Mercedes SLK World banner

SLK 320 2001 Losing Confidence with Mystery Constant crank and trouble starting problem :(

1 reading
185 views 16 replies 9 participants last post by  James280  
#1 · (Edited)
This is my first post on this forum. I’ve used it a lot trying to figure out how to fix my SLK, which I love so much, but nothing I’ve tried seems to be working. I’ve gotten so frustrated that I finally made an account because I really need help figuring this problem out.
My SLK 320, 6-speed manual, has had trouble starting ever since I bought it about 6 months ago.
At first, I thought it might be the fuel injectors, so some friends recommended that I try fuel injector cleaner. It may have helped a little, but honestly, I couldn’t really tell. In the beginning, the car would usually start on the first try, but over time the problem gradually got worse.
I eventually replaced the crankshaft position sensor and one thing I noticed was that, for a while, the car would start every time if I quickly turned the key straight to the crank position without stopping to prime the fuel system. Eventually, even that stopped working.

I then replaced the fuel filter. After replacing it, the car started normally for the first few test starts, so I thought I had finally found the problem. Unfortunately, shortly afterward it went right back to having trouble starting and feels like its actually worse now requiring more attempts at cranking and rarer perfect starts.

I checked underneath the car the following day for any fuel leaks, but I couldn't find anything leaking but i made sure to tighten the clamps down and even replaced one.
One other thing I’ve noticed is that the exhaust smells strongly of fuel after the car finally starts from the exhaust i think . I’m not sure if that’s an actual clue or if it’s simply because I’ve been priming the fuel system 2–3 times before finally cranking it.

At this point I’m stuck and extremely frustrated. I really love this car and want to get it fixed, but I can’t keep throwing money at parts without knowing what’s actually causing the problem.

EDIT:
After checking the fuel pressure and finding out that it holds perfect pressure during crank and idle, I tested the battery in which it came out to be a perfectly normal battery. I was out of luck and almost gave up but decided to throw in a bottle of fuel injection cleaner into my fuel. THAT made the car start on first try for the next couple tries. My injectors were just dirty? Its funny because i remember putting in a fuel injection cleaner months ago when i first bought the car but it still started having problems down the line again.

PLEASE HELP
 
#4 ·
Some background first. On my R171 I played with a system to deliberating delay engine start forcing an extended crank - this to try to build oil pressure before engine start. Initially I blocked the fuel pump from running. No good.

Now to the issue at hand.

I eventually did this by blocking all power to the injectors and the coils for 5 seconds after ignition start selection. This produced no codes btw. Engine started after an extended crank. (did not help re oil pump priming btw).

More focus now. So you can smell petrol so unlikely to be injectors. I would have a very good hard look at the power supply to the coils.
 
#5 ·
Do you think its plausible that its not my fuel pressure or fuel related but more related to ignition coils? I will say I have yet to change spark plugs and I do feel like they are in need of changing soon as i do feel a slight rough idle. I have purchased a set but still need to change them. I have heard that the fuel injectors could leak into my spark plugs and that could cause problems as well. Ive been told that the smell of fuel could also be from my injectors .
 
#6 ·
This is my first post on this forum. I’ve used it a lot trying to figure out how to fix my SLK, which I love so much, but nothing I’ve tried seems to be working. I’ve gotten so frustrated that I finally made an account because I really need help figuring this problem out.
My SLK 320, 6-speed manual, has had trouble starting ever since I bought it about 6 months ago.
At first, I thought it might be the fuel injectors, so some friends recommended that I try fuel injector cleaner. It may have helped a little, but honestly, I couldn’t really tell. In the beginning, the car would usually start on the first try, but over time the problem gradually got worse.
I eventually replaced the crankshaft position sensor and one thing I noticed was that, for a while, the car would start every time if I quickly turned the key straight to the crank position without stopping to prime the fuel system. Eventually, even that stopped working.

I then replaced the fuel filter. After replacing it, the car started normally for the first few test starts, so I thought I had finally found the problem. Unfortunately, shortly afterward it went right back to having trouble starting and feels like its actually worse now requiring more attempts at cranking and rarer perfect starts.

I checked underneath the car the following day for any fuel leaks, but I couldn't find anything leaking but i made sure to tighten the clamps down and even replaced one.
One other thing I’ve noticed is that the exhaust smells strongly of fuel after the car finally starts from the exhaust i think . I’m not sure if that’s an actual clue or if it’s simply because I’ve been priming the fuel system 2–3 times before finally cranking it.

