MAF grams per second per litre: the rule of thumb, and a better check
A warm, healthy, naturally aspirated engine at idle often shows a MAF reading of very roughly 1 g/s per litre of displacement, a rule of thumb many technicians use as a quick sanity check. It is only a first look. The better check is to work out what the engine should be drawing from its displacement, rpm, manifold pressure and intake air temperature, and divide the MAF reading by that. Close to 100 percent is healthy; short at idle only points to unmetered air getting in around the MAF; short and getting worse as airflow rises points to a MAF reading low; above the estimate everywhere points to a MAF reading high.
Where the rule of thumb comes from, and where it breaks
An engine is an air pump. A four stroke engine fills each cylinder once every two turns, so at idle it moves its displacement roughly six times a second, but at a manifold pressure of about 30 kPa, less than a third of atmospheric density. Work that through and a warm engine at idle draws somewhere between about 1 and 1.5 g/s per litre. The four engines in the TrimCase model come out between 1.1 and 1.5 g/s per litre at idle, in the same neighbourhood as the rule.
The rule breaks wherever idle speed, manifold pressure or temperature differ: a high idle, an air conditioning load, altitude, a hot intake, a big cam. A reading 20 percent off the rule can be perfectly healthy, and a leak that costs 10 percent can hide inside the rule's range. That is why the rule only tells you whether to look closer.
The speed density check
The air an engine draws, in grams per second, is: volumetric efficiency x displacement in litres x rpm / 120 x MAP in kPa / (0.287 x intake air temperature in kelvin). Volumetric efficiency (VE) is how well the cylinders fill relative to the manifold; around 0.7 to 0.85 at part throttle is a reasonable working assumption, and the model page lists the reference values TrimCase uses. Divide the MAF reading by this estimate and you have the airflow check every TrimCase case reports.
| Engine | Idle | 2,500 rpm, no load | Cruise | WOT |
|---|---|---|---|---|
| 2.0 L inline four, narrowband upstream sensor | 1.5 | 3.8 | 7.1 | 42 |
| 2.5 L inline four, wideband air fuel sensor | 1.4 | 3.8 | 6.4 | 42 |
| 3.5 L V6, dual bank, wideband air fuel sensors | 1.3 | 3.7 | 5.5 | 42 |
| 5.3 L V8, dual bank, narrowband upstream sensors | 1.1 | 3.1 | 4.4 | 38 |
Some scan tools show airflow in pounds per minute. One pound per minute is about 7.56 g/s, so convert before comparing.
How to check airflow, step by step
- Record the inputs. Warm engine, closed loop. Note rpm, MAP, intake air temperature and the MAF reading at idle and at 2,500 rpm with no load.
- Work out the estimate. Multiply VE by displacement, by rpm divided by 120, and by MAP, then divide by 0.287 times the intake temperature in kelvin.
- Divide the MAF by the estimate. Express the MAF reading as a percentage of the estimate in each cell.
- Add a WOT reading if it is safe. Where it can be done safely and legally, on a dyno or a closed road, record a wide open throttle pull, which shows whether the shortfall grows with flow.
- Read the pattern. Short at idle only means unmetered air. Short and growing with flow means the MAF reads low. High everywhere means it reads high. Short at WOT only with normal trims means the engine cannot breathe out.
- Confirm on the car. Inspect the MAF element and the intake duct, check the air filter, smoke test the intake, or compare with a known good sensor, as the service information directs.
Worked example: a 2.0 L at idle
The model's 2.0 L inline four idles at 750 rpm with a MAP of 30 kPa and an intake temperature of 38 C (311.15 K), with a reference VE of 0.72. The estimate is 0.72 x 2.0 x 750 / 120 x 30 / (0.287 x 311.15) = 3.02 g/s, or 1.5 g/s per litre.
| Engine state | Idle | 2,500 rpm | Cruise | WOT |
|---|---|---|---|---|
| Healthy | 100% | 99% | 99% | 100% |
| Vacuum leak | 87% | 94% | 96% | 100% |
| MAF reading low | 95% | 91% | 89% | 74% |
The vacuum leak's MAF reads 2.63 g/s at idle, 87 percent of the estimate, and recovers to 100 percent at WOT where the leak stops flowing. The failing MAF reads nearly right at idle, 95 percent, and falls to 74 percent at WOT. Both would pass the 1 g/s per litre rule at idle. Only the check across cells tells them apart.
Common mistakes
- Treating the rule of thumb as a pass or fail limit. It is a sanity check with a wide margin.
- Leaving MAP and intake temperature out. Air density is half of the estimate.
- Comparing a pounds per minute reading with a grams per second rule.
- Blaming the MAF for low WOT airflow while the trims are normal. A restricted exhaust also caps WOT airflow.
- Applying naturally aspirated numbers to a turbocharged engine, whose manifold pressure goes above atmospheric.
Practise it
- Every case in the daily drill reports the airflow check in all four cells. Redo one with a calculator.
- Compare the MAF under reporting and vacuum leak fingerprints side by side.
- Read fuel trim against load and airflow on the model page.
Sources
Last reviewed . Standards and regulations are named so you can look them up; the service information for the vehicle always governs.
- SAE J1979, the OBD II diagnostic services standard, and its successor SAE J1979-2 for OBD on UDS. They define the fuel trim parameters (PIDs 06 to 09: short and long term trim for bank 1 and bank 2, scaled from minus 100 to plus 99.2 percent, where positive means fuel is being added) and the mass airflow parameter (PID 10, in grams per second).
- Manufacturer service information for the vehicle in front of you. It sets the real trim limits, when a lean or rich code is allowed to set, which side is bank 1, and the confirmation tests. These vary by make, engine and model year, so no number on this page replaces them.
- The TrimCase engine model, which computes every number in the worked example on this page.
TrimCase is a training aid built on a simulated engine model. It is not a diagnostic instruction for any real vehicle.
Frequently asked questions
What should a MAF read at idle?
Very roughly 1 g/s per litre of displacement on a warm, healthy, naturally aspirated engine, so a 2.0 L near 2 to 3 g/s and a 5.3 L near 5 to 6 g/s. Idle speed, manifold pressure and temperature move it, so check it against a speed density estimate before calling it wrong.
What should a MAF read at 2,500 rpm with no load?
In the TrimCase model a healthy engine draws about two and a half to three times its idle airflow at 2,500 rpm with no load. On a real engine, work it out from rpm, MAP and intake temperature rather than relying on a ratio.
How do I convert pounds per minute to grams per second?
Multiply by about 7.56. One pound is 453.6 grams and a minute is 60 seconds.
Can a vacuum leak make the MAF read low?
Not the sensor itself. The MAF measures the air that passes it correctly, but the leak lets more air into the engine than that, so the MAF reads short of what the engine is actually drawing, most of all at idle.