5.7 vortec missfire

[darrellward1]

i have a 99 suburban with a 5.7 vortec motor. i get a missfire after strong acceleration,usually right after the tranny shifts into overdrive. if i keep my foot on the pedal the truck shakes violently,but if i let off for a second or two it will smooth out. i have had the truck to the dealer to be checked and they told me i had a bad dist cap i replaced it and it still does the same thing.the plugs are new and so are the wires. the check engine light wont always come on but when it does i get a p0 300 or a p0305. the plugs all look good,(no fouling or burning)and the truck runs fine otherwise.i'm stumped on this one.

[DFRI]

I'd say, either a fuel pump or fuel filter. My 98 would shake if I kept my foot deep into the petal for a while. How many miles on it?? Start with the filter.

 

 

 

i have a 99 suburban with a 5.7 vortec motor. i get a missfire after strong acceleration,usually right after the tranny shifts into overdrive. if i keep my foot on the pedal the truck shakes violently,but if i let off for a second or two it will smooth out. i have had the truck to the dealer to be checked and they told me i had a bad dist cap i replaced it and it still does the same thing.the plugs are new and so are the wires. the check engine light wont always come on but when it does i get a p0 300 or a p0305. the plugs all look good,(no fouling or burning)and the truck runs fine otherwise.i'm stumped on this one.

 

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[darrellward1]

I'd say, either a fuel pump or fuel filter.  My 98 would shake if I kept my foot deep into the petal for a while. How many miles on it??  Start with the filter.

 

 

 

i have a 99 suburban with a 5.7 vortec motor. i get a missfire after strong acceleration,usually right after the tranny shifts into overdrive. if i keep my foot on the pedal the truck shakes violently,but if i let off for a second or two it will smooth out. i have had the truck to the dealer to be checked and they told me i had a bad dist cap i replaced it and it still does the same thing.the plugs are new and so are the wires. the check engine light wont always come on but when it does i get a p0 300 or a p0305. the plugs all look good,(no fouling or burning)and the truck runs fine otherwise.i'm stumped on this one.

 

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fuel pump and filter new 6 mo. ago

[darrellward1]

I'd say, either a fuel pump or fuel filter.  My 98 would shake if I kept my foot deep into the petal for a while. How many miles on it??  Start with the filter.

 

 

 

i have a 99 suburban with a 5.7 vortec motor. i get a missfire after strong acceleration,usually right after the tranny shifts into overdrive. if i keep my foot on the pedal the truck shakes violently,but if i let off for a second or two it will smooth out. i have had the truck to the dealer to be checked and they told me i had a bad dist cap i replaced it and it still does the same thing.the plugs are new and so are the wires. the check engine light wont always come on but when it does i get a p0 300 or a p0305. the plugs all look good,(no fouling or burning)and the truck runs fine otherwise.i'm stumped on this one.

 

<{POST_SNAPBACK}>

 

 

 

<{POST_SNAPBACK}>

 

 

 

fuel pump and filter new 6 mo. ago

 

<{POST_SNAPBACK}>

 

 

 

125000 miles

told by gm bad distributor, replaced cap and wires still the same. plugs new too.

[Mr. Smith]

I'd say, either a fuel pump or fuel filter.  My 98 would shake if I kept my foot deep into the petal for a while. How many miles on it??  Start with the filter.

 

 

 

i have a 99 suburban with a 5.7 vortec motor. i get a missfire after strong acceleration,usually right after the tranny shifts into overdrive. if i keep my foot on the pedal the truck shakes violently,but if i let off for a second or two it will smooth out. i have had the truck to the dealer to be checked and they told me i had a bad dist cap i replaced it and it still does the same thing.the plugs are new and so are the wires. the check engine light wont always come on but when it does i get a p0 300 or a p0305. the plugs all look good,(no fouling or burning)and the truck runs fine otherwise.i'm stumped on this one.

 

<{POST_SNAPBACK}>

 

 

 

<{POST_SNAPBACK}>

 

 

 

fuel pump and filter new 6 mo. ago

 

<{POST_SNAPBACK}>

 

 

 

125000 miles

told by gm bad distributor, replaced cap and wires still the same. plugs new too.

 

<{POST_SNAPBACK}>

 

 

 

 

May have a partially plugged injector. IF the ignition is good start looking at fuel delivery.

[DirtyRat]

Probably not what you'd like to hear, but Vortec engines, paticularly at your milage are known for valve problems. The valves hang in the guides, causing a misfire under the harshest conditions. Unfortunately the best fix for this would be to remove the head to be sent to a machine shop for a valve job.

 

You may get by with just installing stonger valve springs which would elimate the need to remove the head(s). But that's only an untested theory, try that at your own risk.

[omegafiler]

Both myself and another friend with Tahoe's (5.7l) had very similar issues. In my case it was due to a fouled plug AFTER using the injector cleaner (it was rather dirty); but for my friend, just running the amsoil equipvalent of seafoam motor treatment fixed everything. This was of course after trying everything else, replacing plugs, wires, cap/rotor, ignition module, coil pack.... etc.

 

You'll just pour/spray this stuff directly into the throttle body. Cleans everything out, including your injectors. You'll just need to change your oil again, and in some cases (if it's dirty enough) your plugs again as well. Because as I found out the hard way, it CAN foul out the plugs. Just means your engine was quite dirty.

 

http://www.seafoamsales.com/motorTuneUpTechGas.htm

[1999 Tahoe 4x4]

Mr. Smith is right. Check for Poppet Nozzle restriction. There is a Technical Bulletin by AC-Delco for Fuel Injector cleaning: TSB 05D-105. I have it in PDF format on my computer so I can't post it. I could e-mail it. It basically states how to correctly clean the fuel injector system in your particular Fuel Injection System. As far as Fuel Injector cleaners Techron is said to be the only one that is really effective. There are different versions of it. You can go to GM Dealer and by the original GM Fuel Injector Cleaner which is essentially Techron. Or you can by Techron at the auto parts store or Chevron Gas Station. There are 3 different kind of Techron out there if you buy it outside of GM. Make sure you get the FUEL SYSTEM CLEANER as this one seems to be the more aggressive one. The AC-Delco TSB describes how to inject the cleaner detergent into the fuel rail. Well I did not have that injection device. I just drove the tank almost empty and poured in a bottle of cleaner. Added 2 gallons of gas afterwards just to make sure the fill in tube/hose does not have any high concentration detergent on it. The problem with this method it that it will solve all the debris in the tank as well. That can be quite a problem. I was not too worried about that in my case because I was almost only buying gas at Chevron, so I assumed (prayed) that my tank is clean and no debris will be dislodged.

 

Another thing to check is if the distributor is positioned properly. Normally the scan tool displays the distributor base timing as degrees. Procedure is to check it with the scan tool at 1200-1500rpm. By it is entirely possible that as you brought the truck to GM they have only checked for codes on your truck. There are NO direct OBD codes that tell the mechanic the distributor base timing is off. He has to check that himself. On the Snap-On MT2500 Diagnostic Scanner this is e.g. on a different page/menu than the codes. So I would check distributor base timing again as well.

[ferggmc98]

had similar problem only my idle was also very rough. did a full tuneup with know change. had a code po304. turned out to be bad fuel injector for cyl. 4.