Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
Hey all, I've been working on a 2000 silverado with a 5.3. Customer complained of a faint misfire he could feel. I felt it as well check the codes and it was showing a multitude for both upstream o2s (I dont remember the codes, I know shame on me). I then went to live data and noticed the o2s were stuck lean and wouldnt switch. Replaced 02s and fixed that issue however I still had high positive ltft on both banks. In fact they were maxed out at +25 and stft was positive 15. I suspected a dirty maf so I cleaned it with no change, so I changed it out. (Throwing that part at it didnt fix it, I was going off the premise of g/s should match liter and I was showing 4.5 g/s at idle). Today I went back up there with my scanner danner book and started using it to revamp my memory some. I started looking at misfire counters and noticed a bunch of randoms I then looked at my IAC and noticed it was at 0. I thought that was odd and matched a vaccum leak. I wasnt sure what theory to use (map or maf since this truck has both). However I pinpointed a pretty significant leak at one of the fuel injector o rings. Changed those out and had huge difference but still a misfire on cylinder 6 that was a max of 8 counts per 200. And my ltft was +10 and +11 with a short term of -3 to +3 on both banks. I started looking for more leaks and only found one when I sprayed around the throttle body housing gasket. So I replaced that and my numbers were perfect. No misfires on the counters but I could still feel a little shake in the car at idle. I took it up the road and come back, let it idle for a min and look at fuel trims and I be dam if at idle their not back ltft of 25 and stft under +10 and switching rich to lean. So I pulled out the spray again looking for leaks and I'm still getting a rich o2 reading when spraying around the throttle body. I cant get any noise and visual of a leak using water but when I spray around the tb I get rich o2 and start getting negative stft. This vehicle had a cheap brand cai and the silicone 90 that connects to the tb I suspect to be leaking some but I'm not sure. Has anyone see one of these cheap cai leak? Also so help with my lean condition. Also a little more info the truck runs better cold as in doesnt feel like a misfire and after all these fixes the fuel trims above idle check out great all within the -10 to +10 paul mentions.
Is there a way you can duct tape or something around the seal on the throttle body temporarily?? Also does owner have stock air box I think your on the right track...
The following user(s) said Thank You: Andy.MacFadyen
Fuel level shouldn't have any effect on most systems --- there are exceptions to this espeically on cars with saddle tanks.
Restriction on fuel filter or fuel ines will cause lean mixture at higher loads and RPM generally not so much at idle.
Low fuel pressure from a bad pump usually affects crank time at cold start but less than 0.5 bar (7psi) drop can have a big effect on fuel trims.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
A skewed O2 sensor is a sensor that is always reading a little higher or lower than normal. I believe if you have KOEO on a cold engine it should be around .450 mv. Someone please correct me or feel free to add anything I left out
how can i confirm my suspicious of the tb coupling? i dont have a smoke machine and it wont suck water put my 02s go rich when spraying brake cleaner around it.
Yes I sprayed all around the intake with water and brake cleaner. No change. Brake cleaner around tb and 02s go rich and fuel trims start reducing fuel. Spray water around tb and nothing. Customer has the truck back and is very pleased with it. However I am now. I'm leaning towards the aftermarket cold air intake not sealing properly around the tb and would like to confirm but changing out for a factory filter box. Any one have another theory or way to verify.
Topic Author
