backfire in mufflers
Thread Starter
Drifting
Joined: Feb 2013
Posts: 1,464
Likes: 48
backfire in mufflers
Hello everybody
My 1989 Z51 6 spd is backfiring, in the mufflers, when I slow down. I have verified that the compression in all eight cylinders is good. The spark plugs that have been in for a couple of thousand miles, I drive the car often, all look the same. The car has new fuel injectors. I checked that the resistance is good, each one is 14.3 Ohms. The wires and cap have a few thousand miles on them. The TPS, CTS, EGR, ECM and memcal are all new. I disconnected the AIR pump electronics and the backfire remains. I set the timing, correctly i.e. I disconnected the Electronic Spark Timing wire, set the timing to 6 BTDC, connected the wire and rechecked the timing. Vacuum at the intake plenum is 21 inches.
The car is running great except for the backfire. The backfire occurs at idle but it is just noticeable. The catalytic convertors are empty.
Well thank you in advance.
My 1989 Z51 6 spd is backfiring, in the mufflers, when I slow down. I have verified that the compression in all eight cylinders is good. The spark plugs that have been in for a couple of thousand miles, I drive the car often, all look the same. The car has new fuel injectors. I checked that the resistance is good, each one is 14.3 Ohms. The wires and cap have a few thousand miles on them. The TPS, CTS, EGR, ECM and memcal are all new. I disconnected the AIR pump electronics and the backfire remains. I set the timing, correctly i.e. I disconnected the Electronic Spark Timing wire, set the timing to 6 BTDC, connected the wire and rechecked the timing. Vacuum at the intake plenum is 21 inches.
The car is running great except for the backfire. The backfire occurs at idle but it is just noticeable. The catalytic convertors are empty.
Well thank you in advance.
Thread Starter
Drifting
Joined: Feb 2013
Posts: 1,464
Likes: 48
Oh and your original question sorry. According to Scott the data indicates I am running rich.
The injector Base pulse width is usually below 1. 0.26/0.27
The injector Duty Cycle is usually 0.3/0.6
The L term-C counts is usually 108 and shaded pink when this becomes 128 the indicator box is white. From what I read this is where it belongs smack dab in the middle of 256.
When the BPW goes above 1.0 the Inj DC also goes into the whole numbers i.e. 3.4 and the L term -C counts is 128
Any and all ideas are welcome. Thank you
Thread Starter
Drifting
Joined: Feb 2013
Posts: 1,464
Likes: 48
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Le Mans Master
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From: Las Vegas - Just stop perpetuating myths please.
Backfire is a symptom of retarded timing or sometimes lean fueling can do that. The only mods I can read that you have is the cats are gutted. Less exh restriction can cause better head flow and higher VE. Higher VE is gonna need more fuel.
Now the $50 question is whether the ECM is in open loop or closed loop when backfire occurs? Closed loop is controlling with the O2 sensor. Open loop is using the fuel map and you should be able to see this when the shirt term fuel trims/integrator goes to a value of 128 and I think the long term no longer changes and remains as is.
Something that can cause lean is even sometimes an exhaust leak that pulls air in one of the header tubes while scavenging.
Hope this helps.
Now the $50 question is whether the ECM is in open loop or closed loop when backfire occurs? Closed loop is controlling with the O2 sensor. Open loop is using the fuel map and you should be able to see this when the shirt term fuel trims/integrator goes to a value of 128 and I think the long term no longer changes and remains as is.
Something that can cause lean is even sometimes an exhaust leak that pulls air in one of the header tubes while scavenging.
Hope this helps.
Thread Starter
Drifting
Joined: Feb 2013
Posts: 1,464
Likes: 48
Backfire is a symptom of retarded timing or sometimes lean fueling can do that. The only mods I can read that you have is the cats are gutted. Less exh restriction can cause better head flow and higher VE. Higher VE is gonna need more fuel.
Now the $50 question is whether the ECM is in open loop or closed loop when backfire occurs? Closed loop is controlling with the O2 sensor. Open loop is using the fuel map and you should be able to see this when the shirt term fuel trims/integrator goes to a value of 128 and I think the long term no longer changes and remains as is.
Something that can cause lean is even sometimes an exhaust leak that pulls air in one of the header tubes while scavenging.
Hope this helps.
Now the $50 question is whether the ECM is in open loop or closed loop when backfire occurs? Closed loop is controlling with the O2 sensor. Open loop is using the fuel map and you should be able to see this when the shirt term fuel trims/integrator goes to a value of 128 and I think the long term no longer changes and remains as is.
Something that can cause lean is even sometimes an exhaust leak that pulls air in one of the header tubes while scavenging.
Hope this helps.
The ECM is in closed loop, the car is at full operating temperature and the Datamaster program reads closed loop. I have tightened up the exhaust at the manifolds. I will check the rest of the exhaust.
I think I will block the AIR check valves. Maybe they are leaking. They are new. Thank you.
The O2 sensor is new. Is there a way for me to check to see if it is working. On the data master software it usually reads 900.
Interesting thing I just noticed is that the TPS voltage rarely goes above 1 volt. Even with the car at 2200 RPM.
The EGR DC is at 100 the TPS voltage is 1.06 the BPW is 2.72 O2 is 900.
Thank you all again.
Burning Brakes
Joined: Mar 2005
Posts: 1,082
Likes: 115
From: Richmond Hill Ontario
Also the TPS should be able to swing from about 0.56 V ( at idle) all the way up to about 4.5V ( 100% throttle opened). There should be a very linear sweep in TPS Voltage from Idle to 100%. I would test the TPS to see if in fact you do get this linear voltage range.
Thread Starter
Drifting
Joined: Feb 2013
Posts: 1,464
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If the TPS voltage is at 1.06 V at idle then this is too high. It should be at about 0.56V.
