missfire code po300

[woody69]

shows random missfires when cold. '02 Taho 5.3 any help would be appreciated.



Thanks Woody

[joecar]

Check if the actual AFR is too lean.
Check spark plugs and wires.

[woody69]

already changed plugs and wires still same . this is a stock taho no hotrod.

[joecar]

Also, check for air leaks, and check your LTFT's.

Also, might try cleaning the MAF (use MAF cleaner; do not use brake clean), altho I don't think this is causing it.

[smslyguy]

Also, check for air leaks, and check your LTFT's.

Also, might try cleaning the MAF (use MAF cleaner; do not use brake clean), altho I don't think this is causing it.

I have the same problum on my 2000 gmc 5.3 I have changed spark plug wires, plugs, cleaned the m.a.f with m.a.f. sensor cleaner ( I did that twice) and still no luck??? Here is a copy of my tune. see what you think?? No vacum leaks either.

[joecar]

The scantool was not able to view the contents of your file, I'm using build 101.

Can you zoom in your view to an area of interest and take a screenshot...?

[joecar]

Check for insufficient spark (check with spark tester, see attached).

Check for insufficient fuel (check fuel pressure, replace filter).

Check for too much air (airleaks at manifold or after MAF).

Check NBO2 sensors.

Look at the midfire counters for each cylinder... which ones are counting up...?

Does MIL flash...?

Perform a CASE relearn and see if you still get P0300.

[smslyguy]

Check for insufficient spark (check with spark tester, see attached).

Check for insufficient fuel (check fuel pressure, replace filter).replace the fuel filter.

Check for too much air (airleaks at manifold or after MAF).no leaks at the manifold or after the m.a.f.

Check NBO2 sensors.nbo2 are switching fine

Look at the midfire counters for each cylinder... which ones are counting up...?number 5 and 6 are always misfiring at idle and then random ones as you rev it up.

Does MIL flash...?it did until i changed the spark plug wires

Perform a CASE relearn and see if you still get P0300.

did that with no results

I have heard of a po101 and a po300 ,which i have had both, being caused by clogged cats. I beleive i have two high flow cats on the 2000 g.m.c. which i would think would be kinda hard to clog, but no sure. My L.T.F.T are really high as well and have been since i have owned this truck. The vehicle has 125,000 miles on it. Is there a test with efi live tunes where you can test out the cats? Number 6 cylinder seems to be having the most misfire's and then number 5.

[joecar]

If you suspect clogged cats:
- do you feel (with your hand) any exhaust pulses out the tail pipe...?
- do you feel more exhaust out the tail pipe when you rev the engine...?
- does engine seem low on power when you rev it...?
- does engine seem RPM limited when you rev it...?
- does exhaust appear to be coming out of the intake manifold...?
- does exhaust appear to be coming out of the crankcase/PCV...?
- if you remove the front NBO2's does the engine now rev easily...?

Clogged cats leads you to think that timing chain has jumped a few teeth.

You would get other DTC's, like NBO2 failure and cat failure.

If the LTFT's high, then the PCM is adding fuel because the NBO2's are indicating lean (possibly an airleak... or faulty NBO2's...).

If the LTFT's are pegging high, then the PCM can only add that much fuel, and the PCM would set some DTC's.

I suspect that your NBO2's are "worn out tired" and need replacement (they're only good for 60K-100K miles)... but you mentioned #6 and #5...

P0101 is MAF fault, look into this, either MAF or MAF wiring... the PCM would failover to the VE table which should run the engine in a reasonable manner.

P0300 tells you that the some cylinders are randomly misfiring... if the counters indicate that #6 and then #5 are seeing the most misfires, then the head gasket may be very likely suspect... although valves/springs may also be suspect.

How much different are the misfire counts for #6, #5 and the rest of the cylinders...?

May need to do a leakdown test. Also use a block leak kit, they check for combustion gases in coolant, they are about $40 from NAPA.

Small amounts of coolant in combustion chamber will cause misfires.
Remove spark plugs and see what's on the tips (what do your old ones show...?).

Are all the injectors firing correctly, are they all clean...?

[joecar]

With EFILive, start logging, then use the DVT control panel to shutoff each injector in turn...

When you look at the log of this, the cylinder(s) with the least amount of RPM drop are misfiring.

You can point an IR thermometer ($60 Sears) at each port of the exhaust manifolds and find the cylinders that are not getting as hot.