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did some upgrades, now i have problems

2K views 18 replies 4 participants last post by  wbrockstar 
#1 ·
ok here it goes ,

1989 mustang, 5.0 automatic. did a gt40 upper and lower, 65mm ported to 70mm cobra TB, and matching spacer, (spacer is new) new shorty headers, and new ignition up grades (wires ,cap . rotor , ) 8.8 wires from accell , EGR was deleted before, i did the upgrades i just used the old egr delete plate on the new spacer,
ok now here is the problem; he engine wants rev uncontrollably , after i try to start it it will hesitate< then rev to 3- thousand RPM before i hurry and shut if off , it dosde it instantly , if i dont shut the engine off it will most certainly blow.

any thoughts , any help is much appreciated , also i am new here , i am from asheville nc (blue ridge parkway anyone lol ):wink:
 
#3 · (Edited)
I see one possible cause,depending on your answer to the following question
When you ported the tb to 70mm,did you remove the tps,throttle plate & the throttle linkage before you began porting it so it would be the same size hole throughout the tb or did you leave those components in place and just port it up to the throttle plate??
If its the first process,youve got a 70mm opening now throughout the tb,but your throttle plate is still the same +/-65mm size.If the opening is 70mm,but the plate is +/-5mm smaller in diameter,air is gonna make its way around the outer edges of the throttle plate, which will have the same effect as opening the throttle.Most of the air coming in during idle gets bypassed around the throttle plate via a passage in the iac valve,not the throttle plate itself.The throttle plate is closed almost completely at idle that way the computer can have total control over idle via the iac valve.If extra air is passing by the throttle plate,because the throttle body bore is now 5mm larger in diameter than the throttle plate itself,its gonna make idle control impossible.If this air causes the idle to increase above 600-625rpm,with the iac harness disconnected,the computer will lose its ability to control idle using the iac valve & when this happens,youll have a high idle or surging idle.The idle set screw on the tb is suppose to set idle at a fixed value of 600-625rpm & after that,the computer will take over idle control at any value thats above 625rpm by opening the iac valve,which bypasses air around the throttle plate via a passage in the iac valve.
If you replaced the throttle plate with the correct size already,that matches the port size(+/-70mm) there shouldn't be any air passing around the plate edges.If youve already replaced the plate with the correct size or you only ported the tb up to the throttle plate,but you still cant get the idle to cooperate,try doing a base idle reset.
You should also make sure the throttle plate shaft is sealed good at the top & bottom of the tb by checking it for a vacuum leak.Since its idling too high to use carb cleaner, to test it for a vacuum leak,just cover the throttle shaft & linkage up on the bottom side of the tb & cover the tps up on the topside by using some duct tape temporarily.Its probably not leaking,but if it is that will prevent air from being drawn in.If the idle drops back down towards normal when you put tape over these two areas,thats an indication there is a leak at those locations.
You would normally test for a vacuum leak by letting the engine idle while you spray carb cleaner at the suspect leak point & if it caused the engine idle to surge upwards,that would ID it as a vacuum leak.However its not possible to use this method at the moment because the engine is already revving up too high as is.


Summarized/Corrected Base Idle Reset Procedure
 
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#4 ·
Double check all of the vacuum fittings screwed into the upper intake & all vacuum hoses.The symptoms are spot on for a vacuum leak,unless youve got the tb bore diameter to tb plate diameter issue present,like I stated in my previous post?? If you didnt get a good seal on the lower intake gaskets,that will also cause a vacuum leak.Did you install the metal filter,grommet & pcv valve correctly in the lower intake & are they the correct parts for the gt40 intake?? Make sure the grommet lip is seated against the block fully and the grommet fits tightly.Does the pcv valve fit tightly inside the grommet??
Did you use a gasket between the egr delete plate & spacer??
Gasket between the iac valve & tb??
 
#8 ·
That suggestion would be dependant on which gt40 intake he installed.If its the 97.5(Feb)+ Explorer/Mountaineer intake,it doesnt have the passages in it to even run the egr system.
 
#9 ·
Did you use the 65mm throttle body off of the Explorer/Mountaineer or is it an aftermarket tb?? If its the first one,did you modify everything correctly,on the tb,like the throttle linkage,tps,breather tube fitting?? The throttle linkage is clocked in the wrong direction for use on the Mustang,so it has to be modified in the opposite direction.

Make sure the plate can be closed fully.Pop the throttle cable off the linkage,manually push the linkage so the throttle plate is fully closed then get an assistant to start the engine to see if its still idling high.If its idling high still,check the set screw to see how far it has the throttle plate opened.If its adjusted too far inwards,the idle will also be really high because the plate can only close as far as the set screw allows it to.If you try to push the plate closed 100%,but the linkage hits the set screw when the plate is only closed 85%,youre gonna have a really high idle because its allowing alot of air to pass by the throttle plate.
 
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