My CEL has been going on and off for a while now and I can't seem to track down the problem. The codes are P0171, P0174, and P0300.

I got a Bluetooth OBDII plugin and the Torque App for my phone today so now I have some scanner data to work with.

The previous owner put a new Fuel Pump and Fuel Pressure Regulator on the truck. I put a new Fuel Filter on it and checked the fuel pressure. 60 psi key turned, engine off / 51 psi engine running / jumps back to 60 when i hit the throttle. These seem to be within the ranges in the Chilton manual.

I have cleaned the MAF Sensor. It reads 4.47 g/s at idle (630 RPM) and about 15 g/s at 2,000 RPM. I also cleaned the Throttle Body and EGR valve. The TPS sensor has 5.4 reference volts and the output ramped up from 0.5 to 4.5 volts as the throttle was opened. I smacked the throttle body and the computer around while taking these readings and there were no breaks in the signal.

I checked for vacuum leaks with both the stethescope method and spraying starter fluid around common leak areas, but found none. With a vacuum gauge, I checked the intake manifold pressure. It read -17 inches Hg at idle and jumped to -20.5 when I opened the throttle. All readings held constant (no sticky valves, etc.)

The following are some O2 Sensor reading screenshots from a few runs down the highway (VALUES ARE FROM 3 SEPERATE RUNS - Going about 55-60 mph at around 1700-1800 constant RPM):

Bank 1 Sensor 1


Bank 2 Sensor 1


Bank 1 Sensor 2 (After the "Y", Pre-Cat)
This sensor was replaced when I first bought the truck because its heater was bad and throwing a code. It seems to show a rich condition while the other ones don't. Does this one have anything to do with calculating the fuel injected, or is it just used with Bank 1 Sensor 3 to monitor the catalytic converter?



The long term fuel trim ranged from +15 to +25 on both banks. The short term fuel trim ranged from 0 to +3.12 on both banks while on the throttle, but went down to -5 volts just after letting off the throttle when idling to a stop.

Changing out both sensor 1's is about the last thing I can think to do, but I'd like to get a fresh set of eyes on this before I throw $100 dollars at the problem. Any ideas or advice is welcome. Thanks for reading.

 

Get the battery tested and double check for vacuum leaks before you spend the money.

 

It has a new battery. The previous owner put it on along with new distributor, plugs, and wires (not sure if AC Delco was used). I guess I could give it a test though. I will definitely test for vacuum leaks again. Does anything work better than starter fluid? Maybe brake cleaner?
Also, I was thinking the EGR might not be opening. But I would think that would throw a code. Is there any way to test this?

 

Here's my problem with your Images. If I was just looking at them, I would say the sensor 1's were showing a rich condition. It would be interesting if you took voltages and put them on an excel sheet and plotted running average, starting at .45
Another thing.. The swings. I've only read mine out when I assumed engine was running right but they plotted between .25 and .6. Maybe mine are old and creaky. According to a couple articles I surfed, they are. Yet I found another that stated between .2 and .7
But it's the crosscounts {crossing .45 per second that matter}

- Now my mind is a little numbed.. because of taking it all the exceptions but it has to be considered, that uncalled for EGR causes false rich and ignition misfire reads as a false lean.

So maybe .. back to basics:
What are your idle and driveability symptoms?

Get the little things out of the way first, just so you dont have to think about them..
When engine warmed, disconnect MAF and drive-test. Remove EGR and check for exhaust one port, vacuum the other. Clean it, yeah, again.. while you have it off. Just because the PCM sees the valve working doesnt mean you have EGR.

Disconnect and plug all vacuum takeoffs from intake, including pcv. If no improvement, disconnect fan belt and IAC connector and go around intake with unlit propane torch. When you hit leak, rpm should change and engine run smooth.
If you use non-flammable spray, monitor MAP, instead. When you hit leak MAP will drop.

Dont replace any O2 while the other symptoms persist. Esp the one GM found was 'dispensible'

 

Pettyfog,

if the EGR has a machanical failure ( stuck?) It would run rich but not setting codes. The ECM will lean down the mixure and you will have a segnificant power loss.

It that correct?

BR
Bob

 

Whatever part of the intake component is exhaust gas contains much less oxygen than air from the throttle. Thus looks 'rich'. {The sensor just looks for oxygen}.
There's a circular effect invloved but simplified.. yep.

If it happened to me, I'd block the egr ports, drive-scan it again and see how it looks. Or I might just clean the crap out of the egr pintle.

 

No power suddenly white smoke from exhaust power is back Dear prettyfog thats happend on my Blazer S10 4.3V6.

