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Discussion Starter #1
My ride is a 2003 525 with 317,000 miles on the clock.

About 4 months ago, it began starting hard which I attributed to the Catalytic converters. It’s been throwing and removing cat codes for a while and they are original.....

Three weeks ago, it began idling rough and two weeks ago, would not idle. I parked it and ordered new cats and O2 sensors. They got replaced last weekend.

The car went from not idling to not wanting to idle. It was throwing misfires from 4 and 6 but would smooth out at higher revs. We disconnected the MAF and the car would hold idle. Still acted like it was missing occasionally, but ran much better. But it’s running rich and mileage is about 14mpg. The only code it throws is for a low MAF circuit (P0102?)

I ordered a new MAF, put it in this morning and initially it did better. But then it started throwing codes for random misfire (P0300), and cylinders 1,2,4,5,6 were either confirmed or pending. Mileage however went up to 24mpg. It idled a little rough, but not terrible.

This evening, it didn’t want to idle. Stalled, and was still throwing codes for misfires. I disconnected the MAF and it smoothed out. I drove it home without issue albeit running rich.

I’ll stipulate that it does need plugs, coils, and a new fuel filter. Maybe a new fuel pump. Those are on tap for the coming weekends.

But it seems like it also has a vacuum leak - although I’m not sure why it doesn’t manifest with the MAF disconnected. Bad MAF?

The CCV was replaced with a catch can. The PCV valve might need replacement - but once again, I’m mystified why it only manifests with the MAF connected.

Any good methods for diagnosing vacuum leaks? Smoke seems best. I’ve ran the hoses many times but can’t find a culprit... I don’t have a smoke machine, but I can find a place that does.

Mostly, I’m looking to know I’m on the right track - and maybe why the car is acting a little counter to what I’m expecting with the MAF.


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3,703 Posts
with the MAF unplugged, the DME defaults to a preprogrammed map. following it may be why the car idle changes with it unplugged. i agree from what ive read here so far that you have a vac leak, random misfires especially. yes have a smoke test run. there may be a bad hose/fitting related to the catch can. what brand O2 sensors and MAF did you put in? what makes you say it needs coils. plugs, fuel filter and possible fuel pump? are they old? if so , plugs and coils id agree with easy enough, but as to the filter and pump, whats the pressure at the rail? a vac leak allows unmetered air (air not accounted for by the MAF), in to the engine, O2 sensors can try to compensate to a point. take a look at your fuel mixtures, both long term and short term, this will most likely give you the best indication of whats going on with the engine.
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