Tech Feature: Valve Timing Terror Tales

Since Pontiac introduced “rubber” timing belts with cogged teeth on its six-cylinder overhead camshaft engine during the late ’60s, anybody who’s made a living fixing cars has experienced the free-winding sound of a starter motor cranking an engine with a stripped timing belt.

Back in the day, timing belt replacements were recommended for most ­engines around 60,000 miles. That interval was recently increased to 100,000 or more miles by many manufacturers to meet various state emissions mandates and to reduce maintenance needs.

While a lot has been written about timing belt replacement intervals, not a lot has been written about diagnosing timing belt and valve timing-related driveability complaints. Because a valve timing-related driveability problem is relatively rare, information about such problems has been difficult to collect.

But, because I’ve been doing more mobile diagnostics lately, I’m beginning to see more driveability problems caused by faulty valve timing. With those thoughts in mind, let’s take a look at how valve timing affects engine performance in this “Diagnostic Dilemma.”

Intake Valve Timing
To understand how valve timing works in modern engines, let’s ­visualize a column of air moving through an intake port on a naturally aspirated engine. When the intake valve opens, a low pressure develops at the valve head that extends the column of air like an accordion through the length of the intake port.

When the valve closes, the moving column of air compresses like an accordion as it slams against the closed valve. At this point, the moving column of air creates a mild supercharging effect by building a slight amount of positive air
pressure in the intake port.

In general, there’s no significant air flow through a valve until it lifts about 0.050” off the valve seat. While effective lift might be less on a standard passenger car engine, the concept of effective lift generally dictates that the intake valve begins opening before TDC.

After TDC when the piston begins moving down in the cylinder, the valve is open far enough to allow air in the intake port to achieve maximum flow as the piston travels downward on the intake stroke. Because air velocity remains high in the intake port and continues to “pack” into the cylinder, the intake valve remains open until slightly after bottom-dead-center (BDC) to achieve optimum cylinder filling.

Keep in mind that valve opening and closing times are critical to engine performance. Advanced intake valve closing times tend to increase low speed compression and torque while retarded intake valve closing times tend to increase high-speed air flow. While exhaust valve timing isn’t as critical, changing the valve opening and closing times tend to produce similar results.

As we shall see later, variable camshaft timing systems on modern engines utilize these effects in combination with tuned intake manifolds to vastly improve the throttle response and performance of relatively small displacement engines.

Exhaust Valve Timing
Let’s look at exhaust valve timing as the piston travels from power to exhaust strokes. During the power stroke, most of the effective cylinder pressure dissipates as the piston approaches 45° after TDC and is nearly gone by the time the piston reaches 90° after TDC. For this reason, and because the valve needs to achieve  effective lift, the exhaust valve begins relieving cylinder pressure by opening before BDC.

Keep in mind that exhaust valve opening times aren’t as critical as exhaust valve closing times. Exhaust valve opening simply releases pressure from the cylinder and allows exhaust gases to start flowing into the exhaust port.

But exhaust closing times do have a great deal to do with fuel economy and power output. As the piston travels upward, the spent exhaust gasses achieve a very high velocity through the open exhaust valve and port. As the piston reaches TDC, this high velocity column of exhaust gas creates a slight negative pressure in the cylinder.

To optimize this negative pressure, the exhaust valve remains open at TDC. Because the intake valve is beginning to open at TDC, this creates a valve timing “overlap” condition between the intake and exhaust valves.

In short, the negative pressure formed in the exhaust port helps draw the intake mixture through the open intake valve. The exhaust valve must be held open long enough to effectively utilize this negative pressure, but not long enough to allow any portion of the intake mixture to flow out the exhaust port.

In summary, engineers improve air flow through naturally aspirated engines by manipulating valve opening, closing and overlap timing. The opening and closing times of the intake valves are critical to low- and high-speed performance. And last, the closing time of the exhaust valve is critical to initiating air flow into the cylinder when the piston is in the overlap position.

