Carburetor EGR port question

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?

Reply to
Simpson
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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.....

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Reply to
MasterBlaster

I don't know that particular carb, but have seen that kind of failure before. When I saw it it was a gasket in wrong or the wrong one in so a notch in the base plate was covered when it was supposed to be open.

The other times I have seen the base plate in upside down. There are notches in some that need to be on the carb side, not the manifold side and if folks don't know this, the plate can be in wrong.

I would figure the port you are using was blocked or it would have one or the other types of vacuum.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 'New' frame in the works for '08. Some Canadian Bush Trip and Build Photos:
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Reply to
Mike Romain

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.

Reply to
Simpson

Could this 'base plate' be a one inch aluminum spacer with a single manifold vacuum port? Or are you referring to the actual throttle portion of the carb? That would be kind of hard to put in upside down. In any case, as in my reply to MasterBlaster, I suspect a leak in the canister purge line, which enters the carb just above the closed throttle plates, same as the EGR port.

It's brand new, straight from the Holley factory, after sitting on some shelf for about 20 years or more.

Reply to
Simpson

Yes it could. Sometimes the kits come with multiple types of base gaskets with different holes or slots.

Or are you referring to the actual throttle

There is an easy test for that though I don't think it can be the trouble unless you were measuring the vacuum downstream vs at the carb nipple, just pinch the line closed.

Reply to
Mike Romain

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.

Reply to
BigIronRam

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.

Reply to
Simpson

Mike, I solved the problem. See my detailed reply to BigIronRam in this thread.

Thanks, for the help. You were on the right track as far as the port probably being blocked goes. Whatever the reason, it just wasn't developing EGR vacuum characteristics and would not even make it to 2 pounds at 2,000rpm.

Reply to
Simpson

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.

Reply to
aarcuda69062

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

Reply to
Simpson

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 ?

Reply to
Mike

Hi Jack, Does this truck have an air pump? The reason I ask is because there is an awful lot of O2 in the exhaust sample. If the truck has an air pump, that would account for it, but unfortunately the dilution from the air pump makes analyzing the gas samples difficult. If there is NO air pump, here;s what I think; too much O2, too much HC, not enough CO2 and not enough CO. That is a lean mixture. Richening it up a bit will drop the HCs, lower the O2 and increase the CO2. A richer mixture doesn't contribute to more NOx typically as long as the other NOx treatments are functioning as they should.

Advancing ignition timing hurts NOx, it tends to hurt HC and CO also, the exception being if the ignition system is marginal and HCs are caused by an ignition misfire, this is because it takes less voltage to ionize the plug gap the farther the piston is away from TDC (advanced). Late ignition timing tends to help HC and CO for the exact reasons given in my first post, hotter combustion chamber wall, hotter exhaust valve and port, but late timing can reveal marginal ignition components.

CO2 is the best indicator of combustion efficiency, higher is better so anything you do that raises CO2 shows you're headed in the right direction. I mention this because your CO2 reading are low, by about 3 percent or more.

An efficient catalytic converter lowers CO and HC and raises CO2 by its very nature, OEM catalysts are usually 90%+ efficient, aftermarket catalysts are usually only 40%-50% efficient. The only way to tell if the tailpipe readings are due to an inefficient catalyst is to take pre-cat and post cat gas samples and calculate the efficiency.

Without an air pump diluting the sample a stoichiometric air/fuel ratio would read .5% CO and .5% O2 give or take .1% either way.

Reply to
aarcuda69062

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

Reply to
Simpson

Yes it does. It injects air directly into the cat.

That's what I thought.

I'm confused. If there is excess O2 and excess HC, what prevents them from combining in a burn to produce more CO2?

That's seems to be in line what what I have read concerning the formation of NOx, which is mainly a factor of a too hot combustion chamber.

I can't quite follow you on this. Did you mean that "it takes *more* voltage to ionize the plug gap the farther the piston is away from TDC (advanced)."?

Plugs, cap and rotor are all very healthy looking, but the resistance of the wires all measure below the lower spec for resistance in the shop manual. 250 ohms per inch is the minimum spec. Mine are at about 200.

At this point, I should probably mention that I have an MSD Blaster Ignition, model PN 5900, installed:

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More precisely, it was installed when the truck failed the test and not installed when it passed, but I don't think it had anything to do with the failure. The non-functioning EGR system definitely played the major role, IMO. I unconnected the MSD unit for the retest because I thought that perhaps the more powerful and longer duration spark it supplied might be contributing to the hot combustion chamber conditions that caused the high NOx reading.

So the truck has an air pump, and given what we have hashed through here so far, I feel that, just to be sure, I should replace the plugs, wires, cap and rotor before concerning myself with the jets or the float level, even if I think they are in good shape.

BTW, do you happen to know if plug wires can *lose* resistance over time?

I hope all is well in your part of the world.

Jack

Reply to
Simpson

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.

Reply to
aarcuda69062

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.

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Scroll down to the yellow hi-lited text

Jack

Reply to
Simpson

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.

Reply to
aarcuda69062

Ahn... you're right.

You referred to manifold pressure, not vacuum.

Reply to
Simpson

I suspected so. The only way to make meaningful determination of the gas readings is to take the gas readings again with the air pump disabled.

At this point, it's nether good or bad until you attempt to use the gas readings as a diagnostic aid.

There's a thing called the Brettschneider equation which allows with the input of 4 or 5 gas reading to accurately calculate the air fuel ratio. The equation works whether or not there is a catalytic converter ahead of where the sample is taken. What the Brettschneider equation can not compensate for however is false air introduced either by leaks in the exhaust pipes or joints or an auxiliary air system such as is on your truck.

Low catalytic converter efficiency. The excess gases are in the wrong ratio.

Ever use a cutting torch? If you set the flame as oxidizing, you won't get the results you need. If you set the flame as carburizing, you won't get the results you need. One size fits all replacement parts... I've seen vehicles fail emissions tests because the air pump was too small, I've seen vehicles fail emissions test because the air pump was too big. (the OEMs rate/size them by CFM)

No, opposite that. If you have a weak ignition system, chances are it will present itself or be more problematic with less ignition advance. if I see low firing voltage on a secondary waveform on a scope and no indication of plug fouling, I first grab my timing light and check ignition timing. You can search for my posts in either of the Chevy truck groups, you want to focus on exchanges between myself and a poster named "snoman" for my attempts to get him to understand why his truck runs better with the ignition timing jacked sky high contrary to the hundreds of identical trucks I've worked on in the last 20 years that would just ping, rattle and set ESC codes when set the same or similar.

Which shop manual specifies ohms per inch?

Again, hot combustion chambers do not contribute to NOx formation. Hotter combustion does contribute to NOx formation. Hottest combustion typically occurs if the mixture is 2 percent richer than stoichiometric.

It's cheap enough to do on that engine. My hunch however is that the carb is a little lean.

Outside of leaking insulation, I can't say I've ever seen it in

37 years in the trade. The typical failure that would account for high HCs would be an open plug wire.

Ask me again tomorrow 8-) they're forecasting 8-14 inches of snow for our area. We've already had over 100 inches this season though most of it had melted in the last two weeks.

Good wrenching and let us know the results.

Reply to
aarcuda69062

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