Hey guys,

Tried searching first. Wasn't sure how to explain what I wanted to search for. Basiclally I have an 04' coupe with a few new mods. Going from completely bone stock, I just had a 5/16 motordyne plenum spacer, z-tube, oem rev-up air box installed. Everything else is stock. After the install, I could feel and hear the difference in a positive way. The only negative is that when driving if I let off on the gas it kinda coasts for a second or 2 and then does a rapid deceleration. It doesnt cut off completely or anything but, Its pretty noticeable. It jerks just enough to feel like I'm braking a bit. Nothing major, but enough to be annoying. I did several ecu resets and throttle body resets. Got the blinking SES light to work though on both resets. Not sure if the ecu reset or not. The throttle body reset definately seemed to work because my rpm dropped in idle from 900 to 600-700. The only thing is that after that I turned my engine off and it went back to 900 again. Was it suppose to stay at 600-700? Do these resets take time to adjust? Is it normal for it to be driving like this after my intake mods? Is there some other issue I'm not aware of?

Thanks for the help.

 

Someone probably installed a Jake brake on your car....

How's your fuel economy been? It could be a stuck brake caliper or worn wheel bearing. Is your car a 6MT or Auto?

 

I signed onto the forum to post about the EXACT same problem. My 6MT has done this a LITTLE bit since it was brand new but recently has gotten much worse. It's also worse when the A/C is on. I cleaned the throttle body over the weekend hoping that would help.... it didn't. Also, my fuel economy has sucked lately. But it really doesn't feel like a stuck caliper, but I guess it's possible.

The worst is when in traffic cruising at speeds less than say 40mph, and have to let off the gas completely. It does the opposite motion as soon as I touch the accelerator again. To keep it from happening, I just have to apply VERY SLIGHT pressure on the accelerator. However, this is obviously not always possible.

 

Do either of you notice a difference in the sound of your car? Could be clogged catalytic converters.

 

What is a jake brake?

Lately my fuel economy hasn't been great actually. This was before my intake upgrades though. The issue with the deceleration is recent....after the upgrades. I don't notice any strange noises though. I tried driving in "cruise mode" at 30mph to 40mph. Normally I don't have any issues, but not driving like it should be. It seems very jerky, like its hard to stay at those speeds smoothly. Hmm...what could this be? And, I am driving a 6mt as well.

 

The sound of the car is just as it has been since day 1. And yeah in cruise control on a slight downhill grade, mine jerks forward/backward quite a bit.... cruise control lets off the throttle completely, which suddenly slows down the car, then it applies slight throttle and it jerks forward a little and the cycle continues. However, it has always been like that somewhat but seems worse lately.

 

Re-read posts #1 and #3. It's a new problem.....

 

Sure, it decelerates But, its almost like tapping the brakes after a 2 second coast. That is with cruise off. With cruise on, its like slight braking and accelerating. Not smooth like it should be. I've owned my G 8 years and believe me, its not like I just bought it yesterday. This is something new.

 

A jake brake is a system that is used on some diesel-powered trucks as a means of slowing the vehicle down by using the power of the engine as a brake. It basically is a metal flap in the exhaust manifold that closes when you let your foot off the gas while the jake brake is engaged, and it causes the motor to not be able to freely push all of the exhaust gases out of the exhaust, since it's partially blocked off. I'm sure you've heard those big Mack trucks going down the highway, when they suddenly start making a really loud engine noise.... that's the Jake Brake.

Here's a sample of a Jake brake in action on a loud 18-wheeler (go to about :41 seconds into the video to see what I'm talking about):



I had a dream that I had a Jake brake on my little regular cab Tacoma the other night and I was going nuts with it in some tunnels in Boston and getting lots of startled looks by other drivers... Haha... it was a good dream!

Your G35 definitely should not feel like you're braking when you let off the gas and leave it in gear. Mine offers so little resistance from the motor that it's almost pointless to compression brake, because the motor doesn't generate enough negative force to make any sort of significant dent in the forward momentum of the car. That's a good thing though.. it means the motor is spinning with very little resistance and probably working pretty efficiently.

 

anybody else have this issue?

 

Just some ideas:

Dirty throttle body, get some TB cleaner and let it work its magic.
Did you pinch the rubber from the oil cap in the spacer when you installed it? Are you getting a good seal?
Clogged cats
Dirty Air Filter

 

Same happening to my car, but I always thought it was because it was a manual, the engine does the braking. No braking of any kind when I shift back to neutral

 

This is true of any manual car. In neutral the rear wheels spin freeeeeeee.

However, the OP is describing harsh and significant braking while in gear, which means his engine is trying hard to slow down. This implies friction in the engine, suction on the intake, blockage on the exhaust, or friction in the tranny.

Can someone more knowledgeable than me chime in on fuel while coasting in gear? I remember reading the fuel can shut off totally if coasting in gear, but I'm not sure if this is true of all or any cars at this point. If it is not true, the OP may have an issue with their fuel injectors, spark plugs, etc.

OP do you have a rough idle?