Ive asked this question on my350z, but hoping someone here might have an idea as well.

I'm experiencing some significant hesitation and just complete lack of power when on the throttle and even bucking around 2200 - 2500rpm when under load; ie. 3/4 to full throttle in 5th or 6th gear at low speed.

BTW my car is basically stock: plenum spacer and z tube as far as the engine goes.

I'm a little stumped as to what might be the problem.
What ive done with no improvement:
-cleaned the maf sensor
-changed air filter
-changed injectors (im at 69k miles btw)

What im considering trying next:
-taking the throttle body to nissan or infiniti to have them check it
-pull the fuel pump and check out the fuel filter/sock
-replace the oem fuel pump with either walbro gss342 or another stock one
-replace the passenger side fuel damper (i read somewhere that they can cause lean conditions as the diaphragm inside gets stuck and doesn't function properly)

Also whats odd, is that i can turn the car off when this is happening for like 10 min, turn it back on and it will seem to run better for a lil bit, but then will go to **** shortly after. Anyone have a guess as to what might be the problem??? I am at a bit of a loss as to what the actual problem might be

 

Name all the mods you have done on your car, including exterior mods that have anything to do with electrical.

First things that come to mind...

Spark plugs
Cam Position Sensor
Throttle Body

They may act up without kicking a CEL for a little while. You should check to see if there is a pending code though.

 

the only thing coming close to electrical would be a full wire tuck, however no wires for the main engine harness were cut, simply re-sleeved and re-routed and this was done maybe 4 months ago with the current issues only appearing recently.

I have noticed one thing. The hesitations and bucking only appears after the car has been running for 10-15 min or rather after its thoroughly warmed up.

 

check codes. i'm going to say you have a bad cam sensor either bank 1 or 2. Hit jp autozone and see what they say when they pull your codes. I had the same damn issue and my ecu never threw a code at me so i replaced the spark plugs and coil packs. Then 4 months later the cam sensors finally failed and threw codes. My idle was terrible and there was definite hesitation wot also it was very lagy. Replaced sensors, cleaned upper intake manifold, cleaned throttle body and she's running like a champ!

 

clean your throttle.

 

For future reference for anyone who encounters similar symptoms the problem turned out to be a leak between the plenum spacer somewhere. After tightening down some of the bolts the problem disappeared, I can't believe I didn't check this before. I was a hair away from pulling the fuel pump, cam sensor, and throttle body, and it turned out to be a lot simpler

Thanks for the tips everyone

 

didn't catch if you were throwing a cel....I would assume the o2 sensor before the cat is the issue.

 

I'm having a similar problem, but not sure its my plenum?

My issue is most noticeable in 1st-3rd once the car has ran for 15-20 mins. On my first startup of the day, even after letting car warm up to temp its perfectly fast. Once I have drove around for 15-20 mins, I can see a significant loss in power on take off. Nothing is slipping, the revs just don't get there as quick as they do in the first 15-20 mins. I feel the same lag hesitation mentioned above when in 5-6th gear as well under load.

I did the same things, replaced plugs, cleaned TB. I even replaced the Knock Sensor as I had a thrown code for that after I replaced the plugs and after talking to a dealer who informed me that those sensors on the 03-04 models got bad without throwing a code by retarding the timing. I figured that was what my issue was, but after replacing it I still have the same problem.

HELP!!! I have gone to the dealer, several other mechanics, I'm at a loss. I know this has to be an ECU issue where the ECU is retarding the timing once at a certain temp??? Only mods on the car is a JWT pop charger and Z-tube.

 

You could try checking your engine electrical ground eyelets. They are located on the front engine cover right by the black bracket that holds the coolant lines in place. It's ver close to the RHS cam cover. Sometimes they corrode over time especially when you live in the rust belt area. Those circuits ground the ignition to the engine electrical harness. Just clean them off with some CLR or metal polish.

Also check your connections to the coil packs. Make sure they are fully engaged. See if there is any damage to the engine harness due to chaffing. It would help most if you could get a DTC reading. If there is one SES light showing.

Hope this helps.

 

I don't have an SES light on, but I checked the codes to see if there was anything pending or stored in one of the banks, but there was nothing.

I have a couple theorys, when I had the plugs changed my mechanic said I had a small oil leak on the bottom plenum. Row2k said his problem ended up being that his plenum had a loose bolt. If that is true, then maybe my small leak is the equivilant to a loose bolt? I was going to install a Kinetix SSV manifold soon anyway, so maybe I will be killing two birds with one stone by getting rid of the stock manifold and replacing it with the SSV. Theory # 2 is I have yet to do a complete re-learn, meaning throttle position, air position, everything. I have only done the ECU re-learn, but not the rest. I'm going to try that first, if no dukes, then time for SSV!

 

YOu said you cleaned your TB, have you cleaned the MAF sensor?

The OP had an issue because of his plenum spacer. I highly doubt that is your problem if you don't have a plenum spacer. HOWEVER, it is possible that you have a vacuum leak which is the same thing.

 

Tollbooth- Yes I did clean the MAF out with some electronic's cleaner. You are correct I don't have a plenum, so yes my prob if it is the prob would be a vaccum leak...

 

My biggest question/ concern is that this "problem" only occurs after the car has been ran for 15- 20 mins. I'd think a fix would be much easier if it was all the time, but it seems the problem is triggered after the engine gets to its peak heat. I'd think a vaccum leak would be constant, but I guess the heat could cause the metal to expand and trigger the vaccum leak? I hate not knowing what a problem is and feeling like I'm going to just have to starting replacing random parts without knowing if they will fix the problem

 

so I am pretty sure this has happened to me = gay. I found a bolt that somehow broke the threads through thermal cycling and now i have an f'n vacuum leak. I have to pull the plenum off and drop a helicoil in = supergay.

Just thought I would vent.

 

Make sure your oil cap 'collar' isn't caught in the plenum. I did that once and it causes the same issue and you end up overtightening the plenum to resolve it... it shouldn't be more than 14lbs of tq that's not much...