Update on rough idle, your help tech please.

[fuzzypuppy]

Ok Tech, we have been discusing this problem of mine for some time now, heres an update, I pretty much trucst your word and advice more than most,
your thought was the temp sensor since the problem get worse as temp went up, so last week i changed the temp sensor, though the old one tested within specs, NO change, i still get the rough idle, plugs tell me it is running lean, performed a vacume test to check valve train etc, this showed running lean as well, the only thing left is the o2 sensor, it varied during the test but not as much as my book said it should, could an o2 sensor cause the problem i have if it does operate, but only within a short range, meaning yes it varies but not all that much, it is a fairly new bosch unit, but during a short 3000 mile vacation i noticed that it not only idles rough but in this low rpm range it as well "hard to explain" runs like a dog, not so much a miss persey but barely giving it a little gas in neutral at a stop light, it just runs like a dog, but again even with this, giving it a little gas to take off from the light she comes back to life, no studder nothing to sugest a fouled or bad plug or cylinder, as well after a particular hard 300 mile run i noticed a definate valve train tic,
"maybe" hard to tell since i still have not replaced the cracked manifold on the drivers side, as i spoke of before i replaced the cam its been about 10.000 ago now, i have no miss at highway speed no lack of power, she pulls great through the power band, could a bad cam lobe cause the bad idle yet perform perfectly in every other way? im very meticluas about torques and adjustments, and my valve adjustment was dead on when i installed the new cam, im from the construction world for the most part where you check your measurments over and over to be sure your dead on, do valves need to be readjusted after initial breakin? since this would explain the valve train tic i got after the hard run i put her through, i was dead on in setting my timing after the cam install, could my timing be off just enough to cause the rough idle and lean condition ?im truely about to give up on my girl, as well i can let her sit in the drive way for as long as i want and idle and it will never idle rough, its only after i drive around a bit that these things happen,
anyways TECH the temp sensor was not the cause of the problem, IAC, is new and as far as tested runs perfectly, the MAP sensor checks perfectly,
the TPS checks perfectly, the radiator may not let it get to temp in my drive way since its been changed to an old school 4 row brass that I had cut and custome fit, i know the o2 sensor takes a temp of 600 deg far, to begin to operate properly, but i figure thatsitting in my drive way it reaches this temp,
I was hopeing that the vacume gauge would show somthing bad in my valve train to explain what in the world is going on, but only shows a lean mixture, which i already figured out from my plugs, could the platinum plugs im running be the cause of my trouble? going to the bone yard to pick up a new ecm is my last ditch effort. which i doubt is the problem since everything else checks out from the ecm, and the truck ran great for a good 5000 after the cam install, i dont know im just stumpped really stumped, i know ive been picking your brain for some time over this and sorry to do so i just have read your past posts and agree on about every piece of advice youve given every other posters problem, so sorry to bug you yet again and thank you so much for all your advice so far.

[ChevyTech]

Hi fuzzypuppy

You said the drivers side exhaust manifold is cracked. There are pulses in the exhaust system that will draw air in a crack as well as leak exhaust out the crack. The air that gets in, will make it so the O2 readings, do not accuratly represent what is happening in the combustion chamber. The O2 senser does not see burnt gas, it only can see unburned air.

A cracked manifold will make it run rich. Even for a experienced mechanic some time it is very hard to tell if it runnig lean or rich, just by listening to it, on a fuel injected engine. When TBI engine runs rich the idle get slow and unstable.

You said the O2 voltage did not vary as much as the book said it should. The cracked manifold, that makes it run rich, could have the O2 sensor coated with soot. Drive it down the road to warm up and clean the sensor before testing it. The voltage should swing from 0.1 volt to 0.9 volt, but the tool that is being used to watch it can make a difference. Some equiptment does not "sample" very often, making it look worse then it is. Watch the voltage for a couple minutes before condeming it.

could the platinum plugs im running be the cause of my trouble?

I always run AC plugs, but I doubt your plugs are a problem.

My guess at this point would be, the manifold is causing some if not all of your problems.

Hope this helps.
ChevyTech