Diagnosing long crank on cold start N52 engine

M

Mangozac

Senior member
 QLD, Australia
Hi all,

My 3.0si has been getting progressively worse on cold starts (first thing in the morning or leaving work in the afternoon). It one day started the behaviour of cranking for an extended period (say 4 seconds), spluttering and then stalling. On second crank it would fire immediately. If I crank for just 2 seconds (so it doesn't fire), stop and then try again it fires immediately. Again, this is only on cold starts.

So it's obviously a problem with the fuel system not retaining pressure when the engine is off. I picked up a fuel pressure tester and did some investigation. The troubleshooting guide in the TIS is for a fuel return style system (not the returnless system used in Zeds) but indicates that the fuel pressure should not drop by more than 0.5bar in 30 minutes. I did this test and it seemed to drop by exactly that amount, which I'm pretty certain indicates an issue. Left overnight the fuel pressure dropped to 2 bar (from the initial 4.5bar the engine operates at).

So there are two potential causes of this problem: the fuel pressure regulator and the fuel pump nonreturn valve. The problem is that I don't think there's any way to determine which one is the cause! Such a problem would likely be caused by gunk stopping the regulator or nonreturn valve from sealing closed. As a diagnostic I did think I could simply remove the pressure regulator and attempt to clean it out. It just sucks that accessing the fuel system components (inside the fuel tank) is such a headache!

Does anybody have wisdom to impart on this topic?

Edit:
The plot has thickened, so here's a new overview of the symptoms. I'm starting to think this is not a fuel pumping/regulation issue but perhaps electronic.
- On cold starts (before and after work) the car won't start on first crank. Starts immediately on second crank.
- If cranked for long enough the first time the engine will fire and then immediately stall. The sound it makes at stall is the kind of sound you get when fuel is cut off.
- Fuel pressure test shows everything to be perfect at 4.5bar, even during cranking. Fuel pressure does bleed off over time when the engine is off but the pumps primes when ignition is switched off and immediately comes up to pressure.
- Waiting a few seconds with the key in position 2 makes no difference.
- No fault codes in the DME. Will check for any other modules like the EWS tonight.
 
Try leaving in pos2 until all the lights go out - might help. I'm not up to speed on the fuel system but assume that as the ign comes on fuel is primed. Worth a shot to see if it helps.
 
I connected the pressure tester again to check the priming. It's definitely priming correctly - as soon as the key goes to position 2 the fuel pressure immediately comes up to spec.

The really frustrating thing here is that I'm unable to duplicate the fault when I have the fuel pressure tester connected. Even if I use the tester to completely drain the fuel line during priming the fuel line comes back to operating pressure and the car starts immediately. This is starting to make me wonder if maybe I'm barking up the wrong tree. The fuel pressure leakdown test from the TIS mentioned a pressure drop of more that 0.5bar and my pressure drop was almost exactly that, so maybe it is actually in spec? It's possible that the 0.5bar test value isn't even valid for the returnless fuel system design. I was going to clamp off the fuel supply line to check for injector leaking but it that line seems pretty rigid so I'm not certain that it's safe to clamp...

Any other ideas?
 
The symptoms sound as though they could be ignition related, have you plugged the car into a scanner?
Crankshaft position sensor could make the car splutter & cut out for example.
Regards
 
I should have mentioned that in the first post. Yep reading codes was the first thing I did, using INPA. Nothing at all flagged by the engine ECU. Everything looked fine.
 
I am still a novice with these cars but I always start with the simple things first. Like what is the air filter like, are there any cracks in the plastic/rubber breathing system, when were the spark plugs changed, have you removed the plugs to see what condition they are in, are they soaked in petrol.
I am sure you will find it is something simple
 
Thanks Mike. I did check the spark plugs early on and didn't see any issues. Air filter is relatively new and I while craked intake components is definitely something to be aware of, I just can't see how that would be causing this issue...
 
Have a look at some threads where the ews unit was at fault.
I did have a case where the car would turn over but not start until I tried it for a second time. It did throw a code though and was slightly different from your problem, but could worth looking into?
 
EWS is a good suggestion too, but I don't think it would the the problem here for 2 reasons:
1. No error codes reported
2. On the long cranks it will eventually fire and then immediately stall. My understanding is that the EWS will prevent any fuel supply.
I will tonight see about checking the EWS module for any errors (rather than just the DME) though, just to be thorough.
 
Update: I did a full error code read and nothing relevant came up (just some things in the airbag and radio modules). As per no_fit_state's advice it could theoretically be an issue with the ignition socket coil antenna but it would be a bit of a stretch. I might try reseating the coil antenna connector just to be certain.
 
Well I've still been having the long crank on cold starts issue and it's still irritating me quite a lot. I'm certain that I've ruled out any problems with the fuel supply (fuel pump, filters and injectors), the ECU immobiliser functionality and the VANOS (solenoids were perfectly clean and working properly). I have been continually on the lookout for other symptoms or ideas but nothing else has arisen.

I was thinking about the problem this morning and it got me wondering if oil pressure could be to blame? My line of thought is this:

The N52 engine has several valve/timing related functions that rely on oil pressure for operation, not least of these are the VANOS and the hydraulic lifters. What would happen if it took longer than expected for oil to come up to pressure upon starting? Without sufficient oil pressure the VANOS cannot adjust the timing to the appropriate position. Likewise if the hydraulic lifters are not pressurised then the valves won't depress fully (please correct me if I'm wrong there). It just takes a few cranks for the engine to bring the oil pressure back to spec and then the engine can start fine.

Could it be possible that the oil pressure nonreturn valve is not operating as well as it should? Similarly, could an unsuitable oil viscosity drain too fast? The last oil service before I bought the car was done at standard mechanic, not even a Euro specialist, so it's very possible that something incorrect has been used. I'm thinking a quick DIY oil change to some known good stuff could be worthwhile.

Can anybody comment whether I'm barking up the wrong tree with this line of thought?
 
Sorry mate, I never did get to the bottom of it. I sold the car a few months later and upgraded to a Z4M!
 
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