Fluctuating idle, hesitation..help please!

[GCB]

2007 Carerra S

Firsly, my apologies for the long account of events, but my car has been in/out of dealer about 6x in 2 months. They can't figure this out.. so i'm hoping someone out there can shed some light on this.. thank you!

 

I have had an ongoing and increasing problem(s) for about last 6 months. Not sure if these issues are related or coincidental, but here goes....

Started out with the car dieing out upon its first startup every morning. It would always start up perfectly the 2nd attempt and then a little rough but then even out and be great after that. Dealership said to run a can a seafoam through the gas every other fill up. This would slight help reduce the problem to intermittant but not complete improvement.

Then I noticed the idle becoming a little bit rough at times and gas mileage reducing from about 16 mpg to about 11mpg without any change in driving habits. I also would occasional notice it difficult to smoothly start moving off from a stop because of a slight engine hesitation and found myself giving it more gas that usual to start off the 'line'. (Never felt like a clutch slipping issue). Also cruising between about 3000-4000 rpm I could feel a slight infrequent 'hesitation' in the acceleration.

Then I started to develop (what dealer calls) a 'hunting' idle when stopped with engine running. I also noticed a drop oil pressue to near minimal to what looked like zero at times on the gauge. Then one day the a/c starting blowing warm air along with significant drop in Oil pressure. Dealer replaced the Oil pump.

The 'hunting' idle continued whereas I would be sitting still with engine running and see the rpm fluctuate from about 750rpm and rise up to 1200 and drop to as low as 300. The odd part is this would happen at EXACTLY 20 second intervals. If i gave it some gas.. it would stop for a few moments then start again. (NOTE.. I NEVER HAD ANY CODES or ENGINE LIGHTS).

Dealership replaced the Mass Air Flow sensor ( MAF) .. of which did not help and 2 days later not only did it still hunt.. but actually died out sometimes while idling.  I took it back in and dealership said it was the Crankshaft sensor causing the hunting and carbon buildup causing the initial startup problem. They did a engine decarbination and replaced the Crankshaft Sensor. 

I received the car back again and STILL had the hunting problem along with the 'hesitation' feel with low speed cruising.

Seemed to be worse when engine was hot. Then one day during the crazy hunting / hesitation issue i pulled into my garage and opened the drivers door and smelled an intense FUEL smell which was coming from the right front fender area.

Took it back to the dealer who swears that there is nothing wrong with the fuel system or vapor canister, etc. (note.. the fuel smell only happened 2 or 3 times and dealer could not reproduce it). HOWEVER, this time they noticed when they gently rested their foot on the gas pedal the RPM when into a crazy constant hunting mode from 750rpm  (up to about 2000 down to about 400). Again a ECU code still has NEVER came up.

Dealer disassembled the Throttle body and cleaned it up, then reistalled. Still had the hunting problem and hard 1st start issue. So they ordered and installed a NEW throttle body. Still no change int the problem. So he put the new Throttle body back on and (oddly enough) discovered it was nearly impossible to remove the oil cap while the engine was running. So he determined it was the Oil Seperator that was bad. NOTE: It never had smoke emmiting from tailpipe or any CODE thrown.

So.. they replaced the Oil Seperator which appeared to stop the hunting problem, rough idle and hard start problem. HOWEVER, THEY NEVER FOUND THE REASON FOR THE FUEL SMELL?

I picked up the car 2 days ago.. This is the current status of things...

The hard initial start is much better.. so far has started up on first attempts... and Resting idle is pinned at about 750rpm and doesn't move.

However, I still feel a slight intermittant hesitation when slowly accelerating or trying to maintain about 3-4k rpm.

AND... at idle.. if i rest my foot gently on the gas pedal.. (for example.. trying hold about 1,000rpm, I still get hunting significant hunting oscillating the rpm from 750 up to about 1200 and down to about 500)

The service manage said this is normal because the car thinks i'm moving and i'm not?? huh??  If i try doing this when engine is cold or warm (just after intial warmup) it doesn't do it... but will do it badly when hot. NOTE this is the Arizona heat!    Service manager feels this is normal and stated it was hunting much higher before with light pedal touch, but says this is normal.

