Hey guys, thanks for all the replies on this ongoing post. As stated earlier, my mpg has dropped below normal and intake manifold fails vacuum test as rpms increase...pointing to a bad or plugged cat. Now another consideration...the EGR valve passes the running rough test as vacuum is applied, but fails the engine off vacuum test (won't hold steady vacuum for 30 seconds, vacuum decreases rapidly instead). EVP sensor, O ring and seal appears good, and EVR appears good. Could the bad or plugged cat cause the EGR inconsistency (passes one test, fails the other)??? Took EGR valve off and cleaned it...it appears to work okay. Also, noticed a puddle of moisture under tailpipe on driveway after I put my tools up...stuck my finger in tailpipe and it came out gray and wet??? Is this normal? I'm wondering which way to go next...change the cat and then recheck the EGR, or change the EGR and recheck vacuum in intake manifold??? Any ideas?

 

TM ,
The Egr should hold vacuum for as long as vacuum is applied . It obviously has a bad diaphram if it loses vacuum that quickly . What Im thinking is ...If the cat is bad , there has to be a reason why , they just dont melt / clog up for no reason . If you do find that the cat has melted and clogged the exhaust , you have to find out why ,You mentioned earlier the sooty plugs etc . Something is causing the engine to run too rich .I know if the Egr is inoperative , it causes combustion chamber temps to rise a little , but not to the point of serious damage .Maybe some of the others know a way to check the condition of the cats w/o removing them ? The grey looking mixture you found mixed with the exhaust moisture kinda sounds like cat material ...its sort of a grey brown looking stuff . Its hard to give definite answers w/o having a look at the truck .Good luck with it .
Paul

 

i would consider looking at the pcv valve because this will cause major problems with idle and exaust issues,. just a suggestion.

 

The grey water mixture is normal. The result of fuel burning is mostly water and unburned fuel. You just are seeing a mixture of this.

I forgot how many miles your truck has seen or where it is located. If you live a very cold climate, then your truck will see a warm-up "rich" situation more often than a southern driven truck. If you have over 100,000 miles combined with alot of rich driving will destroy a catalytic converter by plugging it or causing it to crack. If you want to tell if the cat is working or not here is how I do it (this may not be practical for you).

Make sure the truck is cold. Start the truck. Crawl under the passenger side and, using an infra-red non-contact thermometer (I borrow one from work), note the temp of the cat housing (not the heat sheild) compared to the temp of the pipe directly in front of it. Since the truck is running at high idle, the mixture is rich, that means alot of unburnt fuel is pumping into the cat. If the cat is still good it will start to heat up faster than the pipe, it is a small difference but you can see it with the thermometer. After the truck is fully warmed up, the cat should always be slightly hotter than the pipes ahead of it because the catalytic conversion process is exothermic (get out those thermodynamics texts). Be aware that thermal gradients will eventually balance out all the exhaust temps.

Also, after the truck is really hot and has been driven awhile. Put on some heavy welding gloves and try to shake the cat. If you hear stuff rattling around in there, the cat is shot and will need to be replaced.

There you go. Oh, a good place to replace the cat is from Catco, they sell universals for $99. Might be a good time to think about a true dual exhaust!!

Later.
Randy