98 Vortec miss at 2500 rpm when NOT under load

[98_k1500]

I have a 1998 k2500 5 speed that i have just built. The engine came from the truck in my signature. I ran it for a while on a stock ecm and had problems out of it missing if you revved it to 2500 rpm when not under load (say holding a steady 2500 rpm in 2nd gear). I assumed it was from the built motor. I had the tune transferred to the 3/4 ton ecm, and the problem persists. It runs like a scalded ape if you shower down on it, but it misses and pops out the exhaust if it is under very light load. It is very annoying since the truck is 5 speed and right about the time you get ready to shift it, it starts cutting out if you are being easy on it (in town). It also cuts out for a split second if you are cruising down the highway (4th or 5th) and top a hill. As you ease off the throttle and the load on the engine approaches 0, it hiccups and jerks the whole truck. It only hiccups one time, but does it on every hill. It is extremely hard to reproduce this on flat highway, but sure as you hit a hill, its gonna do it. The tp tested good, and i swapped it for one i had at the shop with no effect. CEL is not on. I have access to a snap-on MODIS scanner tomorrow. Any red flags i should look for?? I'm not 100% sure if the engine did this in my other truck, it was a automatic. I don't recall it doing anything strange tho, and i'm positive it did'n have this hiccup when you top a hill. Are there any parts on the engine (sensors) that are manual trans specific that i need to swap? Its about to drive me insane. I will be changing plugs/wires in it tomorrow also, just because it could use some, I'm sure it won't fix my problem. Any help would be greatly appreciated.

 

[Scrufdog]

might wanna poke around on rockauto.com in the sensor section for the 98 and see if you can find if there are different sensors listed for m/t or a/t. Anything RPM dependant would be the first ones to check.

 

[98_k1500]

I know the tp's are the same, already checked into it. Not sure about the rest. Would anyone think the spider injector setup is trying to take a dump? It starts really easily and runs like a champ all the time except like i described above. Idle is smooth.

 

[98_k1500]

I got the MODIS scanner on it. CEL was not on, but it showed P0340 (Camshaft Position Sensor Circuit Malfunction). This immediately made me suspect the distributor setting, as it does not affect the ignition timing on these vortecs. When i went into the distributor set procedure in the scanner, It showed a cam retard of -45°. The spec on it is 0° ±2°. I loosened the distributor, but turning it counter clockwise did not affect the cam timing. At this point, I assumed the cam position sensor was toast. I pulled it out of the distributor, and found that when i swapped the motor, the plug on the cam position sensor did not engage all the pins on it. One of them was bent and not making contact. I straightened up the pin, cranked up the engine, set the injector time with the scanner, and she runs like a new one. I also changed the plugs in it. My old ones looked perfect. Light tan colored with no deposits on them. They did make it a little smoother but no major difference.

 

[Scrufdog]

Excellent. Figured it was something RPM based. Good find.