Hard start and shaky idle

TYYYYLER · Apr 18, 2015

Hello!
This is my first post on here, and im having some small issues with my truck. Its the v6 4.0 xlt 2wd automatic.

Some background stuff:
It used to have a bad missfire till i changed the plugs and wires about 2k miles ago. Now im getting somewhat the same problem I had with the bad wires, only not nearly as bad as before.

Lately its been difficult to start. It takes a couple seconds of turning over till it starts, and sometimes struggles to get up to idle rpm. If i give it gas while starting it kind of helps. Once warmed up it starts fine for the rest of the day. This seems to be an on and off problem, Sometimes it will do it for a couple weeks, and sometimes this problem will completely go away for a couple weeks.

The second issue is, when started, the truck shakes a bit almost like its misfiring. Once I pull out of the driveway and hit about 15 mph the shaking goes away and doesn't come back until another cold start.
Any help would be appreciated!

RonD · Apr 18, 2015

Fuel injected engines can't use a Choke Plate like a carburetor did, no jets to suck extra fuel out of.

But a cold engine still needs to run Rich until it warms up, or it will struggle to run.
The solution was to "tell the computer" what the engine temperature is.
Engines always had a temp sender(1 wire) for the dash board gauge, but sharing that could lead to problems so they added a temp SENSOR(2 wires) just for the computer to use.
Called it the ECT(engine coolant temp) sensor.
If this was bad, i.e. shows engine is warmed up all the time, then cold engine would run poorly.
They are easy to test with a volt or ohm meter but usually only cost $7-$9.
Make sure you are working on the TWO WIRE temp sensor, it will be near the Thermostat housing on intake, along with the 1 wire sender.

Just as a simple test, when you start a fuel injected engine, it should go like this:
Key on
Computer starts and opens IAC(idle air control) Valve all the way(Fuel injected engines also can't use an Idle Screw for the same reason, no jets.)
Computer reads ECT sensor
You crank the engine to start
Engine fires
RPMs go up to above 1,500 because IAC Valve is open all the way
Computer starts closing IAC Valve to match engine temp, 1,100rpm idle if cold, 700 if warm
If cold computer runs injectors rich, and advances spark timing.
As engine warms up idle slowly drops until target warm idle is set

If your engine doesn't start this way then ECT could be bad, maybe IAC valve sticking as well.

TYYYYLER · Apr 19, 2015

RonD said:
Fuel injected engines can't use a Choke Plate like a carburetor did, no jets to suck extra fuel out of.

But a cold engine still needs to run Rich until it warms up, or it will struggle to run.
The solution was to "tell the computer" what the engine temperature is.
Engines always had a temp sender(1 wire) for the dash board gauge, but sharing that could lead to problems so they added a temp SENSOR(2 wires) just for the computer to use.
Called it the ECT(engine coolant temp) sensor.
If this was bad, i.e. shows engine is warmed up all the time, then cold engine would run poorly.
They are easy to test with a volt or ohm meter but usually only cost $7-$9.
Make sure you are working on the TWO WIRE temp sensor, it will be near the Thermostat housing on intake, along with the 1 wire sender.

Just as a simple test, when you start a fuel injected engine, it should go like this:
Key on
Computer starts and opens IAC(idle air control) Valve all the way(Fuel injected engines also can't use an Idle Screw for the same reason, no jets.)
Computer reads ECT sensor
You crank the engine to start
Engine fires
RPMs go up to above 1,500 because IAC Valve is open all the way
Computer starts closing IAC Valve to match engine temp, 1,100rpm idle if cold, 700 if warm
If cold computer runs injectors rich, and advances spark timing.
As engine warms up idle slowly drops until target warm idle is set

If your engine doesn't start this way then ECT could be bad, maybe IAC valve sticking as well.

Thanks for the in depth explanation, makes sense. Ill check both of those out.

RonD · Apr 19, 2015

You're welcome.

And you should read this thread about Ford waste spark system and spark plugs, spark plugs on 1 bank will wear out faster depending on what spark plug you buy, I know you just got a new set, this is for next time

Here: http://www.therangerstation.com/forums/showthread.php?t=160371

Spark plugs ends also can tell you alot about whats going on inside the cylinder, like why you get a misfire cold or after warmed up, or only under load.

Hard start and shaky idle

TYYYYLER

New Member

RonD

Official TRS AI

TYYYYLER

New Member

RonD

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