Sniper 2 stumbles after warm restart

[Duster360LA]

I have a Holley Sniper 2 and Hyperspark on my moderately-built 360 with a 5-speed. It runs great on a cold startup, but after its runs for 15 or more minutes and it warmed up if I shut it off and restart it (like stopping to fuel up) it will stumble and frequently die when it drops down to idle (almost like a vacuum leak). If it fully cools down and I start it, there are no issues.

Details:
-I run Holleys in-tank pump that doesn't require a return line.
-Fuel line is Holleys 3/8 in hose from tank to EFI
-I have the Sniper PDM module in place
-I get 51 pounds of fuel pressure right at the EFI fuel intake.
-I did have to change the EFI temp sensor to go in a smaller port on my Edelbrock Performer RPM manifold to accommodate my AC compressor. It's a unit made by an aftermarket seller that Holley suggested (and has good reviews).
-Car temp stays at a pretty constant 180 and never climbs over 190.

I am considering switching out the aftermarket temp sensor thinking it may be the issue, but that's just a guess.

Any ideas/suggestions?

 

[Duster360LA]

One more detail. When the stumbling issue occurs, if I give it a fair amount of throttle (or WOT) it screams to life and runs strong.

 

[MV8]

Does the CTS show (under "monitors") what you expect the coolant temp to be cold and 180-190f hot start? If the IAC more than 10% warm idle with the AC off, then the throttle plate angle needs adjusting to open it up more so the IAC can have more control than just trying to flow enough to idle.

AC Kick should be enabled under IAC Rampdown screen. Might change the IAC Parked percentage to be higher (under IAC Startup) . Too high and the idle will be too high momentarily after start up hot or cold. Varies with temp.

 

[Sr4440]

Try data logging the issue.

 

[RustyRatRod]

Does it have a temperature sensor? If so, find the values for it and ohm it out. That could be one issue.

 

[Duster360LA]

Thanks for all these suggestions, I will be working my way through the various checks. Thanks everyone.

 

[daddio4048]

If I'm not mistaken the fuel pressure is suppose to be around 60 psi. They are pressure sensitive.

 

[jimmyray]

also, if you arenot running a return, and depending on how your lines are run, it could vapor lock. I'm I'm running the Sniper 2 with the in-tank fuel pump and the hyperspark. Mine is finally running perfect, but I did a lot of tuning. Happy to share my mapping if you like.

 

[TT5.9mag]

Get the fuel pressure up to where the sniper wants it (58.5) and scale the new temp sender in the software. I guarantee you the scaling of the new sensor is not the same as the Holley sensor. You’ll have to manually measure the new one (if the data isn’t provided by the manufacturer) and enter it in the software.

 

[jimmyray]

from ChatGpt:
An externally mounted EFI pump on a Holley Sniper 2 EFI dramatically increases the odds that this is a heat/fuel delivery issue, especially if the symptoms occur only after 25–30 minutes.

External EFI pumps:

• push fuel well
• pull fuel poorly

They are much more sensitive to:

• heat
• suction restrictions
• vapor formation
• tank venting
• mounting height

Most likely scenario:The pump gets hot, fuel begins to vaporize on the inlet side, pressure falls, and the engine either:

• stalls
• goes lean
• or refuses hot restart.

Very common setup mistakes:

Pump mounted ABOVE tank levelBad.

EFI external pumps should ideally be:

• below fuel level
• gravity fed

If mounted high:

• pump must suck fuel
• fuel can cavitate/vaporize
• hot restart problems occur

Pump too close to exhaustVery common.

Headers and exhaust radiate huge heat.External pumps hate heat.

Even if the pump itself survives, the inlet fuel can boil.

Small feed line or restrictive pre-filterAnother common problem.

Before pump:

• use large line
• low restriction filter

Holley generally wants:

• 100 micron pre-filter BEFORE pump
• 10 micron AFTER pump

Too-fine filter before pump causes cavitation.

Tank venting issueA non-vented tank can create vacuum over time.

After 20–30 minutes:

• pump struggles harder
• pressure drops
• restart fails

Next time it acts up:carefully loosen fuel cap and listen for suction.

Weak electrical supply to pump Hot pumps draw more current.

Voltage drop worsens with heat.

Measure voltage AT PUMP while running hot:

• should stay near battery voltage

If pump only sees:

• 10–11 volts hot

…it may not maintain 58 PSI.

Critical test:Install a fuel pressure gauge you can observe while driving.

Watch what happens as engine warms.

If pressure:

• steadily falls
• fluctuates wildly
• drops during crank

…you found the problem.

Another VERY telling clue:When it won't restart hot:

• pour cool water on the external pumpOR
• cool it with compressed-air duster upside down

If it suddenly starts:the pump or fuel boiling is almost certainly the issue.
Best long-term solution:An in-tank EFI pump.


Modern EFI systems strongly prefer:

• submerged pump
• cooler fuel
• no suction-side cavitation
• quieter operation

External pumps work, but they are much less forgiving.

 

[Duster360LA]

Thanks so much @jimmyray - I would appreciate getting you mapping info. I will definately check voltage at the pump! Will also check on the vacuum issue possibility.

I have Holley's in-tank returnless pump and run a Hyperspark as well. This weekend in leaving a show, it had the stumbling issue on a cold startup (sitting overnight). And when it experience that problem and dies at idle I can always restart it.

Thanks to all!

 

[jimmyray]

my mistake, think got your thread mixed up with another, thought you had an external pump. happy to share my fuel and spark map, how can i get it to you? dont think i can post it here.

 

[jimmyray]

what did you figure out?

 

[Duster360LA]

Still working on it. I located a pinch (with some exposed wire) on my main feed between the battery and starter that I believe might have been all or part of the cause. I've only run the car a few tires since that repair, but each of those startups have been smooth with no issue.