MSPNP only idles well when super lean

Trogdor

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Aug 30, 2018
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I'm helping a friend with his 351 swapped 1991 Notch. The car is running a MSPNP, no MAF, no IAC.

He has an Autometer Pro-Comp Ultra lite Wideband that has a range of 0V=10AFR and 4V=17AFR.

The car starts up and idles but the AFR is 17 on the gauge and in Tuner Studio. If increase the VE table in the idle area once the AFR starts to get to 15ish the engine will begin to surge. I tried timing values between 12 and 23 degrees, but it does not help with the surging. The best idle I got was with the engine lean and timing values around 21 degrees.

When I begin to increase the VE table the engine idle speed increase, so I find myself using the throttle screw to bring it back down.

Why won't the car idle with an AFR close to stoich?
 

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a91what

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I'm helping a friend with his 351 swapped 1991 Notch. The car is running a MSPNP, no MAF, no IAC.

He has an Autometer Pro-Comp Ultra lite Wideband that has a range of 0V=10AFR and 4V=17AFR.

The car starts up and idles but the AFR is 17 on the gauge and in Tuner Studio. If increase the VE table in the idle area once the AFR starts to get to 15ish the engine will begin to surge. I tried timing values between 12 and 23 degrees, but it does not help with the surging. The best idle I got was with the engine lean and timing values around 21 degrees.

When I begin to increase the VE table the engine idle speed increase, so I find myself using the throttle screw to bring it back down.

Why won't the car idle with an AFR close to stoich?
You have the car setup for closed loop idle but do not use an idle valve.. this can make life hard

Lets discuss some of what is happening.. you are running lean and adding air to keep the engine speed steady, this is working because as you remove fuel away from stoich the TQ drops off drastically.
As you increase the VE value and the engine moves to stoich the TQ increases and so does the RPM, more TQ creates an unstable idle in this instance as you have no control over the air valve and you are not moving timing to control TQ output at idle.

You need to remove timing at higher RPM and increase it at lower RPM this creates a timing ramp that holds the engine in a steady state.
instead of using " set value" in the [idle adaptive advance timing] dialog use "adaptive" and set the idle speed you want in the closed loop idle target rpm curve. This way the timing will vary by rpm based on the CLT value.

I modified the tune so you can see one way to configure this, there are many ways to get this done and this is only one of them. If this fails to work I can show you my "you will idle the way i say you will" method :D
 

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  • Trogdor.351.rev.1.0.0.msq
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Trogdor

Active Member
Aug 30, 2018
70
39
28
Anna, TX
Sorry for the late follow up, but I was just able to connect with my friend last weekend. I loaded the file you posted and the car would not idle, I could give it gas and it would keep running, but if I'm not pressing the accelerator pedal, the engine will shut down.

What do you suggest I try next?

Thanks!
 

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor
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One suggestion would be to call a few tuners in your area and see what their hourly rate is. The best part of this plan is that the tuner has everything right in front of them and can walk you through it.
 

a91what

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Or i can log in remotely as long as you have internet and help directly.

How large is the cam? Overlap makes the wideband a liar at idle and a lean reading may be what the engine needs.
Overlap allows oxygen into the exhaust and causes this lean reading even though the engine is running well.
In cases like these we ignore the wideband and tune to the best map signal, adjust fuel to where it pulls the most vacuum and you have the right amount even if the sensor reads very lean.
 

Trogdor

Active Member
Aug 30, 2018
70
39
28
Anna, TX
Or i can log in remotely as long as you have internet and help directly.

How large is the cam? Overlap makes the wideband a liar at idle and a lean reading may be what the engine needs.
Overlap allows oxygen into the exhaust and causes this lean reading even though the engine is running well.
In cases like these we ignore the wideband and tune to the best map signal, adjust fuel to where it pulls the most vacuum and you have the right amount even if the sensor reads very lean.

I completely forgot about the effect that large overlap cams have on the wideband reading at low engine speeds. He has a pretty healthy cam in it. Been too long since I've messed with one.

I'll try and tune for the best MAP signal, that reminds me of how I used to get the best idle for a carburetor, all I needed was a screwdriver and a vacuum gauge.

It will probably be after the Holidays when I'll have the next chance to help my friend out. Thank you for offering to remote in and take a look at it while I'm in the car. Would a mobile hot spot from my cell phone be enough?
 

a91what

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Apr 6, 2011
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I completely forgot about the effect that large overlap cams have on the wideband reading at low engine speeds. He has a pretty healthy cam in it. Been too long since I've messed with one.

I'll try and tune for the best MAP signal, that reminds me of how I used to get the best idle for a carburetor, all I needed was a screwdriver and a vacuum gauge.

It will probably be after the Holidays when I'll have the next chance to help my friend out. Thank you for offering to remote in and take a look at it while I'm in the car. Would a mobile hot spot from my cell phone be enough?
Yes it will, I do this often and a mobile hotspot get the job done 99% of the time