INTERNAL ENGINE CONDITION VS. FUEL TRIMfuel trims. As the exhaust valve leak in-creases, the short-term fuel trims are driven negative enough to force the long-term fuel trims to go

INTERNAL ENGINECONDITION VS.

FUEL TRIM

INTERNAL ENGINECONDITION VS.

FUEL TRIM

30 March 2015

BY THOMAS R. TILLMAN

Driveability symptoms areoccasionally caused by enginemechanical problems. A pulse

transducer provides arelatively easy way to get anidea of what’s going on inside

an engine—good or bad.

Pg_EDIT_Engine:Pg_EDIT_ 2/19/15 3:54 PM Page 1

When diagnosing rich or lean trou-ble codes, we always start by re-viewing the fuel trims. The fueltrims show what the computer istrying to do to correct the prob-lem. Unfortunately, when we

have a rich or lean condition, we usually look forexternal engine problems first. The internal cylin-der condition is just as important, however. Oftenwe even overlook checking the internal cylindercondition altogether.Let’s face it, new engine designs have made it a

lot harder to run a simple compression test. But theinternal cylinder condition still needs to be verified.There are several minimally invasive tests that willverify each cylinder’s contribution to overall engineoperation. However, they really can’t show directlywhat’s causing the fuel trim correction.Using a pulse transducer connected to the ex-

haust tailpipe or the intake manifold can showwhat’s going on inside the cylinders. This is partic-ularly true for the valve operation. Let’s thinkabout this: Can a leaking valve affect the fueltrims? The answer is, yes it can.There are four simple, minimally invasive tests

using a pulse transducer that can be performed totest internal engine condition. They are:•An open-throttle cranking exhaust pulse test.

An example of a high-pressure exhaust test isshown in Fig. 1 on page 32.•A closed-throttle cranking exhaust pulse test.

An example of a low-pressure exhaust test isshown in Fig. 2.•A closed-throttle cranking intake manifold

vacuum test. An example of a high-vacuum intaketest is shown in Fig. 3 on page 34.•An open-throttle cranking intake manifold

vacuum test. An example of a low-vacuum intaketest is shown in Fig. 4.These tests must be synchronized with the cylin-

der 1 spark or injector. This allows us to identify thecylinder with the problem. When synchronizingthe spark, make sure to use the correct firing orderoffset. However, when synchronizing with the in-jector, the cylinder 1 exhaust pattern will line upwith the cylinder 1 injector. The injectors are usual-ly triggered during the cylinder’s exhaust stroke, al-though direct injection is triggered at a differentpoint in time. The synchronization still could beused, after verification of the triggering point.Before we go any further with testing, we must

verify that the oxygen sensors are operating prop-erly. A faulty O2 sensor can lead us to incorrectfuel trim conclusions.Each of these tests can check and verify the

condition of the valves as well as other cylinderproblems. Let’s take each test and relate it to thefuel trims. Keep in mind that fuel trims are ad-

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justed while the engine is running. Acranking test shows only the possiblecauses for the fuel trim corrections.

Let’s start with the cranking exhausttailpipe test. This test allows us to seethe exhaust valve’s ability to manipulatepressure. If one cylinder’s exhaust valvefails to seal, how would that affect thefuel trims? Would you have positive ornegative fuel trims? Also, would bothlong-term and short-term fuel trims beaffected?

First, an exhaust valve leak usuallystarts as an intermittent problem thatmay not be noticeable to the driver orset a code. However, the O2 sensormonitors the amount of unburned air inthe exhaust manifold. As soon as theproblem occurs, the O2 sensor sensesthe change. The computer instantlymakes the adjustments to the short-term fuel trim, which in this case wouldfirst show a negative adjustment. Butthe fuel trims could show positive ifenough unburned air/fuel escapes intothe exhaust manifold.

Fig. 5 on page 36 shows that whenthe Kia Rio’s exhaust valve failed to seal,the burned pressurized gases leaked in-to the exhaust manifold, creating a mo-mentary excessive pressure. A smallcylinder’s leakage will normally showaround that particular cylinder’s exhaustpulse. When the valve initially closes,the actual sealing may not take placeright away. As the leak occurs, some ofthe excess pressure can possibly passthrough to other cylinders that havetheir exhaust valves open. At a certainpoint in time, each cylinder’s valves areboth open. This allows the exhaust gasesto pass back into the intake manifold.Performing the cranking intake mani-fold vacuum test can show the exhaustpressure traveling back into the intakemanifold through a particular cylinder.

