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Discussion Starter · #1 ·
Hi,
I am new to the forum and have a specific problem with my truck that I have been working on for months. Here is the vehicle description, problem description, data, and parts that have been replaced.

Vehicle: 2000 Silverado 1500, 4.3l v6, 190k miles


Problem description: Starts and idles fine, but stumbles for about 5-10 seconds after I give it gas. As soon as it recovers, the truck runs fine. Occurs when engine is hot or cold. Throwing codes 171 and 174(lean fuel banks 1 and 2). Getting about 16MPG, expecting to get 20mpg.


Data: I have a OBDII to USB interface and OBDWiz software on a laptop that collect live data. Here are the reports:


Service $02 - Freeze Frame Report (when code is thrown)
Date: 6/27/2012 1:19:52 PM
PID
Description
Value
Units
0x02
Freeze frame DTC
P0171
0x03
Fuel system status
Closed-loop, using oxygen sensor feedback to determine fuel mix
0x04
Calculated load value
23.92
%
0x05
Engine coolant temperature
188.6
F
0x06
Short term fuel % trim - Bank 1
15.62
%
0x07
Long term fuel % trim - Bank 1
23.44
%
0x08
Short term fuel % trim - Bank 2
20.31
%
0x09
Long term fuel % trim - Bank 2
23.44
%
0x0B
Intake manifold absolute pressure
12.47
PSI
0x0C
Engine RPM
1582.5
RPM
0x0D
Vehicle speed
54.68
MPH
0x10
Mass air flow rate
5.17
lb/min
0x11
Absolute throttle position
36.08
%







Text Blue Line Font Plot

Text Line Font Plot Parallel

Text Font Line Plot Parallel

Text Line Font Number Screenshot






The above charts(except the Freeze Frame) show a trip through to to corner store about 2 mile away(on a hwy) with the engine already warm. Each chart is synced with time and the stats are for this same trip. The Short Term Fuel Trim(STFT) mean is above 10, which causes the 171 and 174 codes. However, O2 sensor mean volt is .5 and .4, which I believe good. Thus, I do not think there is actually a lean condition here. Thus I am stumps to what is causing the codes and presumably, the stumbling.



Parts replaced:
Upper and lower intake manifold gaskets(which were obviously blown and leaking coolant)
EGR valve
Map sensor
O2 sensors(before the cat, ones after cat were replaces no to long ago)
MAF sensor
Throttle position sensor
fuel filter
Fuel pump(including sock filter)
Cleaned injectors(took the spider out and cleaned)
Cleaned entire intake and throttle body.


I have checked for vacuum leaks extensively by spraying card cleaner around the engine and cannot get the rpms to budge or the O2 sensor data to show any deviation.


I was thinking a possibility is that both cats are clogged. The coolant leak was going into cycliders and out the exhaust. However, I would expect data to indicate this. The freeze Fame data for when the codes are thrown shows 12.47psi at 54mph.


I am running out of ideas. Any help would be appreciated.


Randy
 

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If you can, monitor the fuel pressure while this occurs, it may be an issue with the pressure regulator not responding quick enough and starving for fuel.
 

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Discussion Starter · #3 ·
dpeter,
Thank you for your reply. I cannot monitor fuel pressure except with a gauge to the service port on the fuel line as it goes into the injectors(the "spider"). I can monitor fuel rate from the OBDWhiz. I'll do a test run tomorrow and post the results. However, If is was fuel delivery, I would expect it to failure under a heavier demand for fuel than 5mph. That is, after the stumbing occures and passes, then I can drive the vehicle just fine...excpet that it throws codes and get poor gas milage.

I did repalce the fuel pressure regulator about 8 months ago when I was having a different problem. There was enough current to cause the bulb on my sparkplug tester to light up, but not enough to casue a spark. I spent a whole bunch of time and parts tracking down what I though was fuel problem until I opened the distributor cap to find a shorted rotor. Thus, the fuel pressure reg has been swaped. Also at the time, I measured the fuel pressure on my old pump at 60psi, which is the low end of the acceptable range. Thus, I swapped the fuel pump, but I should get the gauge and measure it just to make sure.

Randy
 

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I would agree about fuel delivery under heavy load but my thoughts were more about a momentary lack of fuel then recover to correct levels. From what you have done so far , I would not expect to see fuel issues but it's best to eliminate it and move on from there. So this stumble is just "off idle" and accellerating?
 

