Breaking up on throttle tip in


Apr 28, 2019
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I am having trouble solving a long term problem with my 2000 gts idling and running poorly. The car originally stopped idling in August of last year. It would fairly uncontrollably idle at 2500rpm +/- 500 and generally was dangerous to drive. I replaced the IAC, TPS, and did the intake manifold gasket. After no success I took it in for help and was told the throttle bodies were bad. They replaced the aftermarket throttle bodies with a stock set, compression was also checked and found to be fine. I got it back and it drove fine for the ride home (20mi) and on my next outing that week it started violently bucking and breaking up under light throttle application. I replaced the upstream O2 Sensors (bosch), coils (OEM), and plugs (Champion), rebuilt injectors, and also found 1 vacuum leak. The car no longer is violently breaking up but you can still hear it and feel it under the light throttle application conditions. If you drive it at anything more than 0-5% throttle then it drives fine. It does not sound like it is breaking up buck or act abnormal in those conditions. It is intermittent in the sense that it will drive 20-30miles relatively ok and then go back to its upset state. When starting the car idles at 500 for 10minutes or so then as driving occurs it can become progressively more erratic bouncing between 1250-1500. I can't say this is a hard rule as it has started up and not been happy, idling at 1250 and never reaches anything below 1000.

At this point I have no clue what to do, I would appreciate anyones help on this as it is incredibly frustrating to dump parts into it with no real results. I have a 1 psi smoke machine which easily found the 1 vacuum leak at the IAC gasket. I have been trying every line I can see but maybe somebody has a better strategy or there is one I missed

So far I have smoked the:
1) Evap service port
2) Brake Booster Line toward the manifold
3) Vacuum lines at the front of the manifold
4) Brake booster line toward the vacuum booster

In doing all this all I have found the IAC leak and that there is an ovalized valve on the booster. I tried to get the connection to leak and can't, I am still replacing it however as it is visibly out of round.

Any help or thoughts on tests I can run is much appreciated, I would very much like my car to run as it had for the prior 4 years.


  • vacuum tests.png
    vacuum tests.png
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Nov 10, 2019
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Port Orchard, WA
I had a similar issue to yours, though, not as violent. The car would idle between 1250 and 1500 RPM and throttle tip in was rough. For me it was the vacuum line T that connects both sides of the intake manifold. The plastic T was broken. I think you're in the right search area with the vacuum system. Good luck!