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Related or not?

Condition, cause and correction—connecting the dots
Friday, November 6, 2015 - 09:00
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The F-150

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With new vehicles costing as much or more than the house I built back in the mid-eighties, it’s a no-brainer that people are jumping through hoops to keep their older wheels rolling. I have no hard numbers on this, so shout me down if you disagree. Pickups tend to stay on the road longer than cars because cars don’t have the same “cool” factor and on the practical side, you can’t haul much in the back of a Buick Century or a Ford Taurus. A colleague of mine brought us his 1998 F-150 with an MIL illuminated, suddenly dismal fuel economy and a strange new noise. 

A year ago we had repaired a P0401 code on this same truck after he had attempted to replace the EGR valve on his own and had broken one of the valve’s seized retaining bolts, so rather than drilling out the broken bolt, we had simply robbed the throttle body goose neck off another 98 F-150 engine we had just yanked out of a truck like his, cleaned the EGR passages, installed the new EGR valve he had purchased, and got the gasses flowing again. Well, now his P0401 was back along with fuel trim numbers hovering near +20 on bank 2 (bank 1 stayed near zero), and we also had what sounded like an mild exhaust leak somewhere on the passenger side of the engine.

Applying some vacuum to the EGR valve by grounding the EGR Vacuum Regulator trigger wire, we heard the idle quality deteriorate and saw the voltage at the DPFE change from 0.6 to something like 3.5. While the DPFE should have gone a volt higher with EGR flowing, I was satisfied that the hands-on test exonerated the vacuum feed, the EVR, the EGR passages, the DPFE sensor and the silicone hoses.

Well, the end of that story was that the exhaust noise was the cause of both the P0401 and the high bank 2 fuel trims. One of the exhaust manifold header pipe studs had melted away with age and the header pipe had dropped a bit creating enough of an exhaust leak to reduce EGR flow because of lower backpressure at that point (the tube gets its feed from about an inch north of there) and to allow the O2 on that bank to get a good sniff of fresh air that skewed the fuel trim.

The owner needed the truck for the weekend, and it was the end of the day at the end of a week, so I grabbed an old 5/16 Allen wrench, some all thread, a 3/8 nut and the hourglass-shaped plug plate from a new A/C compressor and applied 10 minutes, the torch and a bit of brazing to build a temporary clamp with the agreement that we’d replace the quick-fix contraption I built by drilling out the dissolved bolt at a later date. That concludes story “related or not” No. 1. The sound, code and fuel trim were the condition, the exhaust leak turned out to be the cause, and fixing the exhaust leak turned out to be the correction.  These three concerns were obviously related.

With my homemade tool installed in place of the O2 sensor we got this reading. Rule of thumb is that there shouldn't be more than 1.5 psi of pressure in front of the catalyst. It would seem that this catalyst would have driven exhaust pressure higher than it did, but this is what we saw when we cut the old cat our of its pipe. This is the graphic of the Mazda's fuse panel superimposed on the schematic with the fuse and relay highlighted both ways.
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