• Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
  • Fox 5.0 Mustang Tech

1990 Cobra Idling Erratically

  • Thread starter Thread starter Whitecobrafox13
  • Start date Start date Oct 29, 2017
  • Tags Tags
    runs rough stumped
Prev
  • 1
  • 2
First Prev 2 of 2

JD1964

there is enough sticking out to grab on to
15 Year Member
Jun 28, 2013
3,427
1,519
194
Maryland
Oct 30, 2017
#21
  • Oct 30, 2017
  • #21
I see in the pictures that you have a breather in the valve cover. There's supposed to be a hose from the valve cover to the throttle body. Instead you have this breather and you have the connection at the throttle body plugged. This can create problems. What you are doing by using the breather is allowing air to be ingested that is not being measured by the MAF. The pcv system takes in enough air to throw things off if it's not included in the MAF measurement. It will mess up A/F mixture.

Loose the valve cover breather and put the hose / pipe back in between the valve cover and throttle body where it belongs. Double and triple check all vacuum lines and connections. Make sure there are no leaks and all connections are tight. Believe it or not, even small leaks can cause problems. But in your case, you already have a big leak. Basically, not having the MAF measuring the pcv air ingestion is a huge leak as far as the ECU is concerned.
 
Reactions: jrichker

Whitecobrafox13

Member
Oct 4, 2017
23
2
13
Tx
Oct 30, 2017
#22
  • Oct 30, 2017
  • #22
John Dirks Jr said:
I see in the pictures that you have a breather in the valve cover. There's supposed to be a hose from the valve cover to the throttle body. Instead you have this breather and you have the connection at the throttle body plugged. This can create problems. What you are doing by using the breather is allowing air to be ingested that is not being measured by the MAF. The pcv system takes in enough air to throw things off if it's not included in the MAF measurement. It will mess up A/F mixture.

Loose the valve cover breather and put the hose / pipe back in between the valve cover and throttle body where it belongs. Double and triple check all vacuum lines and connections. Make sure there are no leaks and all connections are tight. Believe it or not, even small leaks can cause problems. But in your case, you already have a big leak. Basically, not having the MAF measuring the pcv air ingestion is a huge leak as far as the ECU is concerned.
Click to expand...
Will do.
 

Whitecobrafox13

Member
Oct 4, 2017
23
2
13
Tx
Oct 30, 2017
#23
  • Oct 30, 2017
  • #23
Also will be getting rid of this upper and lower tomorrow or the next day and installing my new Holley systemax upper and lower.
 

JD1964

there is enough sticking out to grab on to
15 Year Member
Jun 28, 2013
3,427
1,519
194
Maryland
Oct 30, 2017
#24
  • Oct 30, 2017
  • #24
As for checking fuel pressure, are you checking it with the vacuum line unhooked at the regulator? That's the right way. Set at 39psi with vacuum line disconnected at regulator.
 

Whitecobrafox13

Member
Oct 4, 2017
23
2
13
Tx
Oct 30, 2017
#25
  • Oct 30, 2017
  • #25
John Dirks Jr said:
As for checking fuel pressure, are you checking it with the vacuum line unhooked at the regulator? That's the right way. Set at 39psi with vacuum line disconnected at regulator.
Click to expand...
I was reading it off the gauge in the car. 49 Psi on prime. Regulator still hooked up.
 

JD1964

there is enough sticking out to grab on to
15 Year Member
Jun 28, 2013
3,427
1,519
194
Maryland
Oct 30, 2017
#26
  • Oct 30, 2017
  • #26
Should be 39 psi with engine running and vacuum line disconnected at reg. Obviously reconnect vac line after setting pressure
 

Whitecobrafox13

Member
Oct 4, 2017
23
2
13
Tx
Oct 30, 2017
#27
  • Oct 30, 2017
  • #27
John Dirks Jr said:
Should be 39 psi with engine running and vacuum line disconnected at reg. Obviously reconnect vac line after setting pressure
Click to expand...
Will set Tommorow. Thanks so much
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
27,844
10,520
203
polk county florida
Oct 30, 2017
#28
  • Oct 30, 2017
  • #28
Whitecobrafox13 said:
Also will be getting rid of this upper and lower tomorrow or the next day and installing my new Holley systemax upper and lower.
Click to expand...
I would keep that cobra intake on there at least untill you got the bugs worked out. Most times adding modifications makes the problems harder to fix, besides that intake is better suited to stock engine, even if it is a knock off cobra intake.
 

