Thursday, November 28, 2013

Timing is everything

Here we have a 2005 GMC Envoy 4.2 Liter engine. The vehicle is fairly clean and has 112,361 miles on it. The complaint is the MIL Lamp is on and is setting a P0017 code. This code indicates an issue between Crankshaft and Camshaft correlation. The truck runs well and the shop owner was hoping there was a reprogramming that would solve this. A quick check of present calibration and available updated calibrations yielded nothing for this code. Time to roll up the sleeves. First step is to look at the code and more importantly code set criteria. While we are at it check for pertinent TSB's.

Here is the code P0017. I love the code set criteria. A calibrated amount. What is a calibrated amount? Time to dig a bit. I look for TSB's and PI's on the OE site.

I come up with this document. A world of information including the specified calibrated amount as well as a wealth of causes for this code. A little background on these engines. They utilize a camshaft actuator or phaser on the front of the exhaust camshaft that is loaded to a neutral/base position. When the PCM wants to actuate this actuator it duty cycles an oil control solenoid that will in turn feed oil to the actuator and in this case will retard the exhaust camshaft. Basically, this operation takes the place of EGR operation and it also improves overall efficiency. Like all engines that utilize this type of design it is very reliant upon proper oil level, viscosity, and pressure.

Here is the front view of the engine. The oil control solenoid is in the head right by the power steering pump. The camshaft sensor is also on the front of the cylinder head right by the upper radiator hose.

Here is a close up of the oil control solenoid. I see plenty of issues with these. They clog up, the portion inside the head gets clogged up and doesn't allow the oil solenoid to do its job properly. Typically, when this happens the vehicle runs really poor at an idle but runs decent raced up. Think of a vehicle with a stuck open EGR valve. This vehicle runs rather well at an idle. Scantool data is only going to give me a small portion of what I need to know. My play is to scope crank and cam sensors.

Well here is Crank sensor on channel 1 in yellow and Cam sensor on channel 2 in Green. Is it good? Is it bad? I don't know. This is where it is nice to have a known good. It just so happens that there is another similar vehicle on the lot. This vehicle is running fine and is just in for servicing. Lets take a look at that one and see if we can see any differences.
Hmmm. Waveform interpretation can be daunting at times. I usually zero in on one portion. If you look closely at the crank sensor pattern you will notice a double spike. This is the signature portion of the waveform. The PCM uses this signature pulse to determine piston position. I am going to zero in on that portion of the waveform.

  Notice how the first signature starts after the second trailing portion of the short camshaft sensor pulses and the second signature is on the trailing edge of the second long pulse of the camshaft sensor. Lets look at our suspect pattern.

A definitive difference indeed! Looks like the whole crank pattern is shifted to the left or is the camshaft pattern shifted to the right?

Here is a comparison of the two waveforms. At this point I inform the shop owner of my findings. I tell him definitively that he has a true blue issue with correlation. At this point it could be a stretched timing chain, timing chain alignment, an actuator/phaser not returning to base position, an oiling issue inside the head, an oil control solenoid not operating correctly, etc. I ask the shop owner if he wants me to delve deeper. He refuses citing he has to get approval from customer for more diagnostic time. He was really hoping there was a calibration update to solve this. I advised the shop owner I don't think it is crank endplay or a loose crank bolt. Because I don't see major differences in the amplitude of the crank signal when raced up, etc. Unfortunately, the customer refused more diagnostic time and the vehicle was released. This happens sometimes. What is ironic just last week a buddy of mine called me asking if I had a known good 4.2 liter crank/cam scope pattern that I could send him.

Just a little math here as well. One crankshaft rotation (from signature to signature) took approximately 100ms. That would mean 3.6 degrees per 1ms. The code set was 16.31 degrees which is approximately 4.5ms.  Looking at the bad pattern it is real close to being about 4.5ms out. If memory serves me these cam sprockets have 48 teeth that would yield 15 degrees per tooth. So a tooth out with a little stretch is a possibility here as well.