89 gta sputters only when hot

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Seb

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I have an 1989 Trans Am GTA 5.7L TPI auto. the car has 200 000 kms but the engine has about 60k. the previous owner replaced the short block so the top end is original.
Here is my problem:
When the car gets to full operating temperature and I give it gas slightly hard the engine sputters, pops and backfires really bad. It's like its starving of fuel. I connected a pressure gauge on the fuel rail and checked the pressure when it sputters and it's fine. The computer is giving no codes. I thought it would be the O2 sensor so I replaced it and it's still doing it. What really throws me off is that this problem is only when the engine runs to full operating temp, when it's cold it runs perfect. Does anyone know of anything specific that kicks in when the engine gets to operating temp such as a sensor or something?
Any help whatsoever is greatly appreciated since I've had this problem for almost 2 years now and i'm just out of ideas.
 
check for a vacuum leak?
 
I agree vacume leak or Intake gasket leak. Get a can of WD-40 or Carb cleaner, Put the straw in it & gently spray in limited area while the engine is running. DONOT get on exhaust manifolds! When you spray a spot that has a leak the engine will idle up a little.
Also check any sensors that have diaphrams ie vacume lines going to them.
 
Thanks for the reply guys, do you think a vacuum leak would cause this symptom only when the engine is at full operating temp? If it is a vacuum leak would it not cause the engine to sputter all the time?
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Heat causes expansion which opens up the source of the leak
 
thanks for the help, i'll check for leaks like you said with WD-40. What about connecting a vacuum gauge to one of the vacuum lines?
 
that will only show you that there is a low reading but will not tell you where the leak is. Since you dont have a normal baseline to work from you cannt be sure it is a low reading.
 
Here is what I found out while checking for vacuum leaks, the cable that connects from the throttle body to the vacuum pump doesn't move when I turn the throttle, i'm thinking it's because of a missing clip, view the second image, it shows the throttle body half way but the cable doesn't move. However i'm not sure if this is supposed to be this way. If not this would explain the sputtering when engine is under load since the vacuum pump isn't moving.
Thoughts?
photo.php

http://www.facebook.com/photo.php?p...&id=783088540&ref=fbx_album&fbid=432807513540
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Here is what I found out while checking for vacuum leaks, the cable that connects from the throttle body to the vacuum pump doesn't move when I turn the throttle, i'm thinking it's because of a missing clip, view the second image, it shows the throttle body half way but the cable doesn't move. However i'm not sure if this is supposed to be this way. If not this would explain the sputtering when engine is under load since the vacuum pump isn't moving.
Thoughts?
 
i recommend you have someone sit in the car and step on the gas pedal I think you are looking at the wrong cable.
 
I checked with someone in the car pressing the gas and it's the same thing. If you look at the second picture I posted earlier it show's the brown/brass piece attached to the throttle body, the throttle is half pressed but that brown clip doesn't move.
When you press the gas pedal shouldn't it make the vaccuum pump press in and out? Because right now it doesn't do that.
 
you keep on referring to a vaccuum pump, are you sure it's not the cruise control which would only work when you are driving and engage it?
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