Quote:
Originally Posted by Hib Halverson
Ya know, when I posted above, I didn't really time that video, but Paul must have and I agree with him, if it's six seconds, that's a bit much. My 95's start time after fuel pump shut off, is about the same as Paul's
My 95 has always behaved the same way. Couple of points, though...when the pumps stop running, the fuel system may or may not be at the correct pressure. The pump-cut off is not related to fuel pressure. If you've got a pump out, leaking pumps, leaking regulator or leaking injectors, its possible the fuel pressure is not at a nominal level when the cut-off occurs. The cut-off is related to how long (2 sec.) the ECM will allow the pumps to run without an rpm signal, not fuel pressure. When you turn to "run" the ECM turns on the pumps but, if it doesn't see the engine runningk after two seconds, it turns off the pumps.
Also, if you've got the key in start, the engine is cranking but fuel pressure is not up, the engine will not backfire nor are you inviting it to do so.
Holding the key in "start" after the engine is running is pointless and might not be the best thing for the starter. Intake manifold backfire won't happen once the engine is running.
The infamous backfire occurs if the start cycle in interrupted but the key remains in "run" rather than going to "off". If the engine attains some rotation but not enough run, the ignition is hot and the engine is at a certain position with the right intake valve open, sometimes air-fuel mix in the plenum will ignite. The resulting "bang" sometimes takes out the MAP sensor and/or blows vacuum hoses off the nipples at the front or the rear of the intake manifold.
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I'd heard about the backfires before it happened to me once. The problem, like you alluded (sp?) to occured in that split second where it sounded like it fired, but actually failed to fully catch. Maybe it did fire on a cylinder, but missed the next one or two. In any case, I released the key, and
POP! Just that quick and troubles followed.
Lucky for me it was just the MAP hose (now secured with a small tie-wrap as insurance against that happening in the future) and no further damage. Now I hold the key - perhaps not for a full second - prolly less than that, but I make it a point to hold it in START until I'm absolutely sure it is really running (i.e., more than just a single cylinder or two that fired, is my intention).
I know the bendix on the AC/Delco starters on the gen I SBC/BBCs are designed to "free wheel" when the engine starts and the speed of the flywheel over-runs the speed of the starter. I'm pretty sure it is the same for the LT5 starters, but can't say for sure. But, in reality the length of time I hold the switch in the START position is "under a second" - more like half a second or maybe less before I'm satisfied the motor is really running (for good) and the key is released. So far so good, far as my starter goes, and NO backfires since, far as I know. However, once in a while I will hear a cylinder fire and not be followed by one or more cylinders following. I'm glad I was still holding the switch in START - smoothly passing over until the motor catches for good.
Not being in a big hurry to release the switch from the START position was foretold to me, and then taught first-hand by a few experiences since...YMMV. Just sayin.
P.