Driveability Corner Turbocharging challenges basic assumptions about how an engine works. It’s interesting to see how it affects the readings produced by the sensors designed to monitor the engine operation.

n my previous column, we looked is compensated for variations in air pressure at how a Volkswagen Passat’s turbo- due to elevation and atmospheric conditions. charged engine affected manifold ab- Positively pressurized (turbocharged) engines solute pressure (MAP) sensor readings. generally have turbulence in their intake man- This time, we’ll consider how MAP ifolds. Unlike mass airflow (MAF) sensors, Mark systems should work (based on our as- MAP sensors are not subject to measurement Warren sumptions) and how a 2016 Ford Fusion 1.5L errors due to intake turbulence. MAF sensors IEcoboost engine actually works relative to depend on laminar airflow in the sensor bore; [email protected] those assumptions. For test purposes, I’ve at- the same amount of air is flowing over sensor tached a ¾-in. plumbing gate valve to a large as over the entire bore. A MAP sensor is al- vacuum connection on the intake manifold. ways behind the throttle and a MAF sensor is I can dial in a large, controlled vacuum leak always in front of the throttle. with this valve. Several absolute, inviolable truths form the Before we begin, let’s review several MAP basis for the operation of an engine man- operational characteristics. The MAP sensor agement system’s sensors. First, MAP is the measures true (absolute) air pressure in the opposite of intake manifold vacuum. Second, intake manifold. While MAP stands for man- MAP equals barometric pressure (BARO) mi- ifold absolute pressure, you can also think of nus intake manifold vacuum. Third, vacuum it as manifold air pressure. Absolute means it equals BARO minus MAP. And last, always Fig. 1 remember that vacuum is variable relative to el- evation. Next, let’s review MAP theory so we can under- stand how a MAP sensor should react to vacuum leaks in a turbocharged engine. MAP systems are immune to vacuum leak errors. To a MAP sen- sor, pressure is pressure. MAP systems assume an engine is breathing cor- rectly and does not have volumetric efficiency (VE) problems. Conse- quently, MAP systems are subject to erroneous readings if an engine has VE problems caused by a worn engine, valve timing issues, a plugged exhaust or other engine mechanical conditions.

Screen captures: Mark Warren We’re ready to begin

16 December 2016

Driveability DEC16.indd 1 11/18/16 2:11 PM Driveability Corner

Fig. 2 our baseline testing, begin- ning with the key on, en- gine off (KOEO). Testing occurred in south Florida at darn near sea level. The KOEO MAP reading was 30 in.-Hg and temperature was 75°F. The KOEO MAP reading is a good baseline sensor test. MAP should equal atmospheric pressure. What do we expect to happen next? The engine needs a fixed amount of air to maintain idle speed at these atmospheric and load/ rpm conditions. As I intro- duce the vacuum leak, the throttle position sensor (TPS) reading should close down to compensate for the air bypassing the throt- tle. The engine is getting tem is measuring pressure, and pres- the accelerator pedal position (APP) the same amount of air, some from sure is pressure. If the pressure mea- sensor to the floor (Request WOT). the vacuum leak and some from the surement is correct, then the correct The APP trace is not included in the throttle, and should maintain the amount of fuel should be command- chart. Note that the TPS does not go same MAP reading. Fuel trims should ed and the fuel trims should remain to WOT instantly, as it would with the remain the same, as the engine is get- unchanged (near zero). direct connection of a throttle cable. ting the same air and the same MAP At idle we saw the theory hold. At The TPS takes about 3 seconds to reading. WOT/atmospheric pressure in the achieve WOT and makes the transi- Look at Fig. 1 on page 16. The TPS intake, a vacuum leak should have no tion in steps. starts at 14%. As the vacuum leak effect. Pressure in the intake is the I have a measurement cursor at the valve is opened over 60 seconds, the same as outside the intake. At pos- manifold atmospheric pressure point TPS drops to 9%, a 36% change. The itive pressure (above atmospheric), (30 in.-Hg). Follow that cursor ver- system maintained idle speed and the pressure should be measured cor- tically to see the load readings at the MAP. Short-term fuel trim (STFT) rectly, fuel delivery should be correct atmospheric point. Calculated load was 1% to 2% baseline and moved to and fuel trims should be near zero. and absolute load are both 78.5% 4% to 2% during vacuum leak (no sig- In this test, LTFT remained near ze- here. It looks like VE is 78.5% at at- nificant change). Long-term fuel trim ro and STFT went from about zero mospheric pressure for this vehicle. (LTFT) made no change. Yeah! Our at the point of acceleration, to −20 Manifold absolute pressure (blue theory holds up on this car at idle! STFT over the 13 seconds of accel- trace) peaked at 62.5 in.-Hg, or about Note the MAP at idle is 17 in.-Hg. eration with the accelerator pedal to two times atmospheric. Note that This is interesting. On a normally as- the floor. when this pressure was reached, the pirated engine, I would expect a tight What effect should the vacuum TPS dropped/closed partially and engine at sea level to have about 20 leak have? It should reduce overall then moderated to maintain near this in.-Hg of intake vacuum and there- performance, caused by a loss of in- pressure. All this time the APP was at fore 10 in.-Hg of MAP (30 in.-Hg take pressure from the turbo, but not WOT request (pedal to the floor). It’s BARO minus 20 in.-Hg vacuum affect correct pressure reading and not surprising that the throttle/TPS equals 10 in.-Hg MAP). The higher fuel delivery/fuel trims. It would be command is not directly correlated to MAP/lower vacuum reduces suction interesting to compare fuel trim with the APP WOT request. Follow the vi- throttling/pumping losses. and without the vacuum leak to see if olet vehicle speed sensor (VSS) trace The vacuum leak is wide open in fuel trim change remains the same or to see the load applied. Fig. 2 above. The recording is from if the vacuum leak is the cause of the Keep in mind that the effects of 0 mph/idle to wide-open throttle fuel trim change. our test were recorded on this specif- (WOT). Speed went from 0 to 65 Our test produced some interesting ic vehicle. These results may or may mph and back to closed-throttle de- results and observations. Look at the not carry over to similar turbocharged cel. The theory here is that the sys- TPS in Fig. 2 (orange trace). I had vehicles.

18 December 2016

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