Carburetor EGR port question

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Does anyone have any idea how much vacuum should develop at an EGR port on a carburetor with the engine revved to about 2000rpm?
I installed a brand spanking new Holley 2280 carburetor on my 87 Dodge
Dakota V6 engine and failed the NO part of the California smog test. The HC and CO tests passed well below average. I came back home and checked the action on the EGR valve. No movement on the visible piston when the engine was revved.
The EGR valve checked out okay when tested. It opened with less than 10 inches of vacuum and stayed open indefinitely. The Coolant Vacuum Switch Cold Closed (CVSCC) is operating correctly. The vacuum hoses involved are all in good shape, no leaks.
I checked the vacuum at the EGR port on the brand spanking new carburetor and only got about 1 or 2 inches of vacuum at about 2000 rpm.
Manifold vacuum is 19 inches at a 700rpm idle.
What gives?
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"Simpson" wrote

You sure you're hooked up to the right port?
A few pages of 2280 info a quick search found.....
http://www.slantsix.org/articles/choke-adjust/carb-info/2280-info.jpg
http://www.slantsix.org/articles/choke-adjust/carb-info/2280-info2.jpg
http://www.slantsix.org/articles/choke-adjust/carb-info/2280-info3.jpg
http://www.slantsix.org/articles/choke-adjust/carb-info/2280-info4.jpg
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MasterBlaster wrote:

I'm absolutely sure. I have a spare carb of the same design that is disassembled. There are not many who are more familiar with the guts of this carb than I am. I became knowledgeable about it through necessity. The truck came stock with a Holley 6280, which is the electronic feedback version of the 2280, the main difference being a fuel mixture control solenoid. The solenoid is no longer available as a replacement part and when it goes the only recourse is to locate a used one, which in my experience is no better than the one that went south. So I decided to replace the feedback 6280 with the non-feedback 2280. The problem with the mixture control solenoid first surfaced about eight years ago.
After sleeping on it, I think the problem may be a leak in the purge line to the charcoal canister. This purge line enters the carb through a port in the exact same place as the EGR port, just above the closed throttle plates. This would explain the low vacuum at the EGR port. The lack of EGR function, plus the vacuum leak, which would cause a lean condition, would combine to create a higher than normal temperature in the combustion chamber resulting in the abnormally high NO readings. The ceramic insulators on the plugs have that very white look caused by a lean fuel mix. Soon as I finish my morning cup I'm going to warm up the engine, disconnect the purge line, plug the purge line port in the carb and test the EGR function again.

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Are you absolutely certain you haven't bypassed the vacuum amplifier? As I recall, venturi vacuum only generates around two inches vacuum max and thus needs the vacuum amplifier to operate the EGR valve. Again, if I recall correctly, the amplifier will boost it to around 8 inches.
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BigIronRam wrote:

I found the problem. There are two available ports that tap into the carb just below the closed throttle plates. One develops vacuum and the other doesn't. The one that doesn't develop vacuum is designated in the installation sheet that came packed with this new carb as the 'EGR vacuum line'. The one that does develop vacuum is designated as the 'spark vacuum line'.
On this version of the 2280, the designated EGR port is tied into another port that is much higher up in the venturi via an external vacuum hose with a tee fitting, and this tee fitting is then supposed to connect to the EGR valve. However, the designated EGR port just doesn't develop sufficient vacuum as measured directly with a vacuum meter. With or without the connection to the higher port, the vacuum is the same, less than two pounds at over 2000rpm. The EGR valve begins to open at about 3 pounds and is fully opened at about 5 pounds.
The port designated as the spark vacuum line *does* develop the proper vacuum to operate the EGR valve. The spark vacuum on this truck is supplied directly from the manifold vacuum. I've got the factory manual which shows this in the same vacuum hose routing diagram that is pasted to the underside of the hood.
So whatever the problem is with the designated EGR port on this carb, the designated spark vacuum port has the proper vacuum characteristics to operate the EGR valve: no vacuum at idle, increasing vacuum as the throttle is opened. If I connect the EGR valve directly to the designated spark vacuum port the EGR valve responds to the throttle as it is supposed to. However, if I connect the designated spark vacuum port to the higher port via the tee fitting, the response is sluggish to non-existent. Therefore, in the interest of proper EGR operation and passing the smog test, I am chucking what's in the instructions and going with my gut on this one.
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Not to nit pick but lean air fuel mixtures don't burn hotter. They burn slower which causes associated component temperatures to rise. Lean doesn't cause higher NOx, the extra O2 in the exhaust from a lean mixture makes the reduction bed of a 3 way catalyst less efficient resulting in higher NOx at the tail pipe.
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aarcuda69062 wrote:

