Smoke Opacity Test and SAE J1667 Explained

Short answer: SAE J1667 is the standard snap-acceleration procedure used to measure diesel smoke. The tester puts an opacity meter on the exhaust, the engine is revved quickly from idle, and the meter reads how much light the smoke blocks. Under Clean Truck Check, 2012 and older diesel trucks without OBD get this smoke opacity test plus a visual inspection. The pass/fail limit is set by CARB and depends on the engine model year.

What is SAE J1667?

SAE J1667 is a published test procedure for measuring smoke from heavy-duty diesel engines. It gives testers a repeatable way to load the engine and read exhaust opacity, so results are comparable from truck to truck. CARB uses this kind of snap-acceleration test for older diesel vehicles in its programs.

"Opacity" simply means how much the exhaust smoke blocks light. Clean exhaust lets light through. Heavy black smoke blocks a lot of it.

How does the J1667 snap acceleration test work?

In general terms, the test goes like this:

1. The truck is parked and warmed up. The engine should be at normal operating temperature, with the transmission in neutral and the wheels chocked.
2. The opacity meter is set up. A sensor is placed at the exhaust outlet to read the smoke.
3. The engine is snapped. From idle, the accelerator is pressed quickly to the floor and released, letting the engine rev up and come back down.
4. This is repeated. A series of snaps is done so the reading reflects the engine, not one odd puff.
5. The result is compared to the limit. The measured opacity is checked against CARB's limit for that engine's model year.

The whole thing is done with the truck standing still. No dyno and no road drive. That is why we can run it at your yard, jobsite or home. See smoke opacity testing for how we handle it on site.

How are pre-2013 diesel trucks tested in California?

Under Clean Truck Check, the test depends on the engine:

• 2012 and older diesel without OBD: smoke opacity test plus a visual inspection.
• 2013 and newer diesel: a CARB-verified OBD test instead. See OBD testing.

The visual inspection looks at the emissions equipment to confirm it is present and not tampered with. A truck can pass the smoke reading and still fail if equipment is missing or modified.

What is the opacity limit for diesel trucks?

CARB sets the opacity limits, and they depend on the engine model year. Newer engines, especially those built to use a diesel particulate filter, are held to tighter limits than older engines. You may see different percentages quoted online. Rather than trust a number from a forum, check CARB's current rules for your engine, or ask us at the test and we will explain where your truck stands.

What is PSIP, and how does it relate to Clean Truck Check?

PSIP is the Periodic Smoke Inspection Program, CARB's older periodic smoke testing program for heavy-duty diesel fleets. Many fleet managers still know the smoke test from PSIP.

Clean Truck Check is CARB's current heavy-duty inspection and maintenance program. How PSIP applies today, and how it lines up with Clean Truck Check for your vehicles, is a question for CARB. Check CARB's Clean Truck Check page and CARB's own guidance before you drop any older testing routine. Our Clean Truck Check vs smog check guide covers how the programs are often confused.

What makes a diesel fail the opacity test?

Smoke usually points to something the engine needs. Common causes include:

• A missing, damaged or removed diesel particulate filter (DPF)
• Injector, turbo or air intake problems
• Exhaust leaks that affect the reading
• A cold engine that was not warmed up
• Tampered or deleted emissions equipment, which fails and is illegal

Our how to pass the CARB smoke test post goes deeper on prep, and DPF deletes and Clean Truck Check explains why deleted trucks cannot pass.

How do I prepare for a smoke opacity test?

A few simple steps help:

1. Drive or idle the truck to full operating temperature before the tester starts.
2. Fix known engine problems first. Smoke on a hard pull is a warning sign.
3. Check the exhaust for leaks and make sure the DPF is in place.
4. Park on level ground where the engine can be revved safely.
5. Check your deadline in CTC-VIS so the test lands inside your window.

What happens if the truck fails?

A failed truck needs repair, then a retest. We do not do repairs, but we explain the result in plain English so your mechanic knows what to look at. See failed Clean Truck Check: what to do.

How Diesel Dynamix helps

We perform OBD and smoke opacity tests on site across Southern California, including Los Angeles, the Inland Empire, Orange County, San Diego and El Centro. Older dump trucks, construction trucks and farm trucks often need the smoke test, and we can run it where they work.

• Snap-acceleration smoke testing and visual inspection at your location
• Plain-English results and the next due date
• Help with CTC-VIS questions and reminders before the next test
• Per-truck pricing for fleets. See fleet testing

Read how testing works, then book a smoke test or call or text (858) 342-8328.