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slight backfire issue

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Old 11-17-2012, 07:01 PM
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Default slight backfire issue

Hello everyone, I am pretty sure I had an account on this site a few years back but had no luck recovering a login or pass. Anyway, I just replaced Spark plugs, Wires, Distributor cap, and rotor. When in park or neutral, if I rev the engine, as soon as I let off the accelerator I can hear a very faint series of 3 pops from the exhaust. This problem was there before the tune up, but at that point I was blaming it on a misfire. Normally my next guess on fixing a backfire is a timing issue, but given that the truck has a computer adjusted timing, I'm out of Ideas, any help would be great.

To be clear, these pops cannot be heard from in the cabin with windows closed, open a door and they are faint, and even right at the exhaust tip you have to listen for it to hear it.

Thank you
 
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Old 11-17-2012, 08:44 PM
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Info on the truck, year, model, past codes, problems major or minor.
Welcome!
 
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Old 11-17-2012, 09:55 PM
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sorry, 2000 4X4 blazer. past problems have been a rough idle, shaking wile driving, and a very low idle (~400 RPM). All attributed to a wicked misfire and solved with the tune up. the pop is the only thing the tune up didnt fix. I also have had problems with the fuel cap being pressurized to the point of flying off at the gas station, a fluctuating fuel gauge (just like my last blazer) and its still throwing codes PO440 (new gas cap in place already) and I believe PO452. Im thinking my next step on that is a new fuel pump but ive heard rumors that a wire with insulation rubbed through or cut could be the issue so Im going to check that out first.

as an irrelivant side note all of the problems I have fixed so far are door hinge pins, and somehow at some point in its life the driver side parking brake stuck open and ground the pad down and sheared off one of the brackets.
 
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Old 11-17-2012, 10:33 PM
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I had a 96 with a flowmaster and when reved high and let off the gas completly i would hear a few pops from the tail pipe, not loud but there. I liked the sound, lol. Never viewed it as a problem. Unless it's doing it while driving or at idle i wouldnt worry about it.
 
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Old 11-18-2012, 12:07 PM
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https://blazerforum.com/forum/showthread.php?t=24373
Check out the link for setting timing. Its possible the distributor shaft bushing have worn out and the CMP retard is out of spec (+-2 degrees) and IIRC you won't get a CEL until its off 15 degrees. However, most backfires is the result of runing rich or out of time. Get a capable scan tool and check the 02 sensors to make sure there not getting lazy and the CMP retard. How has your mpg been?
 
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Old 11-18-2012, 02:01 PM
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2000 has the AIR system, (Air Injection Reaction or sometimes called Secondary Air). It is designed to pump fresh air into the exhaust to help the catalytic converter do its job. The problem is on deceleration, the exhaust is slightly rich with fuel, and then the (colder) air is added to it, and the fuel ignites. If you've been around an engine with open headers, you know what I'm talking about.... same thing. Easy way to determine if this is the problem, is to disable the AIR system and listen.
 

Last edited by Captain Hook; 11-18-2012 at 02:04 PM.
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Old 11-18-2012, 10:18 PM
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Gas milage has been pretty low (16-17) but as I said I also had a wi ked miss that replacing plugs and wires fixed, haven't gone through a tank yet so I'm not sure if it's any better now. How would I disable the AIR system? When it comes to mechanical I have a pretty good grasp of what I'm doing but when it comes to electronics, I can rewire an entire truck but computer systems are another language to me
 
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Old 11-19-2012, 01:47 PM
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The A.I.R. pump is located under the radiator on the passenger side. The pump is electrical, it is not driven by the serpentine belt. You can temporarily disable it one of 2 ways: Unplug the electrical connector from the pump or remove the (5/8" diameter???) rubber hose from the pump. With the system disabled, it will set DTC's in memory and turn on the SES light. If the SES light is on before you disable the system, and there are "secondary air" DTC's in memory, there is a problem with the system and it needs to be diagnosed and repaired. If the SES light is not on, and there are no "secondary air" DTC's in memory, the system is functioning properly. A slight backfire through the exhaust on deceleration is considered normal for vehicles equipped with the system. If the AIR system is disabled or inoperative, it will eventually cause the catalytic converter to overheat and melt the substrate in the converter.... not a good situation

http://www.justanswer.com/chevy/1u7p...tion-pump.html
 

Last edited by Captain Hook; 11-19-2012 at 01:50 PM.
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Old 11-19-2012, 05:33 PM
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Between here and other mechanics that have listened to it, I've gathered that it's not something I need to worry about unless it becomes a problem or gets louder. I will eventually get pics up of the truck. thanks for the help on this. its nice to come back to the forum to a warm welcome.
 
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Old 11-19-2012, 06:30 PM
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The constant popping noise is annoying and it tends to shorten the life span of the exhaust system from the concussions. There's nothing you can legally do about it though.
 
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