4.0 OHV hesitation when hitting accelerator
 

Hey team, I got an issue with a 98 4.0 OHV driving us crazy and could use the wisdom of some experts. To start, the engine spun a bearing and we pulled it out to replace it. Found a NAPA rebuilt engine that had 0 miles that had been sitting for 4 years. Dropped the bottom off and everything was brand new. Dropped it in and she cranks right up and runs pretty smooth. The problem is when you hit the accelerator it staggers, a slight drop in RPM's and then skips a little as it comes up to speed. It reminds you of an engine with an air leak or something. Once it comes up to speed, it seems to run smooth. While out running down the road, it will jerk some when you try to push the accelerator when its under a load.

Most everything that was on the old engine was put on the new one; Air Intake, and sensors. The EGR, Ejectors, and Cam synchronizer is from the new engine. New engine came complete with heads, water pump, etc.

What I've checked and done so far:
- Changed Cam shaft sensor
- Changed plugs, wires, and coil
- Changed all air intake gaskets along with the fuel rail bottom gasket
- Changed fuel filter. Fuel pressure is at 65 lbs and holds there. New pressure regulator.
- MAF voltage varies smoothly with engine speed and doesn't throw any codes
- Changed Throttle position sensor. Voltage varies smoothly with throttle position and doesn't throw and codes. I manually move the sensor through its ranges and it works well.
- Changed DPFE and it varies the voltage signal with flow
- Changes idle valve (IAC)
- Changed Throttle boby
- Checked & rechecked all vacuum lines to ensure no leaks. Sprayed some carb cleaner around to see if I could detect and leaks but haven't been able to.
- Spark advance is around 10 but varies as you accelerate.
- I have the ability to graph many variables simultaneously while running the engine so I can watch the performance as you run it.

Possible areas of concern:
- Camshaft position error. Although it has a new sensor, it's throwing that code. It was doing the same on the previous engine. I've done extensive work on this and the ECM is getting the pulses on the connector entering the computer. Still need to resolve this but I don't feel its causing my current problem.
- EGR isn't working correctly. The EGR on the engine is the one that was on it when we purchased it. I can change it but again, there's no reason I know of that it will cause the issue I have.
- O2 sensor voltage is oscillating up and down causing the fuel trim to follow. I have unhooked them to see if it made a difference, the voltage signals settle out as does the trim but the issues persist.
- Per my scanner, both banks appear to be Lean. Of course, this could mean a bunch of things but I've replaced or fixed most of the things I might have thought would cause that - like air leaks on the intake, hoses, etc.
- I tried restricting the air flow from the inlet to see if getting too much air was the issue. Up to the point of killing the engine, I wasn't able to detect any difference in how it ran.
- I removed the TPS and moved it manually, watching the voltage as I did, and even though it registers on the scanner, it doesn't change anything about how the engine runs at idle. Folks have said that the TPS is what told the EMC to apply more fuel but I can't detect whether that's true or not. It's not making a difference for me.
- The injectors are the ones that came in the new engine (sitting for 4 years). I don't know if they were new but I don't think so. But I'm not getting any misfires, no injector errors, etc.

All in all, most of what was making the old engine go down the road at 70MPH before the bad bearing, is on top of the new engine with the exception of the injectors and EGR. Something is out of whack but its hiding itself well from me.

Wisdom form you experts is appreciated!

Mark

TPS(throttle position sensor) seves the same purpose as the Accelerator Pump did in a carb engine

So could cause your hesitation

Why TPS or accelerator pump is needed:
When you open the throttle plate more air comes in, instantly, this will Lean out gasoline ratio, instantly.
So a pump was added to carburetor, accelerator pump, when you press down on gas pedal, throttle plated opened and pump squirted in extra gas at the same time.
This allowed instant acceleration and time for Main Jets to meter more gasoline to match air flow.

Fuel injection is no different, by the time MAF(mass air flow) sensor could react to higher air flow you would have a few seconds of "no go" lean mix.
So TPS was needed, it gives computer instant "heads up" to add more gasoline right now

TPS also saves gasoline

TPS gets 5volts from computer
If it sends back .69 to .99 volts computer knows throttle plate is closed, foot is off the gas.
If you are "in gear" and engine RPMs are above 1,400, and TPS shows under 1 volt then computer will shut off fuel injectors, 0 fuel flow, until RPMs are below 1,400 then it will restart injectors at idle level fuel flow.
So if you coast in gear down a hill there is 0 fuel use
With a carb many would shift to neutral to coast, but with fuel injection leave it in gear with foot off the gas pedal to save fuel.

Also at WOT(wide open throttle) TPS will send computer 4.5volts to 4.9 volts, computer then ignores O2 sensors, and runs engine richer for more power
Thinking is, if driver puts foot to the floor he is more interested in POWER at that time not fuel economy, lol.


Free vacuum leak test
Warm up engine fully
Let it idle
Unplug IAC Valves 2 wire connector, valve will close and engine RPMs should drop to 500, or engine may even stall, either is GOOD, it means no vacuum leaks
If RPMs don't drop you have a leak
OR........someone has messed with the Anti-Diesel screw thinking is was an IDLE SCREW, it ain't, fuel injected engines can't use an idle screw, no Jets

Ron, thank you - you obviously know your stuff. On the TPS, my sensor works as you described. I have 0.9v at idle and she moves up as you push the throttle. I heated it up and pulled the IAC off. Just as you described, she dropped down to about 560 rpm's.

Still looking for the root cause. I'm going to work on the CPS (camshaft sensor) some today but don't know if it would cause the issue I'm having. Your input appreciated.

