1999 GMC Suburban, 5.7l random misfires

[milehighassassin]

Anyone have experience with this?

I've replaced the plugs and wires. Still get a random Misfire. Planning on changing the fuel filter.

What else? Distributor cap? Gear? fuel pump?

 

[Goinboardin]

I've never worked on a 5.7L in a suburban, but in general:
Plugs, cap, rotor, wires, fuel filter, air filter. Still missing? I would figure out which cylinder it is, and swap injectors with a known working cylinder. If the suspect cylinder still misses, check that the injector wire harness has power. If it does, check compression on that cylinder.

In my experience, generally a tune up (plugs, cap, rotor, wires, filters) clears up the misfires.

 

[milehighassassin]

It is not throwing a code for a specific cylinder. It is saying multiple cylinder misfire.

New air filter, new plugs and wires.

New radiator (old one had a small leak), coolant, oil change and filter.

 

[Coldfinger]

Crankshaft Position Sensor? Not sure if the 5.7 has a history of problems but I know my 2002 3.8 Grand Prix has one and they've been known to go bad.

There may also be another sensor or two, like throttle position, fuel pressure, not sure.

 

[AKSNOWRIDER]

distributor cap and rotor..they will arc inside the cap from cylinder to cylinder...replace both and should be good to go....

 

[leadfoot33]

like everyone has said do tune up plugs cap and the gear inside as i believe they dont have the rotor anymore. 02 sensors and clean your mass air flow sensor. does it miss all the time or just at idle if you bring it up to 3000 does it still miss? same when cold or warm?
i do know these things were famous for intake gaskets and i do believe that was causing some peoples mis fires.

 

[locknload]

As aksnowrider said cap and rotor, if you still have issues get the fuel injection cleaner from your local gm dealer, on that engine they use a poppet style injector (under the intake plenum there are injectors with plastic tubes going to the individual cylinders, at the end of the tubes are poppets) the only way to clean them is with the gm cleaner. You just pour it in the fuel tank and put some miles on it. Ive tried cleaning them with a BG injector cleaning tool and cleaner and it just doesnt get the job done. I can get you a part # for the cleaner if you need it.

 

[XXL]

I had a 3.8 giving a random misfire code last summer, only when you got on it to pass, over 3000 rpm. Long, long story short was the catylitic converter starting to plug. You can do a back pressure test by pulling the O2 sensor and installing a back pressure tester, probably rent one from larger Napa store.

 

[AKSNOWRIDER]

As aksnowrider said cap and rotor, if you still have issues get the fuel injection cleaner from your local gm dealer, on that engine they use a poppet style injector (under the intake plenum there are injectors with plastic tubes going to the individual cylinders, at the end of the tubes are poppets) the only way to clean them is with the gm cleaner. You just pour it in the fuel tank and put some miles on it. Ive tried cleaning them with a BG injector cleaning tool and cleaner and it just doesnt get the job done. I can get you a part # for the cleaner if you need it.

try seafoam instead...have never seen a better fuel system cleaner..and i have used them all over the years(including gm's ). you can read more about seafoam here...it flat out works....http://www.seafoamsales.com/motor-treatment.html

 

[milehighassassin]

like everyone has said do tune up plugs cap and the gear inside as i believe they dont have the rotor anymore. 02 sensors and clean your mass air flow sensor. does it miss all the time or just at idle if you bring it up to 3000 does it still miss? same when cold or warm?
i do know these things were famous for intake gaskets and i do believe that was causing some peoples mis fires.

MAF is clean, throttle body is clean. There are not any other codes. Just multiple cylinder misfire. No codes for specific codes, no O2 sensor codes.

 

[milehighassassin]

As aksnowrider said cap and rotor, if you still have issues get the fuel injection cleaner from your local gm dealer, on that engine they use a poppet style injector (under the intake plenum there are injectors with plastic tubes going to the individual cylinders, at the end of the tubes are poppets) the only way to clean them is with the gm cleaner. You just pour it in the fuel tank and put some miles on it. Ive tried cleaning them with a BG injector cleaning tool and cleaner and it just doesnt get the job done. I can get you a part # for the cleaner if you need it.

I would love the part number actually.

 

[milehighassassin]

I did seafoam directly into the intake. I guess I can run it in the tank.


The symptoms act up generally when off throttle. Going down a hill. Once the Service light starts flashing it happens more frequently. I can't hear any knock or ping.

 

[opel]

As said cap/rotor (common). When the SES light flashes it means active misfire/s. Find a scan tool with data stream and watch short term fuel trim and spray carb cleaner or propane around intake base. It should stay around zero if no leak and go to -26 if leak is present. Also watch history snap shot to seeperameters when it was mising and look for a pattern. The spider injectors can be updated if cleaning helps to multi type conversion and also check psi reg as it is under plenum and can cause rich condition. Check LTFT and see what it is reading for fuel trim should also be around zero. Check timing cam retard with scan tool engine must be above 1200 rpm when reading/adjusting should be 0 + or-1. Check fuel psi (50-60) usually and if you can find someone with a labscope watch ignition KV for spikes or unusuall patterns (high KV). Use vacuum gauge ( steady needle). Back pressure test as stated. Seafoam through the intake only goes where air goes has to be in the fuel to go through injectors. We have also seen valve guides cause multiple misfires on the vortec only on decel but can pick that up with vacuum gauge. Thats a good start!

 

[leadfoot33]

yup check fuel pressure, also try unplugging your MAF and just see if maybe its fubaard. otherwise i think you may be looking into going into it further like checking the injectors or intake gasket.

 

[milehighassassin]

Thanks for everyone's replies. Been busy so I haven't had a chance to work on this.

What is the easiest way to check fuel pressure? I don't have a gauge, can I get one with the correct fittings from a parts store (loaner)?

 

[leadfoot33]

that would be my guess, cant say i have ever done it.
speaking of fuel pumps my parents 02 gm 5.3l just gave up teh ghost except it would only fire for a bit and barely idle couldnt give it any gas or anything.

just a thought as well has the fuel filter been changed?

 

[fasta76]

Pull the distributor and check the drive gear. Its a common problem on the 5.7 vortec motors. it wears so much that it throws out the timing.

 

[oldsledsrule]

Yup, cap and rotor.

 

[milehighassassin]

Have yet to do much but I have an MSD distributor and coil ready to go in. Figured at 160k or so miles if the gear is shot on it, might just be ahead to replace the entire thing. After I put this in it will have an entire MSD ignition, wires, distributor and coil. New plugs.

I need to get my hands on and OBDII scanner capable of reading live data so I can phase the distributor with the cam properly. Thinking of the Actron CP 9575. Anyone have experience with this? Any other suggestions?

I can get it at Amazon for $115 no tax and shipped two day for free.


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[Texasron]

If it loads up at a red light and then clears out, and if it seems it has more horsepower and runs great! Then it will be the FUEL PRESSURE REGULATOR, located inside of intake manifold. Remove throttle body, intake cover too, fuel lines,12 or so bolts, buy gasket kit. May as well replace intake gaskets too, if they have not been changed and get it over. I replaced my intake gaskets 3 times in 245,000 miles. Check into BEST NEWEST gaskets so you wont use cheap ones and have to do it next year.