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97 Jimmy 4.3 Misfire Under Slight Load


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Hello,

My 97 4.3 with 101,000 miles has a random misfire at 1800 rpms.It starts perfect with

just a turn of the key,also idles perfect.If you drive it with just slightly pressing gas

pedal it random misfires.If you increase pedal or even floor it its fine.Its throwing code for random misfire.It always misfires at any speed or load with the gas pedal just slightly

pressed always at around 1800 rpm.The fuel system pressure,ignition and vacuum all

perfect.Also, this happens at any engine temp.How do I clean the TB or is it the TB sensor.Any help or ideas PLEASE!

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Hello,

My 97 4.3 with 101,000 miles has a random misfire at 1800 rpms.It starts perfect with

just a turn of the key,also idles perfect.If you drive it with just slightly pressing gas

pedal it random misfires.If you increase pedal or even floor it its fine.Its throwing code for random misfire.It always misfires at any speed or load with the gas pedal just slightly

pressed always at around 1800 rpm.The fuel system pressure,ignition and vacuum all

perfect.Also, this happens at any engine temp.How do I clean the TB or is it the TB sensor.Any help or ideas PLEASE!

 

Check the distributor cap, rotor, and replace them. It's likely there is a crack and/or moisture has built up in there causing problems. My brother's 1990 GMC Safari 4.3 had that problem too. It's a common problem, if that solves your issues. Good luck!

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Hello,

My 97 4.3 with 101,000 miles has a random misfire at 1800 rpms.It starts perfect with

just a turn of the key,also idles perfect.If you drive it with just slightly pressing gas

pedal it random misfires.If you increase pedal or even floor it its fine.Its throwing code for random misfire.It always misfires at any speed or load with the gas pedal just slightly

pressed always at around 1800 rpm.The fuel system pressure,ignition and vacuum all

perfect.Also, this happens at any engine temp.How do I clean the TB or is it the TB sensor.Any help or ideas PLEASE!

 

Check the distributor cap, rotor, and replace them. It's likely there is a crack and/or moisture has built up in there causing problems. My brother's 1990 GMC Safari 4.3 had that problem too. It's a common problem, if that solves your issues. Good luck!

 

 

I have replaced them with delco good stuff.Is there a how to on cleaning throttle body?

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Hello,

My 97 4.3 with 101,000 miles has a random misfire at 1800 rpms.It starts perfect with

just a turn of the key,also idles perfect.If you drive it with just slightly pressing gas

pedal it random misfires.If you increase pedal or even floor it its fine.Its throwing code for random misfire.It always misfires at any speed or load with the gas pedal just slightly

pressed always at around 1800 rpm.The fuel system pressure,ignition and vacuum all

perfect.Also, this happens at any engine temp.How do I clean the TB or is it the TB sensor.Any help or ideas PLEASE!

 

Check the distributor cap, rotor, and replace them. It's likely there is a crack and/or moisture has built up in there causing problems. My brother's 1990 GMC Safari 4.3 had that problem too. It's a common problem, if that solves your issues. Good luck!

 

 

I have replaced them with delco good stuff.Is there a how to on cleaning throttle body?

 

 

What do you mean by "slightly pressed at 1800 rpm"?

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Hello,

My 97 4.3 with 101,000 miles has a random misfire at 1800 rpms.It starts perfect with

just a turn of the key,also idles perfect.If you drive it with just slightly pressing gas

pedal it random misfires.If you increase pedal or even floor it its fine.Its throwing code for random misfire.It always misfires at any speed or load with the gas pedal just slightly

pressed always at around 1800 rpm.The fuel system pressure,ignition and vacuum all

perfect.Also, this happens at any engine temp.How do I clean the TB or is it the TB sensor.Any help or ideas PLEASE!

 

Check the distributor cap, rotor, and replace them. It's likely there is a crack and/or moisture has built up in there causing problems. My brother's 1990 GMC Safari 4.3 had that problem too. It's a common problem, if that solves your issues. Good luck!

 

 

I have replaced them with delco good stuff.Is there a how to on cleaning throttle body?

 

 

What do you mean by "slightly pressed at 1800 rpm"?

