Hemi high idle

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gumper

gumper

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Fired up my new Hemi in the Duster for the first time today. I cannot do anything to get it to idle below 2500. As in even with the idle adjustment screw on the carb turned all the way out the RPM will not go under 2500. You would think immediately it’s a vacuum leak, but I cannot find anything. It has a Holley single plane with a 4150 carb, both brand now. The heads were milled and port matched to the manifold. The intake is torqued down per spec with the oring gaskets in place. Both NPT ports in the back of the manifold are plugged. There’s no ports in the carb.
I am using an MSD Hemi 6 with the cam and crank sensors plugged in. I have the right timing map (built-in) for my coils and harness. Waste spark is turned off. The idle timing should be 10* at 1000 rpm, moving up to 20* at 2000 rpm then 25* starting at 2600 rpm and up. The Map sensor is plugged in, but not in use (it’s zip tied to the harness on the firewall with a vacuum cap covering the sensor). Can a bad MSD box cause the idle to be so high? Can a bad crank or cam sensor cause it? The crank reluctor wheel is the correct one for the box. I would assume with a bad crank sensor it wouldn’t have any spark. It does backfire through the carb occasionally when trying to start it. It acts as if the timing is excessively advanced regardless of the settings.
The MSD was used when I got it. The guy I got it from used it once and he also had an uncontrollable high idle. He thought he had a leak at the manifold, but I am 100% confident it’s not vacuum related.
I ran the engine for a good 10 minutes at 2500 since it’s new and needed to be broke in a bit anyway. Other than the uncontrollable idle it seemed to run fine. Oil pressure was about 50 psi and it held at 160 degrees which is what thermostat I have. It did get up to 200 for a minute before I purged the air from the system and added more water.
 
You need that MAP sensor hooked up. The 6 Hemi uses the MAP for timing, if you're not using the MAP I believe there is setting for manually setting the timing table.
 
Yeah, I was thinking the same thing for the backfire while starting at least. I don't recall if the MSD has a fixed cranking spark, but for sure right after it starts it would have good vacuum that would move the spark around. Having it out of the manifold is really no different than just zeroing the vacuum table.
 
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☆☆☆☆☆
 
gumper said:
Fired up my new Hemi in the Duster for the first time today. I cannot do anything to get it to idle below 2500. As in even with the idle adjustment screw on the carb turned all the way out the RPM will not go under 2500. You would think immediately it’s a vacuum leak, but I cannot find anything. It has a Holley single plane with a 4150 carb, both brand now. The heads were milled and port matched to the manifold. The intake is torqued down per spec with the oring gaskets in place. Both NPT ports in the back of the manifold are plugged. There’s no ports in the carb.
I am using an MSD Hemi 6 with the cam and crank sensors plugged in. I have the right timing map (built-in) for my coils and harness. Waste spark is turned off. The idle timing should be 10* at 1000 rpm, moving up to 20* at 2000 rpm then 25* starting at 2600 rpm and up. The Map sensor is plugged in, but not in use (it’s zip tied to the harness on the firewall with a vacuum cap covering the sensor). Can a bad MSD box cause the idle to be so high? Can a bad crank or cam sensor cause it? The crank reluctor wheel is the correct one for the box. I would assume with a bad crank sensor it wouldn’t have any spark. It does backfire through the carb occasionally when trying to start it. It acts as if the timing is excessively advanced regardless of the settings.
The MSD was used when I got it. The guy I got it from used it once and he also had an uncontrollable high idle. He thought he had a leak at the manifold, but I am 100% confident it’s not vacuum related.
I ran the engine for a good 10 minutes at 2500 since it’s new and needed to be broke in a bit anyway. Other than the uncontrollable idle it seemed to run fine. Oil pressure was about 50 psi and it held at 160 degrees which is what thermostat I have. It did get up to 200 for a minute before I purged the air from the system and added more water.
Click to expand...
Do the box and a timing light agree?
 
Are you tuning with the MSD software on a pc or are you using the handheld device?
 
In the “data editor menu” what are the minimum and maximum allowable timing settings set to?
 
Had an older '90 Caravan acting up with a personality all it's own, could never figure the problem.

Finally tracked it down, put in a new MAP Sensor > runs like a million bucks now.

Computer controlled cars... can be challenging.


☆☆☆☆☆
 
TT5.9mag said:
Do the box and a timing light agree?
Click to expand...
I haven’t put a light on it. Not sure where I’d hook one up with coils on the plugs.
I’ve been through the instructions in manual several times since I acquired the box back in January. The Pro Data won’t play nice with my windows 11 computer, so I’ve just been using the handheld. I have the max timing limits set at 5 and 25. The map sensor is plugged in, and the map timing is set at 0*. The sensor itself is plugged with a vacuum cap so it shouldn’t be getting any readings. There’s no place on the manifold to use it and I had no intention on the map sensor being part of the system. It did have a plot point at like the #6 spot to advance the timing 246* which I turned back to 0 today. I don’t think that’d matter anyway since my timing limit is at 25*, but it was there.
 
George Jets said:
Had an older '90 Caravan acting up with a personality all it's own, could never figure the problem.

