Rough Idle Solved - And Now This... (1 Viewer)

[MountNGoat]

OK wizards of the LC world, hoping you can help me out.

Short story - last May was returning to Cali from a North Rim Grand Canyon trip (pic for attention - taken at Kanab Point), heading north out of Vegas when all of a sudden my '97 started stuttering and losing power on the freeway while doing about 60 mph (huge cross winds). The CEL was NOT on. We stopped several times to try things, and when we deactivated the EGR (disconnected the vacuum hose off the drivers side) it appeared to be running normally again. That got me most of the way home before it started stumbling again to the point of dying, but now just when stopped at an intersection. Deactivating the EGR of course caused a CEL to come on generated the P0401 code (insufficient exhaust gas flow). I changed the EGR valve and modulator to no effect, except the CEL went off and I wasn't throwing any codes. Since the plug wires weren't OEM, I switched those out, along with the plugs. At this point nothing had changed - it was still idling very rough, and when driving it would still want to die when I came to a stop in gear. Lots of things checked, including opening the wiring harness near the EGR valve (all good) and looking at where it comes into the cab (all good).

Then one day I noticed an exhaust leak at the rear Y pipe to exhaust manifold. Investigation suggested that could be messing with the O2 sensor readings downstream and messing with the computer. New gaskets were ordered, installed, and....problem solved. No more rough idle, no trying to die at stop signs, perfectly normal.

Until it had gone through it's drive cycle and then the CEL light came on. Codes: P0120, P0100, P0110, P0115, P0130, P0133.

Getting all of these new error codes that didn't exist before we fixed the exhaust leak was a surprise for sure. Is it possible that the exhaust leak somehow kept the codes from manifesting? Some other reason these codes all showed up? Inquiring minds want to know, thanks for your help.

 

[scottryana]

I would reset them and try again. A lot of those aren't exhaust specific. I see throttle position sensor, IAT, coolant temp, etc.

 

[Rifleman]

You said you checked the wiring harness going pass the EGR, and it looked good. But since you moved that harness, and replaced the EGR valve you now have all these codes that weren't there before. I'm thinking what do all those codes have in common, all the wires that connect to the throttle position sensor, IAT, and coolant temp that are now setting CEL codes, all run down pass the EGR valve, so if it were me, I'd be taking another look at that harness.

 

[REKCUT]

I go with the reset option first like Scottryana suggested. Easiest first. I've had a couple of wierd code issues happen once and never come back. I am assuming its just codes and the truck seems to run fine

 

[baldilocks]

When you first deactivated the egr valve, what you did was remove the heat source from the wire harness beside the egr plumbing. This helped.

But then you disturbed something when you replaced the egr valve. I agree with the theory that you will find problems with the harness once you have all the wire near the egr system exposed.

 

[MountNGoat]

Thanks for all your input. Yes, I reset the codes yesterday, so we'll see what may reappear. After replacing the EGR valve and modulator the CEL still never came on. Opening the wiring harness came several months after that, but like I said inside all of the wires looked very, very good. There are only about 110k miles on the truck. So if it's a wiring problem in that part of the harness it's not easy to deduce.

My theory on how disconnecting the EGR valve seemed to help initially is that the exhaust leak was letting O2 into the stream, affecting the O2 sensor downstream and in turn causing incorrect fuel mixture. It smelled really, really rich. I'm certainly no mechanic, but for extra O2 to get into the exhaust stream it would seem there would have to be a lack of back pressure in the exhaust, and maybe when the EGR valve opened and siphoned some of the exhaust off, this could happen, and with it closed it had enough back pressure to prevent this, until it ate away at the exhaust gasket enough that even that didn't matter any more (when the problem first manifested I could not see the exhaust leak).

Definitely a mystery. I'll wait for the truck to get through its drive cycle again and see what happens. By all external observations, though, the truck is running normally really well.

 

[MountNGoat]

Update to close off this thread:

I'm extremely happy to report that after resetting the codes and driving for a week, the codes have not come back and the system status is normal. New lesson learned about what a simple exhaust leak can do. The hard part was that the exhaust leak wasn't initially visible, so several months of driving and the gaskets further deteriorating were needed to actually see it, while in the mean time we chased all the 'normal' stuff to look at. If these symptoms ever happen again I think I'll take it to a smog shop or somewhere they can use a sniffer to detect an exhaust leak before I go tearing everything apart and throwing parts at it.