Drop in RPM under hard acceleration

[greg0078]

MY 2001 996 Carrera 2. 

I noticed a couple of days ago when i go hard on the gas pedal the tach will increase but then drop 400rpm and then go back up.  Its a split second delay when i/m hard on the gas.   Doesnt seem to do it any specific range, i've notice it in the 3000s in 3rd gear, and in the 4000s in 4th and 5th.  Today i even notice it in second gear.  

 

Any thoughts on what this is?    And if i drive it for a couple of months could i do any additional damage?

[Ahsai]

When was the last time the fuel filter was changed? Do you feel like the clutch is slipping (if it's a stick shift)?

Edited January 10, 2015 by Ahsai

[greg0078]

It is a stick shift but this occurs on the highway when I am not doing a gear change. I could be in 4th or 5th and step on the gas. The car will accelerate but then there will be a slight drop in rpms and it will then accelerate again.

[Ahsai]

Gotcha. Have you tried cleaning the MAF? Is the engine air filter in good condition? Is your car e-gas or cable throttle?

Edited January 11, 2015 by Ahsai

[greg0078]

I haven't tried the maf as yet, but the air cleaner is new. It just had 30k mile service.

[greg0078]

I just spoke to my independent dealer and there first though is that this is clutch related and is showing the first signs.  Tehy said what could be happening is when i stand on the gas the clutch is not engaging, so there is an increase in rpms and then it engages and pull through the rpm curve.  

If it requires a new clutch, i will likely also get the IMS done.  Anyone have any thoughts on this?  How long could i drive the car before the clutch really acts up.  

[dporto]

If the clutch is slipping, the RPM's would increase not decrease...take it to a different mechanic.

[Ahsai]

Agree with dporto. Clarification though, you said "The car will accelerate but then there will be a slight drop in rpms and it will then accelerate again." Does that mean when you give gas, engine will rev up and car will accelerate with the rev (if so, there's no clutch slipping. This is also the most likely place for clutch slipping in this sequence), then rev will drop for 400rpm and car will decelerate with that (almost like fuel cut but smoothly). Then rev will go back up normal and car will accelerate again?

[greg0078]

Let me try and clarify, and thank you all for the feedback.

 

When driving the car under hard acceleration (giving gas)  the rpms will increase and the car is accelerating  but for a split second during acceleration  the rpms will drop.  i  lose forward momentum, the rpms drop (400rpm) and then it accelerates again from that point forward nice and smoothly.  Not sure if i have explained it any clearer does that help

[Ahsai]

So car and engine slow down together for a split second. That sounds more like engine bogging down for a split second as opposed to clutch slipping. If clutch is slipping, engine rev should stay high when car is not accelerating.

Edited January 13, 2015 by Ahsai

[greg0078]

Ahsai - yes car and engine bog down at the same time for a split second

[JFP in PA]

Ahsai - yes car and engine bog down at the same time for a split second

 

That does not sound like clutch slippage.

[Ahsai]

IMHO, the usual suspects in this case will be spark plugs, ignition coils, and fuel pressure related.

[greg0078]

Ahsai, the maf sensor has been replaced, new ignition wires, plugs, not sure on fuel filter need to double check on that.

[JFP in PA]

Ahsai, the maf sensor has been replaced, new ignition wires, plugs, not sure on fuel filter need to double check on that.

 

It would be well worth your while to have the car tested for both correct fuel pressure and delivery as well.

[Ahsai]

 

Ahsai, the maf sensor has been replaced, new ignition wires, plugs, not sure on fuel filter need to double check on that.

 

It would be well worth your while to have the car tested for both correct fuel pressure and delivery as well.

 

^Good point.

[greg0078]

Thank you all for the comments.  The verdict from the independent mechanic is clutch slippage, so im going to get the IMS done at the same time.  I was hoping it would be something trivial but it was not to be.