Holley sniper EFI motor suddenly revving uncontrollably when I blip the throttle

-
RockinRobin

RockinRobin

Well-Known Member
Joined
May 15, 2013
Messages
1,272
Reaction score
950
Location
Corsicana, Tx
It won't do it until the motor is up to temperature, but at that point when I blip the throttle off idle hard enough it will stay at high rpm and will not return to idle.
This is system that has run normally for over a year. I am being told to check for vacuum leak, but I have no smoke tester. Been racing this combo for almost a year now and it has never done this before.
Gen3 hemi 414 stroker with mopar drag pak intake and Holley Sniper 4500 Throttle body Fuel injection.
 
Possibly a sticking IAC (Idle Air Control) .. Vacuum leak wouldn't care hot/cold/on throttle/off... Either the IAC is being commanded to open which it typically is briefly at throttle application to allow the computer to control engine decell preventing the engine stalling... If the engine idles normally before blipping the throttle try unplugging the IAC & blip the throttle... It may stall since the computer won't be able to control the return to idle but if it at least returns to idle you'll know your looking in the right area..
 
BTW IAC looks like this..

543-105_01.jpg
 
1WildRT said:
Possibly a sticking IAC (Idle Air Control) .. Vacuum leak wouldn't care hot/cold/on throttle/off... Either the IAC is being commanded to open which it typically is briefly at throttle application to allow the computer to control engine decell preventing the engine stalling... If the engine idles normally before blipping the throttle try unplugging the IAC & blip the throttle... It may stall since the computer won't be able to control the return to idle but if it at least returns to idle you'll know your looking in the right area..
Click to expand...
Blocked off IAC port and retested. No change.
 
Two things to consider, re-centering the blades if they’re rubbing, or putting an extra wind in a worn out spring on the secondaries:

Re-centering:
1687829549457.jpeg


Winding an extra turn into the secondaries:


Edit:
I guess I should say, check the throttle bores for cleanliness/rubbing first. But I’m wondering if the issue is heat/thermal expansion related of the bores vs plates, which leans toward the re-centering solution…get it warmed up and check then since that seems to be when your issue appears. Then it might be trying to re-center when the throttle body is warm…
 
Last edited:
If there is a PCV attached, disconnect it and plug the port.

If that solves it, and you want to retain the PCV system, replace the valve with a fixed orifice type.
 
PRH said:
If there is a PCV attached, disconnect it and plug the port.

If that solves it, and you want to retain the PCV system, replace the valve with a fixed orifice type.
Click to expand...
I remember reading about this as well…. Good luck
 
My gut leans towards a mechanical problem. throttle blades, linkage, kick down, something like that
 
Things to cause fi idle surge...
IAC commanded to open because the o2 doesn't read enough air or is on it's way out
Popped intake gasket ,maybe from a intake backfire ...aka vac leak
Check the throttle shaft on the throttle body maybe there's a seal maybe the spring tensions too much and bushings worn see if the shaft slides or shifts at all on the bore to create a leak that would be sort of intermittent
 
PRH said:
If there is a PCV attached, disconnect it and plug the port.

If that solves it, and you want to retain the PCV system, replace the valve with a fixed orifice type.
Click to expand...
no pvc system, just a couple of lines from the crankcase going into a puke tank. They do both go through the intake manifold though. Vacuum leak is looking more possible. I need to borrow a smoke tester from someone.
 
I have a vacuum port on the intake. I got the correct sized hose and stuck it in there. Then with a cold engine I blew smoke into the vacuum port. the only place the smoke came out was the top of the Throttle body (engine off). Do I need to do this again with the motor up to temperature? Can you tell I've never done a smoke test before?
 
OK, new information. With the motor running, when I crack it open far enough to open the secondaries they do not close. I think I need to put another turn on the secondary spring, re-center the secondary plates in their bores or both.
 
RockinRobin said:
OK, new information. With the motor running, when I crack it open far enough to open the secondaries they do not close. I think I need to put another turn on the secondary spring, re-center the secondary plates in their bores or both.
Click to expand...

I've run into issues like this, and found I hadn't carefully tightened the nuts holding it to the intake. The torques were all over and it apparently warped just enough to stick slightly (mine is an FItech though). Loosened and then re-snugged, problem went away.
 
UPDATE:
Removed TBI from intake. Could not get secondary plate screws loose, was afraid of stripping them, so gave up on that.
Put an extra turn in the secondary spring and reinstalled. Still hangs open when secondaries come in.
Tried an extra spring. What I had was not strong enough.
The photo shows a temporary solution: filled the slot that secondary linkage is in, this forces the secondaries to close with the primaries.
It is working and is not binding, but I need to find a more permanent solution to block off the slot in the secondary linkage. What you see is a short self tapping screw.
I'm Thinking a grade 8 machine screw and a nylon insert nut, as long as they don't bind on anything.

