Author Topic: EGR vacuum issue  (Read 2168 times)

Offline krigermoose

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  • 1986 GMC C1500 5.7L
EGR vacuum issue
« on: February 22, 2020, 07:52:37 PM »
Hey guys, I failed smog recently trying to get my truck back on the road. 1986 GMC C1500 with a 350, bone stock. One reason is because I am not getting vacuum from my EGR.
Any ideas? I pulled it off and tested if it holds vacuum still...it does...
Would there be a vacuum leak large enough to completely prevent any vacuum at all?
I do know there should not be vacuum at idle, but there is zero even under heavy acceleration.

Offline VileZambonie

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Re: EGR vacuum issue
« Reply #1 on: February 22, 2020, 09:58:36 PM »
Are you saying you get no vacuum signal at the EGR valve and failed emissions because of it? What specifically did you fail for? NOx or a visual inspection?

Validate the plumbing from the EGR valve back to the vacuum source for your application. Reference the appropriate diagram or look at your VECI label if it's still in tact.
,                           ___ 
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              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline krigermoose

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  • 1986 GMC C1500 5.7L
Re: EGR vacuum issue
« Reply #2 on: February 23, 2020, 08:39:06 PM »
Update: I did find the source. The vacuum lines are all in their correct locations. The source is on the top back (towards the rear of the truck). There was no ported vacuum coming from it.
I ended up teeing the line to another ported vacuum source on the carburetor that actually works, so that's my solution for now.
Next thing in line for me is getting a new thermostat because the switch that is between the vacuum source and the EGR only opens when the truck is slightly colder than operating temperature. I'm just assuming that the thermostat is the wrong one, I installed in this last year and never got around to the EGR.

Offline JohnnyPopper

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Re: EGR vacuum issue
« Reply #3 on: February 25, 2020, 08:18:25 AM »
Dude, your post's are clear as mud  :o

Putting a tee is a deviation from the vac system and pretty much fails your visual inspection.

Exhaust Gas Re-circulation is meant to induce exhaust gas into your intake manifold at Cylinder 6 upon de-acceleration or HIGH vacuum.

The idea is that it's better to re-circulate exhaust then fuel under those conditions, when the fuel is just being wasted.

So you can see that vacuum at the EGR valve is conditional, not constant.

Take the 'vile' one's advice and get your plumbing correct first, then see if your 'fail' is actually true.   

1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline bd

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Re: EGR vacuum issue
« Reply #4 on: February 25, 2020, 09:39:07 AM »
JP, the EGR's purpose is to dilute the intake charge with spent (already burned) combustion gases to effectively lower cylinder combustion temperatures, thereby decreasing NOx emissions.  The EGR valve meters recirculated exhaust into the plenum of the intake manifold, typically through the plenum floor, distributing spent exhaust gases "equally" to all cylinders, not just cylinder number 6 or any other particular cylinder.  Since NOx emissions are directly linked to high combustion temperatures, the EGR introduces pre-burned gasses when the engine is under load during periods of acceleration (high manifold pressure).  This is why the EGR vacuum signal is ported (or electronically modulated) and linked to throttle opening (i.e., low venturi pressure - high manifold pressure - engine load).  Capiche?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline krigermoose

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  • 1986 GMC C1500 5.7L
Re: EGR vacuum issue
« Reply #5 on: February 25, 2020, 04:21:47 PM »
JP, my vacuum lines were correct. My carburetor's rear ported vacuum source was not supplying any ported vacuum. I decided to tee because this other source is also a ported vacuum source.

It's true that it is a risk of failing the visual which is why I am reconsidering, but I recently rebuilt that carburetor and the last thing I want to do is pull it off, which is why I'm looking for alternatives.
The guy that smogged my truck will be there to do it again, and he was very lax with the visual inspection which is why I considered fixing my vacuum problem like this.

 I know the vacuum at the EGR valve is not constant, hence me saying I'm trying to find a ported vacuum source.

To VileZambonie: I failed because my timing was too far off, and the visual inspection of the EGR failed. (He tested the vacuum.) The vacuum lines, again, were correct. They were double and triple checked. I attached a vacuum gauge right to the carburetor, where the ported vacuum comes from, and got nothing.

I did not fail NOx.

What I meant to say at the end of my previous post is that my EGR bleed solenoid is only opening right before the engine is at proper operating temperature, then it closes, cutting off the vacuum to the EGR valve.

Offline VileZambonie

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Re: EGR vacuum issue
« Reply #6 on: February 25, 2020, 05:19:40 PM »
The ECM controls vacuum to the EGR valve electronically--through the EGR bleed solenoid.
When the engine is warm the solenoid is de-energized allowing vacuum to the EGR valve.
When the engine is cold the solenoid is energized blocking vacuum to the EGR valve.
The solenoid also decreases the amount of vacuum to the EGR valve when the torque converter clutch is engaged.

EGR Thermal Vacuum Valve, TESTING AND INSPECTION
Drain coolant, as needed, until coolant level is below valve.
Note inlet and outlet ports, disconnect vacuum hoses and remove valve from engine.
Connect suitable vacuum gauge to valve outlet and vacuum source to valve inlet.
Cool valve to a temperature at least 10°F. below calibration value stamped on valve and apply at least 12 inches Hg vacuum to valve inlet.
Valve should be closed and vacuum gauge on outlet should read zero. Leakage of up to 2 inches Hg vacuum in 2 minutes is allowable and does not mean valve is defective.
Leave vacuum applied to valve and slowly warm valve in a pan of water while observing water temperature and vacuum gauges.
When valve reaches calibration temperature (stamped on valve), vacuum passage should open and gauge on outlet should indicate vacuum equal to applied vacuum at inlet.
If TVV or TVS fails to perform as outlined it should be replaced.
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline JohnnyPopper

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Re: EGR vacuum issue
« Reply #7 on: February 26, 2020, 08:26:47 PM »
Nothing like being schooled by bd and the crew :o :o

That was the way it was explained to me back in '79...I like their wrong explanation better though! 8)

Time to go lay down by my bowl...
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline bd

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Re: EGR vacuum issue
« Reply #8 on: February 26, 2020, 09:05:52 PM »
Hey!  What can I say?  Just tryin' to help a buddy out with some edumacation.  Sorry, my explanation didn't rhyme.  Maybe it'll all make sense someday.  I, for one, am still waiting....    :-X
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)