EGR valve testing

[truckin 79]

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

[Matt Man]

Originally Posted by truckin 79

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???

[truckin 79]

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.

[Dead Parrot]

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.

[rpmerf]

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.

[truckin 79]

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

[truckin 79]

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

[rpmerf]

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.

[truckin 79]

So I am tired of messing with my truck and AF gauge. I worked on it again last night. The same thing was happening, in driveway 14.7 and take for a drive 13-13.5 sometimes 12. I thought maybe the transfer slots were exposed to much (info from gregski) so I lowered idle speed in the hopes this would create more vacuum and pull power piston in fat section of metering rods (info from rpmerf). I get there and it changes after a drive again. I was getting frustrated and pulled vacuum line from egr and truck hit 14.7 to 15. I took it for a little drive and stayed 14.7 to 15 when stopping at stop signs with egr vacuum line plugeed. The climb up to 2500 is still giving a reading of about 13 to 13.5 though. I mentioned in previous post that when I rebuilt this carb it had a broken off pencil in vent tube, presumably pushing down on power piston. If truck does not pass today I am going to put a pencil back in there

[rpmerf]

Originally Posted by truckin 79

The climb up to 2500 is still giving a reading of about 13 to 13.5 though.

You might want to look into the power piston spring. I just changed the power piston spring out on my carb.

The spring that WAS in there started enrichment around 8" of vacuum. This meant that below about 25% throttle, it had very little power, then came to life once I hit 8".

I just switched to a 12" spring. I idle around 14-16" in drive, so it is fully seated. Enrichment happens very quickly. As soon as I give it just about any bit of gas. I'm going to have to see how that works out in various situations - mainly highway cruising.

If you go with a weaker spring, you might want to try playing with the Adjustable Part Throttle (APT). The APT sets how far the power piston can close. You can raise this up a bit to help make up for when the piston is in it's closed position, then turn in the idle screws to compensate for the extra fuel from the primaries. It's a balance and a bit of trial and error to find a right combination.

Also, crazy to hear how much of a difference that EGR makes. I've always thought EGR on a carbed motor was a bit odd, turns out it's as bad as I thought. Hard to imagine that even helps emissions.

[truckin 79]

Well it finally passed the smog test what a relief. The good thing is I do not have to run emissions test again unless I sell truck. I can get a collectors plate on it and just renew every 5 years. Now I can spend more money on truck to fix the things it really needs like front shocks, rubber brake lines, fix emergency brake, tint the front windows, swap out a broken leaf in the back, take out dual tank hoses, change drivers fender and maybe add air conditioning.............oh and paint it of course.
I am with your jpmerf, it makes no sense to add exhaust gases back into intake. I guess it makes sense if you are already running really lean and it cools off cylinder and burns gas again. If vehicle is already running rich then it is just adding even more rich fuel in cylinder and then cooling it off. Thanks for all your help everyone and jpmerf

[WorkinLonghorn]

I think you will want to keep that EGR connected. With it connected you can run the stock timing or slightly advanced for optimal performance. With it disconnected or not working you will possibly get spark knock at standard advance setting. I've personally experienced this and then read about it. Talking about a stock truck. -BA

[truckin 79]

I will think I will hook it back up but I think the thermal vacuum switch is bad. The EGR is getting vacuum on start up although truck starts right up even when EGR was hooked up but truck had to sit a couple minutes before I could take off. I usually had to feather the gas pedal to get it going. I think I am going to start from square one with carb and try and reset choke, choke pull off and get secondaries.

[rpmerf]

Some documents I have learned a lot from:

QJet Tuning:
http://www.capitalcorvetteclub.ca/at...ng%20Paper.pdf

QJet Choke:
http://www.vetteclub.org/warehouse/t...ke%20Setup.doc

QJet Common problems:
https://www.digitalcorvettes.com/for...ad.php?t=88376

[truckin 79]

good reads thanks, the more I read on quadrajets the more I relize I need to start from square one on adjustments. I am glad I put on the air fuel gauge though.

