My monster is on the road and racking up some miles. I was having a bit of a "roaming" idle issue. Tracked it down to a bad TPS. Installed a new one, reset base idle and measured TPS voltage to be .9vdc. Right where it should be, and it idles great now and my fuel economy is up.
However, I have a low speed bucking problem. When in 1st or 2nd gear below 2k rpm, the car starts bucking a bit...not horrible, but hard to drive smoothly for my wife. It's almost like a car with too much cam. BTW, stock cam, all stock engine except 1.7 cobra rockers. Timing is set at 10*. Cleaned the MAF, IAC and TB. I don't think it's driveline slop as the shifter doesn't move noticeably when it happens.
A google search shows this to be a common issue for fox Mustangs. Causes have ranged from dirty MAF, bad TPS, dirty TB or IAC, or timing. I've covered all those bases- any other suggestions other than to keep it above 2K?
Car drives great otherwise, and this IS fairly minor. It just keeps the car one notch away from perfect for me.
Cheers,
Brad

 

You will probably have to live with it. That exact same issue is what was causing problems on the emissions dyno for me.

 

Is the VSS hooked up? Without it the electronic dashpot feature of the ECU does not work because it does not know if the car is moving or not. If it is try adding a third bolt through the floor under the passenger seat through the trans crossmember. The crossmember has a tendency to act like a big leaf spring and the bolt eliminates its ability to flex. Lastly, if you used the stock upper Miata differential bushings take them out and replace them with urethane as well as the nose bushings if you just reused the old Thunderbird ones. Also hook up your rear sway bar if you have not.

All of the above eliminated the issue in my cars. #1 and #2 had the biggest effect. The last two had more to do with wheel hop but did help eliminate driveline slop as well.

-Jason

 

I second all of this.

Also, shift into a higher gear. I find that the bucking is much more manageable if I keep the engine under 1200 rpm in traffic. It's not going to lug on you.

 

Yes, the VSS is hooked up and working (-it hangs the idle up for a second when coming to a stop as it should), The rear subframe bushings were replaced with poly during the build.
I was out for a drive last night...it really isn't that bad, but would be nice to make a little smoother. I have 1.7 rr installed. Could that be contributing?
I'm reluctant to do the "bolt through the xmember" mod. The shifter barely moves, so I can't see how it would benefit. But I've been wrong before.

 

The bolt through the cross member is totally worth the effort.

 

Have a bucking issue with mine, change the distributor and problem gone, ritchporter have a nice one for cheap. Just don't want to rebuild my old one. It's take me a long time to figure out that problem.

 

Thanks Seb, new distributer when I rebuilt the engine. The bucking is pretty minor. You car looks good.
Brad

 

Thanks Brad, check your part twice, i buy a bad MAF new in box.