496 Crank sensor intermittent? no code?
 

2001 496MAG HO. Will start when cold and stay running (so far) but will not start after warm. The first time it happened I thought it was bad fuel/filter. When I get the no start condition if I advance the throttle, it will pop/backfire or kind of run on one cylinder. Fuel is good, fuel pressure is good, new spark plugs.... No codes and I can still talk to the computer when it won't start.

So, from searching on OSO, I am thinking it is the crank sensor failing when hot but not enough to throw a code. Any other suggestions?

Also, anything wrong with buying an automotive verson versus going through Merc? Auto is half the price.

Thanks,
Bill

Same part Bill. The early camshaft sensors had two o-rings and would get rusted in the bore and the rust would sometimes lift the sensor just enough to increase the clearance between the Hall effect sensor on the end of the sensor and the reluctor wheel and sometimes the ECM would not allow the engine to start or run properly from this issue. the later sensors only have one o-ring at the top and don't seem to suffer from this problem.
Not saying this is your issue since sometimes when the 496 starts hard or hesitates and pops it can set a camshaft sensor temporary fault and then it will clear on the next start.
If you can borrow a sensor or buy an truck replacement from NAPA, etc. its a simple check out and ridding yourself of the double o-ring early sensor is a good change out anyway.
If this is un-successful look at the fuel rail and injectors as cold versus hot sticky-fouled up injectors will sometimes act like this with a good cold start and a bad hot start.

Best Regards,
Ray @ Raylar

Ray, Thank you very much for your input. I do have a second engine right next to it I can swap parts off of, but I believe I will just pick up a new sensor from NAPA and go from there.

It does seem very strange that when it wouldn't start and with the Diacom software hooked up it didn't trip any codes. From my experience it acts like it is way out of time when it wouldn't start. Which makes me feel fairly confident that it is the crank sensor.

I will report back my findings.

Bill

I replaced the Crank Position Sensor and so far so good. I have tried to duplicate the previous conditions which produced failure twice in a row and so far I have had success. I am fairly confident that that this has solved my problem. Fingers are crossed.

I am confuesd Bill seems to be talking about CRANK sensor and Ray seems to be talking about CAM senso . thought for a while that they might be the same ., just that some call it crank , some call it cam sensor. just looked in my workshop/M and it seems cam/S is at the front near the crank pully and the CRANK/s is on top of the motor at the back ,is this correct? Bill which ones did you replace ,ones near crank pully or ones on top at the back of motor ,

I guess i didn't read Raylar's post that closely and just assumed he was talking about the Crank Sensor as I stated in the original post. It is the Crank Sensor that is located on the back of the engine that I replaced. Maybe Ray will post back and verify if for sure which sensor he was talking about.

its in back of the engine,,,the one he replaced

thanks for clearing that up for me guys , is the Cam/s the same as on truck if so i thought i might as well buy 2 of them from napa ?? as ray says good to change thes over .

I need help on this again. I had no problems the rest of last year and now this year the same problem is happening again.

I noticed a couple of times that the tach and engine would flutter (drop from 4000 to 3000 and then right back up again). Another time out I had it die at 3000 rpm and not restart, I unplugged and replugged the crank position sensor 2 or 3 times with no change, started idling back on the other engine, after a bit I tried it and fired right up. Made it back to the dock where I removed and reinstalled the Crank PS and had no more problems that day. As a preventative, I replaced the Crank PS with another new one. First time out we went a few miles for lunch, come out after lunch and no start (would give me the typical kind of want to fire or backfire so I assume it's the crank PS). I idled back to my dock trying that engine from time to time. Just about to the dock and it decides to come alive. Went for a quick ride and all was well until I am idling back around the dock and it dies with no restart..... That is until it sits for a while - cools off I assume.

I am very disgusted. I can't trust the boat. There are no codes.

Please HELP! Any suggestions or opinions? It seems to be heat related. Could it be the computer or would a bad coil pack cause any of these problems?

The crank sensor can fail and set no codes, the computer just thinks the engine is not turning. Now the hard part is figuring which part is failing. With cars the first thing we do is put a scope on the sensor wires at the computer and crank while it's acting up. The scope should show a nice even wave form from the sensor and that means the sensor and wiring are good. If the pattern looks unusual then you check back at the sensor and see if it looks okay, if it does then you have bad wiring. If not, then you have a bad sensor or some type of issue with the crank and/or bearings. Now, if you have a good pattern at the computer and the Rinda tool shows no or low RPM (should show 300 or more RPM when cranking, then the circuit that interprets the crank sensor in the computer is bad. If you know any solid auto techs they should have a scope, either laptop based or portable that they can check it with.

With no codes being shown for any form of crankshaft failure and the engine restarting and running after cool downs I am suspecting you may actually have a fueling issue which is cutting the engine out and then seeing some backfiring (popping) on attempted restart. Get a fuel pressure guage installed on the problem engine and watch it closely when it first runs and then under load when it stalls or stops. Is the fuel pressure at 43psi key on no start, idlers about 43-41psi and stays above 40 psi when at full throttle and where does the fuel pressure go before or just before it quits or stalls?

Is the engine quitting or stalling when you back off the throttle or quickly reduce speed?

Best Regards,
Ray @ Raylar

I didn't think of using a scope, at least that would tell if the wiring or possibly the computer is bad. I have a hard time believing that 2 new sensors are bad, but who knows. Also, if it was an issue with the crank/ bearings shouldn't I also see other symptoms?

Thank you for your advise.

I never thought about it being a fueling issue. I will get a fuel guage on it and see what happens. Does the computer shut down the engine when it senses low fuel pressure? Would it not set a code?

The engine has never quit or stalled when coming off the throttle. It has quit at idle (after idling for several minutes), it has also quit at 3000 rpm or 4000, doens't seem to have any reason. When it does quit, it has never fired right back up immediately.

Even though everything seems good and tight, I am going to double check all grounds and fuel pump wiring, in fact i will look over all wiring and connections.

Any chance this could be cam sensor related? I'm guessing the computer needs to see both the crank and cam positions to give the spark signal??

Thank you for the help.

Checking all the connections and grounds is also a great idea and it could be an intermittant connection. The cam sensor could be bad also, try swapping between the two engines on the cam sensor and see if the problem moves with the sensor.. The cam sensor is NOT a real expensive part when bought at a auto parts house or GM, so you could just try a new one. The cam sensor won't cause the engine to quit or stall but it will make it hard to start and can pop as described on starting if its bad or intermittant. Usually it will throw a code though for cam sensor issues.

Reason I asked about stalling when pulling back throttles is if an IAC is acting up or bad it will cuase a stall many times at throttle pullback transitioning to idle as that is where it also works to stabilize the engine rpms backing down. Doesnot sound like this is in play here.

Best Regards,
Ray @ Raylar

Here is an update. --And hopefully a conclusion. Possibly help someone else with a similar problem.

I swapped both crank and cam sensors between engines. It took a while but the port (previously reliable) engine gave me the rpm flutter. Not knowing which sensor it was, I moved the cam sensor back to the starboard engine and the problem followed. I replaced the cam sensor and the problem seems to have gone away. Funny it is both the cam and crank sensors on the same engine and the other has been flawless.

So, the symptoms this bad cam sensor has given are engine flutter (tach drop by approx 1000 rpm for an instant and recover), completely die at any given rpm and not restart right away (had it happen at idle and 3000 rpm), sometimes no start when hot. Always started when cold. Never threw any codes which I understand is unusual since a cam sensor usually throws a code.

Thanks to everyone for the input. I am considering it solved.