Converter Causing Misfire?

black77gmc · March 12, 2008

Lets see if anyone can help me. I have a 2000 GMC Sierra 4X4 and 5.3 liter. I have been having trouble with a misfire at idle for quite some time. At one point I cut off the converters and was able to get a little more spunk out of it, but it did not correct or make the problem any worse. I finally took it to a mechanic and he is stating that no converter is the problem, however I dont agree due to the converters were there when it started. Now my question is, it is my understanding that the front two O2 sensors are the only ones that regulate the fuel trim. The second two are to ensure the cats are working, am I incorrect on this? If so then I will purchase a new cat Y pipe and replace them. Is there any down side to running the vehicle without cats? If someone could help me I would greatly appreciate it. I have been chasing this problem for a while now.

Travis Smith

[email protected]

Fortune50 · March 13, 2008

Lets see if anyone can help me. I have a 2000 GMC Sierra 4X4 and 5.3 liter. I have been having trouble with a misfire at idle for quite some time. At one point I cut off the converters and was able to get a little more spunk out of it, but it did not correct or make the problem any worse. I finally took it to a mechanic and he is stating that no converter is the problem, however I dont agree due to the converters were there when it started. Now my question is, it is my understanding that the front two O2 sensors are the only ones that regulate the fuel trim. The second two are to ensure the cats are working, am I incorrect on this? If so then I will purchase a new cat Y pipe and replace them. Is there any down side to running the vehicle without cats? If someone could help me I would greatly appreciate it. I have been chasing this problem for a while now.

Travis Smith

[email protected]

Check the EGR and make sure it's not stuck open. How many miles on the motor? If alot, could be a compression problem (like a burned valve). May want to check compression and see if the cylinders all read close together. Make sure your throttle body is clean. I also used CRC brand MAF cleaner to clean my truck's MAF and I think it helped idle quality a bit. Make sure your fuel regulator is not leaking thru the diaphragm, this can be checked by removing the vacuum hose off it and see if any fuel comes out while the engine's running. If there is fuel coming out, it's bad. Also make sure your truck's tune is up to date using quality parts (I like AC Delco). Also look at the old plugs and see if any have any signs of ignition or fuel problems (black-rich from weak spark, stuck injector, leaking fuel regulator); white-lean from vacuum leak, plugged injector, low fuel pressure).

minibike132 · March 13, 2008

Lets see if anyone can help me. I have a 2000 GMC Sierra 4X4 and 5.3 liter. I have been having trouble with a misfire at idle for quite some time. At one point I cut off the converters and was able to get a little more spunk out of it, but it did not correct or make the problem any worse. I finally took it to a mechanic and he is stating that no converter is the problem, however I dont agree due to the converters were there when it started. Now my question is, it is my understanding that the front two O2 sensors are the only ones that regulate the fuel trim. The second two are to ensure the cats are working, am I incorrect on this? If so then I will purchase a new cat Y pipe and replace them. Is there any down side to running the vehicle without cats? If someone could help me I would greatly appreciate it. I have been chasing this problem for a while now.

Travis Smith

[email protected]

Check the EGR and make sure it's not stuck open. How many miles on the motor? If alot, could be a compression problem (like a burned valve). May want to check compression and see if the cylinders all read close together. Make sure your throttle body is clean. I also used CRC brand MAF cleaner to clean my truck's MAF and I think it helped idle quality a bit. Make sure your fuel regulator is not leaking thru the diaphragm, this can be checked by removing the vacuum hose off it and see if any fuel comes out while the engine's running. If there is fuel coming out, it's bad. Also make sure your truck's tune is up to date using quality parts (I like AC Delco). Also look at the old plugs and see if any have any signs of ignition or fuel problems (black-rich from weak spark, stuck injector, leaking fuel regulator); white-lean from vacuum leak, plugged injector, low fuel pressure).

