I have a brake booster issue with my 2007 Ford Ranger 2.3l. The brake booster passes all functional tests of the brake pedal and the truck brakes fine. However, every time I release the brake, the short term fuel trim spikes and then comes back down. There is no leak other than when brake pedal is moving backward. If I pump the brakes, then fuel trim will continually increase until the engine stalls.
My thought was originally a weak return spring inside the booster allowing one valve to open slightly before the other closed. This would allow an extra bit of air in but the booster would function normally once everything settled out. Or maybe the gasket around the push rod is sticking. However, I started to question how the extra air from the booster is handled in normal operation and now I have questions.
In normal operation, the atmospheric side of the booster is evacuated by the intake vacuum every time the brake is released. This bit of air is added to the intake and is not monitored by the MAF. So, is this amount of air so small that it doesn’t affect engine performance? If so, would this remain true if you pumped the brakes?. Or… Is this amount of air compensated for by the idle air control in some way such as through the brake pedal position sensor?
It seems to me that my booster is pulling extra air on each release of the pedal affecting the fuel trim. However, if the IAC is involved and not monitoring the booster, I could understand why my fuel trims rise every time the booster is evacuated.
So, do I have a bad booster or do I need to look further???
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