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Old 05-07-2019, 11:15 PM
lsmalley's Avatar
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M103 vacuum values

Fuel economy gauge is slightly off even though no ac is on. It stays like this whether in N or D. Looks like a vacuum issue somewhere, but idle is smooth at about 700 N and about 550 D. MityVac connected to vac line on intake shows about 16 - 17 mmhg. This just recently started. Other than economy gauge being skewed, I have not noticed any issues.

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M103 vacuum values-img_20190507_200338.jpg   M103 vacuum values-img_20190507_200321.jpg  

1990 190E 2.6L
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Old 05-07-2019, 11:43 PM
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Gasoline normally aspirated engines should draw 15-20 inches of vacuum at idle.
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Old 05-08-2019, 07:20 AM
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Trevor Hadlington
Join Date: May 2014
Location: Worcestershire in England
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Ism you do have a problem in the loss of vacuum in a line some place . And its running a bit low on the revs . I would mighty vac every line you can find ..
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Old 05-08-2019, 10:16 AM
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My 190E 2.6 with manual transmission idles in neutral at 700 @ 15", which puts the needle against the pin, but just barely. With the A/C compressor engaged it moves slightly off the pin due to the load, and the idle control system maintains 700.

I suspect there may be an issue with your gage. You should check idle vacuum with a reliable vacuum gage both in neutral and drive, and the latter should be less due to the lower RPM and converter drag load.

For those of you who are familiar with vintage Corvettes, the M103 valve timing, duration and overlap, is similar to the 327 CID (5.3L) 350 HP Corvette engine from the mid to late sixties that has a "hot hydraulic camshaft", but the M103 cam is just slightly less aggressive. That's why I sometimes refer to my 2.6 as "half an L-79".

Typical L-79 idle behavior is 750 @ 14-15" with a slight lope. Raising idle speed to about 800 increases vacuum to about 16", which means the absolute pressure ratio between atmospheric and manifold is below the "critical" value of 0.528. This means that flow past the throttle valve is sonic and choked. More vacuum won't create any additional air flow, which is why idle above the critical pressure ratio of 0.528 is smooth.

Below 0.528 flow is subsonic and not choked, so sight variations in manifold vacuum will cause slight changes in airflow resulting in slighty changed idle speed, which is why engines that idle at less than about 14.2" exhibit "lope", and the lower the mean idle vacuum the "lopier" the idle.

To be clear the 0.528 value is the ratio of absolute manifold pressure to absolute ambient pressure - 29.92 " Hg (call it 30) on a standard sea level day minus manifold vacuum.

So, .528 x 30 = 15.8 and 30-15.8 = 14.2" manifold vacuum. Thus below this value the idle will become lopey and smooth out at over 14.2" manifold vacuum.


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