Exhaust touches framerail... Causes issues

So this is another weird one for our 86 245. So imma start with a quick recap.

-1986 Volvo 245 with a b230ft swapped from a 94 940.
-2.5" custom exhaust with new cat, magna flow muffler.
-No codes other than a running rich/lean code (with or without o2 sensor plugged in)
-Trouble shooted crank sensor, maf, coolant temp, no vacuum leaks, and other than that car runs fine/ran fine.
-I believe the wiring is proper between the oxygen sensor and the computers.

So now for the problem...

So for the longest time we've had issues with my exhaust swinging and touching the framerail which somehow caused the idle to start bobbing sporadically until the car just cut off. Of course we "fixed" this by just making sure the exhaust doesn't touch the frame rail.

But now we installed a 16t with the non rotational flange and everything seemed fine but then we started hearing that the exhaust work we did had the exhaust lightly touching the transmission crossmember which brought back our issues. We checked every sensor and wire the best we could and even tried replacing most components with others and still couldn't get rid of the issue.

But the weird part is, what did fix the problem was unplugging the o2 sensor. Obviously didn't run well but stayed on and didn't Bob the idle.

We can't begin to question why the exhaust touching the body of the car would cause this issue and why unplugging the o2 sensor would make it stop so hopefully someone can provide me some insight on this.

Do you have good solid ground wires on the engine block and intake manifold to the frame of the car? Maybe it is finding a better ground through the exhaust or O2 sensor which is throwing the computer off?

^^^ and especially check that both ends of the woven ground cable from the back of the valve cover to the bulkhead are making good contact. Use a little fine grit sandpaper if either end is questionable.

Crappy engine mounts in a ratty old Volvo? Noooooo.... Couldn't be that!

Grounds too. It happens.

rb92673 said:

Do you have good solid ground wires on the engine block and intake manifold to the frame of the car? Maybe it is finding a better ground through the exhaust or O2 sensor which is throwing the computer off?

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all engine grounds happened to be fine, well at least looked fine. Turns out that the chassis ground happened to be somehow damaged but not visibly damaged, we were never able to find this issue before because it also happened to be intermittent for some reason.

This did however put together a bunch of missing pieces for us. Since the chassis wasnt grounded very well, the oxygen sensor happened to be constantly grounding whenever my exhaust touched the framerail or any part of the body. With a multimeter it was actually putting out negative voltage which was very weird to us. Basically all these things combined caused the computer to get relatively pissed and then started screwing with the idle.

Not a very good way to explain how this now 1.5 year issue was caused but i can say we fixed it by stealing a battery ground cable from a p80. All very odd and new to us.

bobxyz said:

^^^ and especially check that both ends of the woven ground cable from the back of the valve cover to the bulkhead are making good contact. Use a little fine grit sandpaper if either end is questionable.

Click to expand...

The engine grounds were perfectly fine, we did sand them just incase but still got the same issue until we checked the chassis to battery ground. But technically everyone was right about it being grounds, it was just a chain of events that led us away from thinking it was grounds.

JohnLane said:

Crappy engine mounts in a ratty old Volvo? Noooooo.... Couldn't be that!

Grounds too. It happens.

Click to expand...

Got rid of my engine mount issues a while ago lol, a nice thick cotter-pin/bolt put through both ends of the mount. Sorta not really quick release.


But anyways thanks guys