All that stuff said you may still be thinking converters . A plugged cat is going to get worse with load not at idle only. Is there a down side to running without cat's, Yep it's illegal and if your asking the gm tech it's illegal for us to advise otherwise. You need to fix the root problem. You say there is a miss at idle. Some more info might help us narrow your possibilities such as is it cold or hot. Have you seen the miss on a scope or scanner and is it on one or more cylinders and which ones. If you do have a scanner lots of readings would help like fuel trim reading, mass air flow, ect, iat...

black77gmc · March 13, 2008

The fuel pressure regulater was changed about six months ago, tune up is up to date with all AC Delco parts, EGR looks fine, I have cleaned the MAF and throttle body, I also ran some seafoam through it a few times in the tank and through vacuum lines. I have not checked the compression yet that was the next thing i was gonna do.

Now for more info, i have a scanner which the fuel trim stays steady on two of them, but jumps all over on the other two. i will hook it up tomorrow and write all of the feedback if it will help. Also this seems like it runs fine until it warms up, which would tell me its the O2 sensors since it is in open loop until it reaches a certain temp, but I changed them and they didnt help so I took them back. But they were bosch and not ac delco, would that make a difference? I will hook the scanner up tomorrow and get the codes if you think it will help me. I did have a buddy hook it up to a very expensive scanner, which one I dont know. Anyways he was able to tell me that cylinder #6 was misfiring more than it should. He worked for a GM dealer and was kinda stumped. The only code that I get from my scanner is a emissions control malfuction which I am also working on fixing so if there is some common problems that could help me there I would appreciate it, I dont know anything about that. I will be back tomorrow, I thought I would leave this to help if someone gets to it. I appreciate the help guys!

Travis

gexline21 · March 13, 2008

Missing converters wont cause any misfire related problems, yes the check engine light will be on for a code p0420,p0430. Id check the compression on the cylinder that misfires the most. The 99-01 trucks had problems with weak valve springs, but that usually caused the cylinder to miss at all times. The injectors had problems at the connectors and what gm called was fretting, which required injector replacment - but this was also a steady skip when it did. Since its at idle and intermittent, i would check for a vacuum leak at the intake to head sealing areas, i'd bet its the intake gaskets leaking, they have a new design at the dealer.

The fuel pressure regulater was changed about six months ago, tune up is up to date with all AC Delco parts, EGR looks fine, I have cleaned the MAF and throttle body, I also ran some seafoam through it a few times in the tank and through vacuum lines. I have not checked the compression yet that was the next thing i was gonna do.

Now for more info, i have a scanner which the fuel trim stays steady on two of them, but jumps all over on the other two. i will hook it up tomorrow and write all of the feedback if it will help. Also this seems like it runs fine until it warms up, which would tell me its the O2 sensors since it is in open loop until it reaches a certain temp, but I changed them and they didnt help so I took them back. But they were bosch and not ac delco, would that make a difference? I will hook the scanner up tomorrow and get the codes if you think it will help me. I did have a buddy hook it up to a very expensive scanner, which one I dont know. Anyways he was able to tell me that cylinder #6 was misfiring more than it should. He worked for a GM dealer and was kinda stumped. The only code that I get from my scanner is a emissions control malfuction which I am also working on fixing so if there is some common problems that could help me there I would appreciate it, I dont know anything about that. I will be back tomorrow, I thought I would leave this to help if someone gets to it. I appreciate the help guys!

Travis

black77gmc · March 14, 2008

I will check the compression tomorrow. All I do is hookup the compression gauge and crank the motor over right? I then should get the highest reading possible. I was not aware of intake manifold gasket problems for this motor, I know they had some issues with the 5.7 liters a few years prior. I will also get the feedback off of my scanner as well while im checking compression to see if it will help. Thanks to you guys trying to help, greatly appreciated!

Fortune50 · March 14, 2008

I will check the compression tomorrow. All I do is hookup the compression gauge and crank the motor over right? I then should get the highest reading possible. I was not aware of intake manifold gasket problems for this motor, I know they had some issues with the 5.7 liters a few years prior. I will also get the feedback off of my scanner as well while im checking compression to see if it will help. Thanks to you guys trying to help, greatly appreciated!

Disable the fuel pump and remove all the spark plugs, that way the engine can spin freely so you can get more accurate measurements. I actually have heard of folks having to replace intake gaskets on the newer LS smallblocks.