I was at a traffic light. I was complaining no power on Autobahn suddenly while I am acceleration, feel like a big relieve, white smoke from exhaust power is back.

I have a MAF failure code stored. But the white smoke out of my exhaust indicates
some oil ran into my manifold.
I was looking on my EGR valve. I cleaned it and it was not poluted at all.

Any ID what it could be. I have an MDI at work ansd can do snapshots.
The failure is not present, so I cannot record the failure mode.

BR
Bob

 

White smoke usually isnt oil.
Dont know what it is or how it relates to sudden performance improvements, but I would be watching coolant system very close.

And you should post a new problem in a new thread

 

Thanks for the reply pettyfog.

As far as drivability goes:
*It fires right up, almost always, with barely a turn of the key. A few times after driving it hard and shutting it right off (like driving up to the local ski hill), I have had to crank it a bit - like it’s flooded – to get it going again. This, along with the O2 data I posted earlier, makes me think it is running rich rather than lean like the trouble codes say.
*It idles ok. Not silky smooth but I wouldn’t consider it rough. If it is cold out, it will start up around 1,000 rpm and drop down to 630 within a minute or so. If it is warm outside, it will start right at 630 RPM. Sometimes when coming up to a stoplight, it will stick at 900 rpm until I come to a full stop and then drop to normal idle.
*It accelerates fine except for one condition. Going from first to second gear (from about 1800 to 2100 RPM) it will buck and stumble sometimes. I’m not sure if this is a transmission issue or a motor issue, but it only does it when it’s cold. I changed the water pump, thermostat, and flushed the coolant about a month ago and these symptoms are much better now (It used to only get to 165 degrees, now it gets all the way to 195).
*Cruising at constant throttle is where I get the most problems. It is most noticeable driving down the freeway. It wants to misfire and buck when cruising at constant throttle. What I end up doing is accelerating up to 70mph then coasting back to 60 or so, and then accelerate back to 70……and so on. This must be extremely annoying for anyone driving behind me . These symptoms are much worse when it is cold outside and the truck has been sitting for a long time. They are also improved since overhauling the coolant system. The P0300 Misfire code has also not come back since then, but I can feel it misfire from time to time.
*I have found that unhooking the battery to reset the PCM also helps it run more smoothly (this resets my Long Term Fuel Trims to 0). The throttle response is better and it just seems to run more “crisp”. Today I hooked up my Torque phone scanner and went for a drive. It’s been a few days since I unhooked the battery, and the LTFTs started out at about +10 on both banks once operating temperature was reached. Even at this point, the truck seems to have good throttle response and power. After driving for 20 minutes or so, the LTFT increased to around +20 on both banks, and the truck seems sluggish.

I disconnected the MAF sensor the other day and I really didn’t notice any change at all (besides the CEL light coming on). It idled the same and drove the same - More evidence that my MAP and TPS are functional. I took some screenshots of the O2 sensors that I could post, but they look the same as the last ones.

I unhooked and pluged the vacuum inlets for the HVAC, Transfer Case, and PCV. It seems to drive the same and fuel trims act the same. I haven't got around to checking for vacuum leaks again or removing the EGR again yet.

I was replacing my tie rods over the weekend and pulled the spark plugs out while I was in the wheel well. They used the cheap copper Autolite plugs when they did the tune up (right before I bought the Blazer). With what pettyfog said about misfires causing a false lean, and the many posts on this site about how finicky these engines are in regards to spark plugs, I thought the plugs might be the problem. If there isn't a strong enough spark to get a proper burn in the cylinder, then there would be oxygen left over for the O2 senors to read. They would think it's running lean and the computer would dump extra fuel into the cylinders. This would explain my fuel trims and bad gas mileage (about 15 mpg). I got some AC Delco Iridium plugs and hope to get them in soon.

Here are some pics of the best looking plug (#4) and the worst (#6). All the others were between these conditions but closer to #4.

 

I got the new spark plugs in, and it ran about the same.

I tested the Ignition Coil - had 0.25 ohms primary resistance and 6,000 ohms secondary. This is within spec and the same values that another one at the junkyard showed.

I cheked the battery and it was fine.

I replaced my sketchy looking alternator wire with a 4 guage cable, and cleaned the battery cable ends and put on new terminals. After this, I got a slight increase in the voltage readout on the dash, but the truck actually seemed to run worse.

Next I cleaned the carburator again, including the IAC channel, and put on a used IAC from the junkyard. I did this to see if I could get it to stop sticking when I come off the freeway to a stop. It runs much better now, and pings less.....have to see if it continues to work. Fingers Crossed!