I am the original owner of an 04 Coupe 6MT currently with 91k miles on it and I noticed that I started losing power during heavy acceleration, it feels like the car hesitates before power is applied. There are occasions that the car runs fine (usually when the motor is cold) so I'm wondering if the car is running lean when it's warm.

The first thing I did was change the aftermarket air filter (I have a stillen intake). Then I changed my spark plugs (this fixed a low idle issue and and occasional multiple cylinder misfire code). I've used various fuel system cleaners (Latest: Chevron 65740 Techron Concentrate Plus Fuel System Cleaner). After that I cleaned the MAF (CRC 05110 Mass Air Flow Sensor Cleaner). Next I'm going to clean the throttle body with the same MAF cleaner and use the 3M 08963 Fuel System Tune-Up Kit. I've reset the ECU various times in between but I'm running out of ideas. Is there anything else I should try before I just take the car in to a shop? And if I do take it in, what should I ask for? Tune-up? Engine diagnostic? Dyno?

Some other notes:
1) My valve covers are leaking oil around the spark plugs. If I replace them, am I going to be in the same boat 30k miles down the road or are the new valve covers better designed? What If I don't replace them? I'm also considering doing the replaceable grommet modification to my existing covers.

2) I installed a z-tube years ago which has the valve cover hose connection on the bottom of the tube (not easily visible). One day I notice that this connection was broken under the intake which means I was essentially running the car with a quarter sized hole in the intake tube and valve cover. I'm not sure how long I was running like this but as soon as I noticed it I replaced the stock intake tube. How worried should I be? This probably contributed to why I'm burning about a quart of oil ever 3k miles

The car is capable of running like it used to and there is no check engine light, just loss of power.

Thanks in advance for any help and suggestions.

 

Here's a pic of my sick baby...

 

typically you will feel a little sluggishness when the car engine gets warm, however a noticeable reduction in the car's power could also be an indicator of something else hampering it's performance...

you listed several issues that could all factor into poorer performance, so I would address all maintenance issues first then look into what is impacting performance.

what I would point out is if you have intake and the hose for the connection is broke off on the intake, it may not be effectively balancing intake air flow. the hole in the intake at the worst is going to allow dirty air to get drawn in without being filtered... unless you are dirt track racing it is a minor issue. I would however suggest replacing the tube and re-establishing proper connections etc. It may also account for drawing warmer air from the engine bay which is mixing with the cold air from the filtered end of the intake & thus lowering air density enough to reduce the performance.

this would likely be more noticeable under heavier acceleration with the motor warm...

while you are dealing with the intake I would suggest typical tune up items & see if that helps...



As far as valve covers go. you don't need new covers, just a gasket set. a new gasket should last you a good long while...

 

Yes, I have already replaced the broken intake tube and reconnected everything properly but there is still a noticeable loss of power.

As far as the valve spark plug gaskets, they are not replaceable. Standard procedure is to replace the whole valve covers. There is a kit available to modify existing valve covers to have replaceable grommets which involves drilling out the existing holes.

What typical tune-up items have I overlooked?

Thanks.

 

how hot is it outside? i know my car is slow as ***** on days over 80 degrees.. haha

 

check for codes..may be knock sensor but it's known to not display an engine light

 

my oem z tube also broke at the vacuum elbow... glued it and broke again... this is why i switched to the Z1 aluminum tube....i thought i read somewhere that we could use vq37 or 35HR valve covers...something about a better design... can anyone enlighten me on this? just in case mine ever start to leak

 

didn't realize the spark plug gaskets weren't replaceable on this car... that kinda sucks ***...

I apologize for the misinformation in that case... was basing my response of my general knowledge of mechanics

 

I haven't had the G35 do this but have had 2 J30s do it, run with the snails and give no code. It's not a fun job to replace.

Good luck!

 

nvm.. i think i was thinking of head gaskets.. my bad

 

Good point. My car used to knock under heavy acceleration. Now, no more knocking but loss in performance. I will definitely check the codes, thanks for the suggestion.