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M156 Intake Manifold Combustion After Head Gasket Job (External Combustion Fun)
Hello,
The M156 in our 08 CLS63 AMG started making ticking noises. We removed the valve cover and verified that on the right side the last cam lobe was extremely worn out. We decided to replace the lifters, intake cams and the original head bolts for both sides.
We changed the head bolts one by one on the left side without completely draining the coolant and oil. Yes, this is a bad idea, we know now. We didn't expect the head to flex enough to let fluid past the gasket. While undoing the lower bolt next to cylinder 5, we heard some fluid moving around like it made its way over the existing headgasket. We drained the oil and coolant before replacing the right side head bolts. Once the job was finished, we ensured the cylinders were fluid free and added coolant and oil and started the car. It started right up without any issues or codes. The car drove fine without issue initially, but eventually started smoking really badly. We think some fluid remained on the head gasket on the left side and ruined the seal once the motor warmed up. We pulled the plugs a while later and found fluid in the left side pistons. The car set for a few days until the parts arrived for a head gasket job.The car was definitely drinking coolant, but at no point did the car overheat.
We decided to do the full head gasket job on the left side. The only thing we did on the right side was remove the valve cover and place the timing tools (straight end piece and the double U brace) because we had two sets anyway.
Our experience so far:
We did the timing using the EWK M156 timing tool (3 pieces). We have two of this tool set. While we were doing the timing on the left side, the right side was locked in using the spare tool set as was mentioned before.
Once the timing was done, we turned the engine over a few times by hand without any problems while spark plugs were not installed.
We cranked it without the spark plugs and with injectors disconnected. We did this for a little bit to get oil on the cams etc. This went very well without any interference.
We did a compression test on all cylinders and they were showing around 180psi. My gauge reads somewhat high, but they were all consistent and did not vary much at all between cylinders.
We cranked the engine while the injectors and the coils are disconnected, but the spark plugs installed. it all went well. No compression issue sounds.
Next, we connected the injectors and coils and cranked the car to start it. It cranked for a while, then attempted to start before it made two extremely loud popping noises from the intake area and stalled on its own. Loud enough to make us think rods snapped. We have never heard rods snap in half, but we imagine that is how loud it would be.
We repeated steps 2-5 to find the engine did not explode.
We noticed we hadn't connected the ground in-between the camshaft position sensors on the left side (driver side) for the ignition coils. We thought the bad ground could potentially cause messed up ignition timing.
When we connected the ground back up and started the car, we heard the loud pop again and stopped the car. We saw some smoke coming from the air intake near MAFs.
We took the left side valve cover and and the front cover off and re-verified the timing by fitting the 3 piece tool. It all fits perfectly.
We took the right side valve cover and and the front cover off and re-verified the timing by fitting the 3 piece tool. It all fits perfectly.
We also read some values using the STAR while the car is being cranked (fuel injectors and coils are disconnected). below is what those values look like. Note that the intake manifold pressure is out of range (too high) and the injection time is out of range. I am not concerned about injection time because the injectors are unplugged. The external combustion might have killed the pressure sensor, but that probably doesn't change ignition timing, so I'm not too worried about that.
We are convinced that the cam timing is correct because we did this before without issue, and the timing tools fit as they should. The valves haven't been eaten either. I think that the coils are firing while fuel is being sprayed into the cylinders.
I've checked the intake cam sensor signals and they look normal to me, but I haven't seen a known good signal. I plan to use the scope to check ignition current with the cam signal to determine if it's firing in a uniform and consistent manner. Maybe the noise is making the computer think the wheel passed by. Maybe the noise is just from my ground - the car is cranking during this after all. Maybe this doesn't even need cam signals and fires based on the crank signal. We noticed that the electrical connections for the camshaft positions sensors (especially the exhaust one) have some oil inside. I imagine that some oil also got into the electrical connections for the coils. We haven't attempted to clean them yet. I don't want to clean stuff without getting before and after waveforms on the oscilloscope. I want to see something that tells me I should try to crank this thing over again with fuel. I don't want to create more pops because I'm concerned about causing damage.
My plastic PCV valve connector clips are broken at the block side, but I don't see how that would cause any of this.
04-26-2020, 05:35 PM
