Idle vs. Cruise BLMs

[mtbraun]

What's the difference between idle and cruising BLM, and is it important? I can't seem to solve a hot idle BLM of 142, but when cruising they stay around 130. I've done all the checks and probably won't tear into anything yet until this winter. My WOT recall O2s are around 750-780 on a new O2 sensor.

I've checked the plugs and they do look a little lean but I guess not enough to concern me.

 

[TurboBuRick]

Sounds like vacuum leak. Try bumping the translator base setting to 10% rich. Bump up wot fuel knob to get the wot o2 number up.

RL

 

[mtbraun]

So that would explain cruise BLM being ok...because the engine is not under vacuum? Is that correct? I feel like I've looked at every vacuum line a hundred times but I'll keep looking.

I'm pretty sure I already bumped up the base setting, but not WOT. Will adding fuel through the chip also help?

 

[TurboBuRick]

A vacuum leak may add allot of un metered air to idle but is only minor when moving higher cfm. Small hand vacuum pumps are a cheap way to pin point the leaks. Sears has them.

Check your chip manual for features. I'm sure it has some adjustments.

RL

 

[turbojd]

Check all vacuum lines and pcv valve and grommet.

 

[TurboDave]

So that would explain cruise BLM being ok...because the engine is not under vacuum? Is that correct?

The engine is still under vacuum at cruise, just different loads and other dynamics. I've never, in 25 years on either of my GN's seen the idle BLM's and cruise BLM's the same.

 

[mtbraun]

The engine is still under vacuum at cruise, just different loads and other dynamics. I've never, in 25 years on either of my GN's seen the idle BLM's and cruise BLM's the same.

Thanks for the info. I did find a vacuum leak but haven't tested on the road yet. The line from the steel vac line to the FPR was popping off. Hopefully one step closer.

 

[mtbraun]

Small hand vacuum pumps are a cheap way to pin point the leaks. Sears has them.


RL

Is this what you are referring to?

http://www.sears.com/lincoln-indust...SPM5890439103P?prdNo=7&blockNo=7&blockType=G7

 

[Ttype6]

There are different cells (as they're called) in the program of a chip. The program has one cell that it uses for idle. As you open the throttle and increase the load on the motor,the program moves to different cells. The person who writes the programs for each cell guesses as to how much fuel your ,particular,combo will need at that,particular,load. Based on feedback from the oxygen sensor (the computer is programmed to maintain an A/F ratio of 14.7/1) the computer adjust up or down from that guess.

The fact that you see one BL number at an idle and another at cruise means nothing.

A better indicator of a vacuum leak is the IAC number. As un-metered air enters the motor,idle speed increases. The Idle Air Control valves job is to lower it down to the programed idle rpm. It does this by closing the valve more and more until the desired rpm is reached. It can,only close so far until it reaches the end of its ability to adjust. When it gets to the lower end of its ability to adjust,the IAC number on the Scanmaster will read zero.

A low IAC number (usually zero) is the best electronic indicator of a vacuum leak.

 

[mtbraun]

This is very helpful. My IAC's at hot idle are between 40-50.

 

[Ttype6]

This is very helpful. My IAC's at hot idle are between 40-50.

You don't have a leak.

 

[3.8L V-8 eater]

You don't have a leak.

Those iac numbers of 40 to 50 are a little high....So what's going on when the numbers are high??

 

[CGASTON]

This is very helpful. My IAC's at hot idle are between 40-50.

Do you have a hard timing getting your IAC to set around 20-30 at idle? Have you tried to spray some starter fluid around gaskets and vacuum lines? If the motor revs when doing so, you have a vacuum leak. I would check that. Cause at idle, that is when your car make most of its vacuum. When you are driving around (cruise) then you vacuum will decrease to about a 1/3 of what it was. Really dependent of cam. What is your fuel pressure? Does it change as you go from vacuum on to vacuum off?

 

[Ttype6]

So what's going on when the numbers are high??