Finally tracked it down, put in a new MAP Sensor > runs like a million bucks now.

Computer controlled cars... can be challenging.


☆☆☆☆☆
Click to expand...
The MSD should only be controlling the ignition since I’m using a carburetor. It’s only purpose to to fire the plugs at the right time depending on the crank position and be a rev limiter.
 
gumper said:
I haven’t put a light on it. Not sure where I’d hook one up with coils on the plugs.
I’ve been through the instructions in manual several times since I acquired the box back in January. The Pro Data won’t play nice with my windows 11 computer, so I’ve just been using the handheld. I have the max timing limits set at 5 and 25. The map sensor is plugged in, and the map timing is set at 0*. The sensor itself is plugged with a vacuum cap so it shouldn’t be getting any readings. There’s no place on the manifold to use it and I had no intention on the map sensor being part of the system. It did have a plot point at like the #6 spot to advance the timing 246* which I turned back to 0 today. I don’t think that’d matter anyway since my timing limit is at 25*, but it was there.
Click to expand...
Step one is to figure out a way to time it with a light and make sure the box and the light agree. Use a long plug wire on the coil and micky mouse a way to fire a light. If you have no idea if the two are synced you cannot move forward.
 
TT5.9mag said:
Step one is to figure out a way to time it with a light and make sure the box and the light agree. Use a long plug wire on the coil and micky mouse a way to fire a light. If you have no idea if the two are synced you cannot move forward.
Click to expand...
This. Putting a wire on a coil on plug is a pretty common trick for timing lights. You may also need to paint timing marks on the front of the motor. Still not a diagnostic step you want to skip here.
 
MadScientistMat said:
This. Putting a wire on a coil on plug is a pretty common trick for timing lights. You may also need to paint timing marks on the front of the motor. Still not a diagnostic step you want to skip here.
Click to expand...
I did it, and it’s firing right where it says it is and not jumping around a bit.

My next option is to remove the intake and put RTV around the ports. Thinking MAYBE the oring gaskets aren’t sealing. I did spray half a can of brake cleaner all around the engine and it did not change the RPM in the slightest.
 
Here is more info on the 2500 rpm fast idle Hemi with MSD and a 4150 Holley.

(For those who wish to look into it further)

* Might want to read this before you consider pulling your intake manifold for inspection.

__________________

An idle speed of 2500 RPM on a carbureted Hemi with a 4150-style carburetor is abnormally high and typically indicates a mechanical restriction, a vacuum leak, or an ignition timing issue, particularly if the idle speed screw has already been turned fully out. In a documented case involving a Hemi engine with a Holley 4150 carburetor idling at exactly 2500 RPM despite the idle adjustment screw being fully retracted, the problem was linked to the ignition system, specifically an MSD Hemi 6 controller, faulty crank/cam sensors, or a MAP sensor that was plugged in but capped (not in use), which caused timing issues.

  • Ignition and Sensor Issues: On engines using electronic ignition controllers like the MSD Hemi 6, a bad box, faulty crank/cam sensors, or improper MAP sensor configuration (such as having it plugged in but vacuum-capped) can prevent the idle from dropping. The MAP sensor is critical for timing on these systems, and ignoring it may require manually setting the timing table.

Hoping you can get things tracked down for a successful outcome.

That 2500 rpm idle is very odd. The way these electronics need to talk to each other can really throw a person a curve.

Good Luck


.
 

Also:

Ignition controllers have been reported to lock into high timing modes if critical sensors are misconfigured.

If the MAP sensor is plugged in but vacuum-capped (not in use), the controller may default to a safety map that advances timing excessively, driving the RPM up uncontrollably.


.
 
Get a $5 hand pump cigar smoke tester or a $50 one that uses baby oil and check for vac leaks.
 
gumper said:
I did it, and it’s firing right where it says it is and not jumping around a bit.

My next option is to remove the intake and put RTV around the ports. Thinking MAYBE the oring gaskets aren’t sealing. I did spray half a can of brake cleaner all around the engine and it did not change the RPM in the slightest.
Click to expand...
Ok, looks like the MSD and its sensors are working. See if backing off the timing - at 2500 RPM first - brings down the idle speed.

If not, make sure the choke isn't stuck on and the idle mixture isn't too rich.
 
gumper said:
I did it, and it’s firing right where it says it is and not jumping around a bit.

My next option is to remove the intake and put RTV around the ports. Thinking MAYBE the oring gaskets aren’t sealing. I did spray half a can of brake cleaner all around the engine and it did not change the RPM in the slightest.
Click to expand...
Good. What’s the timing at 2500? Now compression or leak down test.
 
According to the handled while it’s running at 2500 it’s 22 degrees which checks out with the timing light. I marked 25* on the balancer since the numbers are very tiny and it’s right there.
Compression test shows around 150 lbs. Thought maybe valves were hanging open but that squashes that theory.
Next test would be to seal the carb flange and pressurize the intake and spray soapy water looking for air leaks.
I have a theory that maybe the holes drilled in the throttle blades are unnecessary and causing the high idle lean condition.

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