I rebuilt the carb on my fj-60. Now when the engine is cold - no choke - it backfires. It seems limited to shifting; I haven't noticed it on deceleration. When I take my foot off the gas to shift and push in the clutch it backfires through the exhaust. The carb is performing well, reasonable mileage, good performance, and smooth idle. Thanks - Dan
fj60 - rebuilt carb - now engine backfireing
- Thread starter dansawyer
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Backfiring in the exhaust requires three ingredients: unburned fuel, O2 and ignition heat.
The unburned fuel comes from cylinder(s) that are misfiring for any reason, or a fuel mixture problem (too rich or too lean).
The O2 comes from cylinder(s) that are too lean or misfired, or a leak in the exhaust system.
ignition heat comes from the exhaust port or a cat.
HTH.
The unburned fuel comes from cylinder(s) that are misfiring for any reason, or a fuel mixture problem (too rich or too lean).
The O2 comes from cylinder(s) that are too lean or misfired, or a leak in the exhaust system.
ignition heat comes from the exhaust port or a cat.
HTH.
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It shouldn't be. Coasting decel fuel cut is probably not even a factor on a normal upshift. Engine vacuum has to be pretty high (like coasting down an off-ramp in gear kinda high) for the switch to turn off the fuel.
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The symptoms occur when the engine is warming up. Accelerate with half to three quarter throttle to 2500 or so; then take foot off throttle quickly and disengage clutch. There is one 'backfire' through the exhaust. The mixture could be rich, however nothing should be hot enough to cause ignition.
The manifold vacuum is 5 in on acceleration and rises to 23 or 24 on shifting. The backfire does not occur on deceleration. I will try a 'steady' state, i.e. not accelerating 2500 long enough to burn off any remaining accelerator pump fuel, and then the 'shift'. Dan
The manifold vacuum is 5 in on acceleration and rises to 23 or 24 on shifting. The backfire does not occur on deceleration. I will try a 'steady' state, i.e. not accelerating 2500 long enough to burn off any remaining accelerator pump fuel, and then the 'shift'. Dan
Man- I hope you can figure this out, and it's the same problem I have.
Mine does the same thing since I rebuilt my carb 2 years ago- Afterfire out the muffler when the engine's cold and worse when it it colder outside. I thought it was related to an exhaust leak, but I have a rebuilt intake and exhaust manifold assembly and good gasket.
The Emissions Factory Service Manual says that Muffler explosion (afterfire) on deceleration only is caused by:
1. AI system faulty
2, Deceleration fuel cut system always off.
I do not know if this is the same problem, but so far, I haven't been able to track my problem down.
I also get delayed pinging under load, and adjusting (retarding or advancing) the timing has not seemed to help this.
Mine does the same thing since I rebuilt my carb 2 years ago- Afterfire out the muffler when the engine's cold and worse when it it colder outside. I thought it was related to an exhaust leak, but I have a rebuilt intake and exhaust manifold assembly and good gasket.
The Emissions Factory Service Manual says that Muffler explosion (afterfire) on deceleration only is caused by:
1. AI system faulty
2, Deceleration fuel cut system always off.
I do not know if this is the same problem, but so far, I haven't been able to track my problem down.
I also get delayed pinging under load, and adjusting (retarding or advancing) the timing has not seemed to help this.
- Thread starter
- #7
Hank14 - let me know if you get this. I dropped the exhaust pipe and found the heat riser deflector plate was loose, the small bolts holding it on had broken off. My deflector plate spring retainer was long enough and had threads at the end; I added two nuts to hold it on and tack welded the plate to the manifold in the correct position. The result - no backfire, more, consistent power, and no intermittant pinging. I suspect my plate was overheating the intake manifold and overheating the air gas mixture. I hope there are no small cracks in the intake manifold. - Dan
Hi Dan,
The heat riser deflector plate is certainly a possibility. I have another thread where I rebuilt my intake and exhaust manifold assembly and installed the SOR stainless block off plate between them. To get it to clear, I trimmed the heat riser plate.
