gs300 random misfire

[rambeau]

I have a 1998 gs 300, it sputters and pops randomly while driving, the code that comes up is random misfire, strange thing is it only happens when i'm under half a tank, I don't know if i should change plug and wires, or coils, injectors, injector connectors, or fuel pump, or ?, can anyone help?

[sha4000]

Is your check engine light on? If so go to autozone and they'll read the codes for free. 

[rambeau]

Thank you, the check engine light is on, when they hooked it up to the computer all it said was random misfire, it didn't identify any specific cylinder or any details, it seems to be getting worse, for the first time today it sputtered and popped with a full tank of gas, prior to this it seemed to only do it when below a half tank or so, probably coincidence.

[Jlove1378]

I have a 2000 gs300. It misfired then i had lights come on which once codes were read it turned out to be a mass air flow sensor which im getting replaced soon.  It has almost 200 thousand miles and i used chevron fuel injector cleaner before my next fill up and that fixed the misfiring for me. Maybe its a sensor issue? 

[rambeau]

I'll try the chevron injector cleaner, i did use the cheap wall mart cleaner but there was no change, I hear the wall mart stuff never works well.

thank you

[Jlove1378]

I was pleasantly surprised.  I ordered it from amazon for a good price too. I read 2 treatments is best for high mileage. Goodluck. 

[rambeau]

I'll go online and order that now, I'll post the results. thank you

[rambeau]

Alright, i gave up and took it in, the shop wanted $347.00 for plugs and wires, I bought the factory set myself for $87.00, paid my mechanic $45.00, borrowed the computer to reset everything, and $132 later it runs great ! I will still run the chevron injector cleaner through, I'm sure it could use it. thanks for all !

[Jlove1378]

What did the shop diagnosis the problem as? I just replaced my mass air flow sensor and that helped. Im having misfiring only under 1000 rpms. I just got throttle body cleaner and seafoam, we will see if that helps. Otherwise ill have to get more diagnostics done. 

[rambeau]

The valve cover gasket, the intake gasket, and o rings for the spark plug tubes all need replacing, when we pulled out the spark plugs they had actually been sitting in pools of oil from the leaks, this aside from the plugs being old, so the diagnosis was old plugs, wires, boots, and seals, were all factors, in another week or two i'll be replacing the seals, it all comes together in a valve cover kit for around $150, the mechanic said it should be done within the next two months to avoid the problem recurring, I had a "rebuilt motor" put in last year, turns out they did a motor swap but never rebuilt it so everything I assumed was new was not.