Ive got a problem that started 2 days ago for no reason I can think of. When crusing at 1500 RPM, with or without over drive I am having RPM surges of 200 RPM. Runs strong as hell other than this. After reading all there is to read here I have done the following testing.

Checked for codes and found none. No check engine light has ever come on.

I had the alternator checked out and it appears to be in excellent working condition. I expected that because I performed routine maintenence on the charging system a couple weeks ago after I replaced both batteries. Checked everything out again and all still seems good.

The tranny was rebuilt 3k ago with 2 new governors, new torque converter as well as all bands. Its been working flawless since then.

Checked the fuel pressure and its running at 14 psi using mechanical gauge. The electric gauge seems to be off by 1.5 psi. It reads 10 (11.5?) WOT or 11 (12.5?) crusing and 12.5 (14?) idle. I have been using Opti Lube with injector cleaner and Supertech as additives also.

There were 2 threads with possible causes, one says maybe engine coolant temp sensor but the described symptoms just werent exactly the same because I only have the RPM surge. I chemical flushed, pressure flushed and filled the cooling system last weekend prior to this showing up. I also serviced the rear differential then. The coolant temp sensor is only $8.50 so I will change it as soon as possible.

The other thread says APPS sensor might be the problem or perhaps the VP44. Since the fuel pressure is strong, perfomance is strong, I can duplicate symptoms at will and im not getting codes I felt it to be possible but not likely to be the VP44.

The APPS seems to have identicle symptoms describe by the engine coolant temp sensor thread. Its the one thing I dont have any thoughts on.

Knowing all this, knowing I can duplicate the symptoms everytime I cruise at 1500 RPM, what do you guys feel it to be?

 

Will it do this with OD off? it sounds like issue discusssed in first thread on CumminForum.com. The first suggestion is go back and recheck quality of all heavy electrical connects and grounds.

 

if you can duplicate it at 1500 at will that would suggest to me APPS, so try turning the OD off and running up to 1500, if the surge goes away then I would blame the apps, because at 1500 with OD off your pedal posn will be different than at at 1500 in OD.
other than that I'm outta ideas , unless its just a tac surge and not a veh speed change

 

It does it with or without OD but without OD its higher engine speed and I can only replicate it when slowing down like when exiting the freeway, as soon as I let off throttle, but still 1500 RPM. It can take it 2 or 3 miles before it shows up after stopping. Once it starts though its easy to replicate at will. Seems to be worse with OD off though.

 

The battery cables do look to of seen better days. Ill try the ECT first since its the cheapy, then go to cables, then APPS. The fuel gauge now is reading correctly so thats some relief.

 

Went through and cleaned up all the negative grounds and found the neg cable on passenger side was lose even when fully tightend. Fixed it and now the problem is gone. Like them zero cost repairs!

 

It'll be back.....

 

Problem now fixed using a DTT noise filter at the PCM. Followed all the advice given on other threads prior to doing so. Eliminated everthing else first. Cleaned all grounds, replaced bad batteries, replaced the ECT sensor since was only $8 and had been redesigned, had alternator checked, checked for engine codes. Nothing worked. DTT worked immediately.

 

All grounds are the key. Finally got mine fixed(I think). firewall ground, ecm ground, and both battery grounds. Clean the screws too! Fought this s.o.b for years. Comes and goes but is going good now. Good Luck!!