Highway Acceleration Problem

[redfish0]

I have a 99 gs300 with 112k on it. Best car I have ever owned and I have had no propblems until now. I recently had maintenence performend on my car to include timing belt, serpentine belt, oil/filter, air filter, plugs, transmission fluid, etc. It ran great before but now there is an issue when I drive through the 40-50mph range. At about 40 mph if I try to accelerate gently the car coughs and hestitates and almost shakes. Once I get to 50 mph all is well. If I accelerate vigorously between 40-50 mph I have no problems, just gentle acceleration, like after I coast through a toll plaza causes a problem. Curiously my mpg has gone down as well since the service was performed. App. 415 miles per tank and now about 350. Any thoughts? I disconnected the battery overnight to reset the computer which was ineffective. Any thoughts? I thank you all in advance.

[TurboGS300]

Check TPS, O2 sensors(as it's about time to replace them). Also run a can of seafoam throught the fuel tank and through the vacuum line for the throttlebody. <_< Also check the coolant temp sensor for the ECU.

[ColinBarber]

What plugs were used?

[redfish0]

Check TPS, O2 sensors(as it's about time to replace them). Also run a can of seafoam throught the fuel tank and through the vacuum line for the throttlebody.  <_< Also check the coolant temp sensor for the ECU.

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I used plugs from autozone. they were platinum dual post bosch 2. Should I have used others?

[redfish0]

I have a 99 gs300 with 112k on it.  Best car I have ever owned and I have had no propblems until now.  I recently had maintenence performend on my car to include timing belt, serpentine belt, oil/filter, air filter, plugs, transmission fluid, etc.  It ran great before but now there is an issue when I drive through the 40-50mph range.  At about 40 mph if I try to accelerate gently the car coughs and hestitates and almost shakes.  Once I get to 50 mph all is well.  If I accelerate vigorously between 40-50 mph I have no problems, just gentle acceleration, like after I coast through a toll plaza causes a problem.  Curiously my mpg has gone down as well since the service was performed.  App. 415 miles per tank and now about 350.  Any thoughts?  I disconnected the battery overnight to reset the computer which was ineffective.  Any thoughts?  I thank you all in advance.

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since I am a neophyte, pleae tell me what the TPS is and I will check it.

[Lexusfreak]

Check TPS, O2 sensors(as it's about time to replace them). Also run a can of seafoam throught the fuel tank and through the vacuum line for the throttlebody.  <_< Also check the coolant temp sensor for the ECU.

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I used plugs from autozone. they were platinum dual post bosch 2. Should I have used others?

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I would have stayed clear of Bosch plugs personally & used either NGK or Denso platinums or iridums. The Bosch plugs are certinaly not helping things here. B)

[TurboGS300]

TPS is Throttle position sensor! Irridiuns are the way to go in plugs, however the boschs will work as long as they are cross referenced to OEM. I don't think the plugs are your problem, unless they are too cold a plug.

[redfish0]

Thanks Turbo. I will give it a shot and see if it helps. Is there a computer I could it up to in order to find out exaxctly what is wrong?

[TurboGS300]

Yes, the dealership or any good repair shop should have one. <_<

[chuckb]

I'll bet it's your new plugs. I've heard of the same thing happening when oem iridium plugs aren't used. I would first put oem ngk or nd plugs in and see what happens.

[redfish0]

i have noticed one additional feature of this problem. I want to share it with you all who are knowedgable and see if it sheds further light for diagnostic purposes.

I noticed that the first 10 minutes after strting the cold engine the engine does not cough or hesitate during the 40-50 mph range. After 10 minutes or so the problem surfaces and stays that way the rest of the day. Perhaps it is the plugs and I will start there. Any other thoughts with this new info?