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Carbon Fouling on Exhaust Tips


Rich

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I've recently begun to notice some carbon fouling on my exhaust tips after about 25 miles of relatively high speed running (75-80 MPH for about 30 minutes/25 miles). This must be a recent development as I've owned the car since new, for 12 years now. 1997 Eldorado ETC with just over 100,000 miles now. My driving cycle has changed somewhat over the past few months so I now drive about 50 miles daily (round trip) virtually all highway. I'm a regular advocate of WOT maintenance and the car doesn't blow any smoke ever (not that I can see in my rearview mirror) during WOT applications. The fouling wipes right off with a paper towel, but it now seems to require cleaning daily to look clean. As I said, this appears to be a recent event and I'm wondering if this is something that just develops with high mileage cars. I suppose I can change gasoline brands, I currently use Shell which is a top tier brand I believe. I'm in New York and don't know if this might relate to a seasonal blending issue or something.

Hopefully this isn't a sign of some deeper trouble. I had the dreaded head gasket repair this past February at the dealer (timeserted all 20 headbolts, etc, etc) and the car runs great as it always has. So whats up with the sudden fouling? Any ideas?

Thanks all in advance.

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Hey Rich, have you checked for codes? If you are running rich you will get this. Are you using any oil? What viscosity oil are you using? How is the idle, starting, etc? In closed loop the O2 sensors are striving for a 14.7 air to fuel ratio about 50 mph. The question is what is causing your rich mixture, check your fuel pressure regulator, by pulling the vacuum hose off of it an smelling for fuel any evidence of fuel replace the FPR. You can also idle the engine with the hose off and rev the engine to see if the FPR spits fuel.

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

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Sorry, forgot to mention that there were no codes thrown. I have been using Mobil 1 10W30 for the last 75,000 miles or so. Car starts right up fine on first crank, runs and idles smoothly. No other symptoms I can note. My fuel pressure regulator was replaced years ago, but the symptoms then were difficulty starting, excessive cranking.

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The operative words being "years ago", a bad FPR can have multiple failure modes it can starve the engine and it can flood the engine. Because the FPR is attached to engine vacuum it can have specific failure points. The black sediment implies that you are running rich, but it could be caused by oil bypass also. No codes have been set, so the O2 sensors seem to be happy with the emissions. That said, which O2 sensor did you replace? Maybe it was fouled with oil. How is your oil consumption?

Personally, I would stop using the Mobil 1, there is no need for it in the early Northstars. I once put Mobil 1 in my 91 Seville 4.9 and took a trip to Virginia Beach and was in bumper to bumper traffic on 95 in 100 degree heat. I pulled off into a rest area and my lifters were making noise and the number 1 main bearing knock the engine was known for was loud, the engine NEVER made a sound other than a slight #1 main bearing knock. I checked the oil and it was a quart down, what?, it NEVER USED OIL!!!!

The next morning after I got there, I went out, and started the engine, and it was noisy. I immediatly got the Mobil 1 out and changed the oil and filter, and the engine returned to normal. I will NEVER use Mobil 1 or for that matter synthetic because of that experience in an engine that was NOT designed for it. Rethink your decision to use Mobil 1 in your older engine.

Consider this, I am using Delvac or Rotella 15W40 in my 96 Northstar because it has a higher concentration of zinc, because we have rubbing element lifters not roller lifters and because my case half leak is better and because my oil pressure at idle is good. No you might not need 15W40 because you don't experience the problems I do, but you might consider using 10W30 standard oil to see how your engine sounds and performs..

MY engine?, with 15W40 sounds incredible. Many NSs do not need 15W40 and I do not recommend it UNLESS you have an oil light coming on at idle, but you I think would benefit from a good grade of standard 10W30

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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I agree that the FPR is worth a look, mainly because it's a simple answer that can cause your symptoms without throwing a code. The rich mixture could be happening at a point where the HO2 sensors will not be used, such as engine braking with the TCC engaged.

How is your fuel economy lately? Any change that you have measured or noticed?

How about an oil analysis? That just might tell the tale.

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