Nit picking appreciated! It's always valuable to learn more precisely what is going on. It makes perfect sense to me the way that you explained it. Perhaps you could enlighten me on the effect of ignition timing on emissions.
I just got back from passing the smog test after initially failing it. The failure was caused by improper vacuum to the EGR valve deom the carburetor and a minor manifold vacuum leak. The vacuum being supplied to the EGR valve by the Holley 2280 carburetor was insufficient to open the EGR valve, even at over 2000rpm. This non-functioning EGR valve and the minor manifold vacuum leak caused a measurement of over 3600 PPM of NOx. Anything over 1195 PPM is a failure. Below is the failed test results.
-------------------- Percent of CO2
15mph 1333rpm - 10.8
25mph 1309rpm - 10.7 -------------------- Percent of O2
15mph 1333rpm - 5.9
25mph 1309rpm - 6.0 ------------------- HC PPM
15mph 1333rpm - 34 - PASS (maximum allowable is 134)
25mph 1309rpm - 26 - PASS (maximum allowable is 106) ------------------------- Percent of CO
15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
25mph 1309rpm - 0.01 - PASS (maximum allowable is 1.14%) -------------------------------------------------------- NOx PPM
15mph 1333rpm - 3641 - GROSS POLLUTER (maximum allowable is 1095)
25mph 1309rpm - 3225 - GROSS POLLUTER (maximum allowable is 1140) -----------------------------------------------------------------
After failing the test, I checked the action of the EGR valve and found the condition I described above. After correcting the condition I took the truck back and passed the test with the following results.
-------------------- Percent of CO2
15mph 1333rpm - 10.4
25mph 1309rpm - 10.7 -------------------- Percent of O2
15mph 1333rpm - 6.7
25mph 1309rpm - 6.1 ------------------- HC PPM
*15mph 1333rpm - 132 - PASS (maximum allowable is 134)*
25mph 1309rpm - 65 - PASS (maximum allowable is 106) ------------------------- Percent of CO
15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
25mph 1309rpm - 0.04 - PASS (maximum allowable is 1.14%) -------------------------------------------------------- NOx PPM
15mph 1333rpm - 626 - PASS (maximum allowable is 1095)
25mph 1309rpm - 606 - PASS (maximum allowable is 1140) ------------------------------------------------------
I'm a bit concerned that the truck passed the 15mph test for HC by only 2 PPM. I would like to try to lower the HCs from the tailpipe. The catalytic converter is a brand new replacement. Would changing the ignition timing lower the HC reading? Or changing the air fuel ratio through either resizing the main jets or adjusting the float level? The ignition timing is currently set at 7 degrees BTDC with the vacuum line disconnected from the transducer at the computer, as per the shop manual.
The carb is a new (not rebuilt) Holley 2280. It is the non-feedback version of the Holley 6280 that came stock with this 87 Dakota V6. The mixture control solenoid for the Holley 6280 is no longer available as a replacement part so I tracked down a new 2280 on eBay and bolted it on. This configuration no longer benefits from the input from the O2 sensor, but it was the best I could do on short notice.
The way I see it I have three main ways to lower HCs:
1. Advance or retard the ignition timing
2. Enrich or lean out the fuel mixture by increasing or decreasing the size of the main jets in the carb.
3. Enrich or lean out the fuel mixture by raising or lowering the level of fuel in the bowl via the float setting, which is currently stock.
From the test results, I have more room to increase NOx than HCs so if lowering HCs cause NOX to rise, there is some headroom there.
Please feel free to nitpick away. It would go unappreciated.
Jack
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wrote:

