Extended crank when warm or hot on crate GM L31

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

Pinger

I'm Awesome
Joined
Mar 10, 2020
Messages
3,019
Reaction score
5,952
Location
Scotland.
@Pinger I don't think fuel vapor (such as a leaking purge solenoid valve) would cause a no-start. If you watch a video about GM EVAP systems, the pressures are REALLY small, somewhere around 2 mm of mercury = 0.040 psi. Besides, the compression in the cylinders is much higher. Are you thinking like a vapor-locked engine?
In the fuel supply pipes.
With heat, the fuel can vapourise and a bubble of it be trapped at high spot and the pump not be able to shift it and it impede flow.
It's really only a thing with older low pressure supply lines to carbs as the low pressure suction pumps worsened the situation by dropping the pressure in the line and I don't suppose the intermittency of their operations helped pull a bubble through to where it could be expelled.
The purpose of a pressure accumulator in the engine bay of a high pressure fuel system though is to prevent vapour locks.



What does the compression ratio of an engine mean? 10:1 for example. Is that 10 atmospheres of pressure to 1 atmosphere of pressure? That's like 150 psi? I honestly had never wondered what the ratio was. I'm just a hobbyist, LOL.
Pretty much - but it's the expansion ratio (where the piston is moving down the bore on the power strok-e) that really matters. It is - more or less - a mirror of the compression ratio hence why CR is used as the predominant term. It's not a true figure though as the closing of the inlet valve intrudes and likewise the opening of the exhaust valve on the expansion strok-e though again, they closely mirror each other. Contrast that with how CR is measured on 2-strokes - from exhaust port closing, not BDC.




Edit: looks like compression ratio is volume of the cylinder in the "down" down position to volume of cylinder in the "up" position...
'Geometrically' it is. It is the sum of the cylinder volume and the clearance (chamber) volume divided by the clearance volume.
 

alpinecrick

I'm Awesome
Joined
Jun 12, 2015
Messages
1,614
Reaction score
1,695
Location
Western Slope of Colorado
Lots of good advice. My first thought is the coil.

At night where there is minimal ambient light, with the engine fully up to operating temps, check the plug/ coil wires for arcing.

Possibility of a gasoline smell? Remove the gas cap on hot starts and see if that makes a difference. If it does it might be a venting problem with the tank and can behave just like vapor lock in the fuel lines.
 

Fragment

I'm Awesome
Joined
Aug 8, 2016
Messages
117
Reaction score
80
Location
Brenham, Texas
Lots of good advice. My first thought is the coil.

At night where there is minimal ambient light, with the engine fully up to operating temps, check the plug/ coil wires for arcing.

Possibility of a gasoline smell? Remove the gas cap on hot starts and see if that makes a difference. If it does it might be a venting problem with the tank and can behave just like vapor lock in the fuel lines.

Don't see any stray sparks in the dark... but the rear does smell like gas a little after running it. I replaced the tank and pump and all when I did the engine replacement, and noticed the two "roll-over" vent valves were a little loose when I put them back on and have just attributed the smell to that. I have some new ones sitting on the bench that are waiting to go on.

You really think this could cause the problem though?

Today it did it after only being out of the truck for about ten minutes so that was a shorter time than usual. Crank fire stumble stall... Crank crank crank crank fire and run.

The passlock seems to be behaving normally and I had VATS removed from blackbear anyways. Ordered a tune from them with a core ECU just to rule out a faulty ECU, but it behaves the same with tuned computer and stock.

I have removed the gas cap after running and noticing the gas smell to check for a hissing noise but haven't been able to get one. Don't have any EVAP codes either. Will have to try loosening it when I drive it next to see if it still acts up.
 

Frank Enstein

Best. Day. EVER!
Joined
Jul 15, 2018
Messages
2,005
Reaction score
3,431
Location
Canton, Ohio
You should NEVER smell fuel. There is a leak somewhere. Fix the leak then;

Check the resistance on the coolant temp sensor vs. the manual. The Motors manual at the library had this info.

If the computer thinks the engine is at a different temp than it really is it will hard start.

On my 94 Sub diesel unplugging the sensor will make it high idle because with infinite resistance it thinks it's -40F/C.

Short the terminals and it thinks it's 256 degrees F and it shuts down.
 

Fragment

I'm Awesome
Joined
Aug 8, 2016
Messages
117
Reaction score
80
Location
Brenham, Texas
Adding to the above, what does the scanner show as the temperature (value from the CTS) versus ambient and when warmed up?
Update on all of this:

I replaced the vent valves at the corners of the gas tank and the fumes are gone from the back so that is all good to go.

Ive checked that temp sensor more times than I can count now… I’ve graphed it with a scanner hot and cold and warming up. I’ve never got a bad reading on it. There was one time going down the highway where the graph blipped up about 20deg but never again, and never during a start cycle.
 

454cid

Sooper Pooper
Joined
Sep 5, 2010
Messages
8,009
Reaction score
8,893
Location
The 26th State
I replaced the vent valves at the corners of the gas tank and the fumes are gone from the back so that is all good to go.

What vents at the corners? I've had my tank out and there are no vents other than what's at the pump/fill area.
 
Top