Mustang5L5

That is…until I whipped out my Bissell
Mod Dude
Feb 18, 2001
43,177
17,879
224
Massachusetts
Oct 30, 2017
#29
  • Oct 30, 2017
  • #29
I agree leave the Cobra on there for now unless you suspect a major vac leak


And Fuel pressure vac off should be 39psi
 
Reactions: General karthief

Blown88GT

Founding Member
Nov 13, 1999
2,282
523
164
Palm Beach Gardens, FL
Nov 4, 2017
#30
  • Nov 4, 2017
  • #30
Why does the title of this thread say 1990 Cobra?
Ford never made a 1990 Cobra, they did make a 1993 Cobra.
Putting a Cobra intake on a Fox body doesn't make it a Cobra.
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
27,844
10,520
203
polk county florida
Nov 4, 2017
#31
  • Nov 4, 2017
  • #31
I think it was the Canadians that started that confusion, my insurance card says my 89 gt is a cobra, had a long discussion with them about it, he also may have been told by the previous owner and may not know better, not a big deal,
I hope he keeps us up on what he finds his fix.
 

Whitecobrafox13

Member
Oct 4, 2017
23
2
13
Tx
Nov 4, 2017
#32
  • Nov 4, 2017
  • #32
Just has all the cobra pieces. Might as well call it one. And since repairing the grounds and new tps and fuel pressure adjusted to 39 Psi line off. She runs like a top. So figured its time to start adding go fast parts
 

Mustang5L5

That is…until I whipped out my Bissell
Mod Dude
Feb 18, 2001
43,177
17,879
224
Massachusetts
Nov 4, 2017
#33
  • Nov 4, 2017
  • #33
Whitecobrafox13 said:
Just has all the cobra pieces. Might as well call it one.
Click to expand...

Except it's not, and I only say this because it may cause issues with troubleshooting. For instance, I see it has 19# injectors, so if you call it a cobra I'm going to assume it has the X3W ECU and matching MAF and that's why it's running like junk as true Cobras run 24# injectors. Also being an AOD to T5 swap, there are various harnesses that need to be changed out to work properly.

So really for the sake of troubleshooting, you can't call it that. For the sake of what it looks like, call it whatever you want
 
Reactions: jrichker
Prev
  • 1
  • 2
First Prev 2 of 2
You must log in or register to reply here.

Similar threads

A
1987 5.0 speed density Foxbody crank no start
  • Alan2204
  • May 23, 2026
  • Fox 5.0 Mustang Tech
Replies
10
Views
152
Fox 5.0 Mustang Tech May 30, 2026
Mcmahst
C
Engine Idle cuts out
  • claypoolxx
  • Mar 7, 2026
  • Fox 5.0 Mustang Tech
  • 2
Replies
22
Views
549
Fox 5.0 Mustang Tech Jun 5, 2026
KRUISR
1990 Mustang with no-start condition - Fixed and how-to
  • TTSaleen
  • Mar 22, 2026
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
Replies
0
Views
102
1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- Mar 22, 2026
TTSaleen
I
1994 Cobra - Fuel Pressure Drops Revving at Idle
  • IconicSilverGT500
  • Apr 11, 2026
  • 1994 - 1995 Specific Tech
Replies
16
Views
366
1994 - 1995 Specific Tech Apr 24, 2026
Mcmahst
Engine Popping noise in the intake. Frustrated still.
  • esteban31065
  • Apr 6, 2025
  • Fox 5.0 Mustang Tech
Replies
15
Views
1K
Fox 5.0 Mustang Tech May 23, 2025
gkomo
Share:
Bluesky Email Share Link
  • Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
  • Fox 5.0 Mustang Tech
Menu
Log in

Register

  • Forums
  • What's new
  • Media
  • Resources
  • Contact
  • Sponsor
X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?

X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?