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Discussion Starter · #5 ·
yes, The stumble only happenes after I start the engine and press the gas pedal. After stumbling for 5-15 seconds, the truck runs "fine", but gets poor gas milage and seems bogged down a bit. The stumbling will occure regardless of engine temp when started.
The ODB Whiz software listed a "fuel rate" but I found it to be only a scaling of the MAF(see charts). I did not get to the parts store to get a fuel pressure gauge today. Since I cannot read fuel pressure going down the road, I will have to settle for reving it up in the garage to see if pressure drops below 60ppsi. I will let you know how it come out when I get the gauge.

Here are more charts. I added some parameters. I was trying to analyze the MAP reading. It seems to max out at even a 50% throttle position. However, I admit that I am not sure what to expect as I have nothing to compare.
Again, any help in intrpreting these would be much apreaciated.
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I just love data. Can you change the sample rate? I see they are all about a 25 minute picture, can you get it down to a minutes worth and be able to better see what is happening durring the stumbling period?
 

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Discussion Starter · #8 ·
I went down to the garage and captured a 1 minute chart of the stumbling with the engined reved after a start. Then, before comming back to post the results, I performed another test that was suggested to me. I unplugged the MAF and the truck ran perfect. Below are the charts from a short drive with the MAF unplugged. Short Term Fuel Trim in within bounds(+-10) and the O2 reads good. This is a brand new MAF that I am supposed to belive is not only bad, but bad in the same way as my old MAF. Not sure what to think, but I guess I'm off to the parts store for a new one tomorrw.




 

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Discussion Starter · #9 ·
Just FYI, here are the one minite charts for the stumble(with "bad" MAF plugged in).
revs 2,3, and 4 stumbled. The rest did not. Notice that the STFT kicks in on rev 5 through 7. I guess I don't really know what the MAF should read like.
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Discussion Starter · #10 ·
I got a fuel pressure gauge from autozone and measured the fuel pressure. Its 50psi at idle and shoots up to 67 when I rev it. If memory serves me correct, 50-60 is the proper range for this engine. So, I'm on the lower end.
I got a new MAF and am going for a test drive now.
 

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Hope the drive went well. Like you, I don't know what a properly tuned and well running engine graphs should look like but the engine timing sure looks to be all over the place. Do you have or do you know of anyone with a similar truck that you could use to compare? Where did you get the interface and software and do you find it easy to use?
 

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Discussion Starter · #12 ·
Test drive with the new MAF didn't go well.
Here is what I did. In the off chance that the K&N air filter oil was fouling the MAF, I went a got a Purolator air filter and drove with the new MAF and the STFT was high(range from 0 to 39) and the truck felt bogged down. Unplugged the MAF and the fuel trim was not as high, but still high(range from 0 to 23) and the truck ran a little better, but still bogged down. Then, as a sanity check, I swapped in the K&N filter and kept the MAF unplugged. The truck ran well and the STFT was close to +-10.


So, with the MAF unplugged, the air filter appear to make a huge difference in the STFT. The K&N filter is supposed to be a higher flow filter. It is 4+ years old and last oiled about 2-3 years ago. It does not appear to be dirty. My guess is that the K&N filter flowing better than the “stock” Purolator filter.


I was not sure about the fuel pressure after my test. I was reading 50-57PSI (I typoed and said 67 in a previous port). From reading around the web, it seems that the pressure should be higher, like 60-66psi. Does this sound right? I'm not sure where to find the “real” spec.
 

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Discussion Starter · #13 ·

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Fuel pressure for the V6 is 60-66psi, 48-54 for flex fuel and 55-62 for all others, this is with the key on and engine off. These figures are from a Haynes manual. I recall your pressure was a bit low but came up when you hit the throttle, maybe just not enough pressure at idle and start up. Do you hear the pump come on for a few seconds when you turn on the ignition but not start the engine?
 

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Discussion Starter · #15 ·
You are correct. I also checked my Haynes manual and saw that the spec is 60 to 66psi. Today I disassembled the upper intake, took the regulator out, and went back to Autozone where I bought it 7 months ago. While they were looking up the part number I noticed that their computer screen showed the an operating pressure of 50psi for this regulator...which is exactly what I was getting. Thus, the Autozone computer has the wrong part number cross referenced for a fuel pressure regulator for a 2000 Chevy Silverado 4.3l v6. We looked back to a 1998 and its a different part number with an operating pressure of 65psi. I had them swap pressure regulators. So far, the truck runs good, Short Term Fuel Trim within range and O2 values look good.
I did a lot of work on the truck in the last two weeks tracking the problem down. Much of that work and various parts just simply needed to be done after 190k miles. However, It really sucks that they sold me the wrong part!
 

