99 Gt Jumps/Lurches when put in Gear at Idle

[ebby69k]

This is a 1999 GT. No mods. About 65000 miles. Automatic. Sometimes
after I stop and shut it down say after running into the store I will start it up.
Foot on the brake, put it in gear and it will want to lurch or jump like I gave it throttle. Kinda freaks me out sometimes. Anyone have any thoughts on what may be causing this. My only thought is maybe I am getting some injectors leaking fuel into the cylinders?

 

[wmburns]

Please check the throttle return springs. Should be two.

How high do the RPM's go?

What is the condition of the transmission fluid?

 

[trinity_gt]

What's the idle speed doing?

Is it at ~750 when you put it into gear or is it abnormally high (e.g. 1000)?

 

[007]

What is the condition of the transmission fluid?

Yup. When was the last trans fluid change?

 

[dangrus]

My 00 does the same thing occasionally. It has also done it a few times when I put it in drive after backing out of a parking space. I cleaned my throttle body once and that seemed to help. Mine is stock.

 

[mr-mike]

my 99 does the same thing not all the time but it does happen most times with the AC on ...... was told its a vacum problem no codes ...scary if your not ready for it ! never found out a way to fix it 150,000 + but it started at 75,000 I like the Mustang but like all cars its not without problems ...intake , cops , computer , brake calipers & fuel pump but all in all she has been a decent machine motor is still like new no smoke no leaks & decent MPG .... for the mileage I cant gripe

 

[wmburns]

mr-mike, check the Freon charge level.

If low, the AC will short cycle. The PCM stores IAC idle trim values for AC off and AC on. If the AC is short cycling, the PCM will not be able to learn correct AC on idle trim values.

Try jumping the AC low pressure switch. This will force the compressor to run. A few minutes with your foot on the brake and the AC actually running should improve the learning process.

At the risk of sounding like a broken record, check the throttle return spring. There are two of them. If one is broken/missing, the throttle can be slow to return to closed position. I have had this condition happen to me. Nearly drove through the garage wall. Very scary.

 

[mr-mike]

the AC is perfect , its a convertable & still cold , never added freon .... one of the best AC cars I have owned , but thanks I think one of the trap doors hangs up all fuel inj cars are vacume sensative ..... have a 90 put the AC on & the idle acts up but never surges .....

 

[Dan02gt]

I agree with wmburns on the freon charge level.

My dad has a '00 auto convertible with about 110k on it. It started doing the same exact thing. Scared my step-mom a few times. She would put it in gear or let off the break and the car would jump. Well we started troubleshooting the issue, and found that it only seemed to be happening with the AC was on. The AC compressor would cycle do to slightly low freon and the changing load on the engine would result in a surge in RPMs and if the car was in gear it would jump. We added just a little freon and the issue went away.

The AC seemed to be cold and working fine before we added the freon BTW.

So start your car and let it idle with the AC on, open the hood and check if the AC compressor cycles on and off. It may take a few minutes to tell if it is or not. If it does, have the freon level checked.