1996 GMC Yukon 5.7 4l60e 4x4 multiple misfires but no check engine light

[indexcow]

I have been trying to find the cause of some performance issues. Using Dashcommand I see that most of my cylinders are having a couple misfires on my drive to work but I am not getting a check engine light. On my drive to work I got 4 misfires on cylinder 1, 1 on cylinder 2, 1 on 3, 0 on 4, 2 on 5, 0 on 6, 2 on 7, and 0 on 8.

Does anyone know where I should start to look for the cause of this?

 

[Jcauli82]

My 98 5.7 has been giving me same problems more or less if your check engine light isn't on now it probably will be soon my light comes on intermittently I would start out with checking for a vacuum leak.... Hopefully for you that's the culprit. I don't have a vacuum leak according to my vacuum tester, so I have changed almost every sensor on my upper intake that could be a possible suspect for the p0300 code, and still get random misfires and my truck idles pretty high at start up for a few seconds then is normal again. I also have backfires when I start it occasionally. Also my dash command app says I have no fuel trim even after changing all my o2 sensors and my iac valve isnt giving me a reading and it's brand new, also no evap readings and no egr. I just figured I would share what I've been doing to try and fix mine, to see if that helps......oh yeah also my fuel pressure is a little over 50 psi with key on not running but I have upgraded to mpfi spider injector and heard that was normal.

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[WalkingBlackBear]

I went through something similar sounding a couple of months ago. Engine misfiring like crazy, almost wouldn't run, wanting to stall, etc. But zero codes, zero CEL. Turned out the distributor cap and rotor were borked. When electronics don't appear to be to blame (IE: no codes/lights), look to the basic mechanical points of failure. Plugs/wires/cap and rotor. (FWIW, the engineer who decided the distributor should be placed almost UNDER the dang firewall should be publicly flogged. What a PITA to get to.)

 

[letitsnow]

When my oem plug wires wouldn't stay on the spark plugs anymore - one would fall off and the motor would be running on 7 cylinders (shaking, totally obvious) it still wouldn't throw any codes.

The truck I had before this was a dodge cummins, which would light that damn cel light up every time a bird flew by the truck, or you ran over an ant, or the wind changed directions.... LIMP MODE! Which when pulling a camper in a busy area means that you are now a slowly moving road block.

I am thankful that my old Chevy just runs a bit rougher until I can pull over and fix it.

 

[semperfi]

Check out this video showing the common problem with these GM distributor caps. You are better off buy a new GM replacement rather than an offshore part.
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Doing this from taptalk so not sure if link will display.

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[Schurkey]

I don't have a vacuum leak according to my vacuum tester,

What sort of "vacuum tester" can confirm you don't have a vacuum leak?

That's certainly NOT something a normal vacuum gauge can do.

so I have changed almost every sensor on my upper intake that could be a possible suspect

You'd have been better off to connect a scan tool to see if those sensors were working properly.

for the p0300 code, and still get random misfires

Are those REALLY "random" misfires, or is your scan tool lying to you? Cheap scan tools report "generic" codes instead of specific codes. Are you SURE you don't have cylinder-specific misfires?

my dash command app says I have no fuel trim

Of course you have "fuel trim". You should have a short- and a long-term fuel trim.

my iac valve isnt giving me a reading and it's brand new, also no evap readings and no egr.

I suspect you have a defective scan tool.

 

[Jcauli82]

It's just a bafx obd2 Bluetooth adapter that I have used dash command and torque pro with .....I hope your right because I agree the readings are all screwy but I assumed the obd2 plug in isn't messed up .....does that happen very often ? It's one of the things I have overlooked definitely so far.....

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[Jcauli82]

I get where your coming from with your response and yeah I'm a pretty amateur shade tree mechanic no lie but i have looked at the real time data with dash command and I have seen all of my cylinders at various times misfire .....I get what your saying about the generic p0300 code but I have done my homework for the most part and confirmed my truck is acting crazy lately .....if you got any more insight into this dilemma please let me have it and Im sure I can dig up some screenshots of the real time data showing I have 0 fuel trim all around. Of course I have fuel trim I know this, man, it's just not showing anything on my data feed.....

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[Jcauli82]

What sort of "vacuum tester" can confirm you don't have a vacuum leak?

That's certainly NOT something a normal vacuum gauge can do.




You'd have been better off to connect a scan tool to see if those sensors were working properly.


Are those REALLY "random" misfires, or is your scan tool lying to you? Cheap scan tools report "generic" codes instead of specific codes. Are you SURE you don't have cylinder-specific misfires?


Of course you have "fuel trim". You should have a short- and a long-term fuel trim.


I suspect you have a defective scan tool.

Ok its just a cheap abn vacuum tester from Amazon and when I connected it to the vacuum line that goes to the brake booster the little arrow went to the green "normal" part of the gauge I get it it's not 100% accurate but it can't hurt to have some reference in the whole troubleshooting process .....and yeah I checked all those sensors with my scan tool but you said it might be a piece of junk yourself so I was just replacing what I did replace just to see if it would help and now here I am lol it's a big mess since this is my daily driver and I still haven't pinpointed the culprit yet....

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