at a speed and keep the throttle constant to stay at a speed, for example 60 km/h or 100 km/h.

We thought the 80,000 km major service would fix this but it didn't. New spark plugs did not change anything. Spark plug coils were check and no issue found.

It already had new MAF Sensor in Jan but we replaced this again to be sure and the issue remains.

Engine was checked for leaks including vacuum lines etc using smoke machine and no issues found.

Transmission was serviced in Jan, oil changed. He has also checked the oil again and cannot see any issues.

It may be the transmission but he doesn't think is so due to the above and also that this surge can occur in any gear.

We are out of ideas, and I am losing patience, if anyone has any suggestions that would be great.

My gut feel is look at the Oxygen Sensors.

The problem you are facing seems to be at part throttle, where closed loop operation of the ECU is usually happening, and the accurate information from the O2 sensors are required.

I'm new to Porsche's but I'm just basing my guess from common sense and some experience with other european cars that use Bosch engine management.

Let us know how you get along with the car.

Good luck!

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What size tires do you have on the rear? I had a similar situation with some surging and with the idle hanging at a high rev when I let off the gas that finally got tracked back through the traction control system to the fact that I had 265/40 tires on the rear instead of the correct 265/35.

If this is the case, turn off the traction control and all the symptoms should immediately disappear.

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Thanks for the replies guys.

The car is at porsche specialist and they have checked oxygen sensors and they are all good.

The tyres should not be the issue as they have been on the car since the start. they are 285/? turbo style 18 inch wheels

Car has fairly new idle valve.

More ideas? It may be transmission related, seems engine is ticking all the boxes.

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Thanks for the replies guys.

The car is at porsche specialist and they have checked oxygen sensors and they are all good.

The tyres should not be the issue as they have been on the car since the start. they are 285/? turbo style 18 inch wheels

Car has fairly new idle valve.

More ideas? It may be transmission related, seems engine is ticking all the boxes.

Try unplugging the MAF and driving.

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Thanks for the replies guys.

The car is at porsche specialist and they have checked oxygen sensors and they are all good.

The tyres should not be the issue as they have been on the car since the start. they are 285/? turbo style 18 inch wheels

Car has fairly new idle valve.

More ideas? It may be transmission related, seems engine is ticking all the boxes.

Try unplugging the MAF and driving.

Why do you think we should do this? I had a MAF sensor put in in Feb, when the car starting playing up it was the first thing I thought about. So i took it back to them and said I think the MAF is faulty. The installed a brand new one and it made no difference. They did not charge me for this.

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You need to get the fault codes read - any existing AND pending faults.

Then report them back here.

The diagnotic tools report no fault codes. Thats why mechanic is resorting to tuning issues but cannot find any.

I am taking to Porsche Melbourne (aurthorsied Porsche dealers) Tuesday, I just want the stupid thing fixed.

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  • Admin
You need to get the fault codes read - any existing AND pending faults.

Then report them back here.

The diagnotic tools report no fault codes. Thats why mechanic is resorting to tuning issues but cannot find any.

I am taking to Porsche Melbourne (aurthorsied Porsche dealers) Tuesday, I just want the stupid thing fixed.

Do you mind if I ask what tool he used to check the car?

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You need to get the fault codes read - any existing AND pending faults.

Then report them back here.

The diagnotic tools report no fault codes. Thats why mechanic is resorting to tuning issues but cannot find any.

I am taking to Porsche Melbourne (aurthorsied Porsche dealers) Tuesday, I just want the stupid thing fixed.

Do you mind if I ask what tool he used to check the car?

I am not exactly sure. He just said his diagnostic equipment did not report any codes, in fact he said it was the first thing he did when he got the car.

On one occasion, he road tested the car while in was plugged into a computer of sorts of some tool that reads the cars information while driving. He said there seemed to be a misfire and checked the spark plugs and coils but that did not fix the issue.

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A failing torque converter might cause surging. That's all I can think of in the transmission that would cause anything other than slipping or grinding noises.

Despite your confidence in tire size, try driving the car with the Traction Control turned off. If the problem remains this will eliminate the ABS and traction control system, which can affect the idle air control valve position.

I spent almost a year with my car surging at constant speed and hanging at high rpms during idle (I have a 6-speed) with no codes to go by before I finally tracked it down to the traction control system.

An intermittent misfire or momentary lack of electric power, perhaps from a bad ignition switch, might cause surging, but I think you'd feel it more like a lag instead of a surge.

Good luck.

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A failing torque converter might cause surging. That's all I can think of in the transmission that would cause anything other than slipping or grinding noises.

Despite your confidence in tire size, try driving the car with the Traction Control turned off. If the problem remains this will eliminate the ABS and traction control system, which can affect the idle air control valve position.

I spent almost a year with my car surging at constant speed and hanging at high rpms during idle (I have a 6-speed) with no codes to go by before I finally tracked it down to the traction control system.

An intermittent misfire or momentary lack of electric power, perhaps from a bad ignition switch, might cause surging, but I think you'd feel it more like a lag instead of a surge.

Good luck.

Thanks for your help, it was a failing torque converter that caused the problems. This issue does not show any fault codes so very tricky to diagnose apparently.

Get the car back tomorrow....5 weeks later

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