Sticking1.jpg
 
Just a random thought.....does it have a temp sensor on it for the system?
 
RockinRobin said:
Yes, water temp and air temp.
Click to expand...
So it's a water temp SENSOR? For the computer? That would be suspect to ME for something causing an issue either cold or hot but not both.
 
RustyRatRod said:
So it's a water temp SENSOR? For the computer? That would be suspect to ME for something causing an issue either cold or hot but not both.
Click to expand...
EFI systems use a combination of data to determine how much fuel to put in: water temp, air temp, A/F ratio from o2 sensor, etc.
 
RockinRobin said:
EFI systems use a combination of data to determine how much fuel to put in: water temp, air temp, A/F ratio from o2 sensor, etc.
Click to expand...
I take that as a "yes". That's worse than some of these congressional answers I've heard.

I would check the resistance on that temp sensor. From the sound of it, you'll need to to it while it's up to operating temp.
 
RustyRatRod said:
I take that as a "yes". That's worse than some of these congressional answers I've heard.

I would check the resistance on that temp sensor. From the sound of it, you'll need to to it while it's up to operating temp.
Click to expand...
I'm sorry I was trying to give you a more complete answer than simply saying "yes". I got the impression that you were not familiar with the aftermarket throttle body efi systems. The water temperature sensor that came with the unit was reported as unreliable and I replaced it with the recommended sending unit. I trust that the water temperature it is reporting is accurate enough, I think that the old one was reading about 15 degrees colder than actual. This car leaves the start line at 160 water temp and usually gets no higher than 175 by the time I shut it down in the pits. I'm not going to say that I have had zero cooling system issues with this motor because I don't want to jinx myself!
 
RockinRobin said:
It won't do it until the motor is up to temperature, but at that point when I blip the throttle off idle hard enough it will stay at high rpm and will not return to idle.
This is system that has run normally for over a year. I am being told to check for vacuum leak, but I have no smoke tester. Been racing this combo for almost a year now and it has never done this before.
Gen3 hemi 414 stroker with mopar drag pak intake and Holley Sniper 4500 Throttle body Fuel injection.
Click to expand...
Can also verify that temp sensor is staying wet all the time. We ditched the sniper for sequential. I cannot understand how you open the throttle blades for initial start up and then go back to zero when you verify.
 
I don't know the Holley Sniper, but for throttle bodies in general (both TBI and MPFI), the throttle usually closes off tightly and all air flow at idle is thru the IAC valve. Thus, if you truly plugged the IAC port, the engine should have died, unless getting air somewhere else (vacuum leak from PCV, brake booster, intake gasket). Did you just cover the external hole? If so, that doesn't block the airflow around the throttle plate. You must block the internal hole which the valve plug covers. Speaking of vacuum leaks, my 1996 Plymouth 2.4L began idling way too fast. I figured a leak was overwhelming the IAC authority (full closed). Finally found it by flowing water from a garden hose around the intake. It slowed way down when flowed around the intake to block interface. Hard to see since the intake loops over that. The intake manifold gasket had become brittle and cracked with chunks missing. I had changed it during a head gasket replacement 15 years earlier (MLS gasket to fix a generic oil leak). I recall a name brand like Victor-Reinz but doesn't mean they didn't source them from a Chinese company that used black cardboard.

I had a similar behavior to yours with my 1969 Dart 225 when the Holley 1920 carburetor would run lean at idle (common problem from a plugged idle-metering block). In "P" at idle, if I manually opened the throttle plate briefly and returned it, the engine would speed up and stay idling faster. Shifting to "D", slow the engine way down and would even stall. I suspect the brief rpm increase gave more spark advance, which a lean mixture likes, then it stayed running fast with that more advance. I always thought it idled lean, but 2 rebuilt carbs didn't fix it and no shop could diagnose it (yes, worthless "professionals"). Finally, a 4th carburetor made it purr like a kitten at idle and no more stuttering off the line. Might not relate to your problem, though perhaps general for any lean-running engine. You might be able to diagnose by giving it squirts of Starter Fluid to see if that changes behavior.
 
Last edited:
^^This, "back to basics." An engine MUST have AIR to rev. Whatever the sensors/ electronics/ fuel ratio is off the table, it MUST be getting air someplace. The big difference between EFI and carb is, that if you have an air leak, the O2 will crank up the fuel. But the AIR is what drives it.

Vacuum leak, PCV, loose "somethings", IAC is AFU, stuck throttle blades/ linkage, etc etc.
 
You must log in or register to reply here.
-

Back
Top