I don't know why your HC's went up just from fixing an inop EGR valve ? High HC is from a rich mixture. Did you also mess with the charcoal canister vent line ? I would first try to lower the HC reading by leaning out the idle mixture screws, if that doesn't work then try the float level before you try switching main jets.
One other question, was the engine and converter fully warmed up before the test ?
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Mike wrote:

Hi Mike,
As to the last question, yes, the engine was fully warmed up as indicated by the temp gauge. I don't know what constitutes 'fully warmed up' for the cat. The engine was fully warmed up when the smog tech drove it onto the dynamometer. He did a 15mph test and a 25mph test, during which the engine was turning at about 1340 rpm, as indicated on the test result sheet. I am somewhat mystified by that. I would expect the rpms to be higher during the 25mph test, but it was actually lower by 22 rpms. Another one of life's unexplained mysteries. The test was long enough to warm up the cat, I would guess.
There may also have been a minor manifold vacuum leak. The bolts fixing the EGR valve to the intake manifold were slightly loose. If I had a manifold vacuum leak during the first test, that would have introduced a lean mix. Fixing the leak would richen up the mix. That would explain the higher HC on the second test.
One bolt on the EGR was so loose that when I tried to loosen it with what appeared to my eye to be a 1/2" hex head socket, I thought I must have got it wrong as there was no resistance, so I put on a 7/16" socket and that didn't fit. I tried metric and nothing that fit on the bolt head gripped. That's when I tried my fingers and the bolt unscrewed easy as pie. Being as I am my own mechanic I felt somewhat stupid, but soon got over it, remembering that shit happens. So I probably had a vacuum leak.
I didn't mess with the charcoal vent line except to put a hose clamp (damn, you're good) on where it connected to the carb. But to be honest, I don't think it was all that loose, but I had a couple of extra small hose clamps knocking around and figured what the hey.
As far as leaning out the idle mixture screws goes, how much does the idle circuit figure in at 1350 rpm on the engine and 15 and 25 mph on the speedometer?
Roger on adjusting the float level before changing the jets. I like that because it doesn't require getting new jets, just bending metal.
The Holley 2280 carb that I put on this 3.9L (239 cu) engine was used on Chrysler's 318 engine. It's the same body as the Holley 6280 that came stock on the 87 Dakota, but it doesn't have the mixture control solenoid, which is no longer available as a replacement part. As this part went south on the original carb, and any rebuilt 6280 will have an old mixture control solenoid, those of us who have this carb are left to our own devices. My solution, as was that of others with the know-how, was to install a non-feedback 2280. Just so ya know...
Do you have any light to shed on adjusting the timing either way, advanced or retarded, to bring down HCs? I have found arguments both ways while googling the web and the groups.
Thanks for your sharing your knowledge.
Jack
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Why wouldn't the HCs increase? When the exhaust gas recirculation valve opens, what is it allowing into the intake manifold/ combustion chamber? CO2, CO, O2, NOx and HC.
When the exhaust gas recirculation valve opens what happens to manifold pressure? It goes up. What does the power valve do when manifold pressure goes up? It opens and allows more HC (gasoline) to flow into the venturi in the carb.
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aarcuda69062 wrote:

Yes, it is reasonable to assume that these gases would increase in the exhaust stream over their presence in a non-EGR exhaust stream, but only in proportion to the total EGR gases that are reintroduced to a fresh intake fuel mixture, 5 to 15 percent from what I have read. In my case, the HC content increased 388% at 15 mph.

