Carburetor EGR port question

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I'd think the engine's computer would sense the decrease in fresh air intake and reduce fuel delivery accordingly.
--
Tegger


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Mike wrote:

The air pump is original equipment. I bought the truck new and I have done all my own work on it, including changing the lifters and changing a blown head gasket, two separate operations. So I know the air pump is original. Whether it's working as it should today, I can't say. I tested it a few years ago according to the manual and it seemed to be working then.
What has me puzzled at this point is which of the 4 operations I performed on the fuel and ignition systems contributed to the skyrocketing of CO emissions on the third test (test results reprinted below). HC and NO came down significantly, as did O2, but at the expense of CO.
Only the enrichening of the idle mixture and the widening of the canister purge line ports in the wall of the carb throttle body could have fattened up the mix, which would have resulted in the higher CO and lower O2 readings. Assuming no freakishly timed breakdown of the air pump, it's operation would have been the same for all 3 tests, whether working or not.
If the guy who owns the smog shop will allow it, I would like to run another pre-test and see if clamping down the canister purge hose changes the readings mid-test.
I leaned out the idle mixture screws just a bit after the third test. They were pretty close to lean best idle as it was. I can't imagine that the small change that I made to the idle mix could have such a strong affect on the emissions with the engine running at 1325 rpms. So the enlarging of the canister purge line ports into the venturi just above the throttle plates is the likely suspect, IMO.
%CO2 %O2 HC PPM %CO NOx PPM
15mph
1st test 10.8 5.9 34 .01 3641-FAIL 2nd test 10.4 6.7 132* .01 626 3rd test 12.2 3.6 82 1.02-FAIL 358
25mph
1st test 10.7 6.0 26 .01 3225-FAIL 2nd test 10.7 6.1 65 .04 606 3rd test 12.5 2.9 73 1.14** 191
1st test - 1. EGR run from EGR port on carb resulting in essentially no EGR function 2. MSD ignition hooked up
2nd test - 1. EGR run from spark advance port on carb 2. MSD ignition not hooked up
3rd test - 1. EGR run from modified EGR port on carb 2. Idle mixture enriched slightly 3. The ports entering both barrels from the canister purge hose were enlarged to correspond to those in the original stock carb. 4. MSD ignition hooked up
* passing is 134, measured 132 ** passing is 1.14, measured 1.14

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Mike wrote:

Hey Mike. I'm not trying to prove you wrong but my 1990 GMC Sierra with 5.7L just did the same thing. I failed the emmisions test here in Texas recently. Reason was NOx too high. I put a new EGR valve on her and she passed with the new EGR valve on her. But my HC went up from 64 at high speed (25mph) to 127. On the low speed test (15mph) HC went from 116 to 127. Weird? My truck is now running better than it has in 3 years.
Bob
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No problem Bob. I just don't remember seeing it. My guess would be that I was watching the NOx readings because that was the problem being worked on at the time. Since the new OBD II emissions test no longer uses an exhaust analyzer in this area it has been a while since I have been able to use one. Our emissions testing here is done using the diag connector under the dash, basicly if there are no trouble codes stored and the monitors are run it passes.
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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.
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aarcuda69062 wrote:

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:
http://www.msdignition.com/ignition_1_5900.htm
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
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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.
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aarcuda69062 wrote:

Page 8D-10 of the
Chrysler Motors 1987 Shop Manual
2WD and 4WD Pickups
Dakota Trucks
Like the one shown here:
http://cgi.ebay.com/ebaymotors/1987-DAKOTA-TRUCK-SERVICE-SHOP-MANUAL-2-4-WHEEL-DRIVE_W0QQitemZ260210411809QQcmdZViewItem?hash=item260210411809&_trksid=p3756.m20.l1116
From the manual:
CABLE RESISTANCE CHART
Minimum---------------------Maximum 250 Ohms Per Inch 600 Ohms Per Inch

Right... I went and re-read what you wrote about it in your first post.

I feel certain that the reasoning for this must be in one of your past posts. It appears counterintuitive at first glance, though... a richer mix to lower HCs at the tailpipe.
Would you suggest bigger main jets or a higher float?
Mike suggested leaning out the idle mixture screws to get lower HCs.
At this point I am a bit more knowledgeable, but not quite sure what to do with it. What I need is tailpipe emissions wand like the one at the shop so i can monitor the changes that take place for a given tweak.

Will do, and thanks for all the input.
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Ya gotta do what ya gotta do.
Last fall had a guy bring me one of those kit cars that's a replica of a 20s something Mercedes Benz, the drive train is 78 or 79 Mustang V-6 with a 2 barrel Motorcraft carb. All stock engine wise, all emissions equipment present and functioning. The car couldn't pass the state IM-240 test. The only thing non-stock emissions related was the exhaust, he had one of those small universal catalytic converters on it. The original Mustangs had a 3 cat system. Thing is way rich, CO is sky high. No way to fit two more cats on it. Shoot the moon, pull the carb top off, remove the jets, solder 'em shut and re-drill them .008" smaller. Why .008"? That was the next smallest drill size in my number drill set. Re-assemble and took it to the test station 2 miles away. The car fast passed which means it's running significantly cleaner then it was before. Is soldering and drilling a set of jets a kludge repair? Absolutely! Did it fix the car? Hell yes. It passed the emissions test and he called back 2 days later and said the car had never ran so good.

So, a lean jetted carb is high on HC.

How do you know that? Maybe your HC reading is to the right of the bold black line. The HC graph represented on that chart is "U" shaped isn't it? High rich of stoichiometric, drops near stoichiometric and rises again lean of stoichiometric.

