Do you have access to an OBD-II scanner that will display real-time values of some of the run-time attributes such as Throttle Position?

I wonder if maybe your TPS is on the fritz and maybe it's sending an unstable throttle position value as you push the pedal down through a certain range of its throw. Something like a ScanGauge-II could be used to show real time values of the TPS and you can see if it's dropping. I've had this happen to me on my Tacoma before, but my Tacoma has a physical cable connecting the pedal to the throttle body. Our cars are drive-by-wire... so a TPS error would cause some serious bucking under acceleration, or just flat out lack of acceleration.

Actually... on our cars, the sensor I'm talking about is at the gas pedal itself. Accelerator Pedal Sensor.

 

Speedo sensor/ wiring harness? Have this problem that happens from time to time on my work truck.

Good luck!

 

My best scanner can only record 3 PIDs/second, but I can track data real time. Nothing out of the ordinary with the TPS. If the knock sensor doesn't clear things up, I am going to replace the throttle body next just to be sure. The shop did an analysis of all tracked data earlier this month over a 300 mile stretch. They said the only issue they saw was with the MAF. Replaced the MAF, and no change. The APP (gas pedal/sensor) was replaced in September.

HereKittyKitty- can you expound on the speedo sensor/wiring harness? I'm not certain what you're referring to.

Thanks!

 

This is just a thought, my truck's speedo goes weird while at the same time just revs. There is a wire that goes from the transmission to record speed, millage, give drive to the transmission.

I do not know if this may help you but thought it might.

 

You should check your abs speed sensors and axle rings to be safe. Almost sounds like car thinks it's slipping, cuts power, than traction, power back, repeated. I seen this before and had similar symptoms. Problem was broken axle speed ring. Make sure your sensors are clean too. Good luck!

 

Would you expect to the Slip light to flash if the abs/wheel speed sensors were faulty?

The problem happens parked in neutral too, so I'm wondering if that rules out any speed sensor issues. Thoughts?

Thanks!

 

Has this car ever sustained an accident that you know of?

I once dealt with a car with similar issues. Come to find out, a harness had been pinched in an accident. The car was repaired, butt the harness was not changed out.

The car had ECM issues and ran poorly. Changed out a few ecms and could not get car to run well.

Eventually we started testing sensors against an FSM and found some odd readings. Traced wires and discovered the damaged harness.


Here's the FSM for your car.

http://www.nicoclub.com/FSM/G35/Coupe/2003/

Ec.pdf might be a good read for you

According to page 10, a faulty knock sensor will not trip the CEL. If you click EC-296 on the right, it will take you to page 296 showing info on the sensor and wiring.


You'll just have to make educated guesses and troubleshoot. Another area I would check is MAF wiring.

 

More educated hypothesis...

Bucking while cold and warm would suggest its possible this is not a sensor issue. During cold start, many of the sensors are ignored and the car warms up using preprogrammed parameters from the ecm. Once the engine has reached a certain temperature, it switches to closed loop and begins monitoring sensors.

Which sensors are ignored on a g35 I do not know. Perhaps the EC FSM will tell this info.

That reasoning alone would make me feel this is an issue with wiring somewhere in the car. I'd ask has any of the wiring been altered, damaged, pinched, etc before this problem occurring? Any water damage or rodent nests ever noticed in the engine bay?

 

There haven't been any rat nests or water damage in the G. It's much cleaner than it deserves for 150k miles. I bought it in January from a early 50s financial advisor who owned it since 2003. He lived in Rochester for the first two years, but worked mostly in Chicago. By late 2004, the carfax showed he had hit 100k already. He moved to Northbrook in 2004, and hardly drove it for the next 9 years. Now that we have our history lesson, I have replaced the IPDM even though the original is clean as a whistle with no corrosion (the dealer was throwing parts at it when I first took it in), the ECU, and had the wiring from the ECU to APP redone along with the APP harness. The indie shop had my car for the better part of 6 months, traced every single circuit, and everything checked out. No accidents either, but I do have a healthy amount of rock chips on the front bumper/hood.

I'll replace the knock sensor and harness when they get here on Tuesday, and if that doesn't do it, I'll start diving back into the wiring. That said, I'm really hoping the knock sensor is my problem.

Thanks for the input!

 

You mean the knock sensor?...

 

Yes, knock sensor. Edited!


Thanks!

 

I installed the knock sensor and harness last night - more of a PITA than I had expected! Getting down to the knock sensor requires removal of the upper and lower plenums along with the intake manifold. I thought I may have to remove the fuel rails and injectors to get the intake manifold off, but I was able to just remove all the bolts holding everything together, and the manifold lifted up with plenty of space to work without putting stress on the fuel lines. The worst part was the harness. It's impossible to reach from the top side of the engine, so I had to jack up the car to get to it from the underside. Overall, it wasn't terrible, but just more work than I originally expected to swap out a sensor! I also installed the 5/16" Motordyne plenum spacer while I had things apart.

I put about 50 miles on the car since last night, and can confidently say the knock sensor was my issue. Absolutely no hesitation/bucking/stuttering now, and with the new plenum spacer, it pulls wonderfully. Throttle response is smooth, and the engine runs great.

Thank you to everyone for your assistance and suggestions!

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