You wouldn’t believe it if you tried

Ryan Colley is back, and this month he is getting to grips with a misbehaving SEAT LEON

Published:  24 April, 2022

Following my introduction last month, and the brief description of my business, I would like to continue with the pressure analysis theme, and present to you a recent real-world case study I encountered during a busy week at the workshop.
    
The vehicle in question was a 2012 SEAT LEON 2.0 TDI with a CEGA engine installed. The customer complained that while driving, the vehicle’s engine stalled and wouldn’t restart. The vehicle was then towed to his local garage who is our customer. I often carry out trade work for other garages, offering in-depth diagnostic services along with ECU coding and programming. This can be done while mobile depending on the fault at hand. Therefore, I attended the vehicle at their business address and started my diagnosis process.
    
Firstly, a background investigation was required to determine if any work had already been carried out, and any other information that may be valid. This included driving style and abnormal noises at the time of break-down, among other things. The customer informed me that the vehicle had shredded its auxiliary belt due to a failed belt tensioner. This then allowed the remains of the auxiliary belt to get caught up in the cam cover and subsequently caused the cambelt to detach, allowing the engine to jump five teeth out of time. This is all according to the customer’s report.
    
The technician then informed me he had re-timed the engine and attempted to get the vehicle to start, but was unsuccessful. He then proceeded to remove the cylinder head in hopes of finding that the valves had contacted the pistons. Some, but not all, of the exhaust valves had become bent from contact. No intake valves were damaged, the damaged exhaust valves were replaced.
    
He also mentioned that due to this engine’s specific design, there is a special tool required to correctly time the intake camshaft to the exhaust camshaft. However, they did not have the required tool and subsequently were unsure of the correct installment of the intake camshaft. He was upfront with me, and alerted me to the possibility that the intake camshaft could be 180° out of time. Without the tool, there were two possible positions the intake camshaft would fit into place. See Fig.1.

Developing a gameplan
After being made aware of the situation, I started by carrying out a basic relative compression test. I did so by cranking the engine over and using an oscilloscope:

  •  Channel A is connected to a high amp clamp (around the battery cable)
  •  Channel B is connected to cylinder number one injector control wire.


The purpose of this test is to identify if each cylinder is contributing evenly towards compression. This will easily and swiftly determine if there is an obvious mechanical issue or not.
    
As you can see from Fig.2, there are even and continuous current humps, indicating even compression between cylinders, with an average starter motor current draw of around 278 amps. This indicates good compression.
    
As a rule of thumb, the average amperage reading should be around three times the amp/hour (Ah) capacity of the battery. In this case, that would be approximately 90Ah. The results of this test indicate compression is good. You will also notice that the injector event is firing at TDC of the compression event, therefore, appearing to be in time.
    
At this point, we have ruled out that compression and injector firing was not our issue. A full system scan was carried out, which determined there were no fault codes stored in the engine’s computer relevant to our fault or any others. Next, we checked live data parameters to see if there were any other anomalies present. For instance:

  •  Fuel pressure
  •  Camshaft speed
  •  Crankshaft speed
  •  MAF reading


After viewing the data PIDs, all appeared correct. I then decided to remove the glow plug and using a DITEX pressure transducer, coupled to my oscilloscope, acquired an in-cylinder pressure waveform. Simultaneously, the same cylinder’s injector was captured and synchronized with the pressure waveform. This allowed me to determine where the fuel injector was fired relative to the true top-dead-centre. This will surely determine if there is indeed a cam timing fault.
    
After gaining this capture it was then obvious why the vehicle would not start. See Fig.3. The injector was firing 360° after TDC, i.e. the peak of the compression tower. This was a repetitive firing event occurring each engine cycle, 360° after the compression tower was formed. I then asked the customer if the target wheel had been disturbed or removed. He confirmed the target wheel had been removed to carry out the cylinder head repair.
    
The injector event is often timed from one of the camshaft position sensor signals, usually from a target wheel located on the intake or exhaust camshaft, in some cases both. This vehicle has one target wheel on the exhaust camshaft only. Therefore, a visual inspection of the exhaust camshaft was required to verify its location and correct installation.  

Pinpointed diagnostics
The engine was turned to align the external timing marks. The top timing cover was removed to find the cam timing was correct according to the timing marks and the target wheel was correctly aligned to the exhaust camshaft. On this particular engine, there is a keyway that is part of the exhaust camshaft. This keyway gives the target wheel a point of reference on the exhaust camshaft. See Fig.4.

So, it now appears that the engine timing is mechanically correct and the physical position of the exhaust camshaft target is also set in the correct position. These were the questions I now needed to answer:
 
Why is the injection event 360° too late?
 
Is it possible the compression event for that cylinder is occurring 360° too early?

After looking through ERWIN, the genuine information system for VAG vehicles, I discovered that the intake camshaft has a notch machined into it. The repair instructions insist this notch must face outwards during installation. I quizzed the technician who undertook the cylinder head repair and asked if he had put the camshaft in the correct position, as ERWIN showed. He confirmed he did not. The intake camshaft was installed 180° out as the notch was facing the opposite way. I asked him to remove the rocker cover and re-time the intake camshaft to the correct position, with the notch facing the correct way, as per the repair manual from ERWIN. See Fig.5.
    
The customer, after attempting to install the intake camshaft in the correct position advised that it was impossible, as the camshafts would not correctly align, nor would they bolt down.

I was now suspicious of both the camshaft’s lobes not being in the correct places due to the original valve contact. I asked the customer to remove both the camshafts and compare them with Fig.6, which was found online from a timing tool photoshoot. The picture shows the intake and exhaust cams installed and timed correctly. The important part of the image was that it showed the angles of the cam lobes when the camshafts are correctly aligned.
    
Once both camshafts were removed, I asked the customer to put the camshafts in the same positions on the bench as those as seen in Fig.6. He informed me this was possible, however, and this is the ‘AH HA!’ moment, the keyway for the exhaust camshaft target wheel was 180° out.
    
The key had rotated 180° which now meant when the exhaust cam lobes were in the correct position, the keyway was now facing 6 o’clock, as opposed to 12 o’clock in Fig.6. Knowing this we can now piece the puzzle together.

Suddenly, it becomes clear
If the exhaust camshaft keyway is facing 6oclock when the lobes are in the correct position, then placing it in the 12 o’ clock position to correctly align the timing components, would make the exhaust camshaft also 180° out. This would mean the intake and exhaust camshafts were both 180° out, allowing compression to build 360° earlier.
    
As you can see from Fig.7, which is timed correctly, the intake camshaft notch is facing outwards as described in the repair manual. You will also note the keyway on the end of the exhaust camshaft circled is facing 12 o’ clock.
    
To finalise this and prove this is a factual job, you will find in Fig.8 both the exhaust camshafts (original and replacement) pictured together, showing the keyways and lobes. Once the new exhaust camshaft was installed, this then allowed for the intake camshaft to be installed correctly. Once the engine was reassembled, this vehicle started straight away.

This just goes to show the power of modern vehicle testing using an oscilloscope and genuine technical data. Sometimes the possibilities that exist are not what first comes to mind. It is the ability to learn to trust the data that guides us straight to the fault, every time.


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