Rough idle when engine's cold.

[Rangers n' Hoes]

Hey guys, been having a bit of trouble with a really rough idle lately. Typically I'll start it up and it will idle a few seconds then cut out. After starting it back up it will fluctuate between running normal and sounding like it's about to cut out every few seconds or so. If I drive it around it will often cut out when slowing Down.

HOWEVER once the engine has been running a while (10 min or so) it eventually returns to normal and will stay that way the rest of the day.

I've changed the plugs and wires but nothing changed
Someone suggested I may have a vacuum leak but I dont think that would make sense if my truck runs fine after warming up

Any thoughts or suggestions? Thanks

 

[killj0y]

Hey guys, been having a bit of trouble with a really rough idle lately. Typically I'll start it up and it will idle a few seconds then cut out. After starting it back up it will fluctuate between running normal and sounding like it's about to cut out every few seconds or so. If I drive it around it will often cut out when slowing Down.

HOWEVER once the engine has been running a while (10 min or so) it eventually returns to normal and will stay that way the rest of the day.

I've changed the plugs and wires but nothing changed
Someone suggested I may have a vacuum leak but I dont think that would make sense if my truck runs fine after warming up

Any thoughts or suggestions? Thanks

Need more info man, what year and engine, any other problems? Anything changed out recently, DD or was it parked? Etc...

 

[tmcalavy]

Check all the hoses for a vac leak, especially the air intake hose on the bottom side just back of where it clamps onto the air intake...mine had a hole big as your little finger underneath on the bellows hose and you couldn't see it from up top. Ran rough cold until it warmed up, then it would smooth out and only act rough every now and then. Replaced the air intake hose and it's smooth as a baby's ear at idle now.

 

[Earl43P]

Let's rule out fuel pressure bleeding down.

Before each cold start, turn the key to Run, listen for the fuel pump to hum for about 2 sec. Turn key back to Off.
Repeat, repeat, THEN crank over the starter.

See if that has any effect and report back.

 

[RonD]

Hey guys, been having a bit of trouble with a really rough idle lately. Typically I'll start it up and it will idle a few seconds then cut out. After starting it back up it will fluctuate between running normal and sounding like it's about to cut out every few seconds or so. If I drive it around it will often cut out when slowing Down.

HOWEVER once the engine has been running a while (10 min or so) it eventually returns to normal and will stay that way the rest of the day.

I've changed the plugs and wires but nothing changed
Someone suggested I may have a vacuum leak but I dont think that would make sense if my truck runs fine after warming up

Any thoughts or suggestions? Thanks

Fuel injected engines use an electronic "choke", the computer controls this.
When engine is cold computer will run injectors rich, raise the idle and advance the timing.
So to tell if computer is doing it's part, your idle should go up to 1,500rpm when first started.
Then drop to 1,000rpm after a few seconds.
Then it will slowly drop rpms down to 750rpm(normal warm engine idle) as engine warms up.

If this isn't happening then there could be a few things that could cause this.
The IAC(idle air control) valve is how the computer sets the idle, it is located on the upper intake.
These get dirty and computer can't set idle correctly, they can be removed and clean.
If this valve is stuck or dirty then cold idle would be iffy, but as engine warmed up it would start to idle fine.

The ECT(engine coolant temp) sensor is what tells the computer the engine is cold or warm, this is a TWO wire sensor located near the thermostat housing, this is only used by the computer.
The dash board temp gauge uses a ONE wire sender, usually located near the ECT sensor.
The ECT sensors rarely fail, not never fail but rarely.
These are simple resistance sensors, so can be tested with OHM meter.

IAT(intake air temp) sensor, this tells the computer the temperature of the air coming in to the engine, colder air is denser so requires more fuel in the mix, on the '94 this sensor is on the upper intake, in later years it was combined with the MAF sensor(6 wires).
I would expect a problem with the IAT sensor to show up even after engine was warm.