 

 

I mean just slight throttle pressure to cruise at 55 on flat road.I have seen posts on this same problem in many different forums but no one has solved it.It seems like a vacuum problem but its only at this rpm and steady throttle(highest vacuum)

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Hello,

My 97 4.3 with 101,000 miles has a random misfire at 1800 rpms.It starts perfect with

just a turn of the key,also idles perfect.If you drive it with just slightly pressing gas

pedal it random misfires.If you increase pedal or even floor it its fine.Its throwing code for random misfire.It always misfires at any speed or load with the gas pedal just slightly

pressed always at around 1800 rpm.The fuel system pressure,ignition and vacuum all

perfect.Also, this happens at any engine temp.How do I clean the TB or is it the TB sensor.Any help or ideas PLEASE!

 

Check the distributor cap, rotor, and replace them. It's likely there is a crack and/or moisture has built up in there causing problems. My brother's 1990 GMC Safari 4.3 had that problem too. It's a common problem, if that solves your issues. Good luck!

 

 

I have replaced them with delco good stuff.Is there a how to on cleaning throttle body?

 

 

What do you mean by "slightly pressed at 1800 rpm"?

 

 

I mean just slight throttle pressure to cruise at 55 on flat road.I have seen posts on this same problem in many different forums but no one has solved it.It seems like a vacuum problem but its only at this rpm and steady throttle(highest vacuum)

 

 

When did this start happening? Sounds like a fuel delivery problem, or bad fuel maybe? What if you rev it up past say, 2,500 rpm or, drive it like you're merging onto the highway with 80MPH traffic?

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  • 2 weeks later...

I've got the same problem on my 97 5.7L K1500 GMC PU. 1800 to 2500 part throttle creates significant misfire and will set P0301 trouble code. Part throttle being minimum throttle to maintain 60MPH. Also if I put the truck in 3rd gear I can get this to happen much more frequent and at lower speeds. Like you I've checked fuel system, vacuum, and ignition. I also tried unplugging the EGR. This provide a 90% cure of the symptom (could still discern a little roughness) but of course sets the Service Engine light. I removed the throttle body and cleaned it. I removed the EGR and and cleaned the EGR port in the intake manifold. The EGR head port appeared clear/clean. The EGR, thermostat, and one Catalytic Convertor is new. If I don't operate in the misfire region I get no OBD codes. No issues at idle, coasting (no throttle), or at more than "light" throttle.

 

Sounds like we have got the same problem?

 

Any recent resolution? I'm about to resort to the dealer or tune-up shop.

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  • 2 weeks later...

I've got the same problem on my 97 5.7L K1500 GMC PU. 1800 to 2500 part throttle creates significant misfire and will set P0301 trouble code. Part throttle being minimum throttle to maintain 60MPH. Also if I put the truck in 3rd gear I can get this to happen much more frequent and at lower speeds. Like you I've checked fuel system, vacuum, and ignition. I also tried unplugging the EGR. This provide a 90% cure of the symptom (could still discern a little roughness) but of course sets the Service Engine light. I removed the throttle body and cleaned it. I removed the EGR and and cleaned the EGR port in the intake manifold. The EGR head port appeared clear/clean. The EGR, thermostat, and one Catalytic Convertor is new. If I don't operate in the misfire region I get no OBD codes. No issues at idle, coasting (no throttle), or at more than "light" throttle.

 

Sounds like we have got the same problem?

 

Any recent resolution? I'm about to resort to the dealer or tune-up shop.

[/quote

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Well,

 

I have bought a software OBD package with enhanced functions.I can send codes to

control sensors,make egr open amd close,things like that.Does anyone know where to

get the control codes?GM is supposed to provide them,but I don't where to find them.

My problem is still there and getting worse!Did you get anywhere with your problem?

 

Thanks

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I've got the same problem on my 97 5.7L K1500 GMC PU. 1800 to 2500 part throttle creates significant misfire and will set P0301 trouble code. Part throttle being minimum throttle to maintain 60MPH. Also if I put the truck in 3rd gear I can get this to happen much more frequent and at lower speeds. Like you I've checked fuel system, vacuum, and ignition. I also tried unplugging the EGR. This provide a 90% cure of the symptom (could still discern a little roughness) but of course sets the Service Engine light. I removed the throttle body and cleaned it. I removed the EGR and and cleaned the EGR port in the intake manifold. The EGR head port appeared clear/clean. The EGR, thermostat, and one Catalytic Convertor is new. If I don't operate in the misfire region I get no OBD codes. No issues at idle, coasting (no throttle), or at more than "light" throttl

Sounds like we have got the same problem?

 

Any recent resolution? I'm about to resort to the dealer or tune-up shop.