In modern engine design, valve timing on both the intake and exhaust camshafts has become critical to efficient engine performance.

Valve Timing Caveats
When we discuss valve timing, we should remember that turbocharged and supercharged engines require little or no valve overlap and require less valve-open time to fill the cylinder with air.

As a side note, the reduced dynamic friction of roller lifters also allows engine designers to incorporate even more creative camshaft profiles into their engine designs.

Because the valve can be opened faster with less stress on the lifter and cam lobe, the valve-open time can be reduced to prevent reversion or backflow into the intake and exhaust ports when the engine is artificially aspirated.

Case Study #1
Early this year, I received a call from a local shop about a 2001 Toyota Tundra with about 40,000 miles on the odometer that was equipped with the 3.4L V6 engine.

The Tundra was experiencing an intermittent cranking, no-start complaint with all cylinders misfiring on the driver’s side bank. The complaint occurred as the owner was backing out of his garage. Although the tow truck operator had replaced the Tundra’s original equipment battery, he couldn’t start the engine because of slow cranking speed.

Once in a warm shop, the Tundra started, but idled roughly with a flashing MIL. I’ve seen bank misfires caused by a clogged catalytic converter on dual-cat systems, but this was not the case with the Tundra.

To quickly check bank 2 camshaft timing, we ran a comparative compression test on each cylinder bank. The results were 150 psi on all bank 1 cylinders and 160 psi on all misfiring bank 2 cylinders.

Because the Toyota specification for cranking compression is 174 psi at sea level, I deducted 25% to compensate for our 8,000’ altitude. This calculation would yield about 131.0 psi normal cranking compression for our altitude. This particular calculation indicated that the engine’s compression was about 110-120% higher than normal for the altitude.

The normal failure pattern for timing belts is to lose compression by allowing the camshaft to slip into the retarded position. The fact that the camshaft timing was advanced on both cylinder banks defied the normal failure pattern. And the fact that bank 2 was advanced more than bank 1, again defied logic.

But the answer was relatively simple. While removing the timing belt covers, the shop owner found that, despite the relatively low mileage of the vehicle, the water pump had been seeping coolant onto the front of the engine block. The coolant had dried into a huge, greasy icicle that eventually fell into the timing belt sprockets.
See Photo 1.

Evidently, the crank sprocket had advanced the cam timing during a compression kickback and the cam timing on bank 2 had advanced even further. In short, this was a simple, but generally improbable cause for an intermittent cranking, no-start problem on a V6 engine with a full range of misfires on bank 2.

Case Study #2
Let’s take another case study of a 2002 Hyundai Sonata that stalled as the owner was leaving his driveway. The Hyundai, which was equipped with the 2.0L engine and automatic transmission, had about 103,000 miles on the odometer and on the original timing belt.

The engine seemed to crank normally after the battery was charged and was obviously trying to fire at least two or three cylinders.

Sometimes it’s “the scenic route,” but I still like to begin any cranking, no-start diagnosis by the non-invasive method of polling the various modules for trouble codes and checking the data stream for obvious indications of a component failure. In this case we retrieved two air bag DTCs, but those appeared to have nothing to do with the cranking, no-start complaint.

In truth, I was looking for a cam synchronization trouble code or synchronization error on the shop’s aftermarket scan tool. Since these data parameters either weren’t available or displayed on the scan tool, we had to use the more invasive method of checking for spark and fuel control by removing the spark plugs.

Oddly enough, #1 and #2 spark plugs were drenched with gasoline while numbers #3 and #4 spark plugs were completely dry.

My first thought was that one of the coils had failed on the Hyundai’s waste-spark ignition. But if a single coil had failed, the firing order would dictate that the fuel fouling would have occurred on alternate, instead of successive, cylinders.

