X308 Compression Test
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Joined: Dec 2014
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From: Colorado
Low or no compression - causes and solutions to hard starting
There are many things that can cause hard starting including low compression.
I do not think low or no compression is only a condition of bore wash without any definitive testing in my opinion.
Niklasil cylinder surfacing has been around since the '60s for aluminum cylinder engines and a problem of borewash removing compression "across the board" would be much more widespread an issue across many different aluminum engine manufacturers.
Borewash removes the oil from the cylinders needed to lubricate the ring to cylinder contact and absence of oil causes rapid wear of the cylinders, it does not necessarily remove all sealing capability of the combustion upper rings, and fuel actually asists the upper rings in sealing. I will wager a dry bore has less compression than one with fuel in it, bet?
Compression loss may also be, and is a far more common a symptom of pitted valves and carbon fouling, very typical on these engines for quite a long time seen even at very relative low miles varying only by driving habits.
The difference can be only be detected by a leak rate test to determine exactly where the loss of compression is actually occuring. How to Do Compression Test and Read the Results - AutoRepairPit.com
Either it is leaking past the valves, past the rings / piston or past the gasket or a combination of those.
A test should be made to determine exactly where the compression loss is occuring, not speculated.
Speculation does not help, testing and fault finding determination with a solution does.
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@xalty
There are many things that can cause hard starting including low compression.
I do not think low or no compression is only a condition of bore wash without any definitive testing in my opinion.
Niklasil cylinder surfacing has been around since the '60s for aluminum cylinder engines and a problem of borewash removing compression "across the board" would be much more widespread an issue across many different aluminum engine manufacturers.
There are many things that can cause hard starting including low compression.
I do not think low or no compression is only a condition of bore wash without any definitive testing in my opinion.
Niklasil cylinder surfacing has been around since the '60s for aluminum cylinder engines and a problem of borewash removing compression "across the board" would be much more widespread an issue across many different aluminum engine manufacturers.
bmw m52 m60 jaguar aj26/27
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I have had something similar to the "borewash" symptoms three times on different AJ27 engines. I am not coinvinced it is borewash, but lets call it that for now.
There are programming features in the ECM that if the engine does not start in a certain number of revolutions depending on the throttle position, many things are disabled, and there are typically no useful codes stored during the initial or elongated tickover. There is only a pretest to make sure the sensors are active "ready to start" like crank position sensor and throttle position, injector continuity, oil pressure, but only a couple of sensors are actually briefly logged during tickover, unless you have an IDS attached with proper probes.
So it may be useful to first record the DTC's, then clear the ECM DTC codes including any other modules that inhibit starting like the TCM and ignition / instrument modules, verify you have a "ready to start" signal , meaning you can actually crank the engine with the key engaging the starter.
My opinion is if I cannot see what the various modules are doing, you are pissing into the wind.
The first two times I had a no start with a few cylinders intermittently firing but not enough to start the engine I removed the coils and plugs, cranked the engine to blow out the excess fuel. carbon and then a very short squirt of oil, crank again for distribution and then a short squirt of starting fluid in each cylinder and reinstall the plugs and coils.
On those two cases the engine then started almost immediately. When that did not work I did a compression and leak rate test.
The third time was very different. I had to put two batteries and a charger to crank as it would turn over and almost catch, but not quite start the engine and if the battery voltage goes below 12 volts on tickover you get all sorts of things turning off and it will never start. So after repeating the above sequences, I cranked and it took about 2-3 minutes of cranking continuously while watching the available volts stayed above 12.5VDC before the engine would catch and start. That was very odd, but then after starting, I had no more issues.
There were no leaking injectors, no bad coils, perfect fuel pressure, all sensors had been verified operatonal using the IDS/SDD.
I actually believe there is a particular sequence the ECM gets locked up that is not resettable using simple OBDII tools and needs software capable of interrogating each module in detail with a proper diagnostic power supply attached.
I have seen that too many times where one module error kept the ready to start signal off to crank the engine.
Once my local dealer shop foreman verified my findings, there was a code he was also not able to reset, showed me where to find it, which was when I replaced my ECM with a rebuilt one, started at the first twist of the key. The second ECM I located was for the last series AJ27 motors and after installing that one, started with the first twist of the key, many of the annoying DTC niggles vanished, including reduced performance.
We have to remember that the people programming these engine management systems may not be thoroughly competent engineers, but programmers working for someone operating under a strict production timeline to get something out that "works", and then the rest is troubleshot by the dealer network - that is the way 3 automanufacturers I have done work for have sold their new cars for decades.
Off to go fix my X350
There are programming features in the ECM that if the engine does not start in a certain number of revolutions depending on the throttle position, many things are disabled, and there are typically no useful codes stored during the initial or elongated tickover. There is only a pretest to make sure the sensors are active "ready to start" like crank position sensor and throttle position, injector continuity, oil pressure, but only a couple of sensors are actually briefly logged during tickover, unless you have an IDS attached with proper probes.
So it may be useful to first record the DTC's, then clear the ECM DTC codes including any other modules that inhibit starting like the TCM and ignition / instrument modules, verify you have a "ready to start" signal , meaning you can actually crank the engine with the key engaging the starter.
My opinion is if I cannot see what the various modules are doing, you are pissing into the wind.
The first two times I had a no start with a few cylinders intermittently firing but not enough to start the engine I removed the coils and plugs, cranked the engine to blow out the excess fuel. carbon and then a very short squirt of oil, crank again for distribution and then a short squirt of starting fluid in each cylinder and reinstall the plugs and coils.
On those two cases the engine then started almost immediately. When that did not work I did a compression and leak rate test.
The third time was very different. I had to put two batteries and a charger to crank as it would turn over and almost catch, but not quite start the engine and if the battery voltage goes below 12 volts on tickover you get all sorts of things turning off and it will never start. So after repeating the above sequences, I cranked and it took about 2-3 minutes of cranking continuously while watching the available volts stayed above 12.5VDC before the engine would catch and start. That was very odd, but then after starting, I had no more issues.
There were no leaking injectors, no bad coils, perfect fuel pressure, all sensors had been verified operatonal using the IDS/SDD.
I actually believe there is a particular sequence the ECM gets locked up that is not resettable using simple OBDII tools and needs software capable of interrogating each module in detail with a proper diagnostic power supply attached.
I have seen that too many times where one module error kept the ready to start signal off to crank the engine.
Once my local dealer shop foreman verified my findings, there was a code he was also not able to reset, showed me where to find it, which was when I replaced my ECM with a rebuilt one, started at the first twist of the key. The second ECM I located was for the last series AJ27 motors and after installing that one, started with the first twist of the key, many of the annoying DTC niggles vanished, including reduced performance.
We have to remember that the people programming these engine management systems may not be thoroughly competent engineers, but programmers working for someone operating under a strict production timeline to get something out that "works", and then the rest is troubleshot by the dealer network - that is the way 3 automanufacturers I have done work for have sold their new cars for decades.
Off to go fix my X350
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