99 GTP issues

[ThatSparksKid]

1999 GrandPrix GTP issue.

I have a 99 GTP that when cruising at 70 mph rpms are around 3k where as in my 03 rpms are like 2100 rpms. When letting off the gas in the 99 while cruising, rpms will fall to idle, and when gas is reapplied they Rev back up and, idk how else to explain it but, it feel like it "catches" back up. Kind of like if you're cruising a manual car, push in the clutch, let rpms fall, then let clutch back out. I can basically free Rev the car like it is in neutral while coasting in gear. Seems like the car doesn't go into overdrive like my pictured 03 does. Any suggestions? No codes or anything show. It also guzzles gas way worse than my 03. The 99 has a 3.4 pulley, front plog, 180 t Stat, 3" zzp Downpipe, autolite plugs and zzp tune.

Also, other issue with car is when filling it with gas, when the tank was full fuel spilled out from I'm assuming where the filler neck connects to the tank itself. Is there a seal there I can replace or do I need a new tank

 

[melsg5]

You need a scanner capable of reading transmission codes.
Advance auto shows the neck available but you will have to determine what the source of the leak is.

 

[ThatSparksKid]

You need a scanner capable of reading transmission codes.
Advance auto shows the neck available but you will have to determine what the source of the leak is.

I believe that I do, I have HPTuners. It doesn't show any transmission diagnostic codes, but I can fully control the transmission with the program.

 

[melsg5]

I believe that I do, I have HPTuners. It doesn't show any transmission diagnostic codes, but I can fully control the transmission with the program.

Ask the company that makes it.

______________________________

 

[Queen B]

I had a similar problem with my 98 GTP. Check the throttle body, throttle position sensor, and the TPS wiring harness and pigtail connector for any shorts. After changing my throttle body, the mechanic was careless with the TPS pigtail connector and shorted out one of the wires. At first I thought it was the tranny and took it in to be fixed. The transmission shop ran down the electrical and found it was a damaged wire on the TPS pigtail connector. Very cheap fix if this is an issue. Much cheaper than transmission issues.