Idle speed won't go below 1500 while moving

[The Screwdriver]

99 Mustang GT. Previously had stock air intake, TB, and plenum with 1/2 inch aluminum spacer between Plenum and intake. Idled perfectly - rpms dropped quickly and stayed there. Originally installed spacer and that cured the idle hanging in the stock system for me.

Just installed Accufab 70mm TB and Plenum and JLT air intake (removed 1/2 inch spacer). Power, TB response is great and actually noticeable. Very happy with it except for my idle.

When stopped, car idles Ok at 900-1000, and revs drop ok... not as fast as before, but ok. My main concern is when moving, the car won't idle at less than 1500. If I'm driving and disengage the clutch, the idle speed bounces from 1000-1500 every 2 seconds I can see the needle just bouncing up and down while coasting... very annoying. I've disconnected the battery to reset the computer and even tried restricting the IAC hose with 1/4 inch hole drilled in a pipe cap... no change in behavior. Is this something that the computer will learn to stop doing over the next week? Or is there something wrong here?

Things I can think of to try out are:
1. Replace my intake spacer
2. Try a smaller restriction in the iac hose
3. Replace the MAF screen (JLT said to remove it)
4. Check for vacuum leaks for the millionth time

Thanks,
Jim

 

[MustangLife]

I'm kind of having the same problem. My idle sticks at 1200 rpms when slowly moving but wot and sitting idle or normal.

You removing the maf screen is ok. Thats not gonna cause a problem.

Your car should not be idling at 1000 thats way to high. GT's should idle between 700-900 rpms. Mine is about 800 rpms.

Have you checked your voltage on the tps yet to see how close it is to the norm.

My problems I gave up on cause the tps is ok. No vacuum leaks. No check engine light. Everthing looks good.

Josh

 

[w.neurauter]

Not to hijack the thread but what is the 1/2" spacer you installed? Was that to fix the hanging rpms between shifts with the stock setup? I've heard of and tried both the copper pipe mod and the sheet metal between the IAC and intake mod but have never seen a 1/2" spacer.

Thanks

 

[blkhorse]

acts like a vacuum leak,also make sure the throttle cable is allowing the tb to close fully,also do you have a way to check the tps voltage at closed throttle?

 

[The Screwdriver]

I initially set the TPS to .98 and double checked after tightening it down. I'll check it again along with the throttle cable. If something were wrong with the cable or tps, then wouldn't it be misbehaving when the car is stopped also?

The spacer wasn't necessarily for the hanging rpms... but it happened to help.

Thanks for the replies.

 

[blkhorse]

idle at 900 to 1000 is a little high.try putting your stock intake on, the jlt with the mass air in the fenderwell may cause some of your problem.I know these cars are picky when it comes to mass air postion.

 

[w.neurauter]

Could someone point me to a place where I could find this spacer? Did it make a big difference?

Thanks

 

[steve_stang2003]

I initially set the TPS to .98 and double checked after tightening it down. I'll check it again along with the throttle cable. If something were wrong with the cable or tps, then wouldn't it be misbehaving when the car is stopped also?

The spacer wasn't necessarily for the hanging rpms... but it happened to help.

Thanks for the replies.

Same problem with an Accufab 70mm combo.
Everything was adjusted fine including TPS and throttle cable.
I did NOT touch idle stop screw because Accufab says not to.
No leaks and plates bottomed nice and sharp against stop screw.IOW there was no binding.
Stock unit works fine.

I screwed with it for 2 days and finally sent it back.
My car is an automatic 2003 GT BTW.

 

[The Screwdriver]

Thanks again for the replies.

I don't have a link to the intake spacer anymore. I got it about 4 years ago.

My JLT is the ram air one which doesn't relocate the filter or MAF. Only thing JLT said to do was get rid of the MAF screen.

It's a shame to return it, but I can't live with the idle like this. Thanks again , and let me know if you have any more ideas.

-Jim

 

[Scorcher2005]

do this: http://forums.stangnet.com/showthread.php?t=541046&highlight=braking+effect

 

[The_Screwdriver]

Tried the pipe cap in the IAC line and it didn't work. Is making that plate any different than putting the restriction in the line?

Just double checked the TPSensor and it's fine at .983 when closed.

Also tried adding the 1/2 intake spacer under the plenum... it lowered the idle slightly and kept the rpms from hanging, but it still surges/bounced between 1500 and 1000 when the car is moving and the clutch is disengaged.

I removed the spacer and just left the JLT intake & Accufab TB & plenum with the IAC restriction (3/16) in the hose. It's drivable now, just really annoying.

Hopefully the car computer will figure something out and come to its senses in the next few days. Then I'll try with the stock plenum/TB and see if that fixes it right away.

