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Hello, all....Internet is cool sometimes....
I am A Subaru - Polaris Mechanic..... I own a 90 and 91 3/4 ton 4x4 Suburban...
My Brother owns a 87 1/2 ton 4x4 Suburban....

His is having the issue....

on Start up it lopes at idle real hard then after 10-15 lopes/cycles in cleans up and runs smooth and proper...but idle is high around 12-1500 rpm...It does this every time you start it cold or hot.....

He has replaced throttle body, and all intake sensors trying to get this issue resolved....O2 sensor as well....we pulled the egr unit today to verify it was not plugged or stuck partially open..it is in good shape, and vacuum is good....

ECM trouble lite is giving a trouble code 33 ( low vacuum to map sensor )

I am at a loss at this point as to what is going on...?

I am hoping one of you guys may have had this issue, and can point me down the right road....?

Motor was rebuilt about 45 k ago.....truck has a 700R4 trans, and was getting 17.2-17.9 miles regularly on hwy trips...now it is down around 11-12mpg...?

Thanks, for any info, John in Oregon.
 

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Checked with "smoke' for vacuum leak? What about oil use,fouled plugs

You're an experience mechanic, so you probably already used some sort of fake smoke to check for a vacuum leak under the hood.If the leak was downstream of the air sensor, it would run lean,and would take a lot more RPMs to keep from stalling.
Does it use any oil? If it has developed blowby it might decrease the vacuum.Maybe you could pull a few plugs and see if they are oiled up?
Luck,
Charlie
PS I'm new-4 weeks-to Suburbans/Chevey trucks,so take everything I say with a huge dose of salt.
In the past folks would use WD40 spray or even starter fluid to check for a vacuum leak under the hood.If it was pulled in,the RPMs would change-usually increase.Always struck me as a bit dangerous(the ether/heptane in the starter fluid)
 

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The code is pointing to a vacuum leak, but maybe be a misfire. Keep us posted with what you find.

From: EQUUS (pdf file)
33 Mass air flow (MAF) sensor - signal voltage or frequency
is high during engine idle
Manifold absolute pressure (MAP) sensor - signal
voltage is high during engine idle (Note: Engine misfire
or unstable idle may cause this code)
 

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Maybe a bad plug or wire. Vacuum leak or how about a dirty MAF? Sometimes this may cause a uneven idle. There is a Vacuum switch for the egr that may be at fault too. I think it sits an the passenger side of the intake sort of next to the throttle body. Check those little hard plastic vacuum hoses too one may have an unseen crack in it. Lastly I don't remeber if these have a vacuum modulator on the transmission but this can cause them to act up too.
Hope this helps, keep us posted if you find the problem.
 
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