1996 GMC

I have a 1996 GMC pickup, 5.7 liter engine with throttle body.Service engine light is on. Code is PO300 random miss fire. Have driven truck with scanner hooked up and all it says is miss fire. Have good fuel pressure. Have replaced plugs, wires, coil, cap, and fuel filter. Have started getting gas for it at a different station and have put sea foam in the gas. Any that I may be missing and need to do?
I use the truck to haul my tractor to shows.

check the throttle body for a build of carbon.

I have went through the codes for that year and engine GMC and they show no po300 code but do show a p1300 code which is ignitor ciruit. They say to check wiring for a short. May have wire that rubbed bare or something chewed into wiring.It also said you could have a bad connection.

Forget the snake oils.They are swindles.

Amen to that!

Snake oils and gasket sealers. :>)

Allan

Check for dripping injectors, take the air cleaner off, start the truck, look at the fuel spraying from the two injectors down the throttle bores. It should be a nice even fan pattern, any big drips or droplets means it's time for a rebuilt throttle body.

Does the engine run rough?at idle or driving down the road? Those engines are normally pretty straightforward and trouble free. basicly just an electronic carburetor. A coworker has one in an 89 1500 with 325,000 miles and never been apart.

Kevin

Different engine. ALL '96 trucks got a Vortec engine. Slightly more complicated and less reliable, usually due to the fuel pump.

Check for any vacuum leaks. Could also be EGR valve. Disconnect the EGR valve and take it for a drive. Does it do it at any RPM? Or at a certain temperature. What does fuel pressure check out at? How long does it hold after the truck is shut off?
Get it good and hot, take off the Vortec cover, and slowly pour the bottle of sea foam in, raising the rpm's by turning the butterfly valve. It is all but impossible to truly hydrolock a 5.7L engine doing this. The combustion chamber on a 5.7 is about 2.1 oz.(64cc), meaning that at 2400rpm, you'd have to pour in the entire bottle in, in under a second to hydro-lock a single cylinder. Let alone the fact that it would disperse somewhat evenly among the cylinders. Still use it at your own risk and be smart about it.

Does it seem short on power lately. What kind of mileage is it getting? These engines are infamous for their lower intake gaskets leaking. Is there any water in the oil? Antifreeze on the ground? Antifreeze level dropping? There's a chance it's being burned in the cylinder and causing a misfire. It's also possible that the spider injectors are bad and causing a misfire. Injectors also commonly go bad on these engines.

If it is the injectors, clean the intake manifold while it's apart. Also check the distributor for play. If you pull it, you have to set the cam retard with a scanner.

(quoted from post at 16:38:44 05/31/13) Different engine. ALL '96 trucks got a Vortec engine. Slightly more complicated and less reliable, usually due to the fuel pump.

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e said it was a throttle body engine, if it is a vortech my money would be on a leaking fuel pressure regulator. It is located under the upper intake and a very commonly failed part.

Kevin

Doesn't that engine have a central fuel injector with the eight poppet valves? If it does they had trouble with the poppets sticking, a pressure regulator leaking and I think you need 60 lbs of fuel pressure to make it run right. Could be bad gas, maybe a spec of carbon holding the egr open a hair. If it had a bad miss on one cyl. I'd think your scanner would show like P0301 to P0308 for the cyl.

I know it says throttle body, kind of had to think for a minute if that was possible. The trucks never came with TBI in '96. They were all switched over to CSFI that year. The new injectors are MPFI, simliar to the GMT800 V-8's, except a different style injector. I believe the part # for the updated part is FJ10566 Delphi.

I havn't heard you say that it actually misfires or not. You said the light was on and you checked for codes and it said misfire. Does it actually misfire or is the code just telling you that?

(quoted from post at 18:43:07 05/31/13) I know it says throttle body, kind of had to think for a minute if that was possible. The trucks never came with TBI in '96. They were all switched over to CSFI that year. The new injectors are MPFI, simliar to the GMT800 V-8's, except a different style injector. I believe the part # for the updated part is FJ10566 Delphi.

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I didn't really think they had the tbi motor after obdII started either but I wasn't sure. The injectors are problematic but usually it's only one cylinder not a random/multiple.

Has the truck had an engine work recently? Harmonic balancer or crank sensor been replaced? Engine replacement? ...... It may need a crankshaft variation learn performed. It is a procedure that is done with the tech2 scan tool and it basically tells the computer what the normal crankshaft speed variation is so it can differentiate between normal cranckshaft acceleration/deceleration and a misfire.

Kevin

Engine does have a throttle body and I've checked #'s and it's correct engine.
It does have a miss and happens any speed but mostly about speed limit(55)
Truck get 15MPG and has 373 differential
Fuel pressure, if I remember correctly is around 60 LBS. Intake manifold gaskets were changed when plug,wires,etc. were changed. Can't stay to long as we have a tornado moving into the area.
Hopefully i be able to check back tomorrow.

The Sea Foam contains oil, alcohol, and naptha. The oil should lube up the moving parts I'd think.

Mark