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Old 08-20-2007, 08:29 AM   #1
griesel72
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Help 97 truck popping through the exhaust

I have a 97 GMC 4X4 with the 350 motor, and it has started popping through the exhaust when not under a load. It happens more when the cruise control tries to slow it down (ie when it goes down a hill) but doesn't happen as bad when i have my foot in it.

I have tied to clean the injectors, i checked all of the plastic tubes that run to the injectors, i have checked the timing, all of the spark plugs and wires. I have sprayed ether around the motor the check for any kind of leaks.

I am stumped about what to check next, could it be a distributor issue? It does't seem to miss at all and has all kinds of power, just this popping.

thanks for any advise.

David
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Old 08-20-2007, 01:54 PM   #2
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Re: Help 97 truck popping through the exhaust

Gotta be a sensor that triggers something related to the ignition process. Back in the day, popping through the exhaust was usually because the condensor of a points ignition system was going bad. With the ignition system we have, I don't think that there is anything in the ignition process that would relate to that being a problem.

Has the Check Engine light come on? I am assuming it hasn't, since you didn't mention it. How long has it been since the plugs were changed? Could there be a build-up on the electrodes that is staying hot enough to ignite a stray mist of fuel? Without a code having been set, it's gonna be hit or miss and parts changing. I'd seriously consider the plugs, unless they are new.

Nothing says it couldn't be a fuel injection issue, but (again) I think a code would have to be set to know for sure.

Good luck.
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Old 08-20-2007, 03:52 PM   #3
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Re: Help 97 truck popping through the exhaust

the check engine light has come on, but it just throws the random misfire code and nothing else

thanks
David
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Old 08-20-2007, 06:01 PM   #4
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Re: Help 97 truck popping through the exhaust

Make sure the sensor by the crank and harmonic damper are clean, Spark plugs fail under load when they fail (usually) coils will fail intermidently. popping when not on the throttle going down hill or just under light throttle is usually caused by valve float (weak valve springs). another possibility is a small exhaust leak, when the scavenging effect takes place in the exhaust it creates vacuum which can suck in fresh air if that lights in the mufflers it pops (unburnt fuel plus heat plus fresh air in closed compartment equals boom or just repetative popping) My toyota used to do this all the time sometimes if it was cold enough when I let off the throttle I could get the shotgun effect
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Old 08-21-2007, 11:36 AM   #5
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Re: Help 97 truck popping through the exhaust

Crank Sensor? Gotta be something common to the ignition process for all cylinders.

My daughter had an issue with her Neon, in which it would (according to her) buck and jump. No codes were set, and when I checked it, it was running okay. After a month or so of this, it died and set a code. One plug was misfiring. After a new set of plugs, no more problems. Don't think this is your issue, but there are similarities.

Still, I'm thinking the Crank Position Sensor is either good, or it's bad. Have you changed the wires and plugs, or just checked them? How did you check them? We used to hook up to a Sun Diagnostic machine and know if we had bad plug or wire by the display on the CRT. More resistance created a higher spike. It was easy to look at the displayed firing sequence and find the culprit. Nowadays, we have OBDII but no CRT.

What is the code? I have an OBDII reader and access to their expanded codes, which may help. No guarantees, though.
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Old 08-22-2007, 12:54 AM   #6
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Re: Help 97 truck popping through the exhaust

Had similiar problem not long ago....it was the dist cap! Engine had over 100k and had never been changed. But mines thur a code right away staying no2 misfire. Checked plugs/wires and everything was fine. Cap was pretty well shot. New cap installed and everything when back to normal. Just a suggestion to look into...
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Old 08-22-2007, 03:00 AM   #7
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Re: Help 97 truck popping through the exhaust

Quote:
Back in the day, popping through the exhaust was usually because the condensor of a points ignition system was going bad.
Back in the day when valve deposits built up it would cause popping also.

I would go for a tune up. New plugs, wires, cap, then see what happens. Chances are a plug is causing it or a wire may be bad allowing it to intermittently short on one cylinder.
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Old 08-22-2007, 06:45 PM   #8
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Re: Help 97 truck popping through the exhaust

i am going with exhaust leak
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Old 08-23-2007, 05:22 PM   #9
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Re: Help 97 truck popping through the exhaust

If the crank sensor is dirty it will cause an intermitent misfire since the magnetic pickup wont always read correctly same problem with abs sensors on these trucks ring gets dirty mag sensor reads incorrrectly causing ABS problems when hitting brakes hard. The distributors in these trucks really arent distributors anymore They dont have a pick up coil or condensor in them it gets that from the crank sensor (or reluctor ring)but a cap can still cause a problem excpecially if it is cracked, Tune the thing up if its over a 100 grand see what happens clean the crank sensor, if it still happens look into exhaust and then finally valve springs
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Old 08-24-2007, 12:45 PM   #10
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Re: Help 97 truck popping through the exhaust

update, found out that the distributor had jumped time somehow??
i moved it one tooth, and it runs great now, thanks for all the help
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Old 08-24-2007, 05:31 PM   #11
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Re: Help 97 truck popping through the exhaust

Jumped time, somehow? Slack in the timing chain, maybe?? This may not be over, but glad it's doing better for the moment.
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Old 08-25-2007, 01:14 PM   #12
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Re: Help 97 truck popping through the exhaust

That doesn't sound good expecially since the pcm controls timing, hope it works out, have you pulled the distrubutor since youv'd owned it, and when you pulled it out to fix it was there still a pin in the gear at the end of the shaft.
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http://67-72chevytrucks.com/vboard/s...d.php?t=249435
1997 Bagged XtdCab Vortec350/4L60e
http://67-72chevytrucks.com/vboard/s...44#post2286144

1993 XtdCab Longbed 4x4 6.5Turbo/NV4500 "Daily Driver"

1985 Toyota XtdCab 4x4 22re/W-56/Dual T-cases, Doug Thorley Header, Homebrew front and rear spring packs, Tons of Mods to suspension and drivetrain, 35" boggers, coming soon ARB's and 5.29's, Possible 4.3/700r4 swap

Last edited by Scott68-93-97; 08-25-2007 at 01:14 PM.
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Old 08-25-2007, 08:12 PM   #13
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Re: Help 97 truck popping through the exhaust

I bought this truck brand new in 1996, but traded it in 2000, i just bought it back in May, and it never really ran right, i talked to the guy that owned it, and he had the motor rebuilt in 2001, when the truck had 100,000 miles on it, and (to his knowledge) hasn't had the dist out since. I checked the distributor when it was out, and hit seemed tight and in good shape, do you think that he drove around with it out one tooth for 75K miles and over 6 years????? or do you think that the timing chain jumped a tooth?

thanks
David
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Old 08-27-2007, 12:56 PM   #14
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Re: Help 97 truck popping through the exhaust

If the timing jumped a tooth you would notice and its not common, when they do they usually retard which would kill your low end power. I dont think the vortecs used a nylon gear tooth, and even those dont usually jump he may have drove it around like that would be my guess and whoever did the motor probably was the one who screwed it up
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http://67-72chevytrucks.com/vboard/s...d.php?t=249435
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http://67-72chevytrucks.com/vboard/s...44#post2286144

1993 XtdCab Longbed 4x4 6.5Turbo/NV4500 "Daily Driver"

1985 Toyota XtdCab 4x4 22re/W-56/Dual T-cases, Doug Thorley Header, Homebrew front and rear spring packs, Tons of Mods to suspension and drivetrain, 35" boggers, coming soon ARB's and 5.29's, Possible 4.3/700r4 swap
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