 

Here is an update that might help. I pulled the distributor to examine the drive gear since our symptoms have often been attributed to a worn gear. While it was out I cleaned up some corrosion on the rotor and distributor cap terminals. When I reinstalled the distributor the characteristics of the missfire changed significantly. I suspected the distributor timing since there is no way to set it accurately without a scanner. I figure I could have set it close enough by trial and error but I was already wishing for a scanner so I didn't have to keep going back to Autozone for a scanner read. I purchased the CarCode.com scanner cable with GM enhanced codes for $122. The scanner SW showed the Camshaft Retard Degrees (CRD) was -45. I used to the scanner SW to set the distributor to within the +/- 2 degree spec. The truck runs much better with only a barely perceptible miss under part throttle conditions. The scanner also shows an intermittent misfire on Cylinder 7. I will replace my wires/rotor/cap sometime soon (after Christmas bills :lol: ). For now the truck runs good and I can live with the very minor roughness at part throttle. Its possible the timing wasn't out that far before I removed the distributor but either way I am happy with the results. In the future I may also replace the drive gear since it did have a fair amount of wear. The gear itself is about $45. The motor was rebuilt with a high volume oil pump that some people claim causes premature wear on the distributor gear. I hope this helps diagnose your problems.

 

So far I can recommend the Carcode.com hardware and software. I provided me a lot of insight into the operation of the engine and its sensors. It does show the GM truck enhanced parameters necessary to set the distributor accurately. To get parameters to display you tell the SW which (or all) codes to "Send". I found the way this works a little counter-intuitive when I first ran the SW but its not a problem. If you tell the truck to send all codes, including the enhanced parameters the update rate becomes very slow (>200 parameters). I found I was really only interested in 3 or 4 of the enhanced codes since most other parameters of interest were generic across all vehicles. I haven't tried any other SW or scanners so I can't compare how this works to anything else. I couldn't find many affordable alternatives that provided the enhanced codes but that doesn't mean they aren't out there. This solution requires a Windows PC of some sort as the interface.

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I've got the same problem on my 97 5.7L K1500 GMC PU. 1800 to 2500 part throttle creates significant misfire and will set P0301 trouble code. Part throttle being minimum throttle to maintain 60MPH. Also if I put the truck in 3rd gear I can get this to happen much more frequent and at lower speeds. Like you I've checked fuel system, vacuum, and ignition. I also tried unplugging the EGR. This provide a 90% cure of the symptom (could still discern a little roughness) but of course sets the Service Engine light. I removed the throttle body and cleaned it. I removed the EGR and and cleaned the EGR port in the intake manifold. The EGR head port appeared clear/clean. The EGR, thermostat, and one Catalytic Convertor is new. If I don't operate in the misfire region I get no OBD codes. No issues at idle, coasting (no throttle), or at more than "light" throttl

Sounds like we have got the same problem?

 

Any recent resolution? I'm about to resort to the dealer or tune-up shop.

 

Here is an update that might help. I pulled the distributor to examine the drive gear since our symptoms have often been attributed to a worn gear. While it was out I cleaned up some corrosion on the rotor and distributor cap terminals. When I reinstalled the distributor the characteristics of the missfire changed significantly. I suspected the distributor timing since there is no way to set it accurately without a scanner. I figure I could have set it close enough by trial and error but I was already wishing for a scanner so I didn't have to keep going back to Autozone for a scanner read. I purchased the CarCode.com scanner cable with GM enhanced codes for $122. The scanner SW showed the Camshaft Retard Degrees (CRD) was -45. I used to the scanner SW to set the distributor to within the +/- 2 degree spec. The truck runs much better with only a barely perceptible miss under part throttle conditions. The scanner also shows an intermittent misfire on Cylinder 7. I will replace my wires/rotor/cap sometime soon (after Christmas bills :D ). For now the truck runs good and I can live with the very minor roughness at part throttle. Its possible the timing wasn't out that far before I removed the distributor but either way I am happy with the results. In the future I may also replace the drive gear since it did have a fair amount of wear. The gear itself is about $45. The motor was rebuilt with a high volume oil pump that some people claim causes premature wear on the distributor gear. I hope this helps diagnose your problems.