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Very fortunate that you noticed that!! WOW, it looked like fuel issue to me at first but can always be something else and with the work you had already done it was off to other things. Really glad it is running ok now. Now is the time to take some base line readings and keep them for future reference.
 

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Discussion Starter · #17 ·
Thats a great idea. I'm gonna do a full cold to warm drive, tracking all the parameters as a baseline. The software has a data logging feature to a text file. Then I can graph things in a spreadsheet. Really, for $28, you can't beat the insight this give into the operation of the vehicle.


Thanks for the help!
 

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Hi,
I am new to the forum and have a specific problem with my truck that I have been working on for months. Here is the vehicle description, problem description, data, and parts that have been replaced.

Vehicle: 2000 Silverado 1500, 4.3l v6, 190k miles


Problem description: Starts and idles fine, but stumbles for about 5-10 seconds after I give it gas. As soon as it recovers, the truck runs fine. Occurs when engine is hot or cold. Throwing codes 171 and 174(lean fuel banks 1 and 2). Getting about 16MPG, expecting to get 20mpg.


Data: I have a OBDII to USB interface and OBDWiz software on a laptop that collect live data. Here are the reports:


Service $02 - Freeze Frame Report (when code is thrown)
Date: 6/27/2012 1:19:52 PM
PID
Description
Value
Units
0x02
Freeze frame DTC
P0171
0x03
Fuel system status
Closed-loop, using oxygen sensor feedback to determine fuel mix
0x04
Calculated load value
23.92
%
0x05
Engine coolant temperature
188.6
F
0x06
Short term fuel % trim - Bank 1
15.62
%
0x07
Long term fuel % trim - Bank 1
23.44
%
0x08
Short term fuel % trim - Bank 2
20.31
%
0x09
Long term fuel % trim - Bank 2
23.44
%
0x0B
Intake manifold absolute pressure
12.47
PSI
0x0C
Engine RPM
1582.5
RPM
0x0D
Vehicle speed
54.68
MPH
0x10
Mass air flow rate
5.17
lb/min
0x11
Absolute throttle position
36.08
%







View attachment 41105
View attachment 41106
View attachment 41107
View attachment 41108





The above charts(except the Freeze Frame) show a trip through to to corner store about 2 mile away(on a hwy) with the engine already warm. Each chart is synced with time and the stats are for this same trip. The Short Term Fuel Trim(STFT) mean is above 10, which causes the 171 and 174 codes. However, O2 sensor mean volt is .5 and .4, which I believe good. Thus, I do not think there is actually a lean condition here. Thus I am stumps to what is causing the codes and presumably, the stumbling.



Parts replaced:
Upper and lower intake manifold gaskets(which were obviously blown and leaking coolant)
EGR valve
Map sensor
O2 sensors(before the cat, ones after cat were replaces no to long ago)
MAF sensor
Throttle position sensor
fuel filter
Fuel pump(including sock filter)
Cleaned injectors(took the spider out and cleaned)
Cleaned entire intake and throttle body.


I have checked for vacuum leaks extensively by spraying card cleaner around the engine and cannot get the rpms to budge or the O2 sensor data to show any deviation.


I was thinking a possibility is that both cats are clogged. The coolant leak was going into cycliders and out the exhaust. However, I would expect data to indicate this. The freeze Fame data for when the codes are thrown shows 12.47psi at 54mph.


I am running out of ideas. Any help would be appreciated.


Randy
Can you verify the cam retard & spark advances at start/at cold & warm idle/at rev/during drive cycles? Sounds like the distributor is loose or malfunctioning at the start (for 5-15 seconds) then the pcm starts adjusting & correcting the timing via spark advance & retarding the cam. It is common for the distributor drive gear (engine internal/not the gear at the end of the distributor) to wear out and sometimes it can skip a tooth at startup. It will throw your timing off for a few seconds until the pcm corrects it. If it keeps skipping teeth on the gear it will eventually not start/run because the timing has jumped off farther than the PCM can correct or adjust to it.
 
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