Surging Idle- 94 Cobra

What is causing the surge?

  • Vacuum Leak

    Votes: 2 66.7%
  • Idle Air Control

    Votes: 0 0.0%
  • MAF

    Votes: 0 0.0%
  • Tune

    Votes: 1 33.3%
  • Owner (me) did something stupid

    Votes: 0 0.0%
  • Something Else

    Votes: 0 0.0%

  • Total voters
    3

oilspill

Member
Joined
Nov 20, 2012
Messages
200
Reaction score
227
Location
Atlanta, Ga
If you didn't ignore this because of the title this is a little bit different than the 1000 posts asking you to fix my surging idle. I am past that point and have set up an appointment for Kurgan Motorsports to fix it but for fun wanted to see what everyone thinks they are going to find.

The car- 94 Cobra. Stock cam and bottom end. Trick Flow heads. Kenne Bell. Meth injection. Tuned by kurgan.

What i have done chasing this:
  • Replaced ACT
  • Replaced ECT
  • Replaced O2 sensors
  • Replaced MAF
  • Replaced Throttle body
  • Smoke checked for vacuum leaks (shitty amazon machine)
  • Base idle reset 999 times
  • Checked fuel pressure
  • Replaced TPS
  • Replaced IAC (twice)
  • Replace TFI Module
  • Checked Grounds and added a ground
 

white95

Apex Junky
Admin
Joined
Dec 30, 2007
Messages
20,655
Reaction score
8,889
Location
Prairieville, LA
Possibly a tuning scenario. Mine idle surges when the weather is cool but not when it’s warm. Has something to do with fuel enrichment. I know it’s comparing apples to oranges but idle is idle.
 
OP
OP
oilspill

oilspill

Member
Joined
Nov 20, 2012
Messages
200
Reaction score
227
Location
Atlanta, Ga
Possibly a tuning scenario. Mine idle surges when the weather is cool but not when it’s warm. Has something to do with fuel enrichment. I know it’s comparing apples to oranges but idle is idle.
could be, this is what my old man thinks it is. BUT it ran like a top for a few months after the tune.
 

ttocs

Post Whore
Joined
Oct 9, 2009
Messages
33,012
Reaction score
5,785
Location
Evansville Indiana
have the clips that holds any of the harnesses that clips onto the sensors broken off? I know it doesn't seem like a big deal but I have personally seen where they can cause problems on my own car. I had one of the two clips on my TPS that had broken off, but it still slid on tight and would not come off with the one clip so I didn't think anything of it. I was doing a remote tuning session with my holley and my tuner noticed that it was showing %4 throttle when it was at idle and I was not touching the gas. Then we noticed that if I tapped on the tps sensor that it would go up/down %3-5 so we shut down the session till I could get a new sensor. Two weeks later, we are back to work with a new sensor and once the motor got warmed up we again saw it drift. This time I went and grabbed the harness and jiggled it and it drifted even further this time. When it was cold it was tight enough to hold it but as it got hot and expanded then it would start to lose connection and would show on the sensor. Next time it is surging get under the hood and one at at time go through and grab/pull/wiggle/jiggle every harness that you can get your eyes on and see if the motor changes when you grab one. Don't be afraid to get rough with them and give a very solid pull/tug as they are designed to take this and if it comes apart, it needs to be looked at closer.
 
OP
OP
oilspill

oilspill

Member
Joined
Nov 20, 2012
Messages
200
Reaction score
227
Location
Atlanta, Ga
have the clips that holds any of the harnesses that clips onto the sensors broken off? I know it doesn't seem like a big deal but I have personally seen where they can cause problems on my own car. I had one of the two clips on my TPS that had broken off, but it still slid on tight and would not come off with the one clip so I didn't think anything of it. I was doing a remote tuning session with my holley and my tuner noticed that it was showing %4 throttle when it was at idle and I was not touching the gas. Then we noticed that if I tapped on the tps sensor that it would go up/down %3-5 so we shut down the session till I could get a new sensor. Two weeks later, we are back to work with a new sensor and once the motor got warmed up we again saw it drift. This time I went and grabbed the harness and jiggled it and it drifted even further this time. When it was cold it was tight enough to hold it but as it got hot and expanded then it would start to lose connection and would show on the sensor. Next time it is surging get under the hood and one at at time go through and grab/pull/wiggle/jiggle every harness that you can get your eyes on and see if the motor changes when you grab one. Don't be afraid to get rough with them and give a very solid pull/tug as they are designed to take this and if it comes apart, it needs to be looked at closer.
ive tried that as i have seen you recommend this to others but couldn't really find any that helped.
 