 

So far I can recommend the Carcode.com hardware and software. I provided me a lot of insight into the operation of the engine and its sensors. It does show the GM truck enhanced parameters necessary to set the distributor accurately. To get parameters to display you tell the SW which (or all) codes to "Send". I found the way this works a little counter-intuitive when I first ran the SW but its not a problem. If you tell the truck to send all codes, including the enhanced parameters the update rate becomes very slow (>200 parameters). I found I was really only interested in 3 or 4 of the enhanced codes since most other parameters of interest were generic across all vehicles. I haven't tried any other SW or scanners so I can't compare how this works to anything else. I couldn't find many affordable alternatives that provided the enhanced codes but that doesn't mean they aren't out there. This solution requires a Windows PC of some sort as the interface.

 

 

I have bought the same package and just received today.Thanks for the insight

on enhanced codes.I will scan this afternoon and tell you what I have found.

Thanks

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I've got the same problem on my 97 5.7L K1500 GMC PU. 1800 to 2500 part throttle creates significant misfire and will set P0301 trouble code. Part throttle being minimum throttle to maintain 60MPH. Also if I put the truck in 3rd gear I can get this to happen much more frequent and at lower speeds. Like you I've checked fuel system, vacuum, and ignition. I also tried unplugging the EGR. This provide a 90% cure of the symptom (could still discern a little roughness) but of course sets the Service Engine light. I removed the throttle body and cleaned it. I removed the EGR and and cleaned the EGR port in the intake manifold. The EGR head port appeared clear/clean. The EGR, thermostat, and one Catalytic Convertor is new. If I don't operate in the misfire region I get no OBD codes. No issues at idle, coasting (no throttle), or at more than "light" throttl

Sounds like we have got the same problem?

 

 

 

Got my carcode software and after a learning curve was able to fix the problem.I would read all the sensors and all would show up perfect,so I went for a long ride with the scanner logging all sensors.I would run it to the rpm with the misfire and do a freeze frame.All sensors again wilh the correct values.Did this a couple of times with freeze frame.Sat down at the kitchen table and began to run some tests with the playback

feature.The data looked fine when looking at voltages on sensors.Then I used the plotting graph.I plotted water temp,TPS and fuel trim,BINGO!!!.Slowing down the speed

of replay there it was.Water temp 197 then 0 then 300 then ok.The fuel trim was following,rich then lean and on and on.The misfire has been solved.I replaced sensor

and after a little PCM relearning,The truck has not run this well in a long time.

Thanks for all the help.Happy New Year!!!

 

Any recent resolution? I'm about to resort to the dealer or tune-up shop.

 

Here is an update that might help. I pulled the distributor to examine the drive gear since our symptoms have often been attributed to a worn gear. While it was out I cleaned up some corrosion on the rotor and distributor cap terminals. When I reinstalled the distributor the characteristics of the missfire changed significantly. I suspected the distributor timing since there is no way to set it accurately without a scanner. I figure I could have set it close enough by trial and error but I was already wishing for a scanner so I didn't have to keep going back to Autozone for a scanner read. I purchased the CarCode.com scanner cable with GM enhanced codes for $122. The scanner SW showed the Camshaft Retard Degrees (CRD) was -45. I used to the scanner SW to set the distributor to within the +/- 2 degree spec. The truck runs much better with only a barely perceptible miss under part throttle conditions. The scanner also shows an intermittent misfire on Cylinder 7. I will replace my wires/rotor/cap sometime soon (after Christmas bills :cheers: ). For now the truck runs good and I can live with the very minor roughness at part throttle. Its possible the timing wasn't out that far before I removed the distributor but either way I am happy with the results. In the future I may also replace the drive gear since it did have a fair amount of wear. The gear itself is about $45. The motor was rebuilt with a high volume oil pump that some people claim causes premature wear on the distributor gear. I hope this helps diagnose your problems.

 

So far I can recommend the Carcode.com hardware and software. I provided me a lot of insight into the operation of the engine and its sensors. It does show the GM truck enhanced parameters necessary to set the distributor accurately. To get parameters to display you tell the SW which (or all) codes to "Send". I found the way this works a little counter-intuitive when I first ran the SW but its not a problem. If you tell the truck to send all codes, including the enhanced parameters the update rate becomes very slow (>200 parameters). I found I was really only interested in 3 or 4 of the enhanced codes since most other parameters of interest were generic across all vehicles. I haven't tried any other SW or scanners so I can't compare how this works to anything else. I couldn't find many affordable alternatives that provided the enhanced codes but that doesn't mean they aren't out there. This solution requires a Windows PC of some sort as the interface.

 

 

I have bought the same package and just received today.Thanks for the insight

on enhanced codes.I will scan this afternoon and tell you what I have found.

Thanks

 

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