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6.0L Power Stroke Problems, Part 8: STC Fitting

When Ford changed the 6.0L Power Stroke’s high-pressure oil pump for the ’05 model year, a new problem was introduced. It exists on the discharge side of the HPOP, where highly pressurized engine oil is routed out to the branch tubes. Coined the STC fitting (STC standing for snap-to-connect), its seal is prone to wear, resulting in a slight high-pressure oil leak that grows more and more noticeable. At first, start-ups are normal, but hot re-starts become increasingly more difficult. Then, when the seal is completely gone or (worse) the fitting blows apart, you get an engine that won’t start under any condition.

STC fitting failure is only a problem in ’05-’07 6.0L Power Stroke engines, but it’s a killer and can strike at any time. Other than oil and EGR cooler failures, this is one of the most common failure points on later versions of the 6.0L—and it will definitely leave you stranded. Luckily, Ford released an update for the rampant issue, which solves it, but does call for a fairly laborious install. For more on how, why and where the STC fitting fails, as well as the ultimate fix, keep scrolling.

What The STC Fitting Does

6.0L Power Stroke STC Fitting Failure

On ’05-’07 model year 6.0L engines, the infamous STC fitting is part of the high-pressure oil pump’s discharge. The discharge tube connects to the branch tubes, which route high pressure oil to the oil rails under the valve covers, ultimately delivering the highly pressurized oil that’s needed to fire the fuel injectors. The internal seal and its locking ring is the issue within the two-piece STC fitting. The seal is prone to premature wear that allows vital high-pressure oil to leak past it.

Where It’s Located

Power Stroke HPOP STC Fitting Pump mounting bolts and discharge tube bolts

A new high-pressure oil pump debuted for ’05, which required a few of its surrounding components to be redesigned. This is when the STC fitting was introduced. The STC fitting is positioned on the outlet side of the HPOP, which is next to the HPOP gear at the back (passenger side) of the block. It’s also located under the HPOP cover, which makes accessing it somewhat difficult. Two bolts join the discharge tube and the branch tubes together.

Why The STC Fitting Fails

2005 Ford Power Stroke Branch Tube

As mentioned above, it’s actually the internal seal within the STC fitting that fails. The primary contributor is the branch tubes, which—due to inherent engine vibration and being under extremely high pressure—flex under normal engine operation. Over time, this movement takes a toll on the STC fitting’s seal. Rather than attributing the seal’s failure to poor quality, most (including Ford) have concluded that use of an STC in the HPOP discharge location was simply a bad design idea (Ford worded it more delicately, of course).

STC Symptoms

STC Fitting Failure 6.0L Power Stroke

Hard-starting is a solid indicator that the STC fitting is about to let you down, but specifically it’s excessive cranking on hot re-starts that represents the tell-tale sign. In the early stages of the failure, the engine will first fire up just fine. This is because the engine oil, when cold, will have a higher viscosity and will flow past the worn seal. However, once engine oil has been brought up to operating temp and its viscosity drops, it will leak past the worn seal, culminating in long-crank scenarios when attempting to restart a warm engine.

Complete STC Failure

2007 Ford F250 Power Stroke Diesel

As your 6.0L’s hot re-starts grow longer and longer as the problem worsens, the high-pressure oil system will struggle to build and maintain adequate injection control pressure (ICP). Eventually though, the STC fitting’s seal can blow out completely, and the sudden drop in ICP will abruptly kill the engine and it will not restart until the issue has been addressed. Luckily, Ford came up with an OEM fix that solves the problem for good—and it costs less than $60.

Ford’s Updated STC Fitting Kit

Ford Updated STC Fitting Upgrade Kit

After determining that the STC fitting wasn’t even necessary (which begs the question as to why it was ever incorporated there in the first place), Ford released an update kit to address the issue. The fix carries part number 4C3Z-9B246-F and reverts back to a threaded solid piece fitting similar to what was used on ’03 and ’04 versions of the 6.0L Power Stroke. By eliminating the snap-to-connect fitting, high-pressure oil leaks in this crucial area are history.

Ford’s Update Is 100-Percent Worth The Trouble

Super Duty F350 Cab Removal 6.0L Power Stroke Diesel

Unfortunately, it takes some digging to access the STC fitting to perform the update, and you’ll have to remove the turbo and HPOP cover before you’re there. However, this is why the STC fitting upgrade is recommended any time you’re under the hood (or have the cab off the frame) for other extensive repairs. One last word of advice: there is still an O-ring on the updated, one-piece fitting, so don’t overtighten it during the install (you could damage the O-ring).

2006 Ford F-350 Nitto NT420S

  • Don’t misdiagnose an STC fitting failure with HPOP or injector failure. You can learn more about those issues in Part 5 and 6.
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