Surging idle 95 mustang gt 5.0 bolt ons

Dom5.0

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Jul 25, 2019
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I have a 95 mustang gt 5.0 with stock bottom end, gt40p heads 1.6 roller rockers, comp xe264hr cam, 75mm throttle body, 30 lb injectors, MAF calibrated for the injectors, BBK cold air intake, Trick flow upper and lower, pacesetter long tube headers, custom 3” exhaust, no o2s (yet, bought without them), adj. FPR, around 38 psi. I’ve checked for vacuum leaks, done voltage drops, etc.. , the car will idle fine for a minute or so, surge to 3-4K rpm then drop down and idle and this will continue. Is there anyone else that has ever gone through this exact problem before? I’m new to the site and have been scouring the forums but can’t find a problem similar to mine. I also have done the surging idle checklist. Any help would be greatly appreciated! Also I forgot to add I have an underdrive pulley and the alternator is on the way out I believe because while running battery voltage (optima less than 1 year old) doesn’t go above 12.7 mark at idle.
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
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First thing loose the under drive pulleys or find the right pulley for the alternator to speed it up, get the charging system right first then we can move on to the surging idle if still needed.
Oh, and install 02s
 
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RioRed95Cobra

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Apr 7, 2017
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I have a 95 mustang gt 5.0 with stock bottom end, gt40p heads 1.6 roller rockers, comp xe264hr cam, 75mm throttle body, 30 lb injectors, MAF calibrated for the injectors, BBK cold air intake, Trick flow upper and lower, pacesetter long tube headers, custom 3” exhaust, no o2s (yet, bought without them), adj. FPR, around 38 psi. I’ve checked for vacuum leaks, done voltage drops, etc.. , the car will idle fine for a minute or so, surge to 3-4K rpm then drop down and idle and this will continue. Is there anyone else that has ever gone through this exact problem before? I’m new to the site and have been scouring the forums but can’t find a problem similar to mine. I also have done the surging idle checklist. Any help would be greatly appreciated! Also I forgot to add I have an underdrive pulley and the alternator is on the way out I believe because while running battery voltage (optima less than 1 year old) doesn’t go above 12.7 mark at idle.

Your t.body is too big. So is your exhaust.

Def. fix the alternator.. if you're gonna run the UD pullies get the higher amp one.

Hell my car is built and I don't run that large of a t.body or exhaust.. you are doint more harm than good.

Wait.. there aren't any 02 sensors in the exhaust? Welcome to your computer not knowing wtf is happening. No :poo: it runs like hell. lmfao. Dude....
 
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Dom5.0

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Jul 25, 2019
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Your t.body is too big. So is your exhaust.

Def. fix the alternator.. if you're gonna run the UD pullies get the higher amp one.

Hell my car is built and I don't run that large of a t.body or exhaust.. you are doint more harm than good.

Wait.. there aren't any 02 sensors in the exhaust? Welcome to your computer not knowing wtf is happening. No :poo: it runs like hell. lmfao. Dude....


Well the throttle body and exhaust will help in the future.. we never stop building right? I ordered a stock crank pulley (under drive was on it when I got it) and o2 harness and o2s (got it without either) I should get everything sometime this week and hopefully have it running better so I can get it tuned.
 
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RioRed95Cobra

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Well the throttle body and exhaust will help in the future.. we never stop building right? I ordered a stock crank pulley (under drive was on it when I got it) and o2 harness and o2s (got it without either) I should get everything sometime this week and hopefully have it running better so I can get it tuned.

Um.. it'll be great.. when your motor makes 900 hp. Until then, you're literally hurting your car and performance.. you need smaller stuff. Bro my car is a fully built race ready 331 stroker with 14 lbs of vortech and I don't use the stuff you are.. lol.. 70mm and a 2.5 is fine... by adding so much air to your car you're making it harder for your computer and maf to add enough fuel.. it's trimming wayyy out to adjust.. you simply don't need that much airflow for an un-supercharged/turbo/etc. car.. it's hurting it.. period. My car literally made over 600 rwhp with a 70mm t.body.. you don't need to go above that or you will lose idle and drivability.. for obvious reasons...
 

