Engine Very rough idle, stalls when clutch pushed in when stopping

Black_Hammer

New Member
Jun 17, 2021
13
0
1
61
Myrtle Beach, SC
1988 GT. Cleaned EGR, new fuel filter, set TPS TO .96 vdc, timing set to 12deg BTDC, smoke tested no vacuum leaks.
please listen to video you will see the tach jump all over and ultimately I had to kill the key when it approached 3grand on its own.
This car has not been converted to MAF yet it is still SD set up.. is this possibly the TPS? I don’t want to throw parts at it but will if I need to. I am waiting on an Innova 3145 to pull codes.

thanks, Hammer
 

90sickfox

Wasn't a pretty sight...and I've got big hands
SN Certified Technician
Mar 2, 2015
6,406
5,484
203
44
The idle air control valve controls the rpm. kinda. You can try starting the car without the IAC on the front of throttle body connected. Rpm should stay down. This won't allow the car to go into warm up mode where the rpms start out higher and decrease as the car warms up. Might have to give a little throttle to get it to start and keep it running until warm. This won't help diagnose. Just help getting the car to run without high idle. The IAC is designed to compensate based on sensor imputs to the computer. Several things could be causing your issue.

Make sure the timing and idle are set properly with spout out.

In order to post a video here you have to upload it to YT. Then, click share on YT and copy the link. In the post box paste the link.
 

Black_Hammer

New Member
Jun 17, 2021
13
0
1
61
Myrtle Beach, SC
Hey thanks for the reply. The motor is stock. Don’t laugh I didn’t know it was required to remove spout and unplug IAC to set the idle. Learn something every day.
Would that be why when I attempt to set the idle I cannot get it below 1,000rpm? This is my first speed density Mustang. I will move it to MAF early next year. I used to set my other Mustang’s to about 800rpm for idle. I really appreciate ur help with this.

Hammer
 

KRUISR

5 Year Member
Apr 16, 2015
1,357
602
133
52
My car is MAF not SD. I set my idle (with IAC and spout disconnected) at 650-700. I have a few mods so your stock setup should respond better. Attached is a link to a youtube video of a recent cold start and idle (I have shared the link elsewhere on this site as well).


View: https://www.youtube.com/watch?v=qU6MAVZAcQU


Obviously maintenance items like plugs, wires, vacuum lines and such need to be in good condition. I have my timing set at 10* as well.
 

90sickfox

Wasn't a pretty sight...and I've got big hands
SN Certified Technician
Mar 2, 2015
6,406
5,484
203
44
Make sure the wires are in the correct order. There are two firing orders...one is 5.0 the other is the mustang 5.0 HO.

That car is running kinda bad man. Something that hasn't been mentioned are the salt and pepper 10 pin connectors behind the upper intake. I always take them apart ( carefully ) and clean them with electrical parts cleaner, take a small screw driver and close up the female ends a little, and apply dielectric grease to them. I had an issue with one of my cars running rough. Found out it was the 10 pin connectors causing my issue. With the car running you can sometimes wiggle them to check if it clears up.
 

Black_Hammer

New Member
Jun 17, 2021
13
0
1
61
Myrtle Beach, SC
Good idea and thanks for replying. I’ll take a look at them this weekend.
I also ran the code reader and got three codes.
21 engine coolant temp because the car hadn’t warmed up.
44 thermactor most likely because all the smog stuff was removed prior to me buying.
94 air diverter circuit fault. Not sure what this one means

Hammer.
 

KRUISR

5 Year Member
Apr 16, 2015
1,357
602
133
52
All 5.0 engines in the Mustang from 83-95 were 5.0HO's from the factory. There was not a non-HO option in the Mustang during that time. Other vehicle lines had the non-HO.

One way to tell without complete teardown of the engine would be to pull the valve covers and spark plugs, manually rotate the engine and watch the valves open/close to determine the firing order.
 

KRUISR

5 Year Member
Apr 16, 2015
1,357
602
133
52
302 HO and 351 are the same firing order - 13726548

302 non-HO - 15426378

If your engine is still the factory cam/engine it should be the first firing order.

Like I said above, check by watching the valves or at least by checking by "feeling" the firing order.

Pull all the spark plugs. Put your finger over the spark plug hole for #1 and rotate the engine by hand. When you feel the pressure from the cylinder it is on compression stroke and will want to fire once pressure on your finger stops. Now stop here for a second.

**Side bar thought and explanation - All cylinders in a 4 cycle V8 will fire within 2 rotations of the engine. That means a cylinder will fire every 1/4 rotation.**

Back to the test - we have just hit end of compression stroke on cylinder #1. For the next 1/4 turn of the crank either cylinder 3 or cylinder 5 will be on its compression stroke (pressure against your finger if it is covering spark plug hole). Pick one, cover the spark plug hole and then rotate the crank 90 degrees (1/4 turn). If that cylinder wasn't under compression, rotate engine back around to #1 compression stroke and pick the other. One of the two should be on compression stroke. Which ever one it is, that is likely your cam type. Check the rest of the firing order 1/4 turn at a time just to be sure.

This should solve the question of which cam type your engine has.
 
  • Useful
Reactions: 1 user

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,513
2,851
234
76
Dublin GA
lowendmac.com
@Black_Hammer

You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 250,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
 
  • Useful
Reactions: 1 user