Relative Compression on Diesel
This 2003 Ford 6.0 is all fixed, but I wanted to put this up for the discussion about using relative compression and other electronic means of testing on diesel engines. The pickup came to my shop with a low power complaint. Test driving, it took me about 50 yards to determine this 6.0 would have a problem getting out of its own way. Letting off the throttle got a rather loud "WHOPP, WHOPP" from the intake. This was caused by the turbocharger stalling, then starting again. There was also a small amount of black smoke noticed on hard acceleration, caused by the lack of boost from the turbocharger. The only DTC stored was a P0113 = Air intake temperature fault. Checking the scan data, I can see the turbocharger position relearn has not been done. This engine has no history of overheating, or are there any signs of coolant being blown out of the degas bottle. Checking the coolant level, I found it up to the full mark when cold.
This vehicle has been to another shop and had been diagnosed with a low boost problem. A new turbocharger has been installed, which did not fix the problem, then it had a new Y-pipe installed, which did not fix the problem. At that point, it was towed to my shop to get fixed. My testing started with doing the turbocharger position relearn. Let me back up a bit. This is an early version 6.0, which uses an inferred exhaust pressure input to the PCM. These vehicles must have a turbocharger position relearn performed, or low power problems such as this one is having can happen. Many times, these will learn the position all by themselves, since all that is involved is to bring the engine to operating temperature, remove both negative battery cables for a few seconds to power down the computers, then hook them back up and idle the engine until the procedure completes. This procedure can be watched on the scan tool, but, most times, all that is needed is to back the pickup out of the shop and let it idle for 15 minutes, then go for a test drive. This pickup would not complete a relearn, since a valid MAF value is needed to do the relearn. The problem with the MAF was found to be 3 of the wires to the MAF plug were broken. Repairing this problem allowed the relearn procedure to complete, which fixed the low power, but, now to the rest of the story.
In my testing, I noticed a weak cylinder when cranking the engine. After working on engines for 50 years, I have learned to listen and feel a lot of things. Listening to cranking cadence is one of those things. When using the IDS scan tool, one of the cool functions of it is the relative compression test. All that is involved is to hook up the tool, select the test. It has you put the throttle on the floor, (foot feed for you old guys) :) then crank it for 10 seconds. This is what I found.
The scan tool relative compression test shows low compression on cylinder 7 & 3. My question to you is, "Is the compression really low on these two cylinders"? The engine starts good and idles smooth. There are other scan tool tests that can be done to get the engine to tell me more of what I want to know.
The scan tool has a cylinder contribution test. For you people not familiar with this test, the scan tool draws a histograph of each cylinder. It also allows the idle compensation to be turned off, which means, the scan tool is capable of taking the PCM out of the loop when it comes to controlling each individual injector to smooth the idle out. If perchance there is an injector or two that is lets say, not injecting the proper amount of fuel, and the idle compensation is turned off, the histograph recording will show the engine contribution per each cylinder and will show a drop in engine speed when the low contributing injectors fire. In this case, turning off the idle compensation, and putting the transmission in the drive position would get cylinder #3 to start to misfire. If the transmission was put back in drive, the idle would smooth somewhat, and if the idle compensation was turned back on, the idle would smooth out, which masked the real problem. At this point, I know the low compression cylinder is on cylinder #3. Now its time to fire up the scope and get some raw data. I want to find out exactly where the compression loss is happening. Is it from an exhaust valve, an intake valve, or past the piston rings. I told the customer I wanted to find out why, before I spend any more of his money on something that was going to crash and burn.
My first test is to run a relative compression test with my lab scope. Now, when doing a RC on a diesel, it is a little hard to get a trigger point to determine where cylinder #1 is unless you go to a CMP sensor. If you want to use a trigger from an injector, you will have an engine that runs. If you want to disable the fuel and use the trigger, that is extremely hard on the fuel injectors, since they are running dry, and damage to the injector tips can happen. I opted to find out which cylinder was the low compression, then use my scope to determine how bad and where the air is going.
When hooking up a scope for a relative compression, it is always a good habit to have to gather more than just the RC. Reach out and grab information from the intake manifold, the exhaust and the crank case, if you happen to have enough FLS sensors or pressure transducers. In this case, I have two FLS sensors, so I will get battery current on channel A, intake pressure pulse on channel B & exhaust pulse on channel C.
With this capture I did not find any leaking exhaust or intake valve that would answer my question and give me a direction. I am not ready to pull the cab off and remove the cylinder heads quite yet. Blue trace is RC, Red trace is intake pulse, Green trace is exhaust pulse. Oh BTW, when you need to get an intake pulse from a diesel, you need to add a restriction to the intake manifold. I will usually stick a plastic plug into the intake to make that restriction.
Adding a piston chart to the waveform has not answered my question either. I have one more place to go looking, the crankcase. Is there a chance the compression is escaping past a piston ring? If so, this engine is not worth pulling a head off and is nothing more than a boat anchor.
With the FLS moved to the crank case dipstick, (blue trace = RC, red trace = crank case pressure, the engine was cranked.
When doing testing like this, I have found that cranking the engine will give out more information than starting and idling. Once the engine is started, things happen too fast, and pressure pulses can get out of control, With this information, the waveform shows a higher pressure pulse from each cylinder with the normal compression, then on cylinder #3, which is the low compression pulse on the RC trace the crank case pressure drops.
Now my question to you, Do I have enough information on this engine to make a diagnostic judgement, or do I need to go further before the engine comes apart? What do you think?
Very interesting write up Albin. I'll start by saying that I'm no diesel tech and have learned much just by reading this post. To answer your question, my opinion is, it wouldn't hurt to do a leak down. I'm saying it mostly because I've got a long way to go before i can say I've mastered the art of pressure pulse waveform analysis. I believe that the crankcase pressure pulse waveform is a
Is the intake all coked up? Pulling the EGR valve for inspection is a great place to look for coking faults. The turbo chuffel can be due to excessive coking as well intake valve issues. I have had a few Ford diesels with relatively low compression from the cylinder wall cross hatch being worn but that was typically the top inch or so of travel. Mostly from over filled crank case 6.4's, but
There was nothing wrong with the intake manifold. So far in my experience with the 6.0 I have only seen one with enough coke in the intake manifold to be a concern. I think the reason for this is the way these vehicles have been driven. Around here, they get driven and not idled. getting back to the waveforms, the pressure waveforms do not support any breathing problems, nor cylinder leakage
That would leave a bent connecting rod?
I agree, if there's no leakage that leaves a piston height issue
I probably would have taken a lot longer to get to that conclusion without the waveforms, assuming that was the failure. Mechanical compression test would have been a little low, but I would likely have to disassemble to isolate the fault.
If there's no breathing issue then are these artifacts from the pressure sensor? And if so, by excluding these artifacts you are just looking at this?
I will wrap this one up. I started with the scan tool relative compression test. That led me to using the lab scope with RC, and looking for compression loss past the intake valves, exhaust valves & the piston rings. I didn't find anything significant with these tests, and came to the conclusion of a bent connecting rod on cylinder #3. I tried measuring the piston height by pulling the
Great find, of all the 6.0 heads I have done I have not seen the any cracks. Is this a common failure for anyone else?
This is the first crack like this I have seen on a 6.0. It is common for them to crack around the injector hole, most times on cylinder #3
Cylinder 3 was always the hot cylinder. I frequently see excessive cavitation (pitting) on the heads where the fire ring of the head gasket sits. I think they removed or added a restrictor behind the front cover on the early ones to address a cylinder heat issue on those. maybe in like 2005, I am not sure it made much of a difference.