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

Engine 1989 Fox has hesitation (Stumbles) around 3K and under acceleration

  • Thread starter Thread starter tank76
  • Start date Start date Oct 21, 2022
T

tank76

Member
Oct 21, 2022
32
7
18
Warner Robins GA
Oct 21, 2022
#1
  • Oct 21, 2022
  • #1
Hello All,

This is my first post here. Unfortunately it comes with a problem lol.

So I picked up an automatic Fox body with 78K miles a few months ago bone stock down to the smog pump with the intent of going 5 speed and restoring it to its glory. With that, The car seems to have developed an intermittent stumble.

The car idles like a champ and actually drives very good with the above exceptions

When I picked up the car I did the basic tune up things
-- cap rotor
-- Accel coil
-- plugs
--performance wires
-- 140 amp alternator
-- replaced the fuel pump and filter did a fuel systems flush.
Then I added
--BBK CAI
--3:73 Gears (whole real axel seals and bearings replaced also)
-- off road H pipe
-- FLowmasters

This issue has come and gone since I bought the car but seems to be hanging around now.
under acceleration the car stumbles here and there and seems to lack low end grunt.. at times when i take off it seems to struggle at low end almost like the car is missing, then it goes away slightly or completely at cruising speed, but comes back when the car starts rolling again.
when idling i can rev the car and around 3K then needle bounces and seems to hesitate as if it hits a brief wall and then continues (if needed I can upload a video).


I ran a KOER and KOEO test with an OBD1 scanner and all that was bad was the IAC which I replaced and the code went away.

SO at this point I am at a bit of a loss. I am starting to think the TPI module or Distributor may be to blame

But before I start throwing money at it I'd like to know a few things to check. Even though the car is an automatic, i'd like to think it should have more balls then it does LOL

appreciate any insight from the 5.0 gurus
 

General karthief

wonder how much it would cost to ship you a pair
20+ Year Stangneter
Aug 25, 2016
27,935
10,576
203
polk county florida
Oct 21, 2022
#2
  • Oct 21, 2022
  • #2
First thing I'd do is throw that Accel coil in the neighbors yard and find a stock one, then if that doesn't fix it I'd do a sweep test on the TPS.
 
Reactions: 2000xp8
T

tank76

Member
Oct 21, 2022
32
7
18
Warner Robins GA
Oct 21, 2022
#3
  • Oct 21, 2022
  • #3
General karthief said:
First thing I'd do is throw that Accel coil in the neighbors yard and find a stock one, then if that doesn't fix it I'd do a sweep test on the TPS.
Click to expand...
Lol I did swap the original coil back into it and it did the same thing. Just actually did that after I posted and started thinking about it -- I'll look into the TPS that's an easy kill
 

Potomus Pete

I'm still waiting for the fully nude shots
5 Year Member
Mar 7, 2019
3,308
1,223
153
Sarasota Florida
Oct 21, 2022
#4
  • Oct 21, 2022
  • #4
My issues similar to yours was TPS at one time
 
T

tank76

Member
Oct 21, 2022
32
7
18
Warner Robins GA
Oct 21, 2022
#5
  • Oct 21, 2022
  • #5
Good deal thanks gonna def check that it sounds like that may be the culprit
 

90sickfox

Wasn't a pretty sight...and I've got big hands
SN Certified Technician
Mar 2, 2015
6,945
5,816
213
Oct 21, 2022
#6
  • Oct 21, 2022
  • #6
Check fuel pressure. Pull the codes.
 
Reactions: General karthief

Trogdor

5 Year Member
Aug 30, 2018
195
119
53
Anna, TX
Oct 21, 2022
#7
  • Oct 21, 2022
  • #7
Check your timing with the spout in and rev the engine. Does it increase, decrease or stay the same?
 
T

tank76

Member
Oct 21, 2022
32
7
18
Warner Robins GA
Oct 23, 2022
#8
  • Oct 23, 2022
  • #8
Fuel pressure is good pump was replaced along with filter rail pressure is around 35~40 at idle. Koer and koeo tests performed and no codes
 
T

tank76

Member
Oct 21, 2022
32
7
18
Warner Robins GA
Oct 23, 2022
#9
  • Oct 23, 2022
  • #9
Upon testing the TPS the voltage reading at idle was off a bit suppose to be under 1v reading closer to 2v so I have a motor craft one coming. Gonna start there everything else has either been checked or replaced aside from the timing with the spout in. I did advance it to a conservative 13 from 10 when I did a tune up. Engine didn't mind it at all
 

Mustang5L5

That is…until I whipped out my Bissell
Mod Dude
Feb 18, 2001
43,260
17,945
224
Massachusetts
Oct 24, 2022
#10
  • Oct 24, 2022
  • #10
Acceptable TPS voltage range at idle is 0.5 to 1.25 volts. Anything in that range is good.

I'm leaning this being something in the ignition. PIP, TFI or coil

When i had this issue with my car, it ended up being the Accel coil.
 
T

tank76

Member
Oct 21, 2022
32
7
18
Warner Robins GA
Oct 30, 2022
#11
  • Oct 30, 2022
  • #11
Ok, figured I'd update. Changed out the TPS sensor, with an OEM MotorCraft one, ensured it was calibrated to around one volt. This fixed the problem car has balls again. No stumble at 3k low end torque is back. Thanks for all the help.
 
Reactions: Potomus Pete
You must log in or register to reply here.

Similar threads

C
Engine Idle cuts out
  • claypoolxx
  • Mar 7, 2026
  • Fox 5.0 Mustang Tech
  • 2
Replies
25
Views
769
Fox 5.0 Mustang Tech Jul 24, 2026
alutwon
B
Electrical Why does my fox body keep frying eecs
  • brandyn200
  • Jun 20, 2026
  • Fox 5.0 Mustang Tech
Replies
8
Views
264
Fox 5.0 Mustang Tech Jun 25, 2026
AeroCoupe
A
1987 5.0 speed density Foxbody crank no start
  • Alan2204
  • May 23, 2026
  • Fox 5.0 Mustang Tech
Replies
10
Views
313
Fox 5.0 Mustang Tech May 30, 2026
Mcmahst
1
Engine 91 mustang 302 Low acceleration car backfires pops and acts like it doesn’t want to take fuel
  • 1991Fox5.0
  • Oct 4, 2025
  • Fox 5.0 Mustang Tech
Replies
16
Views
1K
Fox 5.0 Mustang Tech Oct 6, 2025
1991Fox5.0
1
8
Engine Rough idle and aggressive sputtering when accelerating
  • 88vertible
  • Aug 13, 2025
  • Fox 5.0 Mustang Tech
Replies
4
Views
1K
Fox 5.0 Mustang Tech Dec 13, 2025
Jarhead67
J
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?