Turns over won't start AGAIN
 

2007 Chevy Silverado Classic W/T V6 with 134,000 miles. Engine won't start after sitting for 3 days. Engine turns over good, 60+ lbs fuel pressure, good spark. Sounds like it is trying to fire at times but won't start. Mechanic replaced crank position sensor and cam position sensor. ($400 bill) Started good for a few days. Truck sat idle for 2-3 days and problem returned. Mechanic says cam sensor went bad (says only putting out 3.2 - 3.5 volts should be 12V) Replaced cam sensor. Good for 2 weeks and then truck sat in heated garage for 2 - 3 days and problem returned. Mechanic says something may be making the sensor go bad and is ordering a more "heavy duty" cam sensor. No Check Engine Light and No Security Light flashing. PLEASE ANY IDEAS??

Is he doing a crankshaft variation relearn when he's replacing these sensors? There could be something wrong with the PCM too. Don't forget that it's the PCM that controls everything based on what inputs it gets. The crankshaft sensor is an input. If it gets a bad signal, it wont know what to do if it can't tell where TDC is.

Thanks NullHead,
Does this also hold true for the Camshaft position sensor?

Just found out the mechanic is doing the relearn. He called it "synchronizing".

What kind of environment is the truck sitting in normally? Really cold?

It isn't temperature related. Happened during 70 degree weather. Truck is parked in a garage that doesn't get colder than 40 degrees. So far it has happened only after it sits for about 3 days without starting. Strange.

Ah, when you had said you stored it in a heated garage the second time around, I assumed that the outside was then substantialy cold. Nevermind then.

Electrical issues can be hard to pinpoint remotely for keyboard mechanics. I have a hard time accepting that something would cause the sensor to go bad (besides truly bad luck with replacements) with the truck just sitting and no temperature extremes. I wonder if it could be a grounding issue with the PCM. Shooting in the dark here, but electrical contacts can oxidize which affects the conductivity of the wire. Now this is unlikely to be bad enough to affect the computer on the plugs that enter it, but if the chasis ground from the PCM is corroded as well, or somewhere else electrical, then MAYBE this is your problem. When the sensor is replaced or the PCM is unplugged then reconnected, a thin layer of oxidized metal would be stripped off, exposing unoxidized metal and improving the connection (essentially the "Nintendo" fix with those old cartridge games). After some time, especially if you live in a warm area near salt water, the contacts will re-oxidize. This is normal. If, however, there are other bad connections in the system that haven't been experiencing any "cleaning" affects (i.e. have just sat and continued to corrode), they may be affecting the system enough.

Essentially I am guessing that you have some bad connections somewhere in your electrical system. Some of them are "cleaned," likely unintentionally, with these repairs, giving you just decent enough of a connection to start. They would then oxidize as normal, causing the connection to drop a bit, but enough because there could be other, much worse connections elsewhere. Clear as mud?

Get ahold of an electrical schematic and go digging around your vehicle. Clean any grounds you find. Simply loosen the connection, remove any rust or other oxidation (greenish coloring) you see with some sand paper, spray with some electronics cleaner, let dry and reconnect. For plugs (like ones to sensors or the PCM), visually inspect, clean with electronics cleaner, then let dry thoroughly before reconnecting.

It's a bit of a tedious job, but simple and cheap. No promises on it fixing your issue, though. Like I said, hard to diagnose without more thorough information.

What Prometheus is talking about is called "Terminal Fretting" by General Motors and it is a known problem, and dealer technicians (myself included) deal with it constantly. General Motors's fix it to disconnect, check "terminal drag" or in other words, get a dummy terminal from your Kentmoor terminal box that is the proper GM approved P/N terminal to slip into the female side, and make sure it has slight resistance when you pull it back out of the terminal.

That kind of nonsense isn't something the average joe is going to be able to do ... so do like prometheus said. Unplug the PCM connectors, look at them real good, and I'd put some dielectric grease in there. Might sound weird, but I've seen it fix a very strange amount of issues. It helps to cut down on this "terminal fretting" issue that promethius is talking about.

