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Old 05-06-2019, 05:18 PM   #1
truckin 79
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EGR valve testing

I have failed CO emissions test twice and have adjusted idle mixture screws the second time around. I replaced all filters, air, crankcase and canister. It did help but still high HC and CO are way high to pass. I am starting to think either cat or egr might be the problem. The truck does run well for the most part. I can press down on gas pedal pretty hard and truck does not do much. I did stomp on gas pedal and could hear secondaries open up but still no wow affect. The truck is a steady performer though, if I just give it a little gas it will accelerate fine. On a side note what is the temp on cat? I have a temp gun and it was about 350 degress when pointed at cat
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Old 05-06-2019, 05:37 PM   #2
Matt Man
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Re: EGR valve testing

Quote:
Originally Posted by truckin 79 View Post
I have failed CO emissions test twice and have adjusted idle mixture screws the second time around. I replaced all filters, air, crankcase and canister. It did help but still high HC and CO are way high to pass. I am starting to think either cat or egr might be the problem. The truck does run well for the most part. I can press down on gas pedal pretty hard and truck does not do much. I did stomp on gas pedal and could hear secondaries open up but still no wow affect. The truck is a steady performer though, if I just give it a little gas it will accelerate fine. On a side note what is the temp on cat? I have a temp gun and it was about 350 degress when pointed at cat
If you have high hydrocarbons and CO your cat could be on its way out. Is it the original pancake style cat or a newer high flow one? The EGR lowers combustion temperatures which helps NOX emissions. You should measure the temperature of your cat before and after I THINK the temperature should be higher after the cat. Another thing I would check is the condition of your spark plugs if they are sooty or worn out you could have weak spark which will increase hydrocarbons and CO. Adjust the carb to maximum vacuum as well.

EDIT: is the truck the one in your signature? 79 Big 10? I don't think those came with a catalytic converter???
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Old 05-06-2019, 06:49 PM   #3
truckin 79
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Re: EGR valve testing

No it has the new design cat on it now. I think the 78 was the last year a Big 10 did not have to have a cat, if I remember correctly. I adjusted idle mixture with a tachometer. The truck will start with just a turn of the key although sometimes it will start with a bellow of black smoke. I changed oil as well and plugs were a little dark but nothing to write home about. The carb is rebuilt by me about 6 months ago. The distributor is fairly new as well with a accel super coil. I think I am down to egr and cat as the only problems left. I think I will just buy a egr since that is cheaper and see if that helps.
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Old 05-07-2019, 12:21 AM   #4
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Re: EGR valve testing

I would think that high HC and CO might point to a too rich mix from the carb. Dark plugs and the sometimes black smoke also suggest a too rich mix.

Another thing to check is the exhaust riser valve. One stuck shut or mostly stuck will cause a lack of high end performance plus can damage the carb that also results in a too rich mix. If it is stuck shut, most of that side's exhaust gas is forced through the small intake crossover which both limits flow and overheats the carb.
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Old 05-07-2019, 06:33 AM   #5
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Re: EGR valve testing

An important graph for troubleshooting these issues.



There are 2 issues you can have with an EGR - doesn't hold vacuum, and dirty. I use a cheap hand vacuum pump to test that it holds vacuum. If it is dirty, take it off and spray it with carb cleaner.

For the cat, you are checking that the output temperature is hotter than the input, showing that it is burning off the HCs.

When taking your test, make sure your engine is HOT. Choke needs to be off, and the cats need to be hot enough to do their job.
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Old 05-08-2019, 11:25 AM   #6
truckin 79
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Re: EGR valve testing

I forgot I bought a cheap vacuum pump tester/bleeder from H Freight. I hooked it up to EGR and would no hold any vacuum. I did try vacuum pump on exhaust riser valve and that valve is good, held vacuum just fine. I do run constant manifold vacuum to exhaust valve riser though as I don't trust thermal vacuum switch to work correctly and open it up on time. I need to go to parts store to pick up a new EGR, I am hopeful this will cure my problem with emissions and acceleration.
Thanks
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Old 05-13-2019, 12:55 PM   #7
truckin 79
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Re: EGR valve testing

Well it was the EGR valve, the old one held no vacuum. Man what a difference changing that EGR out made. The truck actually has power now that I can feel when I press on gas pedal. Before I could press on gas and could not really feel any acceleration. I still have to readjust idle mixture screws again but could not resist taking it for a drive. I feel like I have a whole new truck. Anyone out there with a truck and still have emissions stuff on their truck and it is sluggish, check your EGR valve. Well after getting back from test drive and all psyched to dial in carb once again, I feel something wet and hot hitting my arm. I thought a bird pee'd on me but nope the radiator was spritzing me with anti-freeze. I cant believe it, not another thing I have to replace. Well I go to parts store and description states brass tanks and aluminum fins. I get the radiator and it is plastic tanks aluminum fins but I need it so I replaced. I hope the plastic one holds up.It cost 150 at Autozone so far the only problem I see is the top transmission line was leaking a little but I cranked on it as much as I dare, not wanting to crack plastic tank. So far it looks like it is not leaking. Still have to adjust carb and hit the emission station. Hopefully I pass this time
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Old 05-13-2019, 01:02 PM   #8
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Re: EGR valve testing

I made the switch to a plastic tank radiator a couple years back. Felt bad going from a brass radiator to a plastic one, but it was half the cost, and has worked well.
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