Have P0174 and P0171 codes on my 2000 Jimmy. All 3 oxygen sensors have been replaced since they were original and would sometimes throw codes for the heater circuit and no activity. I believe it's a vacuum leak because I also have an issue with the HVAC. There's an actuator behind the glove box that controls the door between panel and floor. When I have the output set to floor, the actuator immediately moves when the engine is shut off, and the air blows out the panel vents. Shouldn't the actuators hold their position for at least a little while if vacuum is good? I have replaced all vacuum hoses under the hood, along with pcv valve and grommet, and the line that comes off the pcv valve. The seal was bad where the hat goes over throttle body, so I put silicone around that as was suggested on the forums. I'm not sure where to look next. Would a vacuum leak in the hvac system behind the dash be enough to cause the lean codes?
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Look at fuel trims at idle and at 2500 rpm in park. Then block off the vacuum feed to the brake booster, 4wd and the dash and see if those values change.

George

 

I haven't run it up to 2500 rpm but I have checked fuel trim several times with different vacuum lines capped, except the brake booster. The values didn't change when I capped the lines. What I've gotten so far on fuel trims is around +50 at idle, drops to 0 when I rev it up to 1500 rpm, then goes back up to around 14 if I hold it at 1500 rpms. I've attached a few screenshots. (I can check again tomorrow at 2500 rpm.)

How would you suggest I block off the brake booster? All I have are caps for small vacuum lines. The brakes work good, so wouldn't that rule out a leak involving the booster?

Just FYI...I only have a Bluetooth dongle that I bought to read and clear codes using my phone. Can't do any fancy stuff with it, but I did find fuel trim for each bank and one short term fuel trim. They all read about the same during my testing.

At idle.

Holding at 1500 rpm.

At idle.

 

If you can get your hands on a mighty-vac with a gauge on it, they are great to chase vacuum leaks and have some other uses around the shop too.

 

What scanner software are you using and what BT dongle do you have? We need 4 fuel trim numbers B1 and B2, ST and LT.

Try either plugging the brake boost line or if there is a soft hose suction squeeze it shut with a fuel line clamp or a vice grip using two wooden blocks or other hard flat object on both sides to prevent tearing the hose. That one reading that goes from a very high FT to much lower at rpm suggests a significant vacuum leak. Oh and I forgot to mention the vacuum canister as a source of leaks, its usually in the fender well and can be a pain to deal with.

George

 

I'm using Torque lite. All I found was fuel trim 1x1, 2x1, and a single short term. I know that 50 is very high compared to all the Googling I've done so seems it would be obvious if a vacuum leak. Maybe my app is wonky.

 

Strange results. Unhooked the brake booster and got roughly the same high readings. But after revving it up to 2500 a few times, all values dropped to under +10 at idle and under +5 at 2500 rpm. I reattached hose to booster and readings stayed low, even dipping into the negative. Only other thing I did was fiddle with the brake booster check valve trying to pull it out (didn't get it out, was afraid I'd break it if I tried any harder). And revving it up to 2500 rpm I rarely do (granny driver here). Oh and my dongle is a cheapie off Amazon, brand Kitbest.

 

I never use torque light but I downloaded it to an android device to see what’s going on. Les knows more about these cost-effective apps than I do and perhaps you can chime in if I’ve got this wrong, I use a higher end shop scanner myself. It looks like the fuel trim parameters are kind of messed up. I could not find one that gives long-term trim which is essential to a proper fuel trim analysis. I was able to select what looks like both short term trims which I think are the ones labeled fuel trim bank one sensor one and fuel trim bank two sensor one. Only having those two values is incomplete but can probably still tell us something about the situation. When I say to get readings at idle and 2500 I mean steady state idle and then holding the RPM at 2500 and waiting for the ratings to stabilize, not revving it up . If you see very high positive trims at idle which go down significantly toward zero when you raise RPMs to 2500 steady state then there’s a good chance you have a vacuum leak which can be in the hoses, the vacuum reserve canister, the brake booster, any accessories that require vacuum, the four-wheel-drive function, the intake manifold gaskets, the PCV valve etc. finding that leak is done by pinching things off and spraying something around the leak points looking for an rpm change. The most dramatic is brake cleaner for starting fluid but that’s flammable and a lot of people are uncomfortable with that so the next best thing is water but it does not produce as obvious a result

 

I just checked on the paid version of torque has all four fuel trims displayed properly. I don’t remember how much it cost I downloaded it years ago. Again Les might know if there’s a free app that can do all four fuel trims or not. Interestingly the free Torque Lite displays O2 sensor voltage pretty well in graph form and some better software doesn’t do that very well

 

Sorry. When I say revving it up, I mean I increase the rpms and hold there for readings to stabilize. 😁
The guy who put in my o2 sensors said he sprayed starter fluid all around the engine area and vacuum lines for leaks and found none. I've gone over all the lines under the hood and done the testing I included above. That's why I was wondering about a leak in the hvac lines behind dash, with that actuator losing vacuum as soon as engine is shut off.

But now I'm more stumped, with the fuel trims going down like they did during and after the booster check. I'll start it up tomorrow and see what I get. Also going to try a different app for my Bluetooth.

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