My source tell me you got that backwards. As manifold pressure goes up, the power valve is closed. As manifold pressure goes down, the power valve opens.
My source is "Holley Carburetors and Manifolds" by Mike Urich and Bill Fisher. HPBooks, 1987:
When the engine is called upon to produce power in excess of normal cruising requirements, the carburetor has to provide a richer mixture. Added fuel for power operation is supplied by the power system controlled by manifold vacuum.
Manifold vacuum accurately indicates engine load. Vacuum is usually strongest at idle. As load increases, the throttle valve must be opened wider to maintain a given speed. This offers less restriction to air entering the intake manifold and reduces manifold vacuum.
A vacuum passage in the carburetor applies manifold vacuum to a power-valve piston or diaphragm. At idle or normal cruising conditions, manifold vacuum acting against a spring holds the valve closed. As high power demands load the engine, manifold vacuum drops.
Below a preset point, usually about 6 inches of mercury (in.Hg), the power valve spring overcomes manifold vacuum and opens the power valve. Fuel flows through the power valve and through a power-valve restriction to join fuel already flowing through the main metering system from the main jet. The mixture is richened.
http://books.google.com/books?id=ClyzQHlQbjYC&pg=PA28&lpg=PA28&dq=%22When+the+engine+is+called+upon+to+produce+power%22&source=web&ots=SdPf9kX1c4&sig sxQZoaKdSk_aQ98Kxp41-vp4k&hl=en
Scroll down to the yellow hi-lited text
Jack
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My reply to Mike wasn't based upon your readings, it was based upon his statement. In your case, if the engine is running lean to begin with, the addition of EGR is going to displace any burnable mixture that might have stood a chance of combusting.

Nope.
As manifold -vacuum- goes -up- the power valve closes. Manifold vacuum is a great way to determine engine health, it's a lousy way to refer to how an engine and fuel system operate.
Right now assuming your truck is parked and not running, the manifold pressure is high and the power valve is open. Assuming that you're somewhere below 1000 feet altitude, the pressure inside your manifold parked and engine not running is probably somewhere near 28 or 29 inches of mercury. If you were to then start the engine, the manifold pressure might drop to 10 inches of mercury. If we subtract the 10 inches of mercury running from the 29 inches of mercury measured with the engine not running, we get 19 inches which would be a fairly healthy "manifold vacuum" reading.

I know the book.

Please understand, I'm not referring to manifold vacuum.
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aarcuda69062 wrote:

Ahn... you're right.
You referred to manifold pressure, not vacuum.
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wrote:

Yep, the EGR valve allows exhaust gas back into the combustion chamber, but, it is only a very small percentage of the total combution chamber volume. The exhaust gas was already burnt once in the combustion chamber, how does running it through the combustion chamber a second time cause the HC to increase even further ? Isn't the HC reading from the smog test unburned hydrocarbons ?

Yep, that's how the power enrichment works as well but why such a jump in HC ? This is the first time I remember seeing a NOx problem fixed were the HC reading jumped up like that. Does this have anything to do with the vehicle being equiped with an air pump ? We don't see too many air pumps on the east coast.
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wrote:

So? If the air fuel mixture is lean to begin with, how does adding exhaust gas make the flame propagate better?

See above.

Yes it is. Ever open an EGR valve at idle? Did the engine run better or worse? Do you suppose HC goes up or down if you open the EGR at idle?

Not for me. It's very common on some engines to repair a non functioning EGR for a NOx failure and have the car fail on HC. Typical 4 cylinder where the EGR feeds into the intake ports individually (4 separate EGR passages) and 2 or 3 are plugged. The 1 or 2 that do flow EGR are over fed and cause a misfire. Roto-Root the passages and everything is fine...

The high HC? Maybe yes, maybe no. That's why there's a thing called 'diagnostics.' I'd watch the reading with the air pump disabled. The OP has a 21 year old truck, what are the odds hat the air pump is original? I couldn't get the air pump on my bought new 85 F-150 to last more than three years. Air pumps tend to have a generic universal design, what if instead of a 4 CFM air pump made for a 3.9 liter engine, he got a 10 CFM replacement pump made for a 7.4 liter engine. You -can- pump too much air into a catalytic converter and snuff out the fire.