Your test sample was taken after the catalytic converter. so naturally the numbers would be lower. The specific number aren't what I was pointing out, it's the relationships of the different gasses at different air fuel ratios, i.e., is the HC high because it's too rich or because it's too lean. Some here seem to think that HC can only be high due to a too rich fuel mixture. The only way to know which side of stoichiometric your HC numbers represent is to know what the actual unadulterated O2 measurement is.

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aarcuda69062 wrote:

Got it! Using that way of looking at the graph, especially for CO, CO2 and O2, which are expressed in percentages, I can easily see that my passing test was on the lean side, the CO being a dead giveaway.

Okay, now I see why you place so much store in the O2 reading. I need to richen up the mix a bit. The only tool I have at my disposal to judge The mix is the color of the ceramic insulators on the plugs and they have been pure white of late.
Is plug insulator color a good indicator of proper fuel mix? That's all I have for now. If it's a good indicator, when I get a good color I will take it to the smog shop for a dry run.

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Simpson wrote:

Your issue may be timing. The vehicle is designed to run with EGR. It sounds like your engine performs well without it (ignoring NOX) and with EGR it does not do as well (other than NOX). That suggests you may be able to advance the timing a degree or 2. Doing that will probably increase NOX a little but may bring the other readings back in line.
-jim

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jim wrote:

Considering that in the last test from the smog shop (posted as my last reply to aarcuda69062) about an hour ago showed the NOx well below average, I have room to spare on that measurement. I will try your suggestion of advancing the timing two degrees and leaning out the idle mixture a bit and get another reading, but not today.

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Atta boy!

Plug color is a method, it's naturally not going to be as accurate as a gas analyzer. Hell, it worked for Smokey Yunik for years.
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Below are the results of three different emissions tests on my 1987 Dodge Dakota V6 with a non-stock, non-feedback carburetor, and the differences in the emissions configuration for each test.
The third test brought down HCs considerably for the 15mph test, which was my aim. However, CO measurement skyrocketed and failed at 15mph and barely passed at 25mph. O2 measurements came down by about half for both speeds and CO2 was up 17% in both tests.
The difference in EGR function between using the spark vaccuum port and the modified EGR port was measured using a vacuum meter teed into the EGR vacuum line just before the EGR diaphragm. The spark vacuum port responded more strongly to the throttle and was higher at steady speeds from 15 mph through freeway speeds than the modified EGR port. However, the modified EGR port presented what appeared to me to be favorable EGR vacuum characteristics so I used that port in the third test.
I am not sure which of the three modifications that I made (listed below under '3rd test') contributed to the changes in measurement of the various emissions gases, but I think it is safe to assume that I should lean out the idle mixture to bring down CO. From the looks of the CO line on this graph: http://www.autoshop101.com/forms/h56.pdf I should not have to lean it out much as that line is very steep. I would guess that between 1/4 and 1/2 half turn of each screw would do it. I set the idle mixture by the conventional method of starting with each screw 2 full turns out from fully closed and then screwing each on in until the engine starts to stumble and then backing out 1/2 turn. I backed out each screw from there about another half turn. So 1/4 to 1/2 turn in should do it. I may get another reading after that and then try what jim suggested and advance the timing 2 degrees to see how that changes the readings. That's all for now. I have to go earn some money to pay for all these tests. 30 bucks a pop for a pre-test. Not bad.
Best displayed with Courier, or some other monospace font:
%CO2 %O2 HC PPM %CO NOx PPM
15mph
1st test 10.8 5.9 34 .01 3641-FAIL 2nd test 10.4 6.7 132* .01 626 3rd test 12.2 3.6 82 1.02-FAIL 358
25mph
1st test 10.7 6.0 26 .01 3225-FAIL 2nd test 10.7 6.1 65 .04 606 3rd test 12.5 2.9 73 1.14** 191
1st test - EGR run from EGR port on carb resulting in essentially no EGR function
2nd test - EGR run from spark advance port on carb
3rd test - 1. EGR run from modified EGR port on carb 2. Idle mixture enriched. 3. The ports entering both barrels from the canister purge hose were enlarged to correspond to those in the original stock carb.
* passing is 134, measured 132 ** passing is 1.14, measured 1.14
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File under 'forgot to note':
For the 3rd test, I reconnected the MSD Blaster Ignition. It was present on the 1st and 3rd tests, but not on the 2nd.
http://www.msdignition.com/ignition_1_5900.htm
Simpson wrote:

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aarcuda69062 wrote:

As long as you do the run/ chop the throttle/ and shut down a plug reading is pretty good. The problem is that you don't want it to idle much if you want a good reading. Now if you really want to play with it at idle a gas analyzer works. So does a set of color tune plugs. But on an installed engine where you can't see the plugs real well they are no fun to use. But they do show how well the manifold actually spreads the mix around, or not in the case of some engines.
--
Steve W.

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Simpson wrote:.

One more way, a fresh oil change using conventional oil.
High HC's can come from oil getting past the rings also so a really thin oil or old oil that goes 'thin' can bump them up.
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: http://mikeromainjeeptrips.shutterfly.com
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Mike Romain wrote:

Oil is fresh and the rings seem to be in good shape. I can go all the way between oil changes at 3,000 miles without having to add a quart. I bought it new and changed oil and filter fairly regularly.

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Simpson wrote:

Mine has read high HC's with a lean miss on mine once.

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really lean conditions don't burn hotter please explain why melted holes in pistons don't result from leaned out fuel mixtures?????
aarcuda69062 wrote:

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