Drop in RPM

[STIgone73]

So this is something my truck has been doing for about a month or so now. When slowing down to stop my truck will drop to below 500 RPM. This happens around 40 MPH when the converter I believe unlocks and the truck drops down to idle. Mine will drop to 500 or so then kick back up and smooth out. It doesn't do this every time. Truck is a 2001 4x4 automatic. Mods are DIY intake, 4" straight pipe and PHP 6 position chip. I've tried to see if switching tunes makes a difference. It seems to not do this as much in my 65hp dd setting. Any help would be appreciated. Oh, and I have 2 spare CPS's but haven't changed it yet. Might try that for a place to start....

[Silvercrew]

Mine had been doing the same exact thing for a couple months now. I have a thread in the transmission problem forum, bite no real bites yet.

Have read issues could be anything from the torque converter to an IPR to one guy who had a dead #8 cylinder.

Btw, i have the exact same mods as you except my exhaust is stock size, muffler deleted.

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[STIgone73]

Ya I saw the one about the dead cylinder. Mine doesn't die though so I hope that's not it. And mine seems to make good power still. Hoping it's just something stupid like a sensor. Maybe we'll get some suggestions in one of our posts. Just noticed you have the same mods. I wonder if it could be PCM related. Maybe the connection. I might have gone a tad too far cleaning the connection for mine when i installed the chip. I wonder if vibration could have loosened it some? Been thinking about having mine retinned.

[roaddogjru]

Sounds like the HPOP pressure is dropping off. Could be the HPOP, ICP sensor, or IPR. Find someone with an AE analyzer and look at the ICP voltage. KOEO it should be .20 to .25 volts. Cranking it should be .83 volts min. KOER the ICP pressure should be from 550 to 700. The pressure is a PCM calculated value using throttle position and RPM---not a real pressure reading. Read the ICP voltage for real pressure using the voltage to pressure conversion.
Also look at the Cylinder Rotational Contribution for all 8 cyl. none should be higher than 2.5 to 3%. Shows a bad injector if so but I don't believe that is the issue here.
If the above ICP voltages are within specs there is a chance that the PCM has an issue. If they are something different you may have a HPOP pressure issue that could be a pump issue, the ICP Sensor or the IPR sticking.
Also do a buzz test on the injectors first and then check the IDM for codes---not very likely an issue here. Good luck.

[roaddogjru]

Does your chip control your TQ Converter lock-up and unlock RPM? Is your converter unlocking and shuttering just above idle? If it were unlocking too low it will do this but I have not seen one unlock so low that it pulls the idle down.

[Silvercrew]

Mine, with the chip in it, unlocks the TC at the same point RPM-wise as it did prior to installation. When I get home I will hopefully be ordering my own AE and hopefully get to the bottom of this.

My truck did have a leaky ICP, which was replaced and I was hoping that would correct the issue but no such luck.

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[roaddogjru]

When you get your AE--google Dieselmann's tach pages. Tons of good analitical data that may help.

If you need a system schematic send your e-mail address to jananddeb@embarqmail.com.

Good luck
Jan

[STIgone73]

Mine unlocks around 1k rpm at 40 mph. The problem I have is that once it unlocks it drops too 500 rpm or slightly lower, kicks back up and then smooths out. Driving the truck the last few days I noticed a little more. It seems more prone to this when the truck is at operating temp and more so when running the ac, more load. It seems to not really do it when still cold.

[roaddogjru]

This sounds like one of three things---your HPOP pressure is dropping way down on decelleration or a tuning issue with the chip or your fuel pressure drops off for some reason. You need to drive with a analyzer on it to see where your throttle position, ICP voltage, IPR duty cycle goes to when you lift.
If you are close by I can hook up my AE and check it out.

[STIgone73]

Part of me thinks it's not tuning since this just became an issue within a month or so and I've had the chip for probably 6 months or so. I'm in reno,nv