• Welcome Visitor! Please take a few seconds and Register for our forum. Even if you don't want to post, you can still 'Like' and react to posts.

95 4.0 Rough and random idle


mxboy349

New Member
Joined
May 8, 2012
Messages
18
Reaction score
1
Points
3
Vehicle Year
1995
Make / Model
Ford
Transmission
Manual
I have a 4.0 that idles high, 1100-1200ish rpm, then will drop and rough idle 800-900rpm where it will almost die. I have replaced all vac lines with new, new plugs and wires, new Idle control valve, no dash lights. Where should I look next? Truck has plenty of power, no real hesitation, just rough and random idle when at a stop light or left sitting while idling. One thing I have noticed is my temp gauge doesn't seem to get very warm and I am suspecting a stuck thermostat. Would this cause a random idle issue?
 


franklin2

Well-Known Member
Joined
Dec 24, 2019
Messages
3,354
Reaction score
1,712
Points
113
Location
Virginia
Vehicle Year
1984
Make / Model
Bronco II
Transmission
Manual
Some of the 4.0's had a chronic intake gasket problem. There might even be a old TSB out on it.
 

RonD

Official TRS AI
TRS Technical Advisor
Joined
Jun 2, 2012
Messages
25,292
Reaction score
8,298
Points
113
Location
canada
Vehicle Year
1994
Make / Model
Ford
Transmission
Manual
All gasoline engines need rich mix and higher idle when cold, called Choke mode in fuel injection systems
This is done based on coolant temp
So yes, if your thermostat is stuck open and coolant is not warming up and staying up, 190degF, then computer could be changing mix richer and setting idle too high

Computer doesn't use the dash board temp gauge/sender for coolant temp, that's a separate 12volt system
Computer uses the ECT sensor, a 2 wire 5volt sensor, which can also act up but wouldn't effect the dash temp gauge

Just as a test
Warm up engine or after getting home don't shut off engine
Let it idle
Unplug the 2 wire connector on IAC Valve
RPMs should drop to 500 or so, or engine my even stall, either is GOOD, it means no air leaks, test is done

If RPMs do not drop then there are leaks, and IAC Valve/computer can not set lower idle RPMs because of that
Leave IAC Valve unplugged
Pull off vacuum hoses from intake and plug intake port with finger, repeat with next hose, ect.......RPMs should drop if a leaky system is found
Start with power brake booster and PCV Valve hoses
 
Last edited:

mxboy349

New Member
Joined
May 8, 2012
Messages
18
Reaction score
1
Points
3
Vehicle Year
1995
Make / Model
Ford
Transmission
Manual
Unplugged the IAC when I got home after a 20 minute stop and go drive home. Engine was well warm. The idle definitely changed and was rough, but not down to 500, when I plugged back in the idle smoothed out. I repeated the unplug again after a minute or two and this time it died. Temperature gauge was all the way down to the first mark above the C and on the way home the temperature gauge never really got "warm". I think I am going to try new thermostat along with some new belts and hoses. Can't hurt even if it doesn't fix the issue.
 

RonD

Official TRS AI
TRS Technical Advisor
Joined
Jun 2, 2012
Messages
25,292
Reaction score
8,298
Points
113
Location
canada
Vehicle Year
1994
Make / Model
Ford
Transmission
Manual
So no Vacuum leak

Yes, new thermostat, dash temp gauge has 200degF as center, operating temp is 190deg, so just below 1/2 is what to expect
Just above 1/2 on a long uphill grade or pulling a load

Correct thermostat is 190-195degF, and should have a "jiggle valve" and yes that is its actual name, lol, its a small hole in the thermostat plate to let air out from the engine side, it has a loose metal pin in it that will keep the hole clean of debris, because it "jiggles"
When installing the thermostat the jiggle valve hole should be at 12:00, where trapped air would be
 
Last edited:

mxboy349

New Member
Joined
May 8, 2012
Messages
18
Reaction score
1
Points
3
Vehicle Year
1995
Make / Model
Ford
Transmission
Manual
Well thermostat didn't solve the issue, but it didn't hurt to replace everything as hoses were pretty soft. I am pretty sure heater hoses were original. Still idles rough and randomly drops to 500rpm and almost dies. I am going to do some vac line checking and EGR system checking next.
 

RonD

Official TRS AI
TRS Technical Advisor
Joined
Jun 2, 2012
Messages
25,292
Reaction score
8,298
Points
113
Location
canada
Vehicle Year
1994
Make / Model
Ford
Transmission
Manual
Start with power brake booster and PCV valve, for vacuum leaks
 

mxboy349

New Member
Joined
May 8, 2012
Messages
18
Reaction score
1
Points
3
Vehicle Year
1995
Make / Model
Ford
Transmission
Manual
Well here is where I am at with this.

Still running rough at idle, likes to almost die, smells like it's running rich at idle. I have replaced the following

Temp Sensor - the 2 wire one.
IAC is new
MAF Sensor
Checked for vacuum leaks just about everywhere
New Plugs, Wires.
I have done the "Flood" Test and it doesn't start after so no leaking fuel injectors.

EGR or Fuel Pressure?
 

Sponsored Ad


Sponsored Ad

Staff online

Today's birthdays

Member & Vendor Upgrades

For a small yearly donation, you can support this forum and receive a 'Supporting Member' banner, or become a 'Supporting Vendor' and promote your products here. Click the banner to find out how.

Truck of The Month


Kirby N.
March Truck of The Month

Recently Featured

Want to see your truck here? Share your photos and details in the forum.

Follow TRS On Instagram

TRS Events

25th Anniversary Sponsors

Check Out The TRS Store


Sponsored Ad


Sponsored Ad

Sponsored Ad


Amazon Deals

Top