MAF(mass air flow) sensors need to be cleaned every few years, it is easy to do and will improve mileage and performance.
While I don't think this would cause cold engine issue, it could, and is simple to clean.
Click here:
http://www.therangerstation.com/tech_library/CleanMAF.shtml

 

[Rangers n' Hoes]

Alright guys, so while i was checking for vacuum leaks, i discovered a cluster of wires was unplugged (see picture)

not sure what that particular part does (excuse my inexperience but I'm kinda just learning as i go here) but i plugged it in and started it up and it SEEMS to be running fine, but I'm skeptical. anyone know if that would make sense as being the problem?

as for general info about the truck:

1994 ranger 4.0 v6 OHV automatic
daily driver
i keep up on basic maintenance (oil, brakes, etc.)

Recently replaced:
starter/ starter solenoid (2 months ago)
mass air flow sensor (1 week ago)
plugs and wires (3 days ago)

 

[RonD]

Looks like the cruise control actuator, it uses vacuum to pull and hold the throttle open when cruise control is on.

 

[Rangers n' Hoes]

I see... Well maybe that's why the cruise control hasn't worked since I bought the truck 7 years ago

I drove it around a little more and I could tell nothing had changed. It still has a bit of a tremble. I'll do a more thorough check of the vacuum lines and report back tomorrow

 

[Rangers n' Hoes]

Hey guys! Sou did a check of the vacuum hoses and look what I found. Massive hole on the bottom of the air intake hose.

Thanks to tmcalavy for suggesting that and thanks to everyone else. I ordered a new one on eBay and used a whole bunch of tape to patch it up in the meantime.

Thanks y'all

 

[tmcalavy]

Yeah...this is so obvious it's stupid...and I'm pretty sure I drove my Ranger around for about a year before I found the same damn thing on mine. Only clue I had really was the lopey idle when cold...after it warmed up it was relatively smooth running. Mtnrgr helped me out with a new/used air intake off a 94 that he had layin around. The day I discovered the stupid, but invisible air leak I also pulled the MAF and cleaned it real good. When I filled up today I had run 296 miles on 17 gallons so the old green beast is gettin 17+ mpg in mostly in-town driving. I get to do a road trip at the end of the month, so I'll be able to check road-speed mpg then. Hope you didn't get raped too bad on Schmebay...when I looked they wanted $110+ for a new air intake hose.

 

[tmcalavy]

Was pokin' around under the hood again today and found that the accordion pipe that connects the air funnel right behind the grille to the air box mouth is just as rotted as the old air intake tube....Guess what's next Rangers-n-Hoes? I duct-taped mine...gonna have to do.

 

[Rangers n' Hoes]

I actually checked that one too. Mine was still in pretty decent shape, no holes that I could find. But that air intake was very brittle at the end near where it clamps on. Ended up paying 120 total for the new one. Couldn't find anything cheaper online. I'd rather buy a new one anyways just to prevent getting another hole anytime soon

 

[Mark_88]

If you haven't done so already...I'd suggest giving tmcalavy rep...just click on the icon that looks like a weigh scale at the bottom of HIS post and it will bring up a box...make sure to select the top bullet beside "I approve" and then you can write a comment if desired...

I've been given negative rep by a few people I've helped over the years and...well, my discount at Wal-Mart was denied because of it...lol

 

[Rangers n' Hoes]

If you haven't done so already...I'd suggest giving tmcalavy rep...just click on the icon that looks like a weigh scale at the bottom of HIS post and it will bring up a box...make sure to select the top bullet beside "I approve" and then you can write a comment if desired...

I've been given negative rep by a few people I've helped over the years and...well, my discount at Wal-Mart was denied because of it...lol

ah ive been wondering how the rep system works. ill make sure to do that from now on!

 

[Mark_88]

ah ive been wondering how the rep system works. ill make sure to do that from now on!

It's not mandatory or meaningful other than to say thanks in a small way...just bumps your reputation...