Make sure spark plugs were gapped at .054, big gap but needed with the 4.0l's, smaller gap can be drowned on acceleration, if TPS is working well

And the 1998 to 2000 4.0l OHV had smaller exhaust ports in the head and manifold to increase the speed of the exhaust to warm up Cats faster
If new engine has 1997 and earlier heads you could have restricted exhaust flow using 1998-2000 manifolds

4.0l info here, good read: Rebuilding The Ford 4.0L Pushrod V6, Doug Anderson, Automotive Rebuilder, April 2001

Ron, been working on the CPS. I have a nice 13 volt pulse stream going into pin 85 on the ECM referenced to the negative side of the battery. It varies in frequency as the engine is accelerated. I'm still getting a CPS circuit failure. I'm reading this on the back side of the ECM connector so it sure seems like the CPS is doing its job. And in the past hour, I've disconnected the battery and shorted everything to clear the ECM. Any thoughts?

Cam sensor has a few codes, which one do you have?

P0340 Camshaft Position Sensor Circuit Malfunction
P0341 Camshaft Position Sensor Circuit Range/Performance
P0342 Camshaft Position Sensor Circuit Low Input
P0343 Camshaft Position Sensor Circuit High Input
P0344 Camshaft Position Sensor Circuit Intermittent

Anytime you have a code you need to look at full code list to see what codes the computer did NOT use, that will be helpful in diagnosing why computer chose the code it did use

P340 - circuit fault

So wiring fault, maybe dead sensor or bad computer circuit
If you have pulse at computer then problem is in the computer

I assume this is a 3 wire Hall Effect Sensor, the 2 wire Variable Reluctance(VR) sensor was starting to be used in late 1990's for Cam sensors, these output AC volts not pulses, so totally incompatible signals.

3 wire connector should have voltage and ground with key on
Black/white stripe should be Ground
Light Blue/orange stripe should be 12v(key on)

Dark Blue/orange stripe would be pulse seen at pin85 on computer connector

Disconnect battery, pull off computer connector, they can get water inside if hood gasket leaks, ultimately you may need to pull computer out and open it up, look at circuit board, Capacitors are know to leak and you will see that as discoloration around them, and this can cause odd issues, P0340 being one of those, also could cause your hesitation, the leaking Cap not the P0340 code.

P0340 wouldn't cause engine running issues, Cam sensor is used to fine tune sequential injection and spark timing for best MPG, Crank Sensor runs the show, and computer would just ignore cam sensor if it set P0340, or any cam sensor code.
What is odd is that you are not getting "misfire detection circuit off", but sometimes that doesn't always come on with cam sensor codes.


EDIT: Yes, 1999 was first model year for VR Cam sensors, so 1998 would still use 3 wire Hall Effect

The truck had a 3-wire. New engine came with 2-wire. We swapped it to 3 wire. The signal at the computer is a clear square wave pulse on pin 85.

So 1999 or 2000 4.0l engine, so no exhaust issues.

Yes, looking like computer issue

98 4x4 4.0 OHV is correct. Just pulled one plug to check gap and its right at 0.054 (not exact). We DO have an exhaust leak on the passenger side that appears to be coming from the manifold/head connection. The manifolds had a new gasket on them when we got the engine. We've tightened it down some more but we still got some noise. I'm curious if you think that has something to do with my issue?

Maybe but would more likely just set Lean code on Bank 1, after engine warmed up

Exhaust manifold leaks SUCK IN air, lol, yes they do leak exhaust but between pulses they suck in air.
O2 sensors see Oxygen in the exhaust, too much oxygen is lean, too little is rich
So with a manifold leak the O2 on that bank tells computer too much oxygen(false lean) so computer adds more fuel to get O2/exhaust to correct levels, so that bank is running Richer than it should, while it could cause running issues it would set a Lean code before that happened, big leak

ok, I'm working to find and fix the exhaust leak and see about finding a used ECM to try as next steps. Just found the leak in the exhaust flange, gonna take welding to fix. Question, while at the shop fixing he flange, should I have them cut the Cats out while there? We don't have emission inspections where we are. Don't know if it'll throw a bunch of codes if I do.

1995 and up Rangers do have the 3rd O2 sensor that test the oxygen level after exhaust goes thru Cats to make sure they are working, so you would get a code.

Cats(or mufflers) in no way hurt performance if they are not broken inside, blocking flow, so no reason to get rid of a working Cat

Still the same issue
 

RonD and others...
The last time we communicated everyone seemed to feel my ECM was bad. I replaced it with a new one and all the symptoms are still present. CPS still giving the same error (p340) and still hesitating when you hit the accelerator and sort of stumbling when driving it and try to put it under greater load. We also changed the injectors this afternoon to eliminate those from the equation.

By now, we've about changed everything but not touching the problem. Seems like something we're missing . We're not getting any misfire codes or others that would give us any clues.

Ron - we purchased this long block rebuild from NAPA. I assume there's nothing around timing that can be adjusted on these 4.0's. I know the Crankshaft sensor controls the firing and I assume its not adjustable. Any other thoughts?

You may have done this before or may even seem silly, but did you actually check for continuity from the CSP sensor wires to the PCM.

While checking the wires for breaks and bad connections, move the wiring harness around.

I ohmed out the wiring but I didn't wiggle the wires around while doing it. I understand the point. Honestly, when you set there and watch 13 volt pulses going into the ECM, you sort of say to yourself that the voltages and wires needed to produce a square wave into pin 85 must be ok.

Did you ever find your issue? I'm having these same problems.

No, I have yet to find my failure. I am still working on it. I have replaced all the failed components I have found, but am working on replacing the failed ECU.

My engine swapped 2000 Ranger wants to stall
 

Have you found your issue? I have a 99 motor in my 2000 ranger, I've replaced TPS, PCV, MAF, and I'm going to spend money on plug wires.