MAP sensor reading at idle

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ferrum
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MAP sensor reading at idle

Post by ferrum »

I have Howell TBI on my 401. I'm suspecting vacuum leak from break power booster. High pitch sound in the cab goes away when I disconnect vacuum line to my booster. I have replacement part on order. What is typical MAP sensor reading at idle (~ 700 rpm)? Mine is currently reading somewhere in 35-45 kPa range. I would expect MAP sensor reading to improve once I replace power booster.
'76 Cherokee Chief

Flip
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Re: MAP sensor reading at idle

Post by Flip »

It will depend on altitude, engine condition and state of tune, but that is pretty low.

Here is an online converter so you'll get an idea of what you should see, by converting to in-Hg.
http://www.kylesconverter.com/pressure/ ... of-mercury

Hth,
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Tatsadasayago
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Re: MAP sensor reading at idle

Post by Tatsadasayago »

That high pitched sound is likely coming from the air inlet near the brake rod.
Sometimes spraying brake-kleen around where the rod enters the booster, (With the engine running) while pumping the pedal several times clears out and dirt/gunk that is preventing the valve from seating properly.
Worth a shot in lieu of buying a new booster...

And yes, the vacuum leak with definitely affect the MAP sensor readings.
Pulling the booster vacuum hose and plugging it should get your MAP readings back up to normal.
60-70 is about right for a good engine below 3000', which is equivalent to 20" on a gauge.
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babywag
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Re: MAP sensor reading at idle

Post by babywag »

Tatsadasayago wrote:That high pitched sound is likely coming from the air inlet near the brake rod.
Sometimes spraying brake-kleen around where the rod enters the booster, (With the engine running) while pumping the pedal several times clears out and dirt/gunk that is preventing the valve from seating properly.
Worth a shot in lieu of buying a new booster...

And yes, the vacuum leak with definitely affect the MAP sensor readings.
Pulling the booster vacuum hose and plugging it should get your MAP readings back up to normal.
60-70 is about right for a good engine below 3000', which is equivalent to 20" on a gauge.
60-70 MAP is not what you should be seeing, I think you're confusing the conversion with pressure vs. vacuum.

If stock cam, your MAP reading might drop a tad lower after fixing the vacuum leak, but not much.
Really depends on your engine and tune.
-Tony
'88 GW (aka Babywag)
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Tatsadasayago
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Re: MAP sensor reading at idle

Post by Tatsadasayago »

You are disputing that 60-70 KPa is equivalent to 17-20" of manifold vacuum?
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ferrum
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Re: MAP sensor reading at idle

Post by ferrum »

Manifold vacuum and pressure got me confused a bit until I saw this nice little picture from AA1Car.com site.

Image

I think my break power booster is original and since I have new one coming soon, I'm going to change it out. None of the auto parts store where I live stock this anymore. I hope I can just move my master cylinder out of the way enough to remove booster without having to disconnect break lines. Has anyone change out break booster before without disconnecting break lines from the master cylinder?
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babywag
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Re: MAP sensor reading at idle

Post by babywag »

Tatsadasayago wrote:You are disputing that 60-70 KPa is equivalent to 17-20" of manifold vacuum?
When tuning a vehicle or reading a MAP sensor value 0 is greatest vacuum / 100 is no vacuum
Typically stock engines idle ~30-40
Decel would be ~20, 60-70 would be a slight engine load, 100 would be WOT
-Tony
'88 GW (aka Babywag)
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Tatsadasayago
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Re: MAP sensor reading at idle

Post by Tatsadasayago »

Yeah, if you are careful, you can move the master forward and down without crimping the lines. You only need about 2" space to get the MC clear, but need to get it downward out of the way so the booster can come out.You will find the hardest part is worrying the booster nuts off the studs...long time with a wrench from under the dash. Deep sockets usually aren't deep enough for the first inch or so.
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Tatsadasayago
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Re: MAP sensor reading at idle

Post by Tatsadasayago »

babywag wrote:
Tatsadasayago wrote:You are disputing that 60-70 KPa is equivalent to 17-20" of manifold vacuum?
When tuning a vehicle or reading a MAP sensor value 0 is greatest vacuum / 100 is no vacuum
Typically stock engines idle ~30-40
Decel would be ~20, 60-70 would be a slight engine load, 100 would be WOT
100 Kpa on a MAP sensor is WOT aka: barometric pressure.
I'm not sure where you are getting your 30-40 numbers but we'll agree to disagree.
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babywag
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Re: MAP sensor reading at idle

Post by babywag »

Tatsadasayago wrote: I'm not sure where you are getting your 30-40 numbers but we'll agree to disagree.
From just about every car I've ever seen...if a car has a 60-70 MAP reading @ idle it has some serious issues or one hell of a cam installed.
-Tony
'88 GW (aka Babywag)

Flip
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Re: MAP sensor reading at idle

Post by Flip »

Hey guys,

The only person in this thread who may be wrong, so far, is me. I made an assumption on Howell's strategy without checking (actually that makes me wrong regardless of the outcome).

There are two different ECU strategies. So it would depend on which strategy that Howell follows.
To find out what is correct, hook up a vacuum gauge and compare.
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babywag
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Re: MAP sensor reading at idle

Post by babywag »

Same strategy as a zillion other cars on the road...all the below use the same MAP sensor as typical GM TBI
ACURA 1996-1997
BUICK 1985-2004
CADILLAC 1985-1996
CHEVROLET 1985-2004
EAGLE 1988
GEO 1990-1993
GMC 1985-1998
HONDA 1994-1997
ISUZU 1989-1997
JEEP 1985-1995
OLDSMOBILE 1985-2004
PONTIAC 1985-2004
SATURN 1991-2002
Howell did update to a newer style MAP on newer systems, but it functions the same, just a smaller GM sensor used on newer GM vehicles.
Since a pic is worth a thousand words...and much faster/easier than trying to convince people or explain it...

5" Vacuum
Image

10"
Image

15"
Image

20"
Image

25"
Image
-Tony
'88 GW (aka Babywag)

Topic author
ferrum
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Re: MAP sensor reading at idle

Post by ferrum »

I replaced my break power booster couple of days ago. Took O'Reilly's way too long and a few phone call to get the part that I order in early August. I also had to run over to my favorite nuts and bolts store to get the correct nuts. Booster was shipped with 9/16" nuts that was boogering up the threads on the stud. Studs turned out to be metric 10mm. I was not happy with O'Reilly and I will not buy from them again.

Image

My idle improved quite a bit. I still need to hook up my computer to see the sensor reading yet.
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Re: MAP sensor reading at idle

Post by chevelleguy »

BRAKE- to slow down or stop

BREAK- to separate or cause to separate into pieces as a result of a blow, shock, or strain.
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