Black soot, running rich

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east302

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My 1998 (350, auto, 180k miles) has had this issue for a few years now but now has a new symptom.

1. Under hard acceleration when, for example, merging onto an interstate it will sometimes act like it is misfiring. Years ago it actually threw a misfire code, but I don't remember what cylinder it was. This is when accelerating with 3/4 throttle steadily from 10-mph or so to 60-mph. Once up to around 60, it will stumble and only stop when you let off the gas. It feels like you're hitting a really bad rumble strip. I had both catalytic converters replaced about a year ago (was told they were clogged and I could hear them rattling) and it solved the problem. But, what actually *caused* the converters to be damaged was not addressed so, of course, the problem is back.

2. Under steady cruising at 40-45 mph in 4th gear, it feels like the engine speed is too low for the gear and it seems to lug. It's in the 1100-rpm range I think. If it was a manual transmission, it would be like shifting up too soon. Getting out of that 40-45 range, dropping it into 3rd to bring the revs up or tapping the brake to disengage the torque converter fixes that. When I had the catalytic converters replaced, this problem went away.

3. Spark plugs and wires are ACDelco, with about 60,000 miles on them. The distributor is also a Delco unit and is about a year old.

4. No CEL right now.

5. I have had a gas smell at the exhaust pipes in the mornings. Now that it's cold, I've been warming it up in the mornings and now have black soot stains on the pavement under the exhaust exits.

6. I've had to run 91+ octane in it for years. It pings otherwise.

7. The O2 sensors are all original to the truck as far as I know.

8. All highway, I get about 13 mpg. Around town, 10-11.

9. Exhaust is a true dual setup.

So there we have it. Any thoughts on properly diagnosing this thing? I'm fairly sure that I've ruined or am about to ruin my new cats here, so it'd be good to fix this right this time around. Thanks.
 

Ironhead

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Sure sounds like the mixture is too rich. Two possibilities I can think of without too much effort: bad O2 sensor(s), or leaking injector spider.
 

east302

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Thanks ironhead. I always thought a bad oxygen sensor would throw a trouble code. Or do you only see a code if the malfunctioning sensor causes something else to not work properly?

I'll pick up a pressure gauge to see if the fuel injection is leaking down.


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NAST96

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Have you ever considered replacing the coolant temp sensor? Mine ran pig rich and always sounded like it had a misfire, even at idle. Did plugs,wires, cap rotor, MAF, threw all kinds of things at it. Did a little more research and decided to try the coolant temp sensor that goes in the top of the intake. Turns out mine thought it was -30, so it was just dumping the fuel to it. Just something else maybe to try...
 

east302

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I've heard of it but wasn't really sure what it did. I'll definitely keep the CTS in mind, the one I have is probably original to the truck. Thanks for passing that on.


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east302

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Here are the pressure readings...

-Key on engine off, 57-psi
-Cranked it stays at 57
-Turn off and it jumped to 60 and settled at 57 where it stayed for at least five minutes.

It's an AC Delco pump, installed in 2009. I've put 50k miles on it since then.



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NAST96

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Sounds about normal to me. They say anything over 50psi will overcome the poppets. I have heard guys say that you need 60+psi. I have never got that much even after installed a new pump and filter. Mines always about 58psi or so. Sounds like your FPR is good also beings it isn't leaking off. With you saying it runs really really rich I would be tempted to say it's not a fuel issue. Sounds like your getting plenty of it. So I'd start elsewhere.


There are 2 temp sensors on these trucks. One in the head on the drivers side, and one right in front of the thermostat in the intake. The one in the head goes to the gauge in the cluster, the one in the intake feeds the computer. Therefore you might not be seeing anything "abnormal" on the gauge, but your computer is completely lost. From all of the ones I have seen with a CTS issue, they almost always run really rich and sound like they have a dead miss. Sometimes even at idle. I'm not saying it's your problem, but it would be a real good place to start and possiblly sounds like your issue.

With you saying you have stain's on your driveway, I'm guessing both sides are equally or close to equally running rich, therefore both banks are over fueling, it would be very unlikely you have a poppet issue. Sounds to me more like something is telling the computer to put more fuel to the engine, like its seeing a lean condition, or thinks it's very cold out. If it thought it was a lean condition it could also but not very likely be your knock sensor. It is located on the passenger side of the block between the oil pan and cylinder head. This would also be a possibility as you said it pings some with anything less than 91 in it. The knock sensor could be telling the computer that the engine is lean, and it needs more fuel. So that would be a good thing to change maybe also. Both CTS and Knock sensors are relatively inexpensive so your not going to break the bank changing them. The CTS is the most common thing to try though. And would be a good start.

If you do change the CTS there is no need to drain your antifreeze. Let the truck get warmed up and let the thermostat open, then shut the truck off and let it cool down. Do not open the radiator cap! The cooling system is under vacuume at this point. If you unplug the sensor (make sure you have the new one nearby and ready, they also usually already come with sealant on them) after unscrewing your old one, put the new one in relatively quickly, obviously. You should loose little to no coolant if you do it this way. And let's face it, draining coolant in these trucks suck unless you just pull the lower hose in a "controlled" manor.

Usually an O2 issue comes up with a check engine light, although not always. Do you have access to a scanner that is capable of live data? If so you could go in and check where the O2's are reading.

Goodluck.
 

east302

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Thanks, that sounds like a reasonable next step. It was dripping "tobacco juice" again this morning so it'd be nice to get this wrapped up.

I don't have a scanner (wish that I did) but I think you can rent them from parts stores sometimes.


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east302

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If you unplug the sensor (make sure you have the new one nearby and ready, they also usually already come with sealant on them) after unscrewing your old one, put the new one in relatively quickly, obviously. You should loose little to no coolant if you do it this way. And let's face it, draining coolant in these trucks suck unless you just pull the lower hose in a "controlled" manor.


Goodluck.

Well swapping the sensor took all of thirty seconds. It had that red thread sealant which gooped up. The manual says to torque it to 10 ft-lbs but it isn't seated all the way in as the resistance from the thread sealant didn't help. I guess it's in good enough...it doesn't leak and I'll see if it helps with the issues that I'm having.


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east302

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Just an update, I replaced the sensor and am not seeing the spots on the pavement so I'm thinking that might have been the issue. It still has that lugging feeling when coasting around 40, but I really think that is the result of damaged catalytic converters because that exact symptom instantly went away when I replaced them the last time.

They're probably messed up again since I didn't fix the root cause the first time around. Lesson learned on that one.




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