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olskool720

86 720 failed smog, got any ideas?

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My 86' 720 ST king cab failed smogged on the last test....25mph emissions.  My mechanic worked on some vacuum lines.  Then, the 720 failed again, so I've used my one free re-test.  The testing site suggested the EGR valve.

 

Anyone have any ideas???  Just one test I need to pass ARGH.

 

I should add that I'm not much of an engine guy, so if you suggest something, it'll be for me to pass along to my mechanic, thanks : )

Edited by olskool720
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Did it pass the visual inspection? Why did it fail?

 

NOx too high is common for older mid 80's vehicles and a lot of times , simply replacing the cat does the trick.

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Yes, the CO was too high on the final test at 25mph at 1700rpm.  It passed every test including visual.  By "cat" do you mean the catalytic converter?  The guy at the test shop stated that this wasn't the issue  : \

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It may not be THE issue, but replacing the cat (yes, catalytic converter) is sometimes a quick and easy trick to get them to pass.

 

Ask him if he thinks a new cat will get it to pass.

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Never buy into what the smog guy tells you.I just had my '85 Toyota pickup fail.He says it's running rich.It gets 27 MPG and is properly tuned with new parts so i'm thinking bullshit.I replaced the catalytic converter and what do you know ? almost zero emissions.

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[coming from a non-engine guy..aka me] So my mechanic replaced two hoses and cleaned out some other hoses and did some adjustments on the air filter holder.  That was about it.

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I have had my 84 since new.  still has original carb..  have to test it every 2 years.  Heres   what I do..  I advance timing a little. make sure you mark  original  spot so you  can put it back.  Use a clean rag and , while you rev motor  by hand .put rag over carb and make it suck  air,  rev it preety high.  This kind of cleans out carb or carbon deposits I guess.    then I put  about one can of iso heet  to  half of a tank of gas.   I think I used two bottles one year.  burns  cleaner.   Then pray you get a guy who  knows how to drive a stick shift at the emission place.  most those guys never seen or heard of a carb.  I have the original  catalytic  on my truck..

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7 hours ago, blackbomber said:

I have had my 84 since new.  still has original carb..  have to test it every 2 years.  Heres   what I do..  I advance timing a little. make sure you mark  original  spot so you  can put it back.  Use a clean rag and , while you rev motor  by hand .put rag over carb and make it suck  air,  rev it preety high.  This kind of cleans out carb or carbon deposits I guess.    then I put  about one can of iso heet  to  half of a tank of gas.   I think I used two bottles one year.  burns  cleaner.   Then pray you get a guy who  knows how to drive a stick shift at the emission place.  most those guys never seen or heard of a carb.  I have the original  catalytic  on my truck..

Advancing the timing makes it more likely to fail.

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Mine passed and that's good enough for me.  I'll deal with it again in September 2021 LOL

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9 hours ago, blackbomber said:

I have had my 84 since new.  still has original carb..  have to test it every 2 years.  Heres   what I do..  I advance timing a little. make sure you mark  original  spot so you  can put it back.  Use a clean rag and , while you rev motor  by hand .put rag over carb and make it suck  air,  rev it preety high.  This kind of cleans out carb or carbon deposits I guess.    then I put  about one can of iso heet  to  half of a tank of gas.   I think I used two bottles one year.  burns  cleaner.   Then pray you get a guy who  knows how to drive a stick shift at the emission place.  most those guys never seen or heard of a carb.  I have the original  catalytic  on my truck..

 

Heet is (more or less) just  alcohol and as most gas is 10% ethanol it isn't going to do much. Make sure your engine is at peak temperature before the test and is in good tune. (it should be anyway) Fix ALL emissions devices as they fail. Check all hoses for cracks. Check the EGR tube for leaks that will let in any oxygen and make sure the EGR valve closes at idle... or even remove and wire brush it clean. The later 720 uses exhaust pulses and one way valves to draw fresh air into the exhaust manifold so the catalytic converted has oxygen to burn off any HCs. A new converter won't do much without air to burn with. Look for leaks on the pipe(s) to the air cleaner. Do an oil and filter change.

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