Dom5.0

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Jul 25, 2019
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UPDATE: I installed stock size crank pulley and water pump pulley, ordered a new o2 harness and o2 sensors and installed those. It’s still surging, I disconnected the iac and still surging. Did a smoke test, found 0 vacuum leaks. I’m currently at a loss right now. Any suggestions?
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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First, reset the computer, do the disconnect the battery thing that erases all the crap the computer has stored. Then take it for a drive, 30 minutes or an hour so it will experience a variety of rpms and steady running situations. Then run the codes.
i would replace the 30's with 24s and a matching maf for now but see how it does with the reset first. You may have to adjust the idle. Do you know the procedure for that?
 

Dom5.0

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Jul 25, 2019
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First, reset the computer, do the disconnect the battery thing that erases all the crap the computer has stored. Then take it for a drive, 30 minutes or an hour so it will experience a variety of rpms and steady running situations. Then run the codes.
i would replace the 30's with 24s and a matching maf for now but see how it does with the reset first. You may have to adjust the idle. Do you know the procedure for that?

Yes I know the base idle reset procedure, I also set my TPS voltage, I saw that it was out of range.. I just disconnected the battery so I’m going to let it sit for a bit to reset, I don’t have or know anybody with 24s and the air meter, i figure if i spent the money swapping those out I’d be halfway to a dyno tune and I know they can adjust or those there.. I will report back.. thanks for the consistent help!
 

Dom5.0

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Jul 25, 2019
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So the mustang has been blocked in so haven’t been able to take it on the test drive but I doubt I would even want to, one minute it’s idling good the next it’s not. I don’t know what else to do. I’ve tried disconnecting the iac as well and it will still surge, even while the car is in neutral it will still idle high sometimes. It’s like the computer just does what it wants whenever it’s wants. I’m leaning to just getting it towed to the dyno shop after getting a date. Or should I try the Bama 4 bank eliminator? Money is tight and with moving,the dyno tune would have to wait, but any opinions on bama? Seems really hit or miss..
 

jozsefsz

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I haven't the foggiest how the car can surge with the IAC disconnected unless there's a mechanical problem (as you mention alternator, maybe injectors or fuel pump, maybe an MAF that's marginal or suffering some turbulence where it's mounted). The tune is a bit of a chicken-or-egg situation - you want to make sure you're 100% mechanically solid before you get one, but it may resolve the issues on its own if they're related to the state of tune. Bama's ok but there are just limitations to what they can do - they look at your build and pretty much take a guess as to what will work and fine-tune a little bit from there if you hassle them a lot. Intermittent or stupid issues are going to be hard to resolve that way.

The gentleman who posted above (@Decipha) is unquestionably an expert at tuning our cars. Someone like that can remotely tune your car extremely effectively compared to Bama canned tunes. I also agree with his assessment that there's something mechanical going on - which is much more easily diagnosed with a Quarterhorse (you can view the signals your alternator / maf / o2's / distributor are all putting out). Heck, you've done a lot of mechanical work to the car, you could even learn to diagnose and tune it yourself if you have the time & patience. If you don't, an expert remote tuner's going to get you better results than Bama, perhaps even better results than a few hours of dyno-tuning where their goal is to get you out of the door asap with 'max power' when you're really looking for great driveability too.
 

95stangbang

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Jul 19, 2019
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i just went threw this with my 95 gt it was some :poo: took me a week to finally figure out that i was lacking fuel pressure. tested the system etc and checked out fine so i ruled the fuel system ou. boy was i wrong even tho everything read correct the fuel pump sock had :poo: in it stopping from strong flow
 

559mat

Member
Aug 13, 2019
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Fresno
Hey I have a 95 5.0 to I had the same thing turned out my distributor was bad something in the inside messed it was surging like crazy happened out of nowhere through a 38 dollar eBay distributor and it worked just fine