Truck jerks at 35-40 mph

[WhiteSplash93]

My truck wouldn't start one morning,so I checked everything out and decided that the starter had to be the problem. I replaced the starter,and the truck would start,but it was hard and took a few tries. The truck would then stumble on takeoff and then be fine through third gear and jerk suddenly. The gas pedal wouldn't do anything until I pushed in the clutch for a few seconds,and down shifted, but it does the same thing as soon as I hit 35 in third gear. I'm really getting frustrated with it at this point. I've changed the fuel filter and fuel pressure is fine at idle. How can I monitor fuel pressure under load? I'm missing work and can't seem to figure out why its doing this. Any help would be greatly appreciated.

 

[RonD]

Does the CEL(check engine light) work?
Does it come on with the Key and then go off after starting engine?

Lack of fuel flow/pressure would first be noticed at highest demand, highway speeds, it wouldn't be at lower RPMs or at 30mph and then be fine at 50mph.

Clean MAF sensor, always, anytime there is a stumbling issue.......clean MAF sensor.


Throttle position sensor(TPS), allows computer to know what you want to do.
Air/fuel mix can be calculated just fine using RPMs and MAF sensor air flow, but to be responsive to acceleration computer needs a "heads up" to increase fuel.
On a carb that was done with an Accelerator Pump, it squirted extra gas into the engine when you pressed down on gas pedal, so instant throttle response.
On fuel injection that is done by the TPS, it tells computer to "squirt in extra gas".

Very easy to test these, TPS will have 3 wires
Top is 5volts
Center tells computer throttle position
Bottom is the Ground

You, of course, want to test the Center wire.
Get a sewing pin, use it to pierce the center wire's insulation, while still connected to TPS
Set Volt meter to DC Volts
Ground the meter
Turn on the key, engine off
With throttle closed you should see .69 to .99volt, under 1 volt
Now slowly open throttle and watch voltage
Should go up, and be steady, steady is the key here, no jumps or drops
At WOT(wide open throttle) you should see 4.5volts to 5.0volts
Then close throttle watching voltage

You may have a dead spot on the TPS, so when gas pedal is at a certain position voltage drops and computer "thinks" you took your foot off the gas, so engine stumbles and jerks.

There are other causes for the symptoms as well, which is why you don't just replace sensors, always test them.
A $25 volt/ohm meter will save you hundreds of dollars in unneeded sensors.

DIY fact, 90% of sensors replaced by DIY mechanics were working just fine.
I blame that partly on the way the OBD codes are written, but we are responsible for our own actions

 

[WhiteSplash93]

The tach quit working when I hit a bump too hard. (It came out of nowhere.) Then the CEL came on at the same time. I'll have to try that this weekend, I work 12-14 hours a day.

 

[WhiteSplash93]

So the tps checked out just fine. .96 volts at closed throttle, steady rising to 4.97 at WOT,with no drops or spikes if i hold it in place. What other things could cause my issue? I really need this truck back on the road.

 

[RonD]

Could be the ICM, ignition control module, located on the front side of intake manifold.
Although that is a long shot for the intermittent symptoms.
Many auto parts stores can test these, and the wiring harness for these often frayed or shorted when older, so worth a look.

The Fuel Pump relay could be opening and closing, cutting power to fuel pump momentarily, another long shot.
Inside the cab in the passenger footwell is the Inertia switch, power for the fuel pump passes thru this switch, inside the switch is a heavy metal weight, if there is a sudden stop(accident) or a roll over, the metal weight trips a breaker that cuts power to fuel pump, it has a reset button on the top.
I don't think this switch is the problem, but power for the fuel pump does pass thru this switch in the cab, so if you were to hook up a test light to this switch..............then when driving and engine started to cut out looked at the test light, if it was dimming then fuel pump relay may be the problem.
There will be 5 to 7 volts at this switch when key is on, but low amps, not enough to run the fuel pump, that voltage comes from the computer's monitor circuit, so is normal.
A test light should light up brightly with full engine running power , 13-15volts, is passing thru that switch.


There is a video on youtube of a guy with a 2.3l Ranger that ran a longer hose from schrader valve(fuel pressure valve) out the front and up on the hood then duct taped it there, so he could watch fuel pressure while driving.
he did have a fuel pressure issue.
Sorry couldn't find that video but it is out there.

 

[WhiteSplash93]

Thanks for the replies. After testing the tps I decided to see if it would crank. It fired right up. No hesitation, first try. I then drove it about 15-20 minutes to see if it would act up again,but everything went smoothly. For some reason if I leave it sitting overnight or a couple of days,everything goes fine. However when I come back to it,say after working 14 hours it won't start,or will start and does this jerking and stalling crap at 1am.