multiple misfires (p0300)

getting multiple misfire code (p0300) on 98 GMC 1/2 ton V6 with 135000 miles. chgd the plugs, cap, rotor, wires are ok, Auto Zone tested the coil and it checked out ok - from what they can tell (they're not experts at this). getting spark on all cylinders. it runs like it's only firing 4 or 5 cylinders - really weak / loss of power. any ideas????? please help -this is only vehicle in family of 3 (baby). thank you.

put new air and fuel filters in. Check fuel pressure- key on, engine off. Might want to try some injector cleaner

You'll need to clean the injectors.

GM has a great procedure for injector and top engine cleaning, but you'll most likely need to go to your dealership for that.

I think you'll find that the injectors are your problem.

Hope this helps.

getting multiple misfire code (p0300) on 98 GMC 1/2 ton V6 with 135000 miles. chgd the plugs, cap, rotor, wires are ok, Auto Zone tested the coil and it checked out ok - from what they can tell (they're not experts at this). getting spark on all cylinders. it runs like it's only firing 4 or 5 cylinders - really weak / loss of power. any ideas????? please help -this is only vehicle in family of 3 (baby). thank you.

Read my reply on this thread
http://www.automotiveforums.com/vbulletin/showthread.php?t=230163

I had this problem on my 98 with 5.7 engine. I think same as yours just yours is shorter block. My problem was sticking valves. I did the whole ignition / fuel thing and still had the problem. I would 1st get your fuel pressure checked. Should be 55-60 PSI. I fixed my problem by changing the oil and using a quart of Marvel Mystery oil instead of 5 quarts of regular oil. Then I did a top engine clean using a product called SEAFOAM. You remove vacuum line from brake booster and slowly pour Seafoam into the hose. It gets sucked into intake. As the last of the product is poured in, shut off the engine. Let it sit 4-6 hours ( can says 5 min). Also put 2 cans of Seafoam into full tank of gas. Crank it up and be ready for lots of smoke!!!! Drive it for about 10-15 miles and various speeds. My condition greatly improved in about 100 mikes of driving. Best of luck

Thanks for all the info guys. I'll check the fuel pressure and try the seafoam procedure, but i have a feeling it's probably the injector poppets sticking and may end up replacing the spider assembly.

If you replace the injector assembly, switch over to the MFI system that GM has been replacing the CSFI units in Cali with, these should be the part numbers for the V6;12568332
88894355
17113215
Should be the bracket, upper intake manifold gasket, and spider assembly, includes new fuel pressure regulator
You can check prices at gmpartsdirect.com

I got the spider assembly off. Everything is so dirty in there I can't really tell if the poppets are sticking or not. About this MFI system that you are referring to Jeremy-WI, is it compatible with the existing wiring harness? And how many of the injectors (PN#12568332) will I need? Any pics will help me understand what I'm dealing with here. Thanks.

You need only 1 of part #12568332, its the injector assembly, includes 6 injectors and poppets and the old connector just plugs right in. If your intake doesn't have any wet spots in it, I wouldn't spend the money on the MFI. So how was the fuel pressure, 60-66psi key on engine off, did the pressure fall when the pump shut off?

Here are some pics on another site of a V8 engine getting MFI http://www.pacific-audio.com/performance/messages/231640.html

Thanks Jeremy-WI. I don't have a fuel pressure gauge that i can connect to the fuel line, but by eyeballing the force of the spillage when the fuel line (diagnostic) valve is depressed i'd say it has adequate fuel pressure while running and at key on position. Compared it to a another vehicle with the same system/engine, and it appears to be the same. Again, this is just a novice way of checking (eyeballing) the fuel pressure. I didn't think fuel pressure was the prob because the truck ran and responded on acceleration - as if it was running on 4 cylinders rather than 6 however.