5.0 Ranger, engine does the opposite of surging
 

I have an intermittent issue. when accelerating, mid-throttle or WOT, doesnt matter. The engine does the opposite of surge.

its almost as if I was letting off the gas pedal and pressing it again rapidly. The whole truck jerks. When it does this, I also get a popping sound out of the tailpipe, which makes me think unburned fuel, but I could be way off base. The popping is much worse when "lugging" the motor instead of holding it WOT


The issue is intermittent, but usually happens. I have no check engine light. I can even reproduce this issue while taking data logs for my Tune guy, and he cant see whats happening.

I replaced (and no improvement)
Coils (upgraded to MSD Blaster coils)
PCM
Cam Syncro (sensor)
Water temp sensor
Throttle Position Sensor
Air Intake temp sensor

unplugging the EGR also makes no change. (vacuum line plugged)
Im sure im missing a few.....

The plugs and wires are relatively new and are Motorcraft Platinum plugs, I pulled them all out and they all look good. Nothing visibly wrong with any of the wires

The fuel pump is also a brand new Motorcraft pump when I did the motor swap.

Pulled the valve covers off thinking maybe a rocker broke, but that wasnt the case when I checked last year.

Ive been chasing this issue for 2 years and I am out of ideas or parts to throw at it.

Well there are only two things that can cause that
Spark or fuel, lol.

Just to be clear on what is happening
Engine idles and cruises along fine
When you press down on the "air pedal" the RPMs increase briefly then drop out then come back and it accelerates like normal after first drop out.

Have you checked TPS(throttle position sensor) for a "dead spot"
TPS gets 5volts from computer and sends back under 1 volt for throttle closed and above 4.5v for WOT
In between the voltage varies between the two upper and lower voltages
If there is a "dead spot" then computer "thinks" you did take your foot of the "air pedal" as TPS passes that spot, voltage drops, then comes back up, so "foot off the gas pedal"

Top wire on TPS should be 5 volt key on
Center wire under 1 volt, then goes up as you manually open throttle
Use a sewing needle to pierce the wire for testing
Should be nice steady voltage, no jumps or drops

Don't care if is new test it anyway

And its not really the fuel flow with TPS its the spark advance that changes quickly with TPS voltage drop, hence the popping from exhaust.


Is there an EGR system?

Does it use EDIS module, TFI or Computer for spark control?

Engine idles pretty good. sometimes it stalls right after it starts, but its rare, and I suspect it is linked to the other issue.

its really hard to describe because the problem changes so much. WHen it starts acting up, if I dont move the gas pedal, the problem doesnt go away. pressing more/less can get it to straighten out usually. it has to "work through it" if that makes sense.

If it is the TPS, the whole thing is a dead spot sometimes :/
Ill replace it and report back. I should test it first, but I'm determined to positively eliminate parts because of how much the issue comes and goes.


Its a stock Explorer electronic setup, so the spark is PCM controlled.

and Yes, the EGR is present

Replaced the Throttle Position Sensor with a new one. zero change :(

If I unplug the vacuum from the EGR, would that open or close the valve?

Pull the vacuum hose off the EGR valve and plug it, the vacuum hose
You will get a code eventually but it will go away when vacuum hose is plugged back in

So PCM does both centrifugal and "vacuum" advance spark timing calculations both.
Centrifugal is RPM based advance/retard, "vacuum" advance/retard is throttle position based.

As RPMs increase spark has to be advance, moved farther away from TDC, higher BTDC number, because fuel mix takes same time from spark to full explosion but piston is moving much faster
But when you step on the "air pedal" fuel mix gets Richer so spark has to be retarded, so closer to TDC, because richer mix burns faster.

TPS is a Light dimmer or volume control, variable resistor, and they do get dead spots

replaced TPS, no change.
unplugging the EGR also makes no change. (vacuum line plugged)

also, add the MAF sensor and IAT sensor to the list of new parts recently changed.


its definitely frustrating as heck

Vacuum at the EGR opens the valve, EGR system is there to prevent Spikes in NOX, a toxic gas, NOX goes up with cylinder temps, like when accelerating, so computer will open the EGR valve a bit to add exhaust gases to the intake to cool the cylinders.
Yes hot exhaust gases in the intake cools the cylinders, it slows down the burn rate of the Richer fuel mix, which prevents a spike in temperature, and NOX gases.
I wanted to see if computer was dumping too much exhaust into the intake which would cause misfires

I would have bet money on the TPS, I would still test its voltage.

The added electronics to an engine just makes it more reliable, it doesn't change what it is, a self power air pump.