Can you PLEASE let me know your thoughts on this?  I do not think a light touch of the gas pedal should cause a crazy oscillation of the RPM's. Also the car still feels like its hesitating during slow, smooth acceleration.

Like its trying to take a breath while its increasing speed? 

Does it sound like a fuel issue? They stated fuel pressure was good, but they still do not know the cause of the fuel vapor leak/smell from before?  The gas mileage is still at around 12 mpg (with standard driving)

The one thing the service manager also cant understand is why the ECU still hasn't thrown any codes during any of this? 

Could the ECU be bad?

Can a worn clutch cause any of these symptoms? (its never during shifting)

Why the gas smell/bad gas mileage, etc?

Please... any suggestion would be greatly appreciated!! THANK YOU!!!

[jaybart]

Hi GCB,

 

what a bummer. Looks like you and your dealer looked at just about all intake issues (the AOS is related to intake as the vacuum in the oil filler came from there). So seems you have oxygen coming in. Maybe it's time to look at the other 2 items needed for an explosion - fuel and spark.

 

I recently had an issue on my 997S where i had hesitation and engine stalling that turned out to be a coil that had come loose (both bolts that secure it to the head had literally fallen off the engine and the coil was bouncing around). While something like that is unlikely - you would usually see misfire codes as I did - but worth checking as it is easily done visually/manually (i.e. are the bolts on tight, are there cracks in the coils?).

 

The more likely contender maybe being fuel. Now you said your shop says you have good pressure on the fuel pump. Not sure how hard that is to test as I haven't had to content with that (knock on wood). Maybe someone else on here will chime in on how to hunt fuel supply issues. Your mileage having changed like that - there's definitely something off.

 

 

Good luck and keep us posted,

 

Jay

[Ahsai]

Did the dealer monitor for misfires in real-time (misfire counters for each cylinder) using their scanner when this hesitation is happening?

How old are the coils and plugs and are the plugs stock?

Did they check for leaky injector?

Did they use a smoke machine to check for intake air leak?

Did they monitor the gas pedal position values with a scanner and confirm they look ok? I highly doubt the pedal is the problem since the DME monitors that all the time and it should trigger a CEL.

Edited August 5, 2016 by Ahsai

[jl-c]

The hesitation between 3 and 4K could be the transition of the cam from one setting to the other, it is vastly more pronounced the more "sporty" your exhaust is! It seems it needs a lot of back pressure from the exhaust to smooth the transition out. Can't help with the hunting but monitoring live data would seem a sensible move.

[GCB]

thank you for your input Jay

[GCB]

All the ignition coils and plugs had been changed approx 20k miles ago. They don't think its a vacuum leak as its somewhat intermittant and it hasn't thrown any codes.. Although i'm pretty tired of hearing the 'code' excuse as this issue has NEVER thrown any codes... meanwhile they have changed all those supposed bad items. Thanks Ahsai for your

suggestions... i'll run that past them when i drop the car off again on Monday

[Brainz006]

What you're describing sure sounds like an intermittent vacuum leak. And when unmetered air gets admitted, the ECU will add fuel to compensate (and the engine may rev). And the rev may cause the leak to go away, and then when then leak goes away and your exhaust smells rich (like fuel) because the fuel trims may have adjusted to add fuel all the time to compensate for the leak. You need to smoke test the car and also log your LT and ST fuel trims while you run the engine under the circumstances that cause the problem. You can do the latter yourself with a cheap bluetooth OBD scanner and the Torque App (Google OBD LT ST fuel trim - - there are some great vids on YouTube). Other ideas include failing primary O2 sensors (seems like that would throw a code though), but I'd absolutely rule out vacuum leaks and see what the fuel trims tell you before throwing more money at parts.

[GCB]

great suggestions Brainz006