Hey everyone, let me start off by saying I hope this is in the right section and im sorry for the long read. I have been having this problem for a while now and I have tried different things to try to remedy it and have not found a cure. Im hoping that some of you guys can point me in the right direction.

My 03 350z with 75k miles is performing very sluggish. When the car first cranks and even shortly after it reaches operating temp the car runs like a champ. However after a while if I go WOT the car seems to fall on its face. It accelerates smoothly, but it is a very noticeable and dramatic decrease in power. I cant tell you guys how discouraging it is.

Now for troubleshooting. The car is not throwing any codes and idles smoothly as well. Every now and then it will give a slight hiccup at idle, and I do mean slight. Unless you're paying attention you would not notice it. I have done research and decided to change the plugs. The plugs were the original I believe (75k miles) and were bad (large gaps). I replaced them and thought the problem was fixed. After that it continued. I pulled the plugs and observed that the middle drivers side plug was black and smelled of gas. I replaced the new plugs with the NGK plugs that came stock and hoped that would solve it. It did not. I checked the drivers side middle plug again and it was doing the same thing. The rest of the plugs look good. I borrowed a friends coil pack thinking that it was not sparking, and the problem continued. I replaced that injector today and the problem has continued. I have no idea what the problem could be and would like some insight from you guys. There was no oil in the spark plug tubes when I pulled the coils btw. The only engine mods that I have done is a cat back exhaust.

Cliffs:
1)Car runs like a champ after it is first started, runs very slow and sluggish shortly after warm up
2)No codes
3)Drivers side middle spark plug is black
4)I have new plugs that are gapped correctly
5)Swapped out coil packs on the bad plug, not the problem
6)New injector on the bad plug, not the problem.
7)Only engine mod is AAM cat back. No tune, intake, etc.

 

What was the gap of the plugs you pulled? You say large gap but without a number I hope you didnt just eye it up.

Id do simple things like plugs, fluids, filter changes. Then I'd dyno it to see if its making power. Or do a compression/leakdown test.

It could be that with just your cat-back the tune is off and isnt runing very well. The dyno would be more helpful here. You are using premium gas right? The car isnt low on coolant and the heat blows hot right?

edit-stock cats? Maybe they are getting clogged (how is your gas mileage btw-normal?). Take some electronic parts cleaner and clean the MAF carefully, I used to do that from time to time.

 

Subscribed, I have the very same problem.

 

The gaps were anywhere between .55 and .68 if I remember correctly.

Like I stated earlier the plugs are brand new. Maybe 20 miles on them. All the fluids are changed regularly. Oil, tranny, diff.... the only fluid that I have not changed is the coolant. The air filter looks pretty new. I could always buy a new one just to cross that out as a possibility. As far as the fuel filter, I was told by a Nissan tech that the fuel filter should never clog. Im not sure how accurate that is. I have not performed a compression or leakdown test yet.

I dont see just a cat back reducing the power that much. Its really noticeable. I am using premium. My coolant is at the proper level and the heater does work.

I am on stock cats. I was thinking they could be clogged but would it not rob power all the time? Mine is only doing it after warm up. Im not shooting it down or anything, just trying to offer more information and get a better understanding myself. Ill try cleaning the MAF tomorrow. Im getting pretty good gas milage. Average around 22-24 with city driving.

 

Sounds normal to me. Z's are slow.

 

If you have a cylinder that has new plugs with gap properly set that is turning black and smells of fuel, I would do a compression test and especially pay attention to that cylinder for starters.

 

try the throttle body, it can cause the little hickup at idle and to fall on its face at wide open throttle. but under normal driving conditions it could be fine.
the throttle bodies tend to go bad around 75k miles. mine went at the end of fall (US. model) and it took a very long time to get codes, i didnt get any until it got really bad and started causing a misfire.

cats are more of a constant power loss and usually wont got over 40

 

transmission and differential fluid?

 

Hmmmmm NO.

 

Those are huge! haha, I think you meant .055-.068 but that is still big.

Stock should be .044

But yes, a compression test/leakdown test should be performed.

 

Alright guys, just to add some closure to this thread. Im pretty sure I fixed the problem. I checked for codes (i didnt have a CEL) and I got the knock sensor code. I replaced the knock sensor harness that was visibly not intact, and the problem is fixed. Man sensors can make your car act WIERD. Runs like a rabid ape now.

 

Good to hear, thanks for updating the thread!

 

So basically it was seeing knock all the time and pulled timing.

 

Ive owned about 10 350z's in my time, and i can tell you that they all run differently.
Some have a ton of power, and some are just plain sluggish..

Ive tried everything that you tried.
Sparkplugs, seafoam, injectors.. you name it..
Nothing helped on the slower Z's that Ive owned.

I personally believe that some of the Z's Ive owned had a bad 'break in' period and were probably ragged on from the start..

for Instance, I have two 350z's right now.. One has 100k, and the other only has 50k.
The one with 50k is super sluggish! and i mean it is slow! probably does 0-60 in like 7 or 8 seconds. I have no idea why its so sluggish. I changed everything on it! nothing helped. I even tried putting supreme gas fill ups in case the previous owner was using regular. That didnt help either..

Ok now the 350'z that has 100k miles... that one is a BEAST! It pulls extremely hard.
i bought it from a single owner who babied it. It is very responsive when i got WOT.

So.. in other words, its either hit or miss with the 350z.. you either get a sluggish one thats been ragged on, or a good one with lots of power left..

Im a dealer by the way.. Thats why ive owned so many of them..

Hope this helps

 

Where did you get the knock sensor harness? Im having similar problems and I am pretty sure that is the problem because I have the code P0327

 

sorry to bing back an oldie,....

I'm having the same problems still with mine, threw Camshaft sensor code, had the those along with knock sensors replaced, but now I'm back to the same problem.

I know that the sensors were replaced but noe the harness's, is this most like still the same sensor issue???

 

Idk man. Are you getting any codes? Check and see even if youre not getting a CEL because it wont light up for everything apparently. It could be something completely different than what I had happen. I would start with the spark plugs because they can tell you a whole lot.