Hello guys. So, I got this 2008 Mercedes Benz C300 recently and its been giving me quite a lot of headaches. I thought I was getting a decent deal since the price was pretty low, but now I am questioning that. Car drives good when my foot is on gas pedal, but when I come to the red light and have to wait a bit (my foot on the brake and car is in D), engine light starts flashing and car starts jerking pretty bad. So, I noticed that car has a misfire on Cylinder 2. I swapped coil pack and spark plug from that cylinder to cylinder 3 to see if the code will follow, and unfortunately no. The code stayed with cylinder 2. I was kind of panicking at the moment, so I wanted to rule out valve issues. I went to local Oreillys and got the valve testing tool. Now, I know this isn't the best practice, but I don't have the access to expensive computer tools made by MB. So, I undid all the coils and plugs and started with cylinder 2. I was shocked when the most I got was 60psi. But, then I went on and all the other cylinders had 90 psi. I know that supposed pressure is supposed to be 150 psi, so first thing I am wondering about is if this tool is showing wrong values and 90 is actually 150. It would be really weird if 5 cylinders all have 90 psi. And if that is the case, does that mean cylinder 2 has about 120 psi? Now my next question is, since I already checked the coils and plugs, and since it seems that valve on cylinder 2 is burnt a bit, does that probably mean fuel injector on that cylinder is bad? And would replacing that fuel injector make the car drive better (since the valve isnt burned too much)?
I do not intend to keep this car for too long and also I dont have enough money to take it to shops since I already know it will take hundreds to just get it diagnosed.

 

first question is how many times did you crank it with the pressure gauge? 3-4 cycles should give the full number. then, replacing an injector will not fix a low compression issue. all cylinders should be within 10% of each others number. if cylinder 2 is still a lot lower squirt a few drops of oil in it (literally), cycle the cylinder and put the gauge back on and see if the compression goes up. if it does significantly you have bad piston rings and you have to replace or rebuild the engine. when it comes to used cars always get the best you can, otherwise you get problem cars and end up dumping a lot of money into it. I have the feeling this is an expensive fix... good luck

 

Yeah I think I cranked it enough. I am not trying to fix low compression issue. I am just trying to make it run a little smoother. I suppose if the injector is bad and that is what made the valve burn, replacing the injector could smooth it out quite a bit?