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Discussion Starter · #1 ·
I have a 98 gt that I recently did a complete PI swap and 5 speed swap in. Engine came out of a 2005 crownvic. I bought the engine with a Trick flow track heat or street burner intake (not sure which one yet as i didnt think to get the modle number off of it when i installed the engine) already on it. So car has the intake with a accufab single blade throttle body, jlt cold air, new bbk maf, bbk shortys with pypes off road x, msd coils and FR 9mm wires. Other than that, the engine is a stock PI. I'm having idleing issues. Rough idle and non cosistant. I originally had hanging idle. I made a IAC place with a smaller diameter hole to prevent so much air flow, it fixed that issue but now it is idleing at 1100 sometimes 800. It's never consistant. Ive tried adjusting the idle screw along with testing and adjusting the throttle positioning sensor. If I lower the idling just slightly, the engine won't stay running. Also while driving, say in 3rd gear i let off of the gas and let the car coast down to about 2k i then push the clutch in to downshift and sometimes the rpms drop to 400-500 and come back up to 1100, sometimes it drops completly and car dies. I've also replaces all of the vacuum lines and can't find any leaks. Are these 2 issues related? Could the intake really be causing these issues as it's a lot of air on basically I stock Engine? Would upgrading injectors to 24lb help? ( they probably need replacing anyways ) any help is greatly appreciated. This issue is very annoying and I've exhausted all that I know to check and do.
 

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First and foremost is it tuned? And do you have any means of datalogging?
 

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Discussion Starter · #3 ·
No there is no tune on the car. I do want to get a new tuner/software for data logging. My current SCT is married to other mustangs so I'll need to get another.
 

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The first part was just showing I had pressure set to a steady 10psi. Then I pressurized or tried to the intake tube with the TB attached and the back side blocked off.
 

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sounds like vacuum leak symptoms, possibly around the intake manifold to cylinder head seal.
 

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These comments may or may not help but here they are. Our cars have a high idle at various points in time unrelated to hanging RPMs. (I also did the IAC mod. Instead of making a plate I put a restriction in the hose.) There is a high idle during a cold start and a high idle when slowing and stopping. If you notice the rpms will immediately drop from around 1100 rpm to around 800 as soon as the wheels stop turning. I assume the higher rpm is needed for the brakes and steering.

When I installed my TB/Plenum I had a problem with low rpm/surging rpm/sometimes engine dying when slowing to to turn or stop. I had no idea what was causing it until I noticed the throttle sticking at idle. I could feel it catch when I first pressed down on the gas pedal. The catch wasn't there with the motor off. Long story short the butterfly valve in the TB was closing too tight. The IAC needs a certain amount of bypass air in order to work properly. There is no idle adjustment screw on our cars. Idle is controlled by the IAC. We have a TB stop screw. That is how you adjust the bypass air around the TB butterfly valve. Even stock TB/Plenums sometimes need the stop screw adjusted because of wear and tear.

Cliff Knight has the correct procedures for adjusting the stop screw on his website.
PaladinMicro

I did it a little differently. With the butterfly valve fully closed (just touching the stop screw) I turned the stop screw 1/16 of a turn clockwise until the low idle/idle surging stopped. I think it took three tries or 3/16 of a turn on my car. If you actually change the idle then you have gone too far. That means you opened the butterfly valve to a point where the IAC has no affect and the motor cannot go to low idle. Playing around on my motor it took a little over one full clockwise turn of the screw to affect idle.

P.S. Making the hole too small in the IAC mod will also cause problems.
 

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The test in the above video would also be a vacuum leak. It would make the idle be high as I had that issue along with it leaking lots of boost. He did state he had a accufab TB. Easy to pressure test it with compressed air. Here was how the Stock cobra TB worked with the same test.
 

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Discussion Starter · #8 ·
Some great tips. I'll definitely do the pressure test in the morning and check the throttle body blade and stop screw.
Eagle2000gt what size diameter did you restrict the hose down to for the IAC? The plate I made, I drilled a 1/8 inch hole. That's what I read that the size needed to be but could be to small or to big still?
 

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The idling issue sure sounds like a vacuum leak.
 

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Some great tips. I'll definitely do the pressure test in the morning and check the throttle body blade and stop screw.
Eagle2000gt what size diameter did you restrict the hose down to for the IAC? The plate I made, I drilled a 1/8 inch hole. That's what I read that the size needed to be but could be to small or to big still?
1/8 is only 2/16. I did 5/16. Here is a thread I found where Cliff Knight talks about the testing he did. He said 9/32 causes a lumpy idle and that you should go .300" if a lumpy idle bothers you. .300" is basically 4.8/16. I consider 5/16 close enough.
https://mustangforums.com/forum/4-6l-1996-2004-modular-mustang/309397-iac-gasket-mod.html

This thread has a lot of discussion about the IAC mod. Lots of good information. In this thread Cliff actually says to use 5/16.
http://www.mustangevolution.com/forum/f280/t311890/


As it was pointed out later in the first thread. You will want to disconnect the battery for 15 minutes and then reconnect it. That drains the memory in the ECU. When you start the car give the ECU around 10 minutes to relearn the system.
 
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