quartercenturytruckin

Hi All!

I have a 1999 Chevy Silverado 2500 with 196,000 miles on it. It is new to me and I've only driven it a few times now. When just scooting around town with no load I have no issues.

When pulling a 3000 lb camper with the bed full of camping stuff and the "tow/haul" button pressed on the truck has a flashing check engine light come on. It comes on and flashed for about 60 seconds when I am at 55-60 mph. It does not come on immediately when I get to that speed, but after ~10-15 min of driving that speed. After about 60 seconds the light goes off completely never to return on that particular drive. What I mean is this flashing check engine light only came on the one time each way on a 45 minute drive to and from camp. The only code I am getting is P0300-random/multiple cylinder misfire.

My plan for diagnosis is the following:1. Check air filter, clean or replace

2. Check for intake leaks around air box

3. Check MAF sensor. Live data should be 2-7 gram/second at idle. Check at 2000 rpm and 4000 rpm. Should get progressively higher

4. Check fuel trim with live data. Short term should be -5% - +5%. Long term should be -5% - +5%

5. Check fuel pressure, should be 55-65.

6. Check coil packs/plugs

7. Check o2 sensors

8. Check/clean cam position sensor

Does this seem like a decent plan? Am I missing anything that I should check?

Does anyone have any idea why I am getting this code only under load at this speed?

mountainmanjoe

That's a good start

not just the air box. Vacuum leaks can come from various places: the manifold, brake booster, EVAP system, HVAC, etc.
A smoke tester plugged into the intake is a reliable way to test all of that.

I think it should be a lot more than 2g/s. Post your engine please.


STFT can kind of go all over. Pay more attention to the LTFT



Pull the plugs out one by one and visually verify each one is sparking. If it does then coil and wires should be good too.

clean out your throttle body, throttle plate, IAC valve and channels.
Check exhaust back-pressure
Until then you should stop driving the truck. Raw fuel is being dumped down the exhaust and into your catalytic converters. That's why the manuals tells you to stop when the light is flashing at you.

quartercenturytruckin

Thank you for the advice!

A smoke tester is a great idea, my buddy told me I should just spray staring fluid around and listen for revs to rise.
The engine is the 6.0L V8. I'm not sure what the airflow should actually be on it.
10-4 on the fuel trims.
Do you mean pull the plugs out one by one and start the truck to test for spark? I'm not really clear on this point.
I can clean out the throttle body and plate, I just did that on a 2005 saturn. I'll figure out the IAC valve and channels

Finally, I have no intention of driving the truck until i resolve this problem. I bought it specifically to tow a camper and don't have any other trips planned in the near future.

Thank you so much!

Edit: Looks like I pull the fuse for the fuel pump and crank the engine with the spark plug out but connected and touching a grounded surface to check the spark plugs and wires/coils.

mountainmanjoe

you should see at least 6.0 g/s at idle.

quartercenturytruckin

Thank you. I'll go through this list with the advice you gave me this week and report back when I know more.

quartercenturytruckin

have an update from the code reader.
MAF 6.03-6.1 at idle, 17.4 at 2000 rpm, 29.9 at 4000 rpm
short term fuel trim is b1=0%, b2=-1.6%-0%
long term fuel trim is b1=-7%, b2=-8.6% (both of these went to 0% when I blasted the A/C
O2 b1s1 voltage is varying between. 15-.875
O2 b1s2 is .24-.755 but looked worse on the graph only going between .11-.2
o2 b2s1 is .135-.855
O2 b2s2 is .21-.545 graph confirmed this.

From that I think my MAF sensor is good and fuel trim looks OK. I could have two bad o2 sensors (o2 b1s2 and o2 b2s2)

What do you think?

I would've done more but I took most of my free time replacing the u btlts on the rear leaf springs. Will definitely do some more poking around tomorrow

mountainmanjoe

O2 and fuel trims show rich mixture (expected when there's misfire)

keep looking.

quartercenturytruckin

It hasn't misfired since Sunday and the code was cleared. Would you still expect a rich mixture in those conditions? I will go through and check the spark plugs, clean the throttle body and check for air leaks today.

chem_man

Can your code reader display misfire counts for each cylinder? If it can then look at the current and historical misfire counts for each cylinder. That will clue you in to which cylinder(s) is/are acting up.

Good Luck!

quartercenturytruckin

Unfortunately it does not. Thank you for the tip though.

I did go through and drive it to gather some more data. When I accelerate and the rpms get to ~2000 the MAF sensor shows 38-42 g/s and the long term fuel trim goes up to +25% and my short term rises to about 12-15%. My current theory is that the MAF sensor is faulty, showing artificially high airflow, and causing the engine to run richer than it really needs.

If this is the case, then towing a trailer at 2000-2500 rpms could foul the plugs enough to fail to ignite after about 20 minutes of driving.

If this theory is correct I should be able to clean or replace the MAF sensor and see a drop in readings when driving at 2000 rpms and a corresponding drop in short term fuel trim.
If the drop in MAF and fuel trim does appear as I think it will, I can probably go ahead and reset the long term fuel trims by disconnecting power and draining the capacitors.