Problem is- I think it was backfiring before I rebuilt the manifold. I thought the backfire was due to an intake leak or an exhaust leak. Now I don't have a clue.
One of these days I'll probably get a SMOG legal header and try that- or desmog altogether.
I still have to figure out the pinging issue as well.
It's always something!
Bump. It's cold out and the engine always has afterfire when decelerating until it warms up. I have another ABV valve that I am rebuilding to see if that makes a difference. I temporarily grounded the fuel cut solenoid and that didn't help. My VSVs are working properly.
I may need to get ahold of a professionally rebuilt carb and swab it for mine to see if that makes a difference.
I may need to get ahold of a professionally rebuilt carb and swab it for mine to see if that makes a difference.
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If the truck runs good otherwise, seems unlikely to be a carb issue.
Look to the AI system, ABV valve, exhaust leak, retarded timing....
Look to the AI system, ABV valve, exhaust leak, retarded timing....
It runs very good when the engine's warm. I did have an exhaust leak where there was a crack at the exhaust down pipe flange that connects to the exhaust manifold. I found it after I rebuild my intake and exhaust manifold assembly.
I had the crack welded. I will look for another.
I have tried different timing settings. It is at the factory setting now and runs best there- with the exception that I will get some delayed pinging under open throttle acceleration. I think all the vacuum advance parts are working right (VTV and vacuum advance on the distributor).
Just to update and revive an old thread:
I finally found the cause of the afterfire when my engine was cold- I worked back through the emissions FSM and found that the EGR modulator valve was not working properly. Swapped it for a used, functioning one, and no more problems.
I finally found the cause of the afterfire when my engine was cold- I worked back through the emissions FSM and found that the EGR modulator valve was not working properly. Swapped it for a used, functioning one, and no more problems.
hward1
87 fj60 / 95 fjz80
nice to here updates - hate when im reading and never find out what happened -
Mine backfires only when hot upon deceleration in gear, albeit, rarely. I do have an exhaust leak somewhere though, I can hear it. I don't make much of it, it happens maybe half the time and only after she's well, well warmed up. OH I forgot to mention that i pinched all my air rail inlets and disconnected the air pump hose. The rail was perforated in spots and was whistling like crazy. I guess the lack of after burn air injection to cool off the exhaust gasses might cause my problem. I have a new air rail, I just got to get around to installing it.
What about the check valve on the skinny pipe that comes off the exhaust down pipe? If that is shot, it will allow exhaust to come back up into the ABV/ASV unit. I just replace both my check valves (the other one is on the air rail). To replace the check valve on the down pipe side, I had to remove the downpipe from the vechicle in order to get enough torque on it to remove it. It was on there real good.
When you put the down pipe back on, put a new donut gasket on, too.
Rusto
SILVER Star
Old thread coming back to life... Mine is doing the occasional popping when shifting gears. I think it's because the choke isn't closing all the way, leading to unburnt fuel. Reason I think it's the choke is that when my wife was climbing into the cab to drive it, I had pulled the choke out already to get it ready to start, and her knee hit the choke knob and snapped it off it's mount.
I was able to get it reattached using some superglue to fix the plastic piece that snugs it up to the dash, but it doing so, it's not adjusted quite like it was. As soon as I get a few minutes, I'm going to adjust the choke cable and see if that fixes it. Having a 3 year old and 1 year old in the house, and getting ready to move, my time is limited, but I will get back in here and post if it fixes it.
I was able to get it reattached using some superglue to fix the plastic piece that snugs it up to the dash, but it doing so, it's not adjusted quite like it was. As soon as I get a few minutes, I'm going to adjust the choke cable and see if that fixes it. Having a 3 year old and 1 year old in the house, and getting ready to move, my time is limited, but I will get back in here and post if it fixes it.