tinnocker

Member
Joined
Apr 12, 2021
Messages
153
Reaction score
71
You could try one of these https://lmr.com/item/LRS-9939A/1986-93-Mustang-50L-58L-Idle-Air-Control-Iac-Adjuster . I am like you. I set my idle dozens of times being careful to follow the instructions of different on line remedies and the only one that I could make work was the beer can method, only I used an aluminum tag. I have a 94 GT with cam and aluminum heads and it did not surge after I had it tuned. My problem was the high idle, around 12-1300 rpms. When I brought the idle down, I had the surge. I bought the cam for sound as much as for performance and at that high speed, I could not hear the lope. I used the IAC gasket and cut the aluminum tag out to the shape with matching holes and drilled a 9/32 hole in each side where the larger holes in the gasket were. I had to buy another gasket and sandwich the aluminum plate between them when I installed it. I used one of the online remedies to reset my base idle and the surge was gone and the idle would come down quicker at a red light. Be careful with the online remedies, some are for Fox Mustangs and they have a different computer. I can get my idle down to about 850-900 rpm now without the surge. Any lower and I have the surge again. In warm weather there is no need to warm the engine, the surge is gone. In cold weather it takes about 15/20 seconds and I have to keep my foot on the pedal to keep it running. Then it will run ok. I wish I could get the rpms down to around 600/650 but the computer has a mind of its own. The adjustable piece that LMR is selling was designed by Ford and used to be sold by Ford. Aftermarket jumped on board and I found them online from different companies. The adjustable controller is made to fine tune the holes. I am still not happy with my idle and may order one of the adjustable controllers and see if it will work. Good luck!
 
OP
OP
oilspill

oilspill

Member
Joined
Nov 20, 2012
Messages
200
Reaction score
227
Location
Atlanta, Ga
You could try one of these https://lmr.com/item/LRS-9939A/1986-93-Mustang-50L-58L-Idle-Air-Control-Iac-Adjuster . I am like you. I set my idle dozens of times being careful to follow the instructions of different on line remedies and the only one that I could make work was the beer can method, only I used an aluminum tag. I have a 94 GT with cam and aluminum heads and it did not surge after I had it tuned. My problem was the high idle, around 12-1300 rpms. When I brought the idle down, I had the surge. I bought the cam for sound as much as for performance and at that high speed, I could not hear the lope. I used the IAC gasket and cut the aluminum tag out to the shape with matching holes and drilled a 9/32 hole in each side where the larger holes in the gasket were. I had to buy another gasket and sandwich the aluminum plate between them when I installed it. I used one of the online remedies to reset my base idle and the surge was gone and the idle would come down quicker at a red light. Be careful with the online remedies, some are for Fox Mustangs and they have a different computer. I can get my idle down to about 850-900 rpm now without the surge. Any lower and I have the surge again. In warm weather there is no need to warm the engine, the surge is gone. In cold weather it takes about 15/20 seconds and I have to keep my foot on the pedal to keep it running. Then it will run ok. I wish I could get the rpms down to around 600/650 but the computer has a mind of its own. The adjustable piece that LMR is selling was designed by Ford and used to be sold by Ford. Aftermarket jumped on board and I found them online from different companies. The adjustable controller is made to fine tune the holes. I am still not happy with my idle and may order one of the adjustable controllers and see if it will work. Good luck!
Interesting little contraption there. With all my searching never seen that. At this point I’m done trying things and going to let the shop fix it. If somehow they are stumped I may try it.
 

ttocs

Post Whore
Joined
Oct 9, 2009
Messages
33,012
Reaction score
5,785
Location
Evansville Indiana
Those are good for if when you shift that the idle goes up in between shifts, but I was thinking of surging as something a little different.
 
OP
OP
oilspill

oilspill

Member
Joined
Nov 20, 2012
Messages
200
Reaction score
227
Location
Atlanta, Ga
Those are good for if when you shift that the idle goes up in between shifts, but I was thinking of surging as something a little different.
Mine is certainly something different than that. Up and down 500 or more RPMs and sometimes shuts down if I don’t give it gas. Typically pegged rich on the afr.
 

