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96 suburban 6.5 timing control

T

turboaudi

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Location
upstate NY
hi i have a 6.5 sub that has a problem with controlling timing when i start it in the morning it will run quiet & very smokey it has no timing advance when it warms up it will run perfect sometimes for 60-80 miles when you shut it down for a few hours then restart it sometimes it will advance the timing so far it will almost stall then it will run normal for 500 feet then timing will advance again it may do this 8-10 times then settle down &run good when it does it the desired timing is normal but actual timing will go as high as 19 deg it rattles very loud also when it does this i have put a inj pump ,ecm ,set tdc offset& checked ecm to pump wiring when i use time set it goes to 2.5- 3.0 deg like it should my other truck is also starting to do this although not as bad i have a mastertech scan tool that has dealer software i am a gm tech also i started my own shop a few years ago i do alot of 6.5 work but have never run into this any ideas ? thanks dave
 
It could be the temp sensor in the crossover sending the wrong info to the pcm.
 
coolant temp looks good on the scan tool the crank & cam ref pulse missed shows 0 i will try a crank sensor thanks dave
 
yes coolant temp is ok crank ref missed is 0 cam ref missed is 0 des timing is 10-14 deg act timing is 10-19 deg & hunting then it will run perfect 80 or 90 miles until i let it set for a hour or 2 then it starts over i cant believe the pistons arent in the oil pan it rattles so harsh!!
 
Just a "out there" thought. I would be tempted if this was sitting at my shop to pull out the old oscilloscope and look at the stepper pluses coming from the ecu. Looking to see if they are clean and correct in amplitude and polarity. Grounds?
 
i have also pulled the advance piston out & checked the bore also the valve in the piston all free i pulled the return line &put it in a jug it stopped acting up but it was intermittent so i have to run it like that & see. i dont know what a restricted return would cause its probably not it but i will try it the stepper motor operation seems ok it will drop timing when in time set mode when you take it out of time set it goes to desired timing ok my other truck just did it today but not as bad . i forgot to put in my 1st post that i just put a engine in the sub because it broke a hole in one piston &6 others have strange cracks on the pistons right about the center the truck was doing fine going about 68 mph for 30 mi when i shut the cruise off & stopped for a red light it was running very rough lots of blowby i had the radio on when i was cruising so i think it was doing the timing thing but i didnt hear it so i put another motor in it &here we are thanks dave
 
IIRC on the db2 pumps the advance is affected by the fuel pressure not sure if this is the case on the ds4. sounds like your going to let us know
 
ok it acted up today i drove it about 25 mi yesterday ran good when i started it up this morning it ran rough & quiet / no advance then after running about 2 mins started advancing timing then would back down to normal timing then advance again i unhooked the return line no change then i noticed that when it advances out of control it pumps alot of fuel from the return fitting on the pump then the flow will slow down a lot then timing returns back to normal truck has good power doesnt seem to have fuel supply problems would a weak internal transfer pump cause this ? i would think a weakpump would cause retarded timing this thing is going to get a cummins if it doesnt behave!
 
i would say the stepper motor is trying to correct timing it is moving with the timing noise 2 different ecms all scan tool data looks ok ex the act timing when it acts up otherwise it is ok i can command 0 deg with time set & the act will go to 3-3.5 deg turn time set off goes right to des timing most times it will only act up after a 20-30 mi trip after it has been sitting over a hour to overnight when started warm it will cycle between normal & intense timing noise about 4-5 seconds it will do this 4-5 times then run ok when started up sitting overnight after being driven 20-30 mi cold start is hard &smokey it runs very quiet - no timing adv for 20-40 sec then timing comes in very advanced then will cycle like warm i have checked & cleaned all grounds checked ecm pins. no harness damage i can see . the funny thing is i have 2 trucks doing almost the same thing the other one likes doing it going down the road it scares the hell out of you . i have factory shop manuals for these trucks i have been to all the dealer classes on 6.5s from 92-98 this is the only time i have seen one do this
 
no os filter i have 97 software in the ecu the other truck has a ss diesel tune i will pull the old pump apart &look at trans fer pump & fuel sol adj i already pulled the advance piston it looked ok dave
 
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