4.4 Valve Train, Lash, Overhead Cam, and Timing Drive

Pushrods, rockers, shafts, electronics, and seals

OHV light-duty diesels—6.7 Power Stroke, 6.6 Duramax, 6.7 Cummins—open valves with a cam in the block, followers, pushrods, and rocker arms. Inspect pushrods for bend (roll on a plate), mushroomed ends, and oil holes on engines that feed rockers through the pushrod. A plugged Duramax or Power Stroke pushrod starves the rocker pad and the valve tip. Do not mix pushrod lengths between banks or between intake and exhaust if the engine uses different parts.

Inspect rocker arms for pad wear, roller condition where used, cracked forked pads, and worn bushings. Rocker shafts need open oil holes and a snug fit in the stands. A shaft that spun and blocked an oil hole takes out a whole bank of rockers. Cummins rockers work through the bridges you already adjusted; a rounded rocker pad will hammer a new bridge to death.

Electronic components and wiring on the head are part of this inspection, not a separate “electrical” later:

• Injector harnesses under a Duramax valve cover sit in oil. Swollen connectors and oil-soaked pass-throughs cause misfires that look like mechanical lash problems.
• Glow-plug harnesses and glow plug control modules on Power Stroke and GM pickups cook on the valve cover. Melted loom is a finding.
• Camshaft position (CMP) sensors on OHC EcoDiesel and TDI heads, and CMP or reluctor hardware on OHV engines, belong in the visual. A cocked sensor after a valve-cover job sets correlation codes.
• Valve-cover gaskets and injector-harness seals that leak oil onto the exhaust or into connectors are not “just a leak.” They are fire and misfire risks.

Replace leaking pass-through seals when the cover is off. Do not silicone a diesel valve cover as a substitute for the OEM gasket and locator bushings.

Followers, hydraulic lash, and mechanical clearance

Hydraulic lash adjusters take up clearance as oil pressure rises. 6.7 Power Stroke, Duramax, EcoDiesel, and TDI engines typically use HLAs or hydraulic cam followers. A collapsed HLA ticks, holds a valve slightly off the seat, or both. Pumping a collapsed adjuster with assembly lube on the bench is not a repair; replace it. Debris from a failed oil cooler (classic 6.0 Power Stroke) or sludge from the wrong oil will stick HLAs. Prime oil galleries on reassembly as specified so the first start is not dry.

Mechanical lash is the 6.7 Cummins way: a specified cold or hot clearance at a specified crank position, intake and exhaust often different. Adjust only on a correctly equalized bridge, with the cam on the base circle for that valve. Too tight burns valves; too loose hammers tips, bridges, and pushrods and can throw off injector timing feel on a running engine even though the Cummins cam does not drive the common-rail pump the way an old IDI injection pump was geared.

Inspect cam followers (flat tappet or roller) for pitting, roller flat spots, and retainer clips that keep rollers in the bore. Roller followers that have sat in a failed-oil 6.7 Power Stroke can brinell. Replace them in the sets the OEM requires; do not put one new roller next to seven worn ones.

If the engine uses retainers or keepers on the HLA (some OHC bucket designs), install them. A bucket that rotates out of position on a TDI cam will wipe a lobe.

Overhead camshafts: journals, bearings, endplay

EcoDiesel V6 and TDI inline-four heads are OHC (the EcoDiesel is dual overhead cam on each bank). There is no pushrod story here. The cam journals run in the head or in a carrier with caps.

Measure:

• Journal diameter and out-of-round
• Cap or carrier bore, then calculate clearance
• Endplay with a dial indicator at the thrust location

Excess clearance knocks, drops oil pressure at the cam, and lets the cam walk so the CMP pattern jitters. Excess endplay is a thrust-bearing or thrust-face problem; shimming by guess is not the TDI procedure. Replace the cam, carrier, or head as specified. Score marks from oil starvation after a dry start or a failed chain-tensioner dump are cause to inspect every journal, not to polish one and send it.

Reinstall OHC cams with the locking tools the belt or chain procedure requires. Loosening a TDI cam sprocket without locking the cam is how tooth correlation is lost even if the belt “still has the old paint marks.” Torque cam caps in the published sequence; a cocked cap pinches a journal.

OHV engines still have a cam, but it lives in the block—that measurement set is engine-block work. What you still do with the head off is inspect the drive end, the CMP trigger, and any rocker geometry that the cam lift has to match.

Timing drive: gears, chains, and belts

A9 asks you to inspect and time the drive gear train as gear, chain, and belt systems. Light-duty diesels use all three.

Gears: inspect teeth, backlash, hub-to-key fit, and timing marks or dowel orientation. Assemble with the specified crank and cam positions. A Cummins gear train that is one tooth off may still crank; it will not run right, and valves can meet pistons.

Chains: measure slack or link elongation as specified, replace brittle plastic guides, and do not reuse a tensioner that has no ratchet left. That is EcoDiesel and Ford 3.0 territory—the 6.6 Duramax camshaft is gear-driven behind its front cover, so inspect gear teeth and backlash there instead of hunting for a chain. Duramax front covers still must sit on their dowels; a cocked cover walks the crank seal and skews the camshaft-position trigger.

Belts: TDI belts are a scheduled interference-engine service. Lock the cams and crank, install a new belt, tensioner, and usually the water pump as a kit, rotate the engine two full revolutions by hand, and recheck marks. If the belt was already skipped, stop cranking and perform a leak-down or borescope for bent valves before you install a new belt on a head full of damage.

After any drive job, verify cam/crank correlation with a scan tool where the manufacturer gives an actual-versus-desired angle, and with a cranking waveform if needed. Paint marks on a dirty cover are not correlation data.

Worked examples

Power Stroke 6.7. Tick at the rear of the engine after a clutch job. The rear gear housing was resealed; a gear was a tooth off. Front-cover “timing” habits from a Duramax do not apply. Correct the rear gear timing and confirm CMP/CKP data.

Duramax. Crank/cam correlation code after a water-pump job that never opened the gear train. The front cover was pried off a dowel. The gears did not jump, but the CMP sensor in the cover now reads a skewed wheel. Reseat the cover; do not throw a cam at it first.

Cummins 6.7. No-start after a gear-housing reseal. Marks “looked close.” One tooth off on the cam gear. Hand-turn before first start; you should have felt the valve-to-piston tight spot.

TDI. Belt dust in the cover at 80,000 miles on an overdue belt. Do not crank. Lock what you can, inspect valve pits in the pistons, then decide head versus belt-only.

EcoDiesel. Rattle at cold start, then a correlation code. Chain tensioner and guides are the first mechanical suspects, then cam endplay if the rattle stays after the chain is right.

Valve-train traps

• Treating every light diesel as hydraulic lash, or every light diesel as Cummins lash
• Ignoring oil holes in pushrods and rocker shafts
• Unplugging a Duramax under-cover injector harness full of oil and calling the miss “a bad injector” without sealing the pass-through
• Timing a 6.7 Power Stroke from the front like a 6.7 Cummins, or hunting for a timing chain on a gear-driven Duramax
• Cranking a skipped TDI belt to confirm a start
• Skipping two hand revolutions and a correlation check after a belt or chain job