Musturd

Post Whore
Joined
Feb 12, 2011
Messages
10,615
Reaction score
2,412
Have you checked your plugs at all recently ? Do you have a fuel pressure gauge ?
 

ttocs

Post Whore
Joined
Oct 9, 2009
Messages
33,012
Reaction score
5,785
Location
Evansville Indiana
ok that is what I was thinking when you said surging. Like I said that little gadget is good for when you change out throttle bodies and some designs will make the rpms jump up to 12-1500 rpm while your shifting.

Has it always been surging since the tune was done or is this something new?
 
OP
OP
oilspill

oilspill

Member
Joined
Nov 20, 2012
Messages
200
Reaction score
227
Location
Atlanta, Ga
Have you checked your plugs at all recently ? Do you have a fuel pressure gauge ?
Haven’t checked plugs. They were new with the tune.

I have a gauge that I put on the schrader valve and checked it. Remember the pressure being in line and held after shut down
 
OP
OP
oilspill

oilspill

Member
Joined
Nov 20, 2012
Messages
200
Reaction score
227
Location
Atlanta, Ga
ok that is what I was thinking when you said surging. Like I said that little gadget is good for when you change out throttle bodies and some designs will make the rpms jump up to 12-1500 rpm while your shifting.

Has it always been surging since the tune was done or is this something new?
It started about 2 months after the tune. Been chasing for damn near a year though!
 

dcm0123

New Member
Joined
May 12, 2018
Messages
77
Reaction score
18
This is an alternative to the aluminum tag.
Helps reduce the hunting of the IAC valve when shifting and letting off the accelrator. Start with the small hole and drill out if needed.
It was not getting enough air on a cold start so I drilled it out to increase the idle speed.
Bought on Ebay but I believe also available from some of the online speed shops.
I also learned the idle air bleed / throttle plate opening on the throttle body influences surging as well. Reduce the air flow and see if it helps.

1675302545792.png
 
Last edited:

ttocs

Post Whore
Joined
Oct 9, 2009
Messages
33,012
Reaction score
5,785
Location
Evansville Indiana

This piece that they said to get above is just an adjustable version of what he has shown. As they said they are good for if your rpms jump between shifts or if you hit the clutch but if this is the surging where while idling it revs up a few hundred rpm, then goes down, then back up/down it will not fix that.

Does it do it right from the jump or only when warmed up? I know you said you smoke checked it but I wonder if you got the motor hot and did it if it wouldn't show something that had expanded enough to cause a leak?
 
OP
OP
oilspill

oilspill

Member
Joined
Nov 20, 2012
Messages
200
Reaction score
227
Location
Atlanta, Ga

This piece that they said to get above is just an adjustable version of what he has shown. As they said they are good for if your rpms jump between shifts or if you hit the clutch but if this is the surging where while idling it revs up a few hundred rpm, then goes down, then back up/down it will not fix that.

Does it do it right from the jump or only when warmed up? I know you said you smoke checked it but I wonder if you got the motor hot and did it if it wouldn't show something that had expanded enough to cause a leak?
It’s at its worst when cold started. It gets better sometimes after warming up.

When first cranked a/f pegs rich.

Will be interesting to see what they find, I still think I missed a vacuum leak with the POS smoke machine I bought.
 

tinnocker

Member
Joined
Apr 12, 2021
Messages
153
Reaction score
71
1675302545792-png.23497

This looks exactly like the one or ones that I have made using an aluminum tag. You could use a beer can. The bottom large hole is the same size as the one in the gasket. The top is 9/32". I made one with 2, 9/32 holes but it didn't help the surge as much. I replaced it with one as pictured and the surge lasts lest than a minute on a cold day and then it is gone. I have done a lot of online research and it seems that the surge and high idle can be eliminated on the Fox model (pre-1994) and the surge on the 94/95 ( once warmed up) , but I haven't found anyone able to idle the 1994-1995 computer down to 600-650 rpms if the engine has an aftermarket cam. A good tuner may be able to do it. I can do it until I connect the IAC, then the computer jumps the idle up to around 850-1000 rpms. It sure is an aggravating problem.
 
Last edited:

Forum statistics

Threads
77,669
Messages
1,506,966
Members
15,096
Latest member
CarManTN

Members online

Top