- Jul 11, 2022
- Reaction score
You can also test for vacuum leaks using an unlit propane torch with a little propane flow.
The fuel trims indicate a classic vacuum leak that affects both banks. Plug off the vacuum line to the brake booster and retest trims at idle and 2000 rpm. If they are down more at idle, Bob's your uncle. Great trims are less than ±5% (-5 to +5) when adding ST & LT. Anything more than ±10% is a concern, or should be. EDIT: also check the air plenum between the MAF and the throttle body for leaks. Unmetered air is the culprit.
Add each bank's fuel trims together to get the cumulative fuel trim.
For example, in your first pic, B1 = 16.4 + -4.6 = 11.5.
B2 = 16.4 + -5.4 = 11
Compare that to how great they get at the 2000 RPM. B1 total is 0.8 and B2 is -0.8.
It looks to me like the computer is adding fuel at idle, then the LTFTs show it being taken away when it's not needed, which is why they are negative.
I ran a vacuum test on the brake booster hose and attached images of my scan tool data. STFTs seem right with the hose plugged and I'm thinking the LTFTs are likely deep in the negative numbers per the ECM trying to reconfigure with the leak absent. I presume that after the booster is replaced and the engine runs for a couple hundred miles the LTFTs will settle into healthy numbers, but correct me if I'm wrong.
Any tips on the brake booster job? It looks like I don't even need to empty the master cylinder or bleed the brakes, but I'm also not sure if I should replace the master cylinder in tandem. Considering my brake fluid drop last year, should I toss my current MC?
Below is the best video I've located for a visual on what I expect to do. Please advise.
Additionally, how do my MAF sensor numbers look? Should I be thinking about replacing it, as well?