Also the TPS should be able to swing from about 0.56 V ( at idle) all the way up to about 4.5V ( 100% throttle opened). There should be a very linear sweep in TPS Voltage from Idle to 100%. I would test the TPS to see if in fact you do get this linear voltage range.
Also the TPS should be able to swing from about 0.56 V ( at idle) all the way up to about 4.5V ( 100% throttle opened). There should be a very linear sweep in TPS Voltage from Idle to 100%. I would test the TPS to see if in fact you do get this linear voltage range.
Someone suggested that the distributor could be 180 out. I think the car would run awful at 180 out. It is running, with the exception of the backfire, very well. Getting 20 mpg and accelerating very well. A car 180 out would not be running that well. I think. Besides I was able to time it with the EST wire disconnected.
Thank you
Drifting
Joined: May 2007
Posts: 1,426
Likes: 165
From: Motor City
Backfire is a symptom of retarded timing or sometimes lean fueling can do that. The only mods I can read that you have is the cats are gutted. Less exh restriction can cause better head flow and higher VE. Higher VE is gonna need more fuel.
Now the $50 question is whether the ECM is in open loop or closed loop when backfire occurs? Closed loop is controlling with the O2 sensor. Open loop is using the fuel map and you should be able to see this when the shirt term fuel trims/integrator goes to a value of 128 and I think the long term no longer changes and remains as is.
Something that can cause lean is even sometimes an exhaust leak that pulls air in one of the header tubes while scavenging.
Hope this helps.
Now the $50 question is whether the ECM is in open loop or closed loop when backfire occurs? Closed loop is controlling with the O2 sensor. Open loop is using the fuel map and you should be able to see this when the shirt term fuel trims/integrator goes to a value of 128 and I think the long term no longer changes and remains as is.
Something that can cause lean is even sometimes an exhaust leak that pulls air in one of the header tubes while scavenging.
Hope this helps.
Oil Producer
Joined: Apr 2014
Posts: 9,371
Likes: 2,736
does the 89 still have only 102 sensor on the driver side like my 85? If so if the fuel trims are not showing Rich, then the leak has to be on the passenger side. If you have an air leak ingesting fresh air between exhaust manifold and the O2 sensor, it causes the O2 to think you are running lean and therefore increases the fuel which makes you run Rich.
if the 89 only has an O2 sensor on the driver side and the passenger side header is drawing in the air, the O2 sensor will not see this, and you will get the popping in the cat or the muffler because of a higher 02 content that has not being compensated for by the computer with a higher fuel amount
Melting Slicks
Joined: Feb 2003
Posts: 2,677
Likes: 812
From: Saskatoon Sask.
does the 89 still have only 102 sensor on the driver side like my 85? If so if the fuel trims are not showing Rich, then the leak has to be on the passenger side. If you have an air leak ingesting fresh air between exhaust manifold and the O2 sensor, it causes the O2 to think you are running lean and therefore increases the fuel which makes you run Rich.
if the 89 only has an O2 sensor on the driver side and the passenger side header is drawing in the air, the O2 sensor will not see this, and you will get the popping in the cat or the muffler because of a higher 02 content that has not being compensated for by the computer with a higher fuel amount
if the 89 only has an O2 sensor on the driver side and the passenger side header is drawing in the air, the O2 sensor will not see this, and you will get the popping in the cat or the muffler because of a higher 02 content that has not being compensated for by the computer with a higher fuel amount
This confuses me. If I understand this, previous posts indicate that a leaking header will draw in O2 and cause a false lean condition, thereby causing the ECM to command more fuel. Am I correct in assuming that it is this "extra" fuel that is causing the backfiring. Your post seems to indicate you think it is the extra O2, not sensed by the sensor that is causing the backfiring. I wasn't aware that O2 would "backfire."
Thread Starter
Drifting
Joined: Feb 2013
Posts: 1,464
Likes: 48
This confuses me. If I understand this, previous posts indicate that a leaking header will draw in O2 and cause a false lean condition, thereby causing the ECM to command more fuel. Am I correct in assuming that it is this "extra" fuel that is causing the backfiring. Your post seems to indicate you think it is the extra O2, not sensed by the sensor that is causing the backfiring. I wasn't aware that O2 would "backfire."
Thread Starter
Drifting
Joined: Feb 2013
Posts: 1,464
Likes: 48
First of all. Reading my response the FYI sounds rude. It was not intended to be rude. I thought your question is does the car have one or two O2 sensors. It has one O2 sensors.
Following your discussion from the post. The only way the ECM would detect a lean burn would be from a leak in the drivers side, where the O2 sensor is. And that would have to be almost at the manifold. I think the resonator is immediately after the flange. I have tightened the flange. I will give it another look. Thank you.
Following your discussion from the post. The only way the ECM would detect a lean burn would be from a leak in the drivers side, where the O2 sensor is. And that would have to be almost at the manifold. I think the resonator is immediately after the flange. I have tightened the flange. I will give it another look. Thank you.
Melting Slicks
Joined: Feb 2003
Posts: 2,677
Likes: 812
From: Saskatoon Sask.
First of all. Reading my response the FYI sounds rude. It was not intended to be rude. I thought your question is does the car have one or two O2 sensors. It has one O2 sensors.
Following your discussion from the post. The only way the ECM would detect a lean burn would be from a leak in the drivers side, where the O2 sensor is. And that would have to be almost at the manifold. I think the resonator is immediately after the flange. I have tightened the flange. I will give it another look. Thank you.
Following your discussion from the post. The only way the ECM would detect a lean burn would be from a leak in the drivers side, where the O2 sensor is. And that would have to be almost at the manifold. I think the resonator is immediately after the flange. I have tightened the flange. I will give it another look. Thank you.