The challenge here is a Renault C460 lorry fitted with a D11 engine with the customer complaint of poor performance and a vibration which feels like a misfire. No warning lights on the dashboard is interesting as you would assume a misfire would log a DTC. Complaint was verified and so onto testing. For misfires one of the quickest tests you can do is a relative compression. Whilst not conclusive it can be used to determine to give direction and the best part is the fact you don’t have to remove any parts.
Using the guided test meant the setup and trigger are done for you. All that is required is the engine is prevented from starting, Pico is connected and the current clamp is switched on, zeroed and clamped around a battery cable that carries current for the starter motor circuit. With the engine unable to fire, it was cranked.
Immediately we can see that there is an issue and more importantly it is repeating. By using the rotation rulers, we can see that there is a lack of current associated with a cylinder. We know this because each time a piston comes up onto a compression stroke, the starter motor requires more current in order to keep turning the engine. If there is less or no current required during what would have been a compression stroke, then we must focus on this cylinder to begin with to determine the cause. Next challenge is then how to identify the cylinders.
We tend to say to people practice using Pico on known good vehicles. If it’s just capturing say injector 1 and a camshaft signal, great! If it’s a fuel pump current from inside the fuse box, perfect. What this does is starts to build the confidence in order to start using Pico on the trickier jobs when they come in. Not only this but you’re also starting to build a nice collection of known goods! Uploading these to the waveform library also means you have a secure place to store them but also others can use them for their own diagnostic purposes. The other options for when there isn’t a known good is to refer to the technical information.
The easiest way to identify cylinders is to use an identifier for a known cylinder, or sync. For Petrol engines this could be an ignition trigger, primary or secondary ignition pattern but for diesel you only really have an injector. We pick this as they tend to fire around TDC during the compression stroke. Petrol engines tend to be less picky about whether or not they spark during cranking but diesel engines tend to want to see a certain amount of fuel pressure before they will send a signal to fire an injector. During a relative compression test, we need to engine not to start so the quickest way is to stop the fuel or disconnect the injectors. This means though we lose our sync, what now?
A look through the waveform library found nothing for Renault C however, Renault and Volvo trucks use the same powertrain. Under Volvo we filtered by using same basic information in order to broaden our search. We kept the filter to make, primary fuel and capacity. The D11 is a 10.9L of which removes a lot of the passenger vehicles! Unfortunately, though there were no waveforms we could use. We had found through the technical information that that the timing for both the D11 and D13 engines are the same so we could use a capture from a D13. Amending the engine size to 13L we found the one we were looking for, Injector 1 with the camshaft signal. Looking at this capture using the Camshaft signal and an injector meant we could use the camshaft to determine cylinder 1 and then using the firing order, associate the other cylinders. We do have a presentation on this technique which can be found here on our YouTube channel - https://youtu.be/EsJBOFzETjQ
The next capture to make then was to perform another relative compression test using the same setup but to add another channel, in this case the Camshaft sensor. We knew from the known good that cylinder 1 fires first after the double pulse reference on the camshaft sensor. As we had the option to add another channel, grabbing the exhaust pulse is always a nice addition. Remember the exhaust pulse is the result of everything, intake, combustion and exhaust. We would expect a nice, uniform, waveform if all things are equal.
Clearly, that isn’t the case. We’ve used the camshaft to now identify the cylinder with the lack of current draw during compression. We know that the cylinder 1 is first to fire after the double pulse on the Camshaft and so using the firing order we can identify the remaining cylinders. What we notice in the exhaust pulse is that it is not even and to be honest, at this stage of the diagnosis that’s all we need to know. We could spend another 30mins trying to work out what is going on but ultimately that’s not going to fix the truck. From everything we have gathered so far, we need to be looking at Cylinder No.3.
Removing the rocker cover things become clear relatively quickly.
What we are looking at here is Cylinder No.3 exhaust valve. As you can see the adjuster has moved out much further than the others and in doing so was partially holding open the exhaust valve. What had happened was the lock nut had come loose and in doing so the adjuster had wound itself in.
Removing the adjuster to make sure there wasn’t any damage to adjuster you could see how close it was before the locknut was about to fall off completely.
With no signs of damage, the adjuster was refitted and all the valve clearances were checked and adjusted to the correct setting according to the technical information. The locknuts also received some Loctite to hopefully prevent this issue reoccurring before being torqued to the correct setting. Refitting the rocker cover it was time to carry out a relative compression test to see if the fault was fixed with fingers crossed that the exhaust valve hadn’t hit the piston!
Everything is now much even and the exhaust pulse is also now uniform. Needless to say, the misfire is now gone and the truck is back on the road again.
I hope this helps in someway and sometimes stepping back from the waveform and just seeing it as what’s good and what’s bad is often all you need to give you the direction you need.
What was happening in the exhaust?
With the fault now fixed it gives us the opportunity to refer back to our original captures to better understand and therefore learn what is happening to create the patterns we were seeing. Let’s refer back to the capture with WPS500 in the exhaust.
We know that the dead cylinder is on number 3 but at the same time there is a large pulse in the exhaust. If we think about what is happening in the exhaust at that time when cylinder 3 is coming up on its compression stroke, cylinder 4 is also coming up the bore but on its exhaust stroke. Not only this but also have cylinder 1 beginning its exhaust stroke. We now know that the exhaust valve on cylinder 3 was being held open which means we have 2 maybe 3 cylinders travelling up the bore but all are now contributing to pulse in the exhaust.
MichaelFrey / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
Any form of pressure pulse analysis is difficult as there are often many variables to consider such as manifold designs, VVti control, exhaust after treatment systems and turbochargers. All of which can have an effect on the pressure waveform but getting known good waveforms where and when you can, means looking for the odd one out is made that little bit easier.
I hope this helps in some way and thanks to Lee Sharp (Sharpy) of L&D Commercials for sending it in.
Should anyone have specific questions about those engine platforms feel free to pm me, I worked (12yr) for the North American division Of Mack Truck & Volvo Truck in the powertrain support group.
It seems Volvo at least in the UK anyway don't recognize this as a repair to the problem and their solution is to replace the rocker arm as a complete assy... Even with discount I spent £2500 + vat today just for the parts to replace all 6 on this case study truck as the issue returned and the very same piston had come loose again after 10 weeks. Now at the time I wasn't aware of the tool or the torque specs but I did use loctite and tighten them as much as I could but now I think I could use one of the old piston caps to easily make the tool up for the next one. Cheers for posting the link
https://www.freedomracing.com/88800564- ... -tool.html
Not sure if they would ship internationally, or if it would be worth the money... could be better to make local like you mentioned.