At this point I’m stuck and extremely frustrated. I really love this car and want to get it fixed, but I can’t keep throwing money at parts without knowing what’s actually causing the problem.

PLEASE HELP
Read post #2 and do before you change anything else. SCAN and get the Codes.
 
#7 ·
Have you connected a fuel pressure gauge yet, and watched the readings during starting and running? it should be rock steady.
 
#9 · (Edited)
I replaced the camshaft position sensor (CPS) on my 2004 SLK320 MT several weeks ago when I repaired the CPS connector shell. This past week while troubleshooting the CEL light (and hard starting engine), my Launch x431 scanner reported a P0341 error, camshaft hall sensor electrical fault. Thinking I installed a defective CPS, I replaced it again, cleared the error code, and it returned after the first restart.

I eventually traced the problem to the #3 terminal in the CPS connector that was not making a good contact. Approach I used in this post:
 
#10 ·
I certainly agree with the advice to do a proper scan before "mending" more or less likely faults. While a skilled mechanic can do a lot by skill and knowledge, there are many data points that is automatically checked by modern scanner software that tells a story, from high resistance (where there should be none), proper voltages and proper pressures, including the likelihod of the sensors themselves to be faulty. I am a pretty good shade tree mechanic with non electronic engines of various kinds, but there is not much to do when the electronic systems starts a irritating series of intermittent faults.

That said, these scanners do their best job if all all service points are up to date and in order. If spark plugs are due, change them first. Make sure fuel is OK, all filters are new, oil is fresh (no, the scanner will not check that). Check coolant as well, it should be reasonably clear and mixed with the proper amount of glycol.

As a certified no-cost-no-liability shade-tree-mechanic, i do think that if injector cleaner seem to help, a new fuel filter would be in order. I would also like to remind you of school book physics: Not only pressure, but flow rate is important.
 
#11 ·
When you replaced the CPS, did you use an OE Bosch unit? I've heard these CPS are sensitive to aftermarket brands. I was having a long-crank issue on our 320 and a quick CPS swap to a new Bosch unit totally solved the issue. When I was having the issue I ran codes on my X431 and didn't have any.
 
Save
#12 ·
Rough idle .... on a 320? What does your tach say? Is the needle steady at 550 RPM or so? That should be a clue of why it is hard to start, right? If it has trouble running at 550 rpm then to me the idea of starting to run at 'cranking speed' seems related.

Smelling fuel??? I think James280 nailed it? Why would CLOGGED injectors flood the motor? I suppose a dirty filter might maybe possibly have something to do with rough idle if starving the fuel but how could it rev?

I asked AI if the Mass Air Flow sensor might cause your problem, here is the response:

Yes, a bad Mass Air Flow (MAF) sensor can cause both a hard start and a rich fuel mixture. [1, 2]

How It Causes a Rich Mixture

  • Over-reporting Airflow: If the MAF sensor is dirty or failing, it may trick the engine computer (ECU) into thinking more air is entering the engine than actually is.
  • Dumping Extra Fuel: The ECU responds by injecting extra fuel to match the false high airflow, creating an overly rich air-fuel ratio.
  • Signs of Rich Run: This excess fuel leads to black smoke from the exhaust, a strong fuel smell, and poor gas mileage. [1, 2]
How It Causes a Hard Start

  • Flooded Cylinders: When the ECU over-injects fuel during cranking, it can flood the engine or foul the spark plugs with raw gas, making it difficult for the combustion chamber to catch fire.
  • Extended Cranking: The engine may crank for a long time before finally sputtering to life once the excess fuel clears out or ignites.
 
#14 ·
Wow thats interesting, To be honest I have not even considered the MAF sensor, Ive always heard it here and there but never thought that could be the case,

As for my idle, I dont exactly have a rough idle, my car starts and after warming up settles at 550 rpm and stays there with little to no fluctuation thats the weird thing, after the hard cranks and starts, after it finally comes alive, it STAYS alive, no issues, I have never had a problem with the engine sputtering or having problems

I will note, The car has 240k miles on it so i feel it could be anything.

Another IMPORTANT NOTE
I also have a slight oil leak on my camshaft position sensor, It seems the oring has gone bad and has been leaking oil ever since i got the car, Im planning on just getting a new sensor as with a new Oring but i am curious whether that could be the problem as well.
 
You have insufficient privileges to reply here.