When the number is high,it means that the throttle blade is fully closed or close to it. If you close the throttle blade all the way,the idle will be low. When this happens,the computer will attempt to raise the idle by letting more air into the motor. It does this by opening the Idle Air Control valve. When the valve opens,it allows air to bypass (go around) the throttle valve and enter the motor. A high IAC number means that the IAC valve is open a lot.

If you open the throttle valve,by turning the throttle valve stop screw clockwise,more air will enter the motor through the throttle blade. when this happens the idle rpm will rise. When the computer sees this rise it will close the IAC (lessening the amount of air allowed to bypass the throttle valve) valve as much as it takes to get the idle rpm back down to the programed idle speed. The IAC number will fall. This is what you need to do. Then re-set the TPS sensor.

If you had a vacuum leak,you would have extra air entering the motor. It would cause the idle rpm to rise. The computer would command the IAC valve to close as much as it takes to get the Idle rpm back to the target number. If the vacuum leak is big enough,the IAC will close completely. This is all the adjustment it has in that direction. This is why,when you see an IAC number of zero,you probably have a significant leak. If the number is high,you don't.

 

[3.8L V-8 eater]

Thank

When the number is high,it means that the throttle blade is fully closed or close to it. If you close the throttle blade all the way,the idle will be low. When this happens,the computer will attempt to raise the idle by letting more air into the motor. It does this by opening the Idle Air Control valve. When the valve opens,it allows air to bypass (go around) the throttle valve and enter the motor. A high IAC number means that the IAC valve is open a lot.

If you open the throttle valve,by turning the throttle valve stop screw clockwise,more air will enter the motor through the throttle blade. when this happens the idle rpm will rise. When the computer sees this rise it will close the IAC (lessening the amount of air allowed to bypass the throttle valve) valve as much as it takes to get the idle rpm back down to the programed idle speed. The IAC number will fall. This is what you need to do. Then re-set the TPS sensor.

If you had a vacuum leak,you would have extra air entering the motor. It would cause the idle rpm to rise. The computer would command the IAC valve to close as much as it takes to get the Idle rpm back to the target number. If the vacuum leak is big enough,the IAC will close completely. This is all the adjustment it has in that direction. This is why,when you see an IAC number of zero,you probably have a significant leak. If the number is high,you don't.

s

Thanks for the great explanation!!

 

[Ttype6]

Thank
s

Thanks for the great explanation!!

Were you aware that IAC stood for idle air control.?

 

[mtbraun]

Ttype6 - thanks for the explanation...very helpful.

 

[mtbraun]

The fact that you see one BL number at an idle and another at cruise means nothing.


A low IAC number (usually zero) is the best electronic indicator of a vacuum leak.

So another potential issue is a pre-turbo exhaust leak, causing the hot idle BLM to be 142. Am I on the right track?

 

[Ttype6]

So another potential issue is a pre-turbo exhaust leak, causing the hot idle BLM to be 142. Am I on the right track?

A pre turbo exhaust leak would mainly effect spool up and boost pressure,but it could allow air into the exhaust stream that would make the O2 think the motor was running lean. It would have to be a substantial leak. do you have one? My BL number at an idle is 147. At cruise it's 128. The higher than 128 number doesn't necessarily mean something is wrong.

 

[mtbraun]

A pre turbo exhaust leak would mainly effect spool up and boost pressure,but it could allow air into the exhaust stream that would make the O2 think the motor was running lean. It would have to be a substantial leak. do you have one? My BL number at an idle is 147. At cruise it's 128. The higher than 128 number doesn't necessarily mean something is wrong.

Based on the amount of welding and planing I've done to my headers I don't think there is a leak. I've smoke tested and used seafoam so i think it is ok. I'm going off what previous posts have suggested. Maybe I'm making much of nothing with the high idle BLM. Everyone seems to suggest they need to be at 128 for hot idle, so I'm just trying to get the number down.

But my hot idle BLM has always been the same (142) before and after the header work.