Check engine light has been on / off for the past 6mo, vehicle seems to run great w/ plenty of power. Vehicle belongs to my father in law which lives out of my area. 80K miles on the odometer. Has had the APP replaced as well as the Upstream O2 (he doesn't know why the O2 was replaced a couple years ago other than it had a check engine light on then). Made a little road trip bringing my shop's scanner, MAF cleaner & a portable smoke tester. LT fuel trim was +18 @ idle but dropped above 2000rpm. Thought must have a vacuum leak. Was a little deflated when I opened the hood and found it was not a MAF fuel system but rather a MAP speed density. Smoked system and only found one little puff on the IAT sensor grommet. Upstream 02 sensor activity is fine w/ a slight bias lean @ idle (.5-.75) but I am able to bring it above .75 by whacking the throttle on / off. Reviewed sensor data ie. ACT , CTS, MAP etc all looked ok. So my father in law asked what was the next step? My brain is still fixated on false air but I didn't pay attention to the throttle angle - I have little experience w/ throttle angles @ idle. My father in law asked what would be the next step? I am still fixated on false air but the smoke test… I also mentioned possible dirty fuel injectors. Wondering if anyone else has been down this road w/ an old Trailblazer? Thanks.
I think that engine has a pcv valve. Might be worth checking. Have a look at map voltage at idle. If it were below .5 volts that might start it correcting rich. Seems like I dealt with one that had a map signal skewed. That’s all I can muster for now. Good hunting. Chris
I would check the intake manifold bolts you may find them loose if so you need to replace the intake manifold gaskets I have seen that on that motor.
I found quite a bit of info. on loose intake bolts & air injection check valve issues. The vehicle doesn't have a MAF or Air so not sure how a vacuum leak is going to effect fuel trims.
Check the little tube beneath plenum. Squeeze it with fingers and look for cracks. and or tiny cracks in exhaust manifold above upstream o2
Is the engine speed at idle correct? If it was a vacuum leak I would expect a MAP car to idle higher than normal. Another thing to consider is the evap system. Are the canister purge and vent valves working correctly? You could perform a purge and vent seal test with the scanner if that model supports it.
Most of the GM's that have come through our shop generated codes for faulty Vent or Purge valves. Easy enough to check though. The Purge did pass during the smoke test on this vehicle.
Since this is a speed density system, you can rule out any air leak, since an air leak will only cause a high idle, and will not effect the fuel trims, unless the air leak is very close to one of the intake ports. Since you have positive trims, Those vehicles can and will trim quite heavily from the rear oxygen sensors, pay close attention to them. If you do suspect a rear trim issue, you can…
I did not know the downstream played a role in fuel trim. I do know the upstream was biased lean because it was running lean @ idle.
Downstream sensors have been used for trim since 1996 model year. Some vehicles use more, some less. I still remember a trim issue from a 1996 Saab 9000, where a very small air leak in the gasket of the rear sensor was causing a +45% trim correction.
How about a top engine cleaning for carbon on the valves. Your father-in-law may not get it out on the highway too often I would guess? Could be the injectors a bit dirty too.
We had the same vehicle with P0171 and high fuel trims (+18% or more). Found a TSB that stated possible clogged fuel injectors, recommending a fuel injector clean. Although we never had much luck with cleaning fuel injectors, We decided to give it a try. After the injector cleaning the fuel trims came down to +5%. The code has not returned and it has been about 6 - 8 months. We have seen the…
I came across the fuel injector TSB as well. Awhile ago we had a Silverado setting a P0300 that was still under warranty. Came back from the dealer and found they performed an “Active Pressure Test” on the injectors. The test was above the standard pressure drop, here is the tool used : gmtoolsandequipment.com/en-US/Pages/It…. I will entertain an injector cleaning, hopefully our…
I seem to recall these having cracks at the exhaust manifold right near the O2 sensors. Might want to look closely at that area. Sometimes they are not nearly as loud as you would expect.
An exhaust leak near an o2 sensor could could cause a false lean condition. One way to try and confirm this is, with the engine idling, graph the short term fuel trim and block the tailpipe as best you can with a towel or handful of rags. If the trim starts to go negative you have confirmed an exhaust leak. The pressure buildup in the exhaust can prevent air being drawn into an exhaust leak.
I have found leaks in exhausts by putting my smoke machine from the tailpipe.
Stuart, It's not just about finding a leak, the idea is to prove if a leak is causing the fuel trim issue. That's what that procedure can do for you.
I'm not disputing you but I have had a vehicle where I suspected a leaky exhaust for lean conditions and blocking the exhaust didn't change the fuel trims but i heard the leak and when I smoked the exhaust found the manifold had a crack.Replaced the manifold and still had a lean condition.Cleaned the injectors and problem was solved.Did the exhaust have…
I ran into a similar condition it never set a lean code but had a hesitation on acceleration. I had found the secondary air valve stuck open. I would start there. Very easy to inspect.
I found info. on the secondary air check valve but this year doesn't have Air inj.