MassHoe04
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This was a good explanation of what fuel trim is about...
What did you do to your NBS GMT800 Tahoe/Yukon Today?
- Thread starter vwbeaner
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MassHoe04
Full Access Member
This was a really good write-up. Hope this helps!
www.mechanic.com.au
Here are the key summary bullet points (see below)...
Below are some tips and tricks to diagnose the most common faults /defects / failures /malfunctions that cause rich air/fuel mixtures that result in high negative fuel trim values-
Understanding Short Term and Long Term Fuel Trims
www.mechanic.com.au
Here are the key summary bullet points (see below)...
Below are some tips and tricks to diagnose the most common faults /defects / failures /malfunctions that cause rich air/fuel mixtures that result in high negative fuel trim values-
- Excessive fuel pressure; test fuel pressure with a dedicated fuel pressure gauge
- Leaking fuel injectors; test injector operation by performing a spray pattern and volume test
- Defective MAF sensors can under report the volume of air passing over them, thereby causing rich running conditions
- Exhaust leaks; repair all leaks found since exhaust gas contaminates the ambient air that oxygen sensors use for reference purposes
- Loss of some compression on one or more cylinders; loss of compression causes poor or incomplete combustion, which results in unburned hydrocarbons “fooling” oxygen sensors into reporting a rich mixture
- Marginal misfire; not all misfires are bad enough to set a misfire code, so check or replace suspect spark plugs, and check the operation of all ignition coils with an oscilloscope
- Marginally defective oxygen-, or air/fuel ratio sensor(s); sensors may be stuck at reading rich conditions as a result of not reacting to switching signals from the ECU, or may be slow to respond to switching signals
iddqd
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I'd also added a leaking fuel pressure regulator
Tonyrodz
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What's a good way to test for this?
TheAutumnWind
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I have yet to get around to an external fan filter. I wasnt aware of the TSB for another pan magnet. Thanks.
TheAutumnWind
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It certainly seemed like a lot.
clandr1
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Hook up a fuel pressure gauge. When you turn the key on (not starting), the fuel pump will prime the pressure. If the pressure starts to bleed off rapidly after the pump stops running, you have a bad FPR.
SnowDrifter
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I have a magnefine inline w/ my trans cooler
TSB:
iddqd
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Tonyrodz
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Thx.
TheAutumnWind
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Nice simple solutions. Need to drop the pan sometime soon for a filter so I'll have to do these. Thanks for the info! I always planned to do a screw on filter. I have had these laying around for an embarrassingly long time: https://www.amazon.com/gp/product/B0014BHVRO/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
NoReverseYukon
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I'm curious as to why they chose only 2002-2009. I don't think there was a design change from 2001 and earlier.
clandr1
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Strange, my FPR is integrated into my fuel pump and is nothing like what's shown in the video. In older GMT800's, the FPR was located on the fuel rail close to the firewall and was easy to access and replace. I had to replace my fuel pump around 120k miles because the FPR was failing, and since it was built into the fuel pump assembly I didn't have a choice but to replace the whole thing.
Well I’ll be. Guess I’ll be getting a magnet.
Edit: no I won’t. That TSB doesn’t apply to my 02 Denali. Also, my magnet was round not square like stated in the TSB.
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Your FPR should be in the pump assembly. The one on the fuel rail was moved there in 03. You won’t be able to access it.
Your graph on misfires showed 0 cycles. It resets at 100 cycles, so try to get a pic of it close to 100 cycles so we can see how many there are.
For a few years after '03 the Flex Fuel engines kept the FPR on the fuel rail. Not sure what @Tonyrodz has...
Tonyrodz
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Mine is on the rail.
Forgot the FF ones did that. They kept the filter on the frame rail too, didn’t they?
Yup.
clandr1
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For what it's worth mine is a 2005 flex fuel and does not have a separate FPR or a (serviceable) fuel filter.
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