4.3L V6, Emissions failure, 1986 Caprice, Parissiene

It seems like my question deteriorated into a bunch of personal attacks and advice that the engine was trashed. I rather doubt the engine went from running well to having a nasty case of blow by during the last ten
percent of it's use. The engine has been maintained with regular oil and filter changes with mobile one. So, I'd like to ask again in hopes of getting some more helpful answers. This time I have the data to post.
The car has 226,000 miles or so.
At around 19x,000 miles the car got a new catalytic converter becuase the old one plugged up and the car's performance dropped dramatically and suddenly. We had this around 160,000 miles on an '86 thunderbird so this seems par for things. BTW, the Thunderbird went 245,000 miles before it was driven to be donated away, and was running strong. Mobile One is good stuff...
At 205,000 miles in fall 1998 the data was:
HC 0.26 state limit 2.00 CO 2.2 state limit 30 NOx .5 state limit 3.0
At around 220,000 miles in fall 2000 the car failed. I had a new O2 sensor installed, new plugs, pcv valve, air filter and retested. The car passed but it is clear from the readings that something was deteriorating.
HC .7332 state limit 1.80 CO 19.38 state limit 19.38 NOx 1.16 state limit 2.8
At 226,000 miles it failed emissions under a different test last month:
15 MPH
HC 105 state limit 108 CO .062 state limit .61 NOx 1389 state limit 823 *fail
25MHP
HC 137 state limit 105 *fail CO .12 state limit .64 NOx 1457 state limit 750 *fail
I've read that high NOx is caused by catalyst failure or EGR circuit failure. I also read that the 4.3L engine (comments I read are about the GMC Jimmy, so probably most versions) has issues with the EGR valve and possiblly the EGR passage in the intake plugging up. So, I'm thinking the EGR system is the most likely suspect.
On the GMC Jimmy, I read that there is a new engine computer prom that opens the EGR wide open periodically to keep it clear. Is there such a prom available on the '86 model year for chevy/pontiac or was this specific to the Jimmy later on?
Does anyone agree that the EGR circuit is the issue, or should I be looking elsewhere? The check engine lamp used to come on after an hour of freeway driving before the new O2 sensor was replaced. It used to ping slightly under light load and high speed, also suggesting an EGR problem or a carbon build up in the cylinders.
I'd like to hear opinons and experiences concerning the use of Sea Foam cleaner or Mopar combustion chamber cleaner for cleaning out the carbon deposits.
I'd like to know if the EGR passages in the intake can be properly cleaned without removing and replacing the manifold.
I'd be very interested in peoples advice who have experience specific to this engine. Generalities such as flush the oil with kerosene don't help much. I've already got a Dodge 225 /six that burns a quart per 500 miles after I used that commercial "engine flush" product years ago. It's bad stuff.
Thanks all, and lets not attach the trolls. ;-)
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wrote:

intake charge resulting in lower temeratures. Therefore if the EGR is clogged you would get high NOX readings.. however the EGR is usually closed at low speed to ensure that the engine runs smoothly.
A lean mixture causes high NOX, low CO and normally low HC, however if its too lean you may get a miss and HC will go up.

A lean mixture also causes pinging. In the maintenance list above I did not see a fuel filter. That would certainly clog over time. Also the fuel pump itself wears internally and you may no longer be getting full fuel pressure. These types of problems may fit in with your increase in emmisions of time
I would suggest unplugging the O2 sensor and checking the voltage with a multi meter when the engine is running. If the mixture is correct, the voltage should occilate around 0.5 volts. If its between .5 and 1.0 its rich and if its always less than .5 its lean. On most cars unplugging the O2 sensor will cause the engine managment system to run open loop, which is normally results in the mixture being a bit rich.
If this test indicates that the mixture is lean, change the fuel filter next. If that doesn't improve things have the fuel pressure checked. The pressure relief valve may have failed or the pump is not delivering enough pressure.

I have not tried these. We used to pour water an brake fluid down the carb untill it stalled then let is sit for 30 minutes and restart and burn it out.

good luck
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Simon wrote:

Simon,
Thanks for all the hints, especially about the open loop test of the O2 voltage. I didn't know they put out a voltage without being connected to the ECC. That should work well as a poor man's exhaust gas analyzer. Thumbs up on that one!
Could be the fuel pump system too as you speculate. I hadn't given that much thought. Probably have the original fuel filter in there too. My friend who has a GMC Jimmy says that the pump went on his 4.3L system and it's probably the same part. I've got plenty to go on now and my friend with the Jimmy has cleared out this carbon path on his car and volunteered to fix mine. I'm betting on the EGR being partially blocked and partially stuck open with a flake of junk. We will see.
no one
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EGR will cause high Nox. So will a bad rad or thermostat causing the engine to run too warm. A bad fan motor will do it. A bad Cat will also cause Nox failures, but generally Nox is not as high as you are seeing, even pre-cat if everything else is good. Is the 86 an electronic EGR valve? If OBD1 the scanner will tell you what position the EGR is at, and what it should be at. If not OBD1, that test is not available to you.

No
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