Truck missing and bucking when cold

cp2295 · Aug 30, 2014

My truck misses and kinda jerks/Bucks when its cold, like a carbureted engine when no choke is on when its cold (acts like there's not enough fuel). Im also getting pretty bad mileage like 14mpg. Is this the coolant temp sensor or the fuel pump? (or maybe something else?)

cp2295 · Sep 1, 2014

I also just got a check engine light, p0174, so I looked at my fuel trims and both long terms are at 25% but when I rev it they Lower to 17 or so percent. Is this indicative of a vacuum leak?

cp2295 · Sep 4, 2014

TxGasRacer · Sep 4, 2014

http://troubleshootmyvehicle.com/ford/4.0L/troubleshooting-P0171-and-P0174-page-1

AgPete139 · Sep 4, 2014

Most Common Causes of P0171 (right side, passenger) & P0174 (left side, driver):

CAUSE 1: PCV Tube's rubber elbow (where it connects to the intake manifold) is torn open and causing a major vacuum leak.

CAUSE 2: Intake manifold gaskets that are leaking vacuum.

CAUSE 3: Fuel pump that's going BAD, but hasn't completely fried yet.

CAUSE 4: A dirty (contaminated) MAF sensor.

CAUSE 5: An exhaust leak right before the pre-catalytic converter oxygen sensors.

CAUSE 6: BAD pre-catalytic converter oxygen sensor.

AgPete139 · Sep 4, 2014

But seriously, check that tube that goes into the intake manifold. My used to pop off, and it gave me that same reading.

The second thing I ALWAYS do on my vehicles is clean the MAF after the air inbox. Do this just for peace of mind anyways, because it's probably dirty and this is easy to do.

The next thing would be to carb spray the passenger side gaskets near the intake manifold.

cp2295 · Sep 4, 2014

I've sprayed carb cleaner all around those manifold gaskets, cleaned my maf twice in the past 6 months, I'll double check the pcv tube for cracks it's not unplugged, I'm gonna change the pump with a Bosch one tomorrow, I don't think the o2 sensors are bad because there voltage fluctuates all the time (0.1-1.0v), I don't think I have an exhaust leak? Doesn't sound like it but I'll check into that.. Doesn't act like a vacuum leak truck idles at 750-800 pretty good.

However my idle takes a long time to lower, and when I sit and idle the idle will start raising to 850-900. Is that a sign of a vacuum leak? Plus the readings I got with the fuel trims..

Thank you for the replies btw, very helpful information

cp2295 · Sep 4, 2014

Plus the bank 1 is really close to 25% (the threshold if I'm not mistaken?) So I should pretty soon get a p0171

cp2295 · Sep 11, 2014

I found that the o2 sensors were very slow. They would go in their full range (.1-1.0v) however they bounced very slowly as if they had no heaters in them. I'm going to replace them and see what happens.

I also found my fuel pressure is at 55 psi and the 4.0 is rated at 65, should I change the pump or the regulator?

Peenar · Sep 11, 2014

i will be following this thread i am having these same issues

downer · Sep 18, 2014

i think my truck runs about 35psi at idle.

sounds like you are using a code scanner of some sort, does it display the coolant temp? have you checked that the indicated coolant temp when cold and up through warm up is correct?

cp2295 · Sep 18, 2014

Yeah it's 188.6 when hot and about the temp it is outside when it's cold. Not doing it anymore am for some weird reason but I got a return less style should be 65-75 psi. Return style should be 35-45 I believe you probably for return style fuel system.

Truck missing and bucking when cold

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