So we checked the fuel injectors for resistance and for correct operation by testing with a lab scope and with a common ’noid light. To our surprise, injector #1 and #2 were drenching the spark plugs by staying open for 31.5 ms and 21.5 ms, respectively, while injector #3 and #4 inhibited fuel flow by staying open only for 10.5 ms and 5.6 ms, respectively.

During any diagnostic process, it’s pretty easy to bark up the wrong tree and, in this case, the injector on-times were the wrong tree to bark up. A compression test revealed a cylinder compression of 90, 83, 70 and 89, respectively.

The sea-level specification was 178 psi, which would equal about 134 psi at 8,000’ altitude, so it was obvious that one or both camshafts were running in a retarded position.

A cylinder leak test ranged from about 20% on the higher compression cylinders to about 70% on the lower compression cylinders, which indicated that at least two valves weren’t seating correctly. The uneven compression and the wide range of cylinder leakage indicated that several valves weren’t seating correctly.

After removing the timing cover, we discovered that the cogged teeth were individually peeling away from the timing belt. See Photo 2.

These missing cogs caused the exhaust camshaft to run in the retarded position but did not cause a catastrophic failure that would have resulted in a complete loss of cylinder compression.

Now, getting back to the wide variation in injector on-times, I suspect that shape of the cam and crankshaft shutters has something to do with how the PCM controls the fuel injector pulse width. See Photo 3.

A scope pattern showed that the shutter remains “on” for 136 milliseconds (ms), “off” 136 ms, “on” 72 ms, “off” 207 ms and then repeats the cycle.

When the signals generated by two shutters on the camshaft position (CMP) signal are combined with the signals generated by the two shutters on the crankshaft position sensor, it’s easy to see why the PCM becomes confused about how long to keep fuel injectors open. See Photo 4.

So, not having the correct operating strategy for the PCM at hand, I’m going to speculate that retarded exhaust camshaft timing caused the faulty injector pulse widths that contributed to this particular “Diagnostic Dilemma.”

Case Study #3
Sometimes we have an engine that runs well, but exhibits a performance complaint. In this instance, I was called upon to look at a 1999 Ford Contour equipped with the 2.0L engine and automatic transmission.

The vehicle came in with the MIL on and a P1383 DTC stored in the PCM’s diagnostic memory. The complaint was that the engine lacked low-speed acceleration.

The P1383 DTC indicates that the exhaust camshaft is operating in the retarded position. Typical causes listed in one source include, “wiring, timing belt, CMP actuator.” Another source included “slipped cam sensor reluctor, stuck variable cam timing solenoid or sprocket, and incorrect camshaft timing.”

It was difficult to visualize a timing belt problem because the engine idled perfectly. But the scan tool data indeed showed that the cam timing was retarded -30° as the DTC 1383 indicated. See Photo 5. Disconnecting the camshaft position sensor (CMP) yielded this data:

Photo 6 is consistent with the default data for warm idle speed, which is released in Ford’s TSB 03-15-14 covering DTCs P0340, P1380, P1381 or P1383. TSB 03-15-14 is a rather lengthy document that should be read before diagnosing any of the above DTCs.

Because the valve timing values changed, the default data indicates that the CMP is sending a valid signal to the PCM. Subtracting default data of -15° from real-time data of -30° indicates that the exhaust camshaft timing is 15° or about one cog retarded.

Removing the upper timing belt cover revealed that the retarded exhaust camshaft timing was caused by individual cogs stripping away from the timing belt. See Photo 7.

Why didn’t a retarded exhaust cam cause a rough idle? Let’s remember that exhaust cam timing isn’t as critical to low-speed engine performance as intake cam timing. Grounding the black/red wire at the cam timing solenoid located at the front center of the cylinder head immediately caused a rough idle because the camshaft was retarded well beyond normal parameters.

But closing the exhaust valves about 15° later due to a missing timing belt cog by itself wasn’t enough to cause a rough idle. So, here again, we have another Diagnostic Dilemma caused by a timing belt with a few missing cogs.