Thanks,
Jim

 

[orangestang35th]

maybe a solution..?

99 Mustang GT. Previously had stock air intake, TB, and plenum with 1/2 inch aluminum spacer between Plenum and intake. Idled perfectly - rpms dropped quickly and stayed there. Originally installed spacer and that cured the idle hanging in the stock system for me.

Just installed Accufab 70mm TB and Plenum and JLT air intake (removed 1/2 inch spacer). Power, TB response is great and actually noticeable. Very happy with it except for my idle.

When stopped, car idles Ok at 900-1000, and revs drop ok... not as fast as before, but ok. My main concern is when moving, the car won't idle at less than 1500. If I'm driving and disengage the clutch, the idle speed bounces from 1000-1500 every 2 seconds I can see the needle just bouncing up and down while coasting... very annoying. I've disconnected the battery to reset the computer and even tried restricting the IAC hose with 1/4 inch hole drilled in a pipe cap... no change in behavior. Is this something that the computer will learn to stop doing over the next week? Or is there something wrong here?

Things I can think of to try out are:
1. Replace my intake spacer
2. Try a smaller restriction in the iac hose
3. Replace the MAF screen (JLT said to remove it)
4. Check for vacuum leaks for the millionth time

Thanks,
Jim

I had the same problem except mine would go all the way up to 2000 sometimes. i bought that MAF cleaner by CRC and lots of fuel injecter cleaner and it fixed my problem.

 

[The Screwdriver]

Swapped back to my stock TB/plenum and problems solved immediately! No computer reset or week of driving or weird workarounds. 100% back to normal idle speed (700-800) and no haning RPMs.

I noticed that my stock TPS is set to .94 volts which seems low according to the Accufab documentation. I couldn't physically get the new TPS on the Accufab unit to rotate below .98 volts. I tried drilling out the holes to get more rotation but still couldn't go below .97 and the idle was still high. Now I've also ruined the brand new TPS by drilling out the holes so I can't return it. Expen$ive lesson learned I guess.

I don't know what else to do about this. Is it just me, or wouldn't you consider this defective? Are they all like this or did I get a "special" one that won't cooperate with the TPS on my car?

Thanks,
Jim

 

[steve_stang2003]

Swapped back to my stock TB/plenum and problems solved immediately! No computer reset or week of driving or weird workarounds. 100% back to normal idle speed (700-800) and no haning RPMs.

I noticed that my stock TPS is set to .94 volts which seems low according to the Accufab documentation. I couldn't physically get the new TPS on the Accufab unit to rotate below .98 volts. I tried drilling out the holes to get more rotation but still couldn't go below .97 and the idle was still high. Now I've also ruined the brand new TPS by drilling out the holes so I can't return it. Expen$ive lesson learned I guess.

I don't know what else to do about this. Is it just me, or wouldn't you consider this defective? Are they all like this or did I get a "special" one that won't cooperate with the TPS on my car?

Thanks,
Jim

I'm wondering if you got the 70mm combo that I sent back to AM a couple of weeks ago? Did you purchase your's from AM?

Aside from that, I read a thread someplace that you need a tune to set the "idle decay" or "return to idle" number in the PCM. I forget the exact term, but on another board this was being discussed. Why some cars have the problem and others don't is a mystery however automatics seem to be more prone to it.

I had same problem. Stock worked fine. Accufab hung.

 

[steve_stang2003]

Swapped back to my stock TB/plenum and problems solved immediately! No computer reset or week of driving or weird workarounds. 100% back to normal idle speed (700-800) and no haning RPMs.

I noticed that my stock TPS is set to .94 volts which seems low according to the Accufab documentation. I couldn't physically get the new TPS on the Accufab unit to rotate below .98 volts. I tried drilling out the holes to get more rotation but still couldn't go below .97 and the idle was still high. Now I've also ruined the brand new TPS by drilling out the holes so I can't return it. Expen$ive lesson learned I guess.

I don't know what else to do about this. Is it just me, or wouldn't you consider this defective? Are they all like this or did I get a "special" one that won't cooperate with the TPS on my car?

Thanks,
Jim

Also being discussed is that the TPS voltage really doesn't matter much as long as it is reasonably close because PCM takes the base voltage when you turn the car on and uses that the determine the throttle plate angle. As long as there is enough room to count up from the base, it should be fine. This is why your sotck unit worked fine a .94 volts.

 

[The Screwdriver]

Thanks for the info. I have seen those threads, and it makes a lot of sense. I did actually get mine from AM. They've been unresponsive to my emails over the last 3 days asking for help on this issue though... I guess I'll have to call them to get the return sorted out.

Thanks,
Jim