There have been several posts lately referring to potential grounding issues which got me thinking about broken grounding straps on my exhaust pipes. These very short braided wire straps, about 1/4" wide, have been broken off from my exhaust pipes for years. I was just reading some information that indicates they have a role in assuring the O2 sensors do not get compromised my "electrical noise" as they do what they do, fluctuating in a range of less than 1V. My Pathfinder has no grounding issues I can find as there is nowhere on or around the engine I can find that measures more than 3mV. However, on those exhaust pipes I can measure as high as 7mV. That doesn't seem like much but it is more than twice the measurements everywhere else. Are these small grounding straps really that important in assuring O2 sensor accuracy?

No, they're meant to provide backup grounding if the system gets so rusty that the pipes no longer have good contact with the block. If the pipes are 7mV with it running then you have plenty of contact. No worries.

Let me add this, Md... in the short time I've been here I've quickly come to appreciate both your knowledge and curiosity. Wish I had a whole pile of customers just like you!

VStar650CL wrote: ↑

Sun Nov 29, 2020 10:26 pm

No worries.

Thank you. That's what I needed to here. I could not track down those OEM ground straps to save my life and all aftermarket ground straps are too long, too big, and have the wrong size ring connectors. I did track down Falconer Electronics in NY that can custom make those straps for $2.50 each plus $8 to ship them so I went ahead and did that. Maybe I'll cancel the order tomorrow or just let it ride and put my Pathfinder back the way it was meant to be. It's not likely to make any difference but it's at least something to play with.

You're most welcome! You might try a starter/alternator shop if there's one near you, they generally use steel braid for brushes and can usually crimp a variety of ends on them.

I installed the new exhaust pipe grounding straps and found a measurable difference in STFT's and the O2 sensor modulated voltage signals. The differences were measurable, repeatable, but insignificant. The LTFT's remained unchanged at -1.6% and +0.8% before and after the ground straps were installed. The STFT's had always been consistently ranging from about -3 to -6% on bank 1 and -4 to -6% on bank 2 at idle and were still at those ranges just before I installed the straps. After the straps were installed the ranges dropped considerably, relatively speaking, to an average of -2.6% and +1.2%. More significantly noticeable, were the O2 sensor 1 signals on both banks that were now very normal, smooth and regular modulations, peak to peak, as shown with the graphing feature on my scan tool. Before the straps were installed, the modulations were a bit choppy and irregular, but only at idle. Even the O2 sensor 2 signals now have less variation at idle after the straps were installed. The measurable ground voltage also dropped from 7mV to 5mV after the ground straps were installed. All these numbers essentially add up to nothing really. The differences are improvements and measurable but not likely to make any noticeable efficiency change in fuel economy, yet I hope it may at least be enough to recover the $13 I paid for the straps.

Nice work and good data, but we have a saying in the business that the front HO2's (or A/F's) are for the ECM, the rear HO2's are for the government. I feel safe in guaranteeing you won't see any mileage difference from the rears, because all they do is monitor catalyst efficiency. No car actually needs or uses them for anything else. The fronts are a different story, but the straps making a difference implies there's a significant amount of crud underneath your manifold bolts.

VStar650CL wrote: ↑

Sun Dec 06, 2020 7:03 am

... the rear HO2's are for the government. ... there's a significant amount of crud underneath your manifold bolts.

Nicely done deduction! I had valve cover gaskets that were allowing oil to seep through for years. Not enough to drip but enough to make the exhaust manifold mounting points kind of cruddy. That's all been recently cleaned up, on the outside, when I finally got around to replacing those gaskets. What's been baked in underneath will remain a character flaw until it gives me grief. At least I can say the government should be pleased with the work I put into it. Maybe I can justify the $13 as a tax write off.

Good one, I'll buy that!

Interesting. I'd never considered exhaust corrosion as an engine management or emissions problem. The logic is clear, I just hadn't thought of it. On the later R50s, I'm assuming all the 3.5s, the front O2 sensors, which are the important ones, are 3 wire and would be subject to problems with corrosion and resistance in the circuit. Oddly, the rears are 4 wire which means they have their own ground, so they should be unaffected by corrosion/resistance.

BTW, seat of the pants, do you notice any difference? Diverging a bit, I once replaced a MAF sensor on a BMW of mine with a generic one. Worked ok for 11.5 months when it failed. Rockauto did replace it free of charge, but I decided to replace it with a VDO, which is an OEM. And there was a difference. Subtle, but there. Mostly smoother, might have been a little more responsive. No CEL prior to the failure, but I wonder if this might be the same thing, not dramatic enough to set a code, but enough to make a difference.

$13 shipped? For a custom part? That's crazy cheap. I'm thinking I'll solder or braze a wire to the exhaust if it's missing, and I've neither seen it or looked for it, but for $13 even this cheapskate might just buy it new.

So you took the mv readings between the exhaust and a chassis ground?

The ECM generally has its own chassis ground but the 3-wire sensors reference the block ground, so determining exactly what the ECM "sees" can be a bit complicated. The only 100% reliable method is to take Vdrop readings to the negative battery post from the ECM ground pin and from the manifold in the vicinity of the front sensor, then subtract. That gives you total offset. However, that kind of precision won't matter much in real life, the fronts just need a solid connection to the block and the block needs a solid connection to the post. To keep things optimum, if the whole glom is under 30mV then you're gonna be fine, 30~50mV you might want to do some cleaning, over 50mV and something is definitely crappy.

Mike W. wrote: ↑

Sun Dec 06, 2020 11:21 pm

So you took the mv readings between the exhaust and a chassis ground?

No. I measured from the negative battery post to the exhaust pipe that holds the downstream O2 sensor, which is where the grounding strap is connected, while the engine is running. One curious note I should mention: I found the old grounding strap had broken off on the passenger side but it did not appear that there ever was one on the driver side. I installed a new strap on both sides.

I’ve been wondering about that strap since I got the car (passenger / Bank 1 side). Couldn’t figure out where the end had originally attached thinking it wouldn’t make sense to attach it to an exhaust, but now I know better.
I didn’t see a matching one on the drivers/bank2 side though.
I suppose drilling a hole through a crimped over exhaust flange somewhere with a stainless bolt and some copper anti-seize will do the trick ?

https://imgur.com/SNCl74w