"its almost as if I was letting off the gas pedal and pressing it again rapidly. The whole truck jerks. When it does this, I also get a popping sound out of the tailpipe, which makes me think unburned fuel, but I could be way off base. The popping is much worse when "lugging" the motor instead of holding it WOT"

The "pop" makes me think its not a fuel issue, but a spark issue.
But computer controlled spark is super reliable, TFI wasn't a computer, lol


Is there a tachometer and does it drop when the jerk happens?

Because if you are running an automatic trans, problem could be there not the engine at all
Manual trans couldn't cause that.

I'll test the voltage to eliminate a wiring issue.

the jerking happens so quickly, theres not enough time for the tach to react. its almost as rough and fast as the rev limiter.

It rarely pops when the engine is ambient temp cold. I figured the pop is unburned fuel hitting the hot cat and igniting

Trans was recently rebuilt, new torque converter, and shift kit.
if the trans was causing this, I would not think I would be getting the popping, nor the loss in engine power. THis issue has persisted for around 5000 miles of driving

I need to add to the original problem description.

If I take off from a stop at two thirds throttle (or most any throttle position)

As the rpms climb , with out moving the throttle, it starts the throttle chopping, works through it (sometimes much worse than others) then it will shift , rpm climb, throttle chopping starts again. It seems to me most pronounced at mid range rpm with moderate throttle application.

So it is more RPM specific than throttle specific, or more load specific.
As you(transmission) shifts gears the RPMs remain constant, slightly climbing maybe, but the load changes, less load at top of gear ratio then load increases again as it shifts to higher gear, lower ratio, like "lugging" the engine but not that extreme
So it almost reads like at a certain load the engine stumbles

Have you tried running without MAF sensor plugged in
Warmed up engine

The computer knows its running a 5 liter engine so knows how much air will come in at any given throttle position and any given RPM, so it knows how much fuel to add to that air flow.
MAF and IAT(air temp) sensors adjust the computers air:fuel mix for the airs WEIGHT, heavier air needs more fuel added, lighter air less fuel
Air weight changes based on altitude/elevation and temperature, thats what MAF and IAT are used for.
Not saying sensors are bad, test is more to see if air:fuel mix is the culprit vs spark

What year is the computer(PCM)?
Its the first two digits in part number
i.e. F8 = 1998, 1L = 2001

Explorers didn't use a MAP or barometric sensor as far as I know, but the computer does calculate/infer BARO pressure from other sensors, MAF being one of those.
159Hz would be correct for sea level(29.92"), 144Hz for places like Denver, mile high city(25")
You can read what computer "thinks" the baometric pressure is with OBD reader, in Hz
Go here to see actual pressure in Toledo, Ohio: http://www.localconditions.com/weath...do-ohio/43601/
And then here for the conversion, second chart: http://www.modularfords.com/content/...ressure-Sensor

We are looking for obscure reasons for the drop out, and BARO pressure it pretty obscure, lol, but related to engine Load

This PCM is an F8. The original was a 2000 year. Both were NRT1 strategies.

3 laps around the block with the engine warmed up.

MAF plugged in. Idle is fine. low RPM is fine. Midrange a bit jerky. WOT was pretty smooth tooday (sometimes it is NOT)

MAF unplugged. idle fine. Low RPM was terrible (right off idle to 1300 ish RPM). Midrange was a bit better. WOT was a bit better.

THe problem varies so much at times, its hard to tell with small changes in behaviour, but the low RPM definitely suffered badly. cleared up immediately with the MAF plugged back in.

No CEL with sensor unplugged.

Checked compression on one cylinder, got about 110 PSI. havent had time to do them all, but 110 seems LOW

My 96 E150 van is doing the same shit. It's a roller 351, GT40 heads, Cobra rockers, shorty headers, no cat, full 3" ingle exhaust. It started this crap after a rear gear swap to 4.10's from 3.55's. If you give it partial throttle, it doesn't act up, but nail the go pedal and that's when it happens. I'm going to change the TPS next. Changed the TFI module last weekend, no change in that, but mileage did go up a bit. Also going to see if the EGR is stuck open.

I changed the TPs on my van today and that cured the issue. No more stumble/popping/backfiring when you nail the go pedal from a dead stop or slow roll.

I have replaced this sensor already.

thinking maybe wiring, but the guy doing my tune cant see an issue with the TPS feed

Seems I recall the plugs can go bad. Did you go back with a Ford sensor ? I did on mine. After changing the speed sensor with an aftermarket unit when I went through the rearend and getting unstable readings from that, I figured it was worth the few dollars extra to stick with Ford parts on the sensors.

I used an aftermarket TPS. But if the issue was the TPS, the chances of the new one being defective in the exact same places the old part was failing is very unlikely.

going to change plugs/wires to eliminate that possibility.

I'm really beginning to suspect a tuning issue. My SCT and laptop isnt picking up anything though, so my guy cant fix it over Email. And since he is a 14 hour drive from me, having him get it on a dyno is not feasible