Rough idle.


New Member
Aug 25, 2016
Harrisburg Il
I drove my Stang to get gas the other day. When I started it back up I heard what sounded like a miss with rough idle. I limped it home and popped the hood. I heard a ping from driver's side. Got the valve cover off and noticed the number 8 exhaust rocker arm just laying there. Replaced the rocker arm stud. Pushrod looked good. Still have a miss. Maybe a stuck valve?
  • Sponsors(?)


StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
Dublin GA
Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Notice the brass fittings on the picture below – they screw into the spark plug home and then the test gauge screws into them

Your local auto parts store may have a compression tester to rent or loan if you have a credit card. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.

With the engine warmed up, remove all spark plugs and prop the throttle wide open with a plastic screwdriver handle between the throttle butterfly and the throttle housing. Crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 PSI. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

I generally use a big screwdriver handle stuck in the TB between the butterfly and the TB to prop the throttle open. The plastic is soft enough that it won't damage anything and won't get sucked down the intake either.

A heavy duty battery charger (not the trickle type) is a good thing to have if you haven't driven the car lately or if you have any doubts about the battery's health. Leave it connected while you are cranking the engine and it will help keep the starter cranking at a consistent speed from the first cylinder tested to the last cylinder.
Thread starter Similar threads Forum Replies Date
93silverlx50 Engine Car in limp mode, fuel spitting out of tailpipes, missing rough idle Fox 5.0 Mustang Tech 9
C Engine Gray smoke from exhaust. Smells like gas. Fox 5.0 Mustang Tech 15
R Engine My 95 gt idling rough 1994 - 1995 Specific Tech 5
MSTNGGRL1995 Rough, loping idle and random attempts to die 1994 - 1995 Specific Tech 1
K Engine New Built Engine - Rough Start and Idle Surge Fox 5.0 Mustang Tech 3
E 2010 Mustang GT - randon, intermittent, fresh hard starts with rough idle. 2005 - 2014 S-197 Mustang -General/Talk- 0
B Cold Start Rough Idle Fox 5.0 Mustang Tech 9
Xenogenesis Engine Idles rough, misfires at idle, smells rich, but codes say it's lean on both banks SN95 V6 Mustang Tech 9
S Installed new intake and now car idles rough and turns off 1996 - 2004 SN95 Mustang -General/Talk- 7
T Engine 1994 mustang gt Rough idle, misfire under acceleration 1994 - 1995 Specific Tech 2
evintho Engine '93 running rough, especially at low rpm's and idle. Fox 5.0 Mustang Tech 1
D 1994 V6 rough idle after head rebuild 1994 - 1995 Specific Tech 0
J 04 GT New intake manifold and car now has a rough idle 1996 - 2004 SN95 Mustang -General/Talk- 5
P Just arrived with a 1965 289 and a 1973 Mach I - and a vacuum leak solved The Welcome Wagon 1
0 2003 Ford Mustang V6 3.8L gets around 12mpg and seems to shake a bit when started cold and has a bad rough idle when warm SN95 V6 Mustang Tech 5
L Engine Rough Idle at Start, Possible CMCV Vacuum Leak 2015+ Specific Tech 0
N Engine Rough idle Fox 5.0 Mustang Tech 1
P 1995 GT 302 5.0 with rough idle and weird behavior (with video) 1994 - 1995 Specific Tech 1
D Engine Vacuum problem Fox 5.0 Mustang Tech 14
COramprat 2002 GT Intermettant rough idle and shut down SN95 4.6L Mustang Tech 4
S Rough idle fixed! 2005 - 2014 S-197 Mustang -General/Talk- 2
L 88 gt rough idle smell of fuel white smoke out of exhaust Fox 5.0 Mustang Tech 13
8 Rough idle and Steering Column vibration at idle and start. Fox 5.0 Mustang Tech 32
H Mustang running rough 1996 - 2004 SN95 Mustang -General/Talk- 3
D 98 Rough idle and ride after motor and tranny mounts SN95 V6 Mustang Tech 2
C 2003 3.8 rough idle and shake 1996 - 2004 SN95 Mustang -General/Talk- 1
M Engine Foxbody 5.0L idling rough/backfires after HCI swap Fox 5.0 Mustang Tech 3
9 Help with random rough idle 1994 - 1995 Specific Tech 5
J rough idle and very slight miss SVT Tech Forum 2
A 98 GT 4.6L SOHC Rough Idle/ Misfire (Vacuum Leak) Help Please. 1996 - 2004 SN95 Mustang -General/Talk- 4
P Engine 2002 mustang V6 rough idle in gear at low RPMs SN95 V6 Mustang Tech 2
9 Need Help with a Rough Idle/Coolant Loss Problem SN95 V6 Mustang Tech 3
I 96 Mustang V6 Help SN95 V6 Mustang Tech 0
F Engine Need Help Tracking A Rough Idle On 2003 V6, Not Iac Valve. SN95 V6 Mustang Tech 2
C Rough Idle Runs Rich No Codes Fox 5.0 Mustang Tech 10
S 99 V6 Mustang Loss Power Rough Idle And Drive SN95 V6 Mustang Tech 0
Whitecobrafox13 1990 Cobra Idling Erratically Fox 5.0 Mustang Tech 32
T 89 5.0l Iac Valve Operation Fox 5.0 Mustang Tech 7
T Running Rich, Rough Idle. Carb Swap Fox 5.0 Mustang Tech 2
1 92 Gt Rough Idle On Cold Starts (long) Fox 5.0 Mustang Tech 2
PJalbanesi Rough Idling Fox 5.0 Mustang Tech 9
G 2002 Mustang V6 3.8l Rough Throttle From Idle SN95 V6 Mustang Tech 0
PJalbanesi Idle Surging 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 15
BOBCAT TECH Rough Idle Fox 5.0 Mustang Tech 24
J Please Help Can Not Figure Out Electrical Problem 1996 - 2004 SN95 Mustang -General/Talk- 1
P 1994 Mustang Gt Rough Idle, Major Power Loss 1994 - 1995 Specific Tech 25
S Rough Idle 2010 - 2014 Specific Tech 2
S Rough Idle While Cold 1994 - 1995 Specific Tech 3
D 1991 Gt Foxbody Rough Idle/stalls 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 8
sen2two Rough Idle After Head Swap 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 7
Similar threads