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south texas | Can anyone post a link to a good source for stc explanation or post here? I am wondering how the system works. From what I can gather the oil pressure from the engine is regulated in the stc valve and sends a portion of the pressure to the inj to adjust timing of the fuel to the cylinder. Does the inj work like a normal inj with out oil prssure or need full pressure to inject corectly at full load? My cummins books have about as much info as a trucks owners manual on stc. Wondering if my lowpower issue could be from injectors placing fuel to late in the compression cycle. Maybe the plastic tube was cracked during installation and allowing pressure to escape the timing control. Does the oil enter the inj from the oil line on top of the head, what should this pressure be related to eng oil pressure? We get about 40 psi at op temp on engine, could place a gauge on stc line and see if it is low. Thanks for all you patience and help. I really like to know my engines since we tend to keep them a long time. If I cant figure this out will be first diesel mech on farm since 2009. |
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| I have a stc in a 1989 "rated at 365
When they did the engine in 1997 they "rebuilt " the stc box
I don't know the pressures
maybe these guys would have a old mechanic
they redid my pt pump couple years ago
http://www.superiorturbo.com/ |
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Leeds, North Dakota | I wish I could help, gonna give you a # Daves Diesel in Minot, ND 1-701-838- 8302, Dave used to work for Cummins, has his own Cummins diesel shop, get cpl and 8 digit # for Dave along with # and name off turbo, with all info, tractor etc, Scott. |
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| cummins engines has a hotline to answer questions about their engines 1 800 DIESELS I have gotten valuable information from them that was unavailable anywhere else |
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| Years ago Pittsburgh Power had a booklet that adressed the stepped timing system. No current info on their site that I can find. Might be worth a call. |
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Iowa | what engine model? are you sure its STC and not top stop injectors |
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Iowa | You say low power. did you adjust the valves and injectors? did you use the OBC method. STC engines get adjusted with obc method. |
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Dearfield Co. | If it has the braided oil lines running from cyl head to cyl head it will be a stc----------------------There is plastic oil galleys that run from the braided into the inj. inside the valve cover/rocker box.Its been years since I messed with one but it seems that they were extremely low pressure oiling but high volume.It seems the little lifter on top the inj link just made the biggest difference at start up basically allowing a oil fed timing setup instead of the air operated like the big cam 2 and 3 475s and california emissions.I do know the one I had required more frequent oil changes to keep the step time control box operational and the thing acting right.I sold the truck and the driver refused his new one and quit as he loved that engine.Since he was one of my better guys the thing was as trouble free as any engine I have owned and we didnt have to do much other than the little plastic oil line updates and inj updates which were frequent back then as well as costly. |
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Mount Vernon, WA | Here you go, straight from Cummins (sorry, pictures will not copy):
Step Timing Control, commonly referred to as STC, controls the engine timing in an effort to minimize white smoke at cold engine start-up.
Refer to Step Timing Control Familiarization, Bulletin 3387380, for addition information on STC.
STC has two stages of injection timing. The engine operates in the ADVANCED mode of injection timing during starting and light engine load conditions and at NORMAL timing during medium to high engine load conditions.
STC offers many advantages. During ADVANCED injection timing, it:
•Improves cold weather idling characteristics
•Reduces cold weather white smoke
•Improves light load fuel economy
•Reduces injector carboning.
During NORMAL injection timing, STC:
•Controls cylinder pressures
•Reduces nitrogen oxide emissions.
The STC system consists primarily of:
•STC injectors
•STC oil control valve
•STC plumbing and check valve.
These components control injection timing based on fuel pump rail pressure (engine load).
In the injector, injection timing is controlled by the STC hydraulic tappet. The tappet has an inner piston (plunger) and an outer piston (sleeve). These tappet components work together with the injector plunger to control injection timing.
In NORMAL timing, no oil is in the tappet. As the cam follower starts up the camshaft injection ramp, the injector rocker lever begins to force the inner piston downward. Because no oil is in the tappet, the inner piston must make direct contact with the outer piston before the injector plunger can begin its downward travel.
In ADVANCED timing, the tappet is filled with oil and the injector is metering fuel. As the cam follower starts up the camshaft injection ramp, the injector lever begins to force the inner piston downward. Since the oil between the pistons forms a solid link, the downward pressure is immediately transmitted to the outer piston and the injector plunger begins its downward travel earlier than it does in NORMAL timing. This causes the fuel to be injected earlier.
Engine oil flows from the STC oil control valve through the oil manifold to the tappets. Whenever the oil pressure in the oil manifold exceeds 70 kPa [10 psi], it moves the tappet inlet check ball from its seat and fills the cavity between the inner and outer pistons.
During the injection cycle, the oil is held inside the tappet by the inlet check ball and the load cell check ball. When the rocker lever forces the inner piston downward, the solid link of oil causes the injector plunger to contact the fuel earlier; therefore, the injection timing is in the ADVANCED mode. At the end of the injection cycle, injection force increased the oil pressure in the tappet and holds the injector plunger firmly in the cup.
This increased pressure moves the load cell check ball from its seat. The oil drains past the load cell check ball and through the drain holes in the injector adapter and returns to the oil pan through drain passages in the cylinder head and block. Meanwhile, with continued cam lift, the inner piston makes mechanical contact with the outer piston and maintains injector plunger seating force.
The STC control valve uses fuel pressure and spring force to control the position of an AFC style plunger. The position of the plunger dictates whether the oil passage to the hydraulic tappets is open or closed. Fuel pressure acts on the piston end of the plunger.
During ADVANCED timing (low fuel pressure), the spring opposes the fuel pressure and holds the plunger in the open position. Pressurized lube oil flows to the tappets and initiates ADVANCED engine timing. As fuel pressure increases, the spring holds the plunger in the open position until the fuel pressure rises above the certified switching pressure.
At this certified level, the higher fuel pressure overcomes the spring. This action shifts the plunger and closes the oil passage. The oil supply to the tappets is interrupted and the engine begins to operate in the NORMAL timing mode.
The control valve supplies oil to the STC rocker housing connection through the STC valve oil outlet line (1).
A check valve in the elbow fitting (2) prevents the oil from draining back into the engine when it is shut off. This prevents any delay of oil to the tappets during cold starts.
An internal oil manifold connects the oil supply to each STC injector in the rocker housing.
Fuel pressure to the STC valve is provided by a hose (1) between the fuel inlet passages in the cylinder head and the STC valve. The internal spring cavity of the valve is vented to the engine crankcase by the crankcase vent line (2) in order to allow the plunger to cycle freely.
The oil control valve is calibrated to a specific flow and pressure using a fuel pump test stand. Tampering with the valve or plumbing will result in the loss of both fuel economy and engine durability. Correct valve operation is necessary to maintain acceptable cylinder pressures and white smoke levels and to assure optimum fuel economy.
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south texas | Thanks for the stc stuff. Its very handy if we ever have to find a problem with the stc. Seems by just reading once the stc system could be pluged to test engine at full load since it should not be working. |
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cummins stc system (step timing control)
