Equipment:
1998 Dodge Dakota Sport Std Cab
Engine: 3.9L V6 Magnum
Transmission: 42RE
Axle: 8.25"
Gears: 3.55

Been doing this and that to my truck, still in "maintenance mode" after buying it, changing the little things. Never know what a previous owner has done to it. Got some random questions and random ramblings.
Seems my random misfire code and all bank 1 misfire codes (P0300, 301, 303, and 305, without any actual misfire) have disappeared since I corrected the purge valve solenoid... Previous owner had it on upside down. Still keeping an eye on this before marking this "fixed". At the moment: No CEL or pending codes.

That aside, I do have a problem that's bugging me since I bought the truck... Sometimes when I come to a stop at a light, while holding the brake and idling, the engine lopes once pretty hard (idling ~700RPM, lumps to ~200 for a second). Feels almost like the transmission decided it was go-time and engaged, dragging the engine down with it against the brakes (think popping the clutch on a manual with brakes applied). Only for one second though, just bloop, then back to fine idle.
This happens maybe 30% of the times I stop or less, and only while in drive, if I'm just idling in park or neutral, it's fine, not even a rough idle. Usually it'll just lope once, rarely twice. Only twice since I've owned it has it hit hard enough to actually kill the engine, but fires right back up fine.
Could it possibly be the output speed sensor on the transmission? If it's going bad and signals the engine it's moving + no throttle to cut gas to the cylinders or that the transmission SHOULD be moving?

Sadly, the truck doesn't burn the tires when you drop the hammer in drive (only sometimes will it break loose and give a good spin, usually in open loop cold start). It does have a powerful takeoff though, wonder if this is attributed to the 3.55 and optional wider tires. However, reverse roasts tires all day long with only half-throttle (makes it hard to back up a slanted yard without tearing it up and leaving tracks... ;p)

Only other issue is the 1-2 shuttle shift dance at 10mph. This started after the transmission rebuild (and me burning it up twice on their dime, check my other thread for those details). Reading up, this can be caused by a bad governor pressure sensor and they seem to be fragile and can be DOA brand new. When I go back in two weeks for them to double-check their work, I'll have the owner take a ride to experience this. All other shifting seems fine, and it doesn't shuttle if you take off hard (sucks in parkinglots and slow traffic).

Things I've done to the truck since purchase:
Rear ABS sensor (the reason they sold the truck, trolol.)
Denso Platinum/Titanium TT plugs
Denso Wires
Brass terminal Dist cap + button
Oil change > 800 miles > 200 with seafoam in crankcase > 2nd oil change
MAP sensor
IAC
Coil pack
Air filter (CAI cone + 3" tube)
Removed and cleaned TB
Checked intake pan for oil (no pooling oil, just normal funk, magnet stuck so stock plate, magnet was clean when pulled out)
Transmission rebuild (TC died, Odo at 169k, went with full rebuild since it was time anyway)
Seafoam decarb through vacuum line (before the new plugs/wires) + two in tank treatments.
Flipped vapor purge valve.
Fixed A/C (high side valve was stuck open)
Reversed TPS change, old one idled 14% throttle, new one idled 21% throttle and acted strangely. On original again.
PCV Valve
Tailgate Handle

ToDo:
O2 Sensor (looks factory... Grabbed a new Denso O2)
PCV Valve (Parts store was out of these... srsly, have to try another)
Tailgate handle (first thing I did was broke this... go me)
Grill (tabs broken on the fins, causing it to catch latch and tear them up)
Tonneau cover

Any thoughts on the random Stop Light Lope (tm) would be nice. I'll take other suggestions on how to make the engine better without really doing mods as well. I don't have the cash, time, or willpower to stuff in performance mods to destroy my gas mileage further. (I'm also not really a mechanic, don't feel like tearing the engine out)
I'll also take thoughts on a random click-clack it has when idling after it reaches temp. (sounds like an idler pulley inconsistent rattle) Cold start runs quiet, read up that it could be a timing chain, but it doesn't really sound like a metallic slap.

 

I'd start with cleaning the throttle body thoroughly. Take it off the manifold and make sure the IAC passages are clean.

 

 

Do it again

What brand TPS did you use for a replacement?

Describe exactly how it does this in relationship to the brake being applied. It could be due to the vacuum booster.

 

It's a Mopar TPS, reading the values with an OBDII scanner, at idle it's 13%, at WOT it's 74% and from what I've read is a normal range. Slowly pushing the pedal it raises as expected without erratic movement.

Doesn't really have a relation to the brake pedal. I can idle for 10 seconds or 30 seconds without any movement of the pedal or truck. It's also very random and inconsistent, the worst kind of problem to track down. I've thought about the vacuum booster though.

 

Can you throw a vacuum gauge on it?

 

Just for shts, grins, giggles, laughs, and everything you can do when not getting laid: check the throttle blade gap.

Also as Dan said, put a manifold gauge on it (mechanical or OBDII or both, I prefer OBDII, I use an Ultragage).

Lope is typically caused from an alternating rich/lean condition. The sensors see rich, so they lean out (RPM drops), sensors see lean so they richen up (RPM rises), sensors see rich again.

First place to look is EVAP, booster,HVAC if 99-, IAC, and MAP.

 

Booster seems fine, holds at about 20hg. When holding the brakes, pressure decreases a few, but remains steady and doesn't drop while holding. Pedal isn't stiff or mushy, holds firm. Haven't gotten a reading when it drops, with how infrequent it is, hard to. Especially since it bloops only once then goes back to idle.

 

Are you sure it's not the AC compressor engaging?

 

Just going to wait until I get the O2 sensor changed this weekend to test anything else, not giving any codes, but it looks to be the factory one... Which means it's going to be so much fun to remove.