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This may get long but I need HELP!!! I have a 98 GT that I have put a rebuilt engine in with PI heads, intake, 75mm AccuFab throttle body & plenum, underdrive pulleys, MAC straight shot air intake, Ford shorties and 3.73's. I took car down a year ago to Alternative Auto Performance for a custom chip and it ran great. Recently the headers developed a leak so I took it down to them to clean up the headers a tighten down with new bolts. After getting car back it now was hesitating and not developing any rpm's past 3500(probably fuel pump), so I returned to them and they put on 2 new O2's in front (rears are turned off) and a fuel filter, still it was not running good. They checked fuel pressure and it showed the pump was indeed bad so I returned a third time(it's a little over 400 miles round trip) to have fuel pump installed. They installed a 255 lph and I was back on the road home. Almost home I stopped at a friends and while sitting at a light my idle would stay up for what seemed longer than usual then when it went down, it went all the way down and my car died. I re-started and took off to my friends. After leaving I went back on highway to get home and I got on it pretty good which then brought on my engine light. I got home, disconnected battery for 5 minutes(per Lidio Iacobelli at Alternative Auto Performance) then went back on road for 10-15 minutes of normal driving so computer could reset,. Went on highway, no engine light, but the rpms do not seem to build as fast, especially when approaching red line. This car used to "fly" to the red line, now it doesn't seem to want to get to 6,000 rpm's. Also, the idle still stays up for a while before coming down and it seems to "lope", it comes down and goes below normal idle rpm and then spikes back up to about 1500 and back down until it settles at about 900 rpm. Where do I start looking for my problem and anyone in Michigan, particularly in the Grand Rapids area know of any good mechanics. Thanks in advance for any help you can provide. I know this isn't the greatest of cars but I like it and it is fun to drive(or it used to be when it ran well).
 

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Check for the following:

Idle -> seem to have a bad IAC (It is MonkeyBoy's favourite Part:winks )

RPM / Power -> Check for vakuum leak, hose not attached. It could also be a leak at the headers. If the headers pull additional air, the motor will run to rich, because the ECU thinks there is to much unburnt oxygen in the exhaust gases, and therefore corrects it to run more on the rhich side.

Driving on highways, "check engine light" -> Bad EGR Valve (solenoid), that black pcs of plastic with an electric connector, regulating vacuum for the EGR Valve on the plenum.

Good luck buddy
 

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engineer76 said:
Check for the following:

Idle -> seem to have a bad IAC (It is MonkeyBoy's favourite Part:winks )

RPM / Power -> Check for vakuum leak, hose not attached. It could also be a leak at the headers. If the headers pull additional air, the motor will run to rich, because the ECU thinks there is to much unburnt oxygen in the exhaust gases, and therefore corrects it to run more on the rhich side.

Driving on highways, "check engine light" -> Bad EGR Valve (solenoid), that black pcs of plastic with an electric connector, regulating vacuum for the EGR Valve on the plenum.

Good luck buddy
Believe it or not, I'm not going to blame the IAC valve this time. Sounds to me more like a bad tune. It's more likely that one problem is causing all of these symptoms rather than a bunch causing all of them.
 

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blackgt98 said:
This may get long but I need HELP!!! I have a 98 GT that I have put a rebuilt engine in with PI heads, intake, 75mm AccuFab throttle body & plenum, underdrive pulleys, MAC straight shot air intake, Ford shorties and 3.73's. I took car down a year ago to Alternative Auto Performance for a custom chip and it ran great. Recently the headers developed a leak so I took it down to them to clean up the headers a tighten down with new bolts. After getting car back it now was hesitating and not developing any rpm's past 3500(probably fuel pump), so I returned to them and they put on 2 new O2's in front (rears are turned off) and a fuel filter, still it was not running good. They checked fuel pressure and it showed the pump was indeed bad so I returned a third time(it's a little over 400 miles round trip) to have fuel pump installed. They installed a 255 lph and I was back on the road home. Almost home I stopped at a friends and while sitting at a light my idle would stay up for what seemed longer than usual then when it went down, it went all the way down and my car died. I re-started and took off to my friends. After leaving I went back on highway to get home and I got on it pretty good which then brought on my engine light. I got home, disconnected battery for 5 minutes(per Lidio Iacobelli at Alternative Auto Performance) then went back on road for 10-15 minutes of normal driving so computer could reset,. Went on highway, no engine light, but the rpms do not seem to build as fast, especially when approaching red line. This car used to "fly" to the red line, now it doesn't seem to want to get to 6,000 rpm's. Also, the idle still stays up for a while before coming down and it seems to "lope", it comes down and goes below normal idle rpm and then spikes back up to about 1500 and back down until it settles at about 900 rpm. Where do I start looking for my problem and anyone in Michigan, particularly in the Grand Rapids area know of any good mechanics. Thanks in advance for any help you can provide. I know this isn't the greatest of cars but I like it and it is fun to drive(or it used to be when it ran well).

MAF, more than likely. Spray it with computer contact cleaner. don't touch the wires.
 
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