Dunno why, they were pretty common in the day that the OPs truck was built. Now they're likely to be electric.
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Yep.

Didn't run at all, they don't run well on an inert gas.

Being the engine wouldn't stay running I would say it went down. ;) If it would stay running it would have a lean misfire that would lead to an increase in HC.

Yes, I'd like to see the results with the air pump disabled.

Yep, I see your point here. My experience has been that aftermarket emission parts are junk.

They sure were, but living in the rust belt they don't last very long and are long ago rusted away.

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wrote:

Next time, don't open it so far. Try a 10% or 20% command on your scn tool instead of 80% .

No, it wouldn't be a lean misfire. Lean misfire comes from too much air and not enough fuel. Opening the EGR does not add more air. The exhaust gasses displace what would have been combustible mixture in the combustion chamber, the actual ratio of that combustible mixture doesn't change. If you have an engine with a leaking EGR valve at idle, fattening up the mixture doesn't improve how the engine will idle. Comparing secondary spark lines on a ignition scope, a lean mixture looks completely different than a stuck open or leaking EGR valve.

Yup.
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aarcuda69062 wrote:

It does add gas which displaces air. That would mean less air going into the cylinders. At least that is what would happen in the first fractions of a second. So for a very brief period of time you get a rich condition.

How can the mixture not change if you just took away some of the air. Of course in about the time it takes you to blink the dynamic system that controls fuel, air, timing and idle speed is going to react to those changes - so any conclusions you draw from this experiment are more than likely to be completely wrong.
-jim

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wrote:

Displaces air -and- fuel.

Less air -and- fuel going into the cylinder.

What makes you think that EGR only takes away air. What makes you think that EGR takes away anything? EGR displaces a volume in a cylinder.

None of what I've said are mine nor are they conclusions. IOWs, you assume wrong again They are the physics of the internal combustion engine, documented and published (but probably not in Popular Mechanics).
Don't take my word for it, feel free to run Simpson's non operating EGR valve gas readings and his operating EGR valve gas readings thru a Lambda calculator. You'll find that the air fuel ratio changes by .01 which is well within the expected sampling error of the type of equipment being used for his tests.
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aarcuda69062 wrote:

One can only guess what it is you are talking about. Let's assume you were talking about a modern car with fuel injection. Air gets displaced meaning less will enter intake (at least for an instant). The fuel at the same time doesn't get displaced. There is nothing physical to cause the fuel to be displaced. If the end result is less fuel is delivered it is only because the engine's control system delivers less fuel (but it could deliver more air or both).     On a vehicle with carb it's different because airflow is part of the physical process that delivers fuel. But that still doesn't mean the air/fuel ratio will stay the same without some engineering effort to make that happen.

Maybe or maybe not. All depends how a particular system is designed. Hopefully it's designed well enough that it won't change the air/fuel ratio very much in your scenario since EGR failures are not rare occurrences.

That was your terminology. Air is a gas. So is the exhaust coming from EGR - fuel is not. If you say some of the volume of gas is displaced that equivalent to saying some is taken away.

It sounded like it was your experiment. It also sounded like your conclusion or at least you hoped others would reach from the experiment. The only reason your experiment would lead to that erroneous conclusion is because that is the way it is engineered to work. If the controls are working as they should then it won't affect air/fuel because it is designed not to.

I suspect that Chrysler had a good bit of data on the vacuum controls for EGR in the 80's also. And that data is what they used to design EGR controls so that didn't throw the air/fuel ratio out of wack. But when you start doing your own design on an engine as the OP is doing it is extremely unlikely that you will end up not changing air/fuel ratio if you just slap any EGR control onto the system any which way and simply hope.
-jim
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