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Old 01-19-2018, 07:02 PM
mdelfunt mdelfunt is offline
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Default 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 didn't it would causing my current problem. I'm willing to listen to arguments
of why I might be wrong on this point.
- 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!


Last edited by mdelfunt; 01-19-2018 at 07:59 PM.
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