I know its been posted ad nausea, but as i have lurked and searched the other related topics, i have yet to find a cause/solution. so here goes.
-TB is clean
-no cracks/voids along intake
-wires are new ac delco brand
-TOAD is reading no misfires in any cylinder
-no CEL, no DTC's
-propane torch along intake, no changes in RPMs
symptoms
-rough idle in drive, at stop lights, stops signs, etc. No effect on drive performance/ no hard shifts
-rough idle in park ONLY when AC is on full blast, turn AC off, rough idle goes away but only in park.
-AC check shows it is fully charged, but it takes about 20 mins of actual driving to start to feel cold.
im in the process of checking for AC leaks, none found yet. what am i missing here?
Hello Members,
I only recently joined the forum so I just saw for the first time that rough idle is a frequent problem. Sorry I didn't see it sooner. I've had lots of experience with rough idles in our 1999 Yukon that may match your problems even though great difference in vehicle model dates. So, I can give you some very real things to watch for--and may apply to some pickups as well. From initial purchase--probably a mistake--the idle has ranged from a slight lope when we bought it used to dead smooth now.
Of three major problems we had, the first was big, not just a lope--and expensive--diagnostics by highly experienced technician. Cost a lot to have him look down through and think out all the DTCs leading to suggestion that there may be a problem with one or two of the injector nozzles. So, I bought new DELCOs and tore it down. Surprise--maybe you too--the existing nozzles were merely loaded with ash in the skirts below the nozzle orifice. This area has the skirt only about 1 mm high and about 5-6 mm in diameter. What had happened is the GMC dealer who sold us the vehicle had been using trash oil of some sort on our four oil changes. Then I changed oil using good, clean, detergent Mobil Super and all the garbage poured forth. We made it about 10 miles before the engine was really, clearly sick. After tear down, cleaning the loose crud out of the skirts took about thirty seconds each with a wooden toothpick. On this 1999 5.7 model the PCV discharge points directly at the ports of cyls 5 & 7 and those were the dirty nozzles.
----So might you be having this problem??!!
Next, and this also was REALLY big, the plug for number 7 always ran cold plus the engine always consumed slight amounts of coolant. So after one final try of snugging the one left, rear accessible lower manifold tie-down bolt, the manifold/head gasket squeezed out of place dumping coolant straight into the valley. We shouldn't have tried to drive it home, but, at that time my wallet and bank account were both on empty. Tear down again found the wayward gasket--right at the rear of number 7 where the head has a port, but, the lower manifold doesn't. Machinist straight edge showed the head straight/flat, but, the manifold face warped about 1/32" upward. (Can't explain this!!) I cautiously used a brand new flat mill file to dress out the crown of the bow forward to about half way to the front of the manifold, hoping for enough 'spring' in the manifold to take care of the rest. Roughed up the gasket faces, back together, had to reset all timing to factory defaults---and it idled dead smooth. Had to look at the tachometer to make sure the engine wasn't already running. (Our 2017 Terrain does this check for you just in case someone's mind is on their cell phone, not the car!!
---Are your symptoms in here?
Finally item three, I've been using fuel system cleaners--best results with Gumout and Berryman's Chemtool--to help the nozzles stay clean and make full pattern flow at idle. Really does work well. Then, I used a bit~ half pint~ of that magic Ellixer, Marvel Mystery additive to try to clean up the lifters. It did that plus stopped burning oil, plus mostly stopped dripping oil. Doesn't use oil now! I've become sloppy about the fuel system cleaners because it doesn't seem to need them to idle smoothly. I'm thinking that, along with eliminated oil blow-by, it's doing less compression blow-by and slowing down the crankcase vent flow that's aimed at 5 & 7 ports.
Couldn't be happier with the car's performance. The past 70K miles have been pretty good. 207,000 on it now, and it's really looking like rust will be the fatal element.
If I can help anyone with clearer details, please ask. Once I cleared all the flimsy parts off the beast, it's been well worth having---good utility, rides like a buckboard, keeps you awake on long, back road trips!!
Regards,
NEECAPR
