Hot Start Stumble - E38/LC9 5.3 - Locked Throttle

[325_53]

Having a weird issue that I presumed was boiling fuel in the lines/rails or fuel related, but after wrapping some foil heat wrap around the fuel lines and finally running a log, finding out there might be more to it. The car runs well outside of this issue.

- Can runs perfect until it has been shut off and attempt to restart within 5-15 min. after a hot run.
- After a track session, requires nearly 30 min of cool-off before it will restart
- The car stumbles and throttle is unresponsive and is barely idling
- The issue only happens after the car has been run hot and underhood temps are high.
- While logging the issue, I found the injectors are running between 6.8-7.0 ms when typically I would see 2.0ms or less at idle.
- Throttle is not responsive (no movement in the scanner when the pedal is pressed)

I'll post the log up tonight when I get home, but has anyone encountered something like this on the Gen4 platform?

Running:

- Stock E38 ECM
- Flashed with 2007 Corvette M6 OS - 6.0L
- Tune is still being worked out, but car is driveable and runs well otherwise
- injectors are stock LC9 (flex fuel) injectors, injector info was copied over from the original avalanche flash file pulled from the ECM

I was sold on the boiling fuel theory until I saw the high injector PW and dead throttle pedal when this occurs.


Issue will clear itself up after 3-4 attempts to restart and opening the hood to cool the air off for 5 minutes or so.

poor-start-53-325-001.hpl

[325_53]

Still chasing this issue, not sure if anyone has run into this before or has had experience with vapor lock in EFI systems due to underhoot heat?
Just can't sort out why the car will run perfect until it's been shut down, and the issue only crops up on restart.

[blindsquirrel]

Describe the fuel system (all plumbing, components) please.

[325_53]

Fuel system is laid out as follows:

- Walbro 255lph pump retrofit into stock BMW E46 Pump housing/Tank
- Factory BMW lines (coated hard aluminum lines) supply fuel from the tank which are 8mm (0.315") ID
- Lines run to a Corvette (ACdelco Brand) Fuel Filter/Regulator Running 58 PSI Connections are rubber hoses with gear clamps
- Return from the filter to tank is OEM BMW lines again 8mm (0.315") ID
- From the filter/regulator I have a -AN adapter on the outlet converting OEM push-to-connect to JIC thread which then feeds -6 AN Black Braided line
- Braided -6AN line runs along the frame rail from the filter, up beside the header on the driver's side (approx. 1" Clearance between header and -6AN Line)
- Header is wrapped in Kevlar header wrap and the -6AN line is wrapped from the filter to the fuel rail in fiberglass/reflective adhesive heat shrouding
- AN line comes across from the frame rail and up the front of the engine connecting to the front driver's side fuel rail. Fuel pressure gauge in-line with this connection point holds steady 58 PSI, sometimes creeps above after the car has been running hot (66PSI or so).
- Fuel rails are 1/2" Billet aluminum rails with a -8AN Braided Crossover line running across the back of the intake to the passenger side rail.
- Fuel system is returnless from the filter forward and ends at the front of the passenger side fuel rail.
- Injectors are factory flex fuel injectors for a 5.3L Gen 4 engine and all

I realize the fuel lines may seem a bit light, however with the current setup the car was dyno tuned at 346 WHP and sustained consistent AFR's throughout the entire process with no indications of fuel starvation. I feel that heat is my main enemy here, but couldn't understand what would cause the massive injector pulse-widths and no throttle pedal response on hot restarts.

ECM is in the engine compartment as well in a separate corner, unsure if too much heat to the ECM can cause issues on restart with injector drivers.

[blindsquirrel]

Issue will clear itself up after 3-4 attempts to restart

Curious if that would be the same if you just did 3-4 key cycles (with enough time inbetween so that it runs the pump prime), or if it's related to actual cranking.

Semi-related, if I were hands-on with this, what I'd do is hook up a fuel pressure gauge with a bleed/sample port. Install gauge, bleed it out, then do whatever it takes for the issue to show up, make it not start. Then take a fuel sample. Is it nothing but vapor, or is there actual liquid fuel in the rail when it won't start?

If there's liquid in the rail, and fuel pressure is still in the acceptable range, then you can rule out fuel system issues and look for heat affecting something electrical. Probably.

[blindsquirrel]

What's your ECM Service Number, please?

[blindsquirrel]

<looks at log> Oh. Have you looked into the cause of your P2176?

[325_53]

Heh.... I feel dumb.
I'd bet money that P2176 is heat related.... Time to dig into that.... can't believe I didn't pick-up on that. That code is not usually active and may be the heat related "electrical" component I was chasing.
Guess having a check engine light wired in would be helpful.

I'll definitely add a bleed port to the front passenger side of the fuel rail to confirm that isn't a secondary issue... but I think you've helped me nail the issue by pointing out that P2176. Can't believe I missed that.

Appreciate you taking a deeper look into the logs. I'll update this thread after swapping on a new throttle body and see if it resolves the issues.

[325_53]

Just wanted to follow this up for anyone researching a similar issue, way late to the response as this was resolved for the beginning of last motorsports season.

blindsquirrel nailed it....
Replaced the throttle body with a new unit and the issue was resolved. Multiple HPDE's and heat soaked engine bay events with no recurrences of it.


Thanks again for your help on this.