Zero Fuel Pressure = Instant Startup WTF

[compyelc4]

Can someone please explain to me how I can have zero fuel pressure on a cold start and get an instant startup? This is not the whole story so please allow me to explain, and sorry for the long post.

I localized my "long crank til startup" (LCTS) problem to warm days only. I have 12,000 miles on my '07, but I've had this problem since just after the warranty expired. For a while I thought Techron cleared up the problem, but this summer the problem returned with a vengeance. When the weather is below 60-70 degrees F, whether on a cold or hot engine, she fires right up, time and time again. Above 60-70 degrees I get a mostly an intermittent delayed 3-4 second startup (I know, this sounds like a short time interval, but believe me it seems like an eternity, and it is embarrassing). I have found if I keep the accessory side of the start button depressed for about 6 seconds I can hear the the fuel pump run for a second or two, and then WHETHER HOT OR COLD OUT I get an instant startup.

Now listen to this!!!!........I checked the fuel pressure at the rail yesterday. It was about 50 degrees F out. The C6 had been sitting all night. The static fuel pressure was 0 pounds! I hit the ignition and within 1/2 second the fuel pressure shot up to a steady 62 pounds and I had an instant startup. I cut the ignition and watched the full pressure immediately start dropping. It took about 3 minutes to reach 0 pounds!!! I tried another startup and again the fuel pressure instantly shot up to 62 pounds and I got another instant startup. I let it sit all night. Today, when I got home from work I did the identical procedure, experiencing an immediate startup with an initial 0 pounds of pressure. PLEASE EXPLAIN HOW THIS CAN BE?! I have heard on several forums that low fuel pressure at startup is indicative of a fuel leak and will cause a long crank til startup. My LS2 just laughed at this theory. What are the physics involved when the ambient temperature is above 60-70 degrees F, I have 0 pounds of pressure at startup and I get a long crank rather than an immediate startup, then under the same conditions below 60-70 degrees F ambient I get an immediate startup?

BTW, I have no soft or hard codes set. A Tech 2 shows everything is within operating limits. Even on a long crank startup there is no black exhaust, no stumbling idle or acceleration. Once she fires she runs like she should.

I believe that losing fuel pressure that fast to one or more leaky injectors would result in one smokey, stumbling engine after she starts (and I don't have that), so I think I may have a drainback problem in the tank but why in the heck does this pressure leakdown to zero pounds in the rails not result in a long crank til startup irregarless of ambient temperature?

PS- She's all stock, no mods, babied and drinks only top-tier fuel only.


You may have read something similar from me in another forum, but I cleaned it up a bit and posted it here, to my home forum.

 

[Hib Halverson]

Do you have the Factory Service Manual?

 

[compyelc4]

To Hib re: C6 Zero Fuel Pressure After Sitting

Hib,

I can't respond directly to your post on this thread as it does not appear.... weird. Via email through this site, you asked if I have a service manual. Unfortunately I don't. I did not expect this type of expensive problem with my low mileage C6 so I have put off ordering one until an expensive problem crops up. It looks like I now have an excuse to get one. I wish this thing were as bullet-proof as my '95 C4. My bro is a certified auto mechanic and he does have access to on-line shop manual type data on all cars, including the C6, so please steer me in the right direction if you have experience with these types of problems. I have a lift and the skills to drop the tanks but I'm not so sure at this time if I have to go that far.

I'm still amazed that I can get an immediate startup with zero fuel pressure. That fuel pump must still be quite strong to pressurize the system to 62 lbs. in 1/2 second! When I do get the extended crank times I wonder if the pressure is 62 lbs. Now that would really puzzle me.

 

[Hib Halverson]

Sounds like you may have a fuel supply problem and the fuel pressure tests in the Service Manual are invaluable in diagnosing such trouble.

That your fuel pressure bleeds down quickly is indicative of a problem. Could be injectors, the FPR or the fuel pump.

The info in the Manual will help you decide where the problem is before you go buy parts.