Also, I've seen corroded/shorted out under hood fuse blocks cause some crazy stuff. Might be something to look into.

Just curious, but what code is he getting when he is replacing the sensor? And I'm not doubting your mechanic, but I'm just wondering how far he's dug into it, like whipped out the oscilloscope and watched the signal to make sure the tone ring is good, or does he see the code and slap a new sensor in it.

I've also seen REALLY weird issues with resistance problems on sensor wires for throttlebodies. If you use a butt connector, solder or crimp any kind of wire repair into the TPMS harness, you'll continuously set throttle body codes. It's been said that the only way to fix this is to rebuild the throttle body harness with all new wires and terminals from the throttlebody to the PCM. I'm not saying that's what is going on with your truck, but just keep in mind that there is some crazy stuff out there and never overlook obvious.

Nullhead -- No codes have displayed before this:
Last Friday I got into the truck to start...it started very hard and FINALLY a CEL code displayed. P0102 - Mass or Volume Circuit Low Input. After searching, it looks like this code is pointing at the Mass Air Flow sensor. This is the first time a code has ever appeared.
Drove the truck home and it sat over the weekend in the garage again. This time only a day and a half without starting. Started VERY hard!! But it DID start finally. I am wondering if the truck may have sat for several more hours between starts, if it would have been worse and not started at all as in the past. I don't know why, but it seems it has to do with the truck sitting without being started for long periods. (a couple of days). I know this sounds weird..just reporting the history.
I can change the MAF sensor for less than $100 but don't really want to keep throwing money at it if this was a fluke.
Thanks to all for your help in this!

Are you sure the battery is getting plenty of charging time? I know with some repair jobs they recommend charging the battery afterwards. As they work on the vehicle, doors are left open (as the mechanics are in and out of the vehicle) so the battery drains. If you haven't driven the vehicle very far, the battery may not have had a chance to receive a decent charge. Hook it to a car charger or run it for a while and let it sit and see if that helps. Or try starting it, then hook another vehicle via jumper cables to see if that improves how the vehicle starts.

Try unplugging the mass airflow sensor - MAF sensors have the capacity to sometimes cause a no start condition. If it set a code for a MAF sensor, that's the direction I would look. I understand that your mechanic has been putting crank sensors on for a reason, but I would honestly be hesitant to put one on unless you have a trouble code for it. Remember, the computer knows what's going on in nearly every situation on a truck like yours. Chances are, if the crank sensor wasn't doing its job, or the crank senor was falsely reporting cranking RPM, the computer would know something is up and set a DTC most likely.

Thanks Prometheus. The battery doesn't seem to be the problem. The engine is turning over very good but it won't fire as if there is no spark. But the spark has been checked and there is 60+ lbs. of fuel pressure.

Did it again. No codes showing up. Turns over very good...just not firing. Changed the MAF (Mass Air Flow) sensor about 3 months ago and all seemed well...until today. Truck sat for 2 and 1/2 days.
Can smell fuel coming from exhaust.
This is very frustrating.
On the plus side....Now sitting at home unable to go to work.
HELP!!!

Okay, so you smell fuel ... do you have spark?

Took it to the GM dealership yesterday. They are telling me the cap and rotor are bad. They were both replaced about a year and a half ago. I don't doubt that they may need replacing since they were not Delco brand, but it doesn't make sense to me why the problem always shows up after sitting for several days....in a garage out of the elements. Picking up the vehicle this AM and will see what happens. I will post the results but it may take a while before I start popping the cork off the champagne bottle. Thanks Nullhead!!

Spark has never been an issue in the past...it always had spark...that's what is weird.

Natural condensation in the cap can cause corrosion and high resistance in the cap its self. I can see the cap causing these problems but what else can be an issue is the little spring in the cap/on the rotor. The spark travels down the center point and into the rotor, if that contact inside the cap that goes to the rotor isn't good, then you'll have exactly that problem.

Glad you got it fixed!