The intake pressure pulse would beslightly higher (positive pressure) onthat cylinder than on the rest. The ex-haust gases now will displace theamount of air in the intake manifold.Once these gases get reburned, theamount of remaining air entering theexhaust becomes minimal. The O2 sen-sor reads it as a rich condition. Thecomputer, in turn, makes a negativetrim adjustment. A minor intermittent

leak will show only on the short-termfuel trims. As the exhaust valve leak in-creases, the short-term fuel trims aredriven negative enough to force thelong-term fuel trims to go negative aswell. Remember, even with a small leak,a cylinder can still be ignited. An exam-ple of this is an engine that misfires atidle only.

In the case of a major exhaust valveleak, as shown in Fig. 6 on page 36, a

large amount of unburned pressure isblown out during the compres-sion/power stroke. This causes the ex-cessive pressure to appear on the com-panion cylinder’s exhaust pulse. Thehigh loss of pressure would also causethe cylinder to misfire. The amount ofunburned air/fuel being pumpedthrough forces the fuel trims to becomepositive. When the actual leaking cylin-der’s exhaust valve opens, the pulse will

INTERNAL ENGINE CONDITION VS. FUEL TRIM

This pattern actually shows the manipulation of the pressure by the valve. This is a typicallow-pressure pulse. Larger engines, like those with six or eight cylinders, will be different,but part of this pattern will still appear and the diagnostics are the same.

32 March 2015

Fig. 2: Closed-Throttle Cranking Exhaust Pulse Test – 1998 Pontiac Sunfire 2.2L

This is a typical 4-cylinder exhaust pulse pattern. This vehicle has 17,000 miles on the clock.

Fig. 1: Open-Throttle Cranking Exhaust Pulse Test – 2005 Mazda 2.3L


appear smaller than normal.The open-throttle cranking exhaust

pulse test creates a high-pressure pulsein the exhaust manifold, making theleaking exhaust valve easier to spot.Once the cylinder and valves are identi-fied, the closed-throttle cranking ex-haust pulse test is performed.This test creates a low-pressure pulse

in the exhaust manifold. The valve pulsepattern is smaller, and more detailed.The high-pressure test can also forcethe valve to close and seal, so that pat-tern could show intermittently. Thelow-pressure test allows the leak to be-come consistently visible. If the leak is alarge blowout, then the low-pressuretest will still show the leak on the com-panion cylinder exhaust pulse.Next, let’s look at the cranking intake

manifold vacuum test, which checks theintegrity of the intake valves. Again, ifone cylinder intake valve fails to seal,how would that affect the fuel trims?Just like a problem with an exhaustvalve, an intake valve usually starts as anintermittent problem that may not benoticeable or set codes. But this timethe O2 sensor would see a lean condi-tion. The computer will make a positivefuel correction. The short-term fueltrim will be instantly corrected at first.If the leak is large enough, the long-term fuel trim will also be affected.How does a leaking intake valve cre-

ate the positive fuel trim adjustment?As shown in Fig. 7 on page 37, whenthe intake valve fails to seal, the com-pression and power stroke force pres-sure back into the intake manifold. Oneor more of the other cylinders will re-ceive this pressure, creating a turbo ef-fect in the intake and on the other cylin-ders. The cylinders receive a lot moreair than the computer has calculated towork with. The added air entering thecylinder forces the O2 sensor to see alean condition.The closed-throttle cranking intake

manifold test can show which cylinderis leaking and which cylinders are beingaffected. The pressure pulse would riseover the next cylinder’s intake strokes.The open-throttle cranking intake man-ifold test would show the leaking valveonly. The other cylinder’s valves willshow a normal pulse at this point.

There are extreme cases where thecranking exhaust or intake patterns canbe confusing. An example is shown inFig. 8. At this point, additional testing isneeded to identify the cylinder with theproblem.An old Chevy Nova came in to the

shop with a broken exhaust valve spring.When the closed-throttle cranking ex-haust pulse test was performed (Fig. 9,

page 38), the cylinder with the brokenexhaust valve spring showed twice onthe pattern. What’s taking place here isthat the bad cylinder’s pulse is appear-ing on its exhaust pulse as well as on thecompanion cylinder pulse. The valve isremaining slightly open at all times.You’d think this was caused by two deadcylinders instead of one.Synchronizing cylinder 1 would nar-


The firing order offset on this vehicle is +2. This waveform identifies the actual cylinder pulse.

This pattern is synchronized with the spark from cylinder 1. The possible opening point ofthe intake valve is noted here. The closing point can’t be determined here, due to cylinderpulse overlap. This test generates less than 1 in.-Hg of vacuum. Even with that smallamount of vacuum, a pressure a pulse is recorded. The pulse has been amplified for clarity.

