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Been trying to figure out my own rough idle lately as well. Same issue as others (sometimes its fine, sometimes it's rough for a few seconds and then is ok for a minute or two). Here's the odd part.... At one point it threw codes P0171 and P0174 (both banks to lean), the then on another occasion it threw P0172 and P0175 (both banks to rich). I've taken the MAF out and cleaned it (the values on ODBII look steady). This morning I watched the short term fuel trim and both banks are going positive 10% then negative 10% then it will hover around 0%. When it gets to around +/-10% that's when the rough idle occurs. Both banks are around the same (one isn't inverse of the other).
Thoughts?
I'll take a look at the EGR as others have recommended.
One other thought: When I when to put the MAF back on after cleaning it I noticed the elbow it plugs onto was loose. I could turn it left and right and even down a bit. Is that supposed to be tight? Maybe I have a vacuum leak?
The other thought I've been having is maybe a fuel delivery issue. Pump? My cheap Bluetooth ODBII wont report the fuel pressure so I cant determine it.
I replaced my EGR valve with a used one and the problems went away for a few months. Recently, I've gotten an "EGR valve circuit error" code in my engine warning light, but that might be a loose connector (I lost the metal clip that secures the plug on it). I'd recommend replacing the EGR, they're under $100 used.
error code aside, the car has run fine since replacing the valve.
I thought it might be a vacuum leak from the (too lean) codes. But what about the fact that its swinging both ways (lean and rich). You think that could still be a vacuum leak?
I'd really like to know if the elbow on the bottom of the MAF is supposed to be loose.
Last edited by TechSuperstar; Feb 5, 2018 at 11:40 AM.
For anyone else who suspects a bad EGR valve causing a rough idle, just remove the engine cover and unplug the vacuum hose where it connects the the diaphragm (red circle). It'll throw an error code, but it stops the EGR valve from actuating, so if your rough idle goes away, you know it's a bad EGR valve.
TechSuperstar, my problem never threw lean/rich codes, so perhaps the EGR valve isn't the issue. still, it's an easy test to try.
1: Dirty air filter clogged with dirt or oil.
2: Spark coil going bad on a cylinder - but typically the CHECK ENGINE light comes on for that.
3: Spark plug wire old - rotted rubber, going bad, or shorting out.
4: Fuel pump and injector issues. Specially - an injector over a cylinder causing a misfire. Starting to go bad.
Either case it could be traced down a fuel or spark issue. Pull the FAULT CODES from Advance Auto - it's free.
Almost forgot - another reason for rough idle could be from (ready for a major headache?) that EGR exhaust gas re-circulation valve sticking OR any one of the myriad of rubber hoses attached to the engine. In the old days, the V8's typically had one small rubber hose sending vacuum to the distributor to advance the spark....or one to the valve cover that kept a vacuum inside the oil pan. Those were the only 2 rubber hoses that could possible present a vacuum leak.
Today - it's like a salad buffet bar - take your pick. 20-30 rubber hoses all sizes and diameters sending signals from computer sensors, coming from the EGR valve, you name it.
Used to be easy to work on these motors - before ELECTRONICS invaded them. Now you need a diploma from a college in electrical engineering just to diagnose codes and trace the problems down.
Originally Posted by cstevenson
For anyone else who suspects a bad EGR valve causing a rough idle, just remove the engine cover and unplug the vacuum hose where it connects the the diaphragm (red circle). It'll throw an error code, but it stops the EGR valve from actuating, so if your rough idle goes away, you know it's a bad EGR valve.
TechSuperstar, my problem never threw lean/rich codes, so perhaps the EGR valve isn't the issue. still, it's an easy test to try.
My guess is a vacuum leak caused by a broken hose. The rubber and plastics in the vacuum system tend to get brittle.
Rodney, Going back on older posts to share resolutions in case others find it helpful...
The misfire on Cylinder 2, ended up being a bad camshaft magnet solenoid (exhaust side, Bank 1) that was causing an excessive amount of fuel to the injector of cylinder 2. For some reason, it didn't set off a code but gave me a single misfire. This was discovered after having a good Indy perform an extensive fuel trim diagnostic.
Lesson learned for other forum members reading this... swap out camshaft solenoids as part of your troubleshooting methods when narrowing down a misfire. Prior to figuring this out, I swapped coils, spark plugs, and injectors.
I ended up with a spare manifold in case you know someone wanting to purchase for the m273 engine (2007-CLK550).
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