MBMB

Driving home from the office the other day I lost power and lost bwah at the same time -- it felt like I was driving a Civic and sounded like I was driving a stock 928 (not rough, just quieter than my 89GT should be with its RMB).

I had the multimeter hooked to the O2 sensor and ground (trying to diagnose my overrich-when-hot problem); it read 0, which, if the sensor is working, indicates a lean condition.

Shut down and restart and the bwah is back and the O2 sensor reads .6-.8 (correct range).

Leave it overnight, install fuel pressure gauge. Start up, same problem -- Civic power, muted exhaust. Fuel pressure is good.

The 89 GT has the ignition control circuit, which is functional. Neither LED is alight.

I suspect a clogged cat (because of the aforementioned overrich-when-hot problem. I try the vacuum gauge cat diagnosis. Vacuum rises and stays up, which does not indicate clogged exhaust. Not sure if exhaust is muted when I do this test because I've gotten used to the muted exhaust.

Suggestions? Ideas? Tests?

Thanks,

Thaddeus

I'll bet your honeycomb lattice in the cat has partially collapsed.

MBMB

The vacuum test should show me if I have a blocked cat. I guess what I need to do is make sure I'm seeing the symptoms while I do the vacuum test.

MBMB

By the way, why would my O2 sensor read lean when I have a blocked cat?

Garth S

While almost anything is possible, a CAT with increasing differential pressure (failing/blocked/melted/core shift/etc.) is not expected to recover: If it's plugged, it stays plugged.
What often happens with standard ceramic honeycomb types is that the ceramic core breaks up, and blows downstream to plug the resonator/muffler. This can lead to signifigant engine problems; however, I do not know if the 928 is this style, OR a ceramic deposited on a stainless steel support that prevents this: knowing the cat replacement cost, you would think that the mesh support is in placefor 928s.

MBMB

Maybe I should pull off my resonators and see what shakes out?

John Struthers

Just in case, take her out for about ten minutes at night stop somewhere DARK with no dry grass and take a peek under the car.
If the exhaust has that high-pro Glow you have a failed cat.
Partial failure might not give you that GLOWING indication so to be safe make sure you have a fire extingusher handy.
BTW
If the cat(s) are clogged and the exhaust system isn't flowing properly you basically have a high temp oven with a restricted exhaust path.
Initially, while the system is relatively cool you should have a rich reading then as it heats up the reading should appear to be lean.
I read somewhere if you cook the O2 sensor at that kind of temp for long that it is scrap.
Good luck on this one...

Lizard928

time for an ott-x pipe and a rmb

MBMB

Already got the RMB. The neighbors will be at the door with pitchforks and torches if I just add an Ott X-pipe, so I'll probably have to switch back to the rear muffler.

Tomorrow I'l take the resonators off and give 'em a good shake to see what comes out, and report back.

The 02 reading does go from stoich to very lean as the engine warms up.

What could the muted exhaust note be other than blocked exhaust???

MBMB

I admit: I'm a slow, methodical mechanic. I finally got both resonators off today, and nothing shook out. She started up fine with the cats only, and the O2 sensor didn't plummet, so I'll run her a bit longer tomorrow, then maybe take her for a spin.

Any idea what the muted exhaust note could have been other than blocked exhaust? What are the symptoms of an engine actually running lean?

Thanks.

Drewster67

Maybe it's not the exit (exhaust) but rather the intake?.

A shot in the dark on my part -

GooD Luck!

ViribusUnits

Could it be carbon build up on the frount side of the cat?

You were running for a long time, very rich. All that excess hydrocarbons should be coked all over the exauste system, expecaly on the frount side of the cat. It should also be coating the O2 sensor pretty good.

If the O2 sensor doesn't have free and clear access to the exaust gasses, it should fail, and provide you with a false lean.

Pull the O2 sensor, and see if it's caked with carbon, and other deposits.

IIRC, the vacuum thing isn't exactly 100% accurate. It should catch it, but depending on how badly the exauste is cloged, it may not.

MBMB

Andrew -- the intake is clear as far down as I can see without serious disassembly (to the mouth of the MAF). One of my early guesses was plastic bag sucked into the air cleaner. Alas, not that simple.

VU -- If the O2 sensor doesn't have access to the exhaust gases, it should provide a false rich, shouldn't it? After all, the greater the differential between the O2 in the environment and the O2 in the exhaust, the greater the voltage produced by the O2 sensor.

Anyway, I'll pull the cats tomorrow and inspect them and the O2 sensor.

MBMB

VU -- BTW is there a solvent that will dissolve coked-on carbon without dissolving catalytic converters?

ViribusUnits

No, the O2 sensor doesn't measure O2. It measures CO. It's a little misnomer. It's all right though, because CO at the temprestures in the exaust will normaly spontanously combine with O2 to make CO2. This little chemical reaction is what gives the O2 sensor it's power to make the electric current.

The richer the condition, the more CO, the higher the voltage the little cell in the sensor creates. It's kinda like chargeing a battery up.

If something gets in the way of the CO, then it's like a battery where the plates are corroded over. The electrodes don't touch what they need to touch, and the voltage goes down.

Therefor, if your O2 sensor is caked in carbon, the voltage returned will go down, and it will look like a false lean.