[rpmerf]

Yes, the air / fuel gauge takes the tuning to a whole new level. I installed a cheap narrow band system in my truck years ago. Air / fuel gauge from my turbo car, and a $20 1 wire O2. I'm working on harvesting a wide band I never got working from a non running project.

I've heard that A/F might is everything. With 10-15% ethanol gas, you might be better off around 14.1 instead of 14.7. You still have to go by feel a bit to figure out where it is happy. The A/F helps you understand what it likes and what it doesn't. Cruise, you still want to lean it out til it isn't happy, then richen it up a bit. I've heard between 16:1 and 17:1. WOT, you want to richen until it stops getting faster, usually between 12.0:1 and 13.0:1.

As you've seen, carburetors like when everything is consistent. Things like air temperature can really throw off a tune. If you are tuning at 95* with 60% humidity, it will be leaner at 50* and 30% humidity.

I'm going through a lot of this myself right now. I found out I've had a decent size vacuum leak since I got the truck 8 years ago. It's running super rich now, like pegs the narrowband once I give it gas. The jets and rods are unmarked, so I need to shoot in the dark a bit to estimate the jets and rods I need. It's been 90*+ during the days and the air feels like soup, so that really doesn't help. I need to get it out at like 6 AM when it is like 75-80*.

[truckin 79]

I only had till the end of the month and tags were going to expire. When I finally got the wideband gauge I tried tuning when it was 55 degrees outside with a little rain. The past weekend it was 90 degrees and very hot under the hood. The good news is I finally passed emissions, the bad news is I did not need it done in the first place. I go to the county clerks office for a second time and she tells me, I did not need a emissions test. I told her the first time I was there that lady over there told me I had to have one. Oh she was wrong, that will be 230.00 (good for 5 years) for the new date tag. Oh well on the bright side I learned more about q-jets and air/fuel mixtures. I definitely want to get the truck to launch a little better from a dead stop.

[rpmerf]

Always fun dealing with the MVA. Here we have tag and title shops. They are like a middle man for the MVA. You pay them like $20, but they are much more knowledgeable and will work with you, because they want your business. When historic tags changed from 25 years to 20, I went down and got historic tags for my car that just turned 20. They were telling me they would get in arguments with the people at the MVA and cite the law directly, because the people at the MVA weren't aware.

I get so tired of hearing one thing form one person and another thing from another. Seems almost commonplace. Part of the reason "I want to talk to a manger" has become so common.

For getting off the line you want to focus on a couple things. Rear ratio has a huge effect. Torque converter can have a huge effect. If that grabs too low and isn't matched to your cam, it can really kill power off the line until you get into the power band. Like having a stick and always trying to launch at 1600 RPM.

As for your carb, focus on primaries. Lars gets into it a bit in the tuning document. Secondaries aren't going to kick in until you are higher up in RPM. Launching is all about the primary jets. When you up your primary jets, you need to up your primary rods to keep the same cruise area. When you are changing both of these, you might want to change your APT to set your fuel usage in the area between when the power piston is seated and when it starts to open. Once the power piston is all the way up, you will be at full enrichment, which is where you want WOT level enrichment. I think I mentioned this in a previous post. A weaker power piston will enrich earlier, but can have you at full enrichment pretty early. A weaker spring will delay that a bit and might work better for cruise. Since you have a 79 carb (I assume), APT should be easy for you to tune.

Another thought is the accelerator pump. If you go WOT from a stop, it should spring to life, no lag or bogging. Accelerator pump is used for throttle changes to throw some fuel in before the manifold vacuum / carb can respond to the throttle change.

I made a post a while back that has an HTML file that you can use for changing jets and rods. You type in your current jet / rod combo and it will show the percentage change for different jets / rods at WOT and cruise.

Sorry if i'm rambling a bit much. I've been doing a lot of research into this.