34 March 2015

Fig. 4: Open-Throttle Cranking Intake Pulse Test – 1998 Pontiac Sunfire 2.2L

Fig. 3: Closed-Throttle Cranking Intake Pulse Test – 1998 Pontiac Sunfire 2.2L


row the problem to two cylinders—thecylinder with the defective valve and itscompanion cylinder. The defectivecylinder’s pulse pressure is exceedingthe pressure of the companion cylinder.Performing a cranking relative com-pression test would correctly identifythe actual dead cylinder’s location. Thecranking closed-throttle intake test can

also help identify the correct cylinder(Fig. 10).

The fuel trims for a broken valvespring would be extremely positive onboth short-term and long-term trims.Of course, this would depend on theseverity of the leak. If the cylinder hadthe same condition on an intake valve,then the fuel trims could be negative.


This vehicle had a code P0301 (cylinder 1 misfire) that was felt at idle only. The cylinder 1exhaust valve was not sealing, as shown in the waveform.

This vehicle had a code P0306 (cylinder 6 misfire). This leak occurred on the cylinder 6 com-pression/power stroke and appeared on the cylinder 1 exhaust pulse. The leak traveledthrough the manifold and overwhelmed the cylinder 1 pulse.

Circle #17

36 March 2015

Fig. 6: Open-Throttle Cranking Exhaust Pulse Test – 2001 Mercury 4.6L

Fig. 5: Open-Throttle Cranking Exhaust Pulse Test – 2001 Kia Rio


This is because the air/fuel mixture isbeing forced back into the intake mani-fold upon ignition. Cylinder identifica-tion would still be an issue here as well.Again, performing the cranking relativecompression test here would display thecorrect cylinder.The pulse sensor can also be used to

conduct an intake or exhaust valve test

on a running engine. Problems with thevalves would still be displayed. Howev-er, ignition of the air/fuel mixture wouldinfluence the pattern as well. All thisneeds to be considered when we evalu-ate the recorded pattern. We need tokeep in mind that cylinder condition isonly one contributing factor that affectsthe fuel trim correction.

37March 2015

Circle #13

The cylinder 4 intake valve on this vehicle failed to seal. This caused the pressure to beforced back into the intake manifold. The cylinder 3 intake pulse was already halfwaycompleted; however, cylinder 2 received the majority of the pressure.

Cylinder 4 shows a constant leak at this point. It appears twice in the same trace.

Fig. 8: Open-Throttle Cranking Intake Pulse Test – 2004 Jeep Liberty

Fig. 7: Closed-Throttle Cranking Intake Pulse Test – 2004 Jeep Liberty


An example of this is a ’99 Ford Ranger that came inwith the MIL on. The code retrieved was a P0301 (cylin-der 1 misfire). All the cranking test results looked good.However, the running intake vacuum test did suggest anintake manifold problem. The cylinder 1 intake pulse pat-tern showed more turbulence and was different from theother cylinders (Fig. 11, below). Also, the cylinder 1 spark

pattern showed a lean misfire. The fuel trims recorded inthe freeze frame were: Bank 1 Sensor 1 Long Fuel Trim12% and Bank 1 Sensor 1 Short Fuel Trim 3%. Sprayingintake cleaner around the manifold at cylinder 1 causedthe misfire to disappear momentarily. Unfortunately, thecustomer declined further service due to the age andmileage of the vehicle.

As you can see, there’s a direct relationship between cylin-der condition and fuel trims. If the computer is making a fueltrim correction, then you definitely want to verify cylindercondition. These tests can be performed very quickly, and youcan save yourself a lot of time and frustration by checkingcylinder condition first. Remember, an engine can leak inter-nally as well as externally.


This article can be found online atwww.motormagazine.com.

Circle #18

38 March 2015

Cylinder 5 has a confirmed broken valve spring. With the valve remain-ing slightly open, the pulse shows on cylinder 5 as well as on cylinder 2.

Fig. 9: Open-Throttle Cranking Exhaust Pulse Test – Chevy Nova

Code P0301 (cylinder 1 misfire) was set. Cylinder 1 shows more tur-bulence than the other cylinders. Also, cylinder 1 spark showed alean misfire. Spraying intake cleaner around the cylinder 1 intakerunner momentarily slowed the leak and corrected the misfire.

Fig. 11: Running Intake Pulse Test – 1999 Ford Ranger

The cylinder 5 intake pulse shows a slight increase in pressure as com-pared to the other cylinders, confirming the broken valve spring.

Fig. 10: Closed-Throttle Cranking Intake Pulse Test – Chevy Nova


Documents

INTERNAL ENGINE CONDITION VS. FUEL TRIMfuel trims. As the exhaust valve leak in-creases, the short-term fuel trims are driven negative enough to force the long-term fuel trims to go