Very rough idle, ~400 RPM, 2005 Corolla

[Crimbo]

Hi All,

I have a 2005 Corolla that was idling very rough this morning. I have searched the forums and most cases seem to be a dirty MAF or a bad intake manifold gasket. Well this winter I got the P0171 code during two separate cold snaps so I cleaned the MAF and replaced the faulty intake gasket. I'm thinking that this is a different problem both because those issues were recently dealt with on my car and I'm idling at ~400 RPM which I don't recall happening while I had a bad gasket.

Anyway, just yesterday I had the intake manifold off the car again to replace a coolant hose, Water By-Pass Hose No. 1, part here:
http://www.toyotapartsoverstock.com/...62610D102.html

The replacement hose turned out to be not quite as long as the original. The original was hooked under another part below it, but I couldn't run the new hose under the hook as well
First pic gives some context:


A closer look. I replaced the hose on the right and it used to run under the upside down J hook right below it:


Any idea if this is related to my idle problem?

Also, the roughness goes away as soon as I step on the gas pedal. And inside the car, the blower fans pick up when I step on the gas and then drop down in idle, which I don't remember happening before either.

[invader]

Can't see the link or photos... That was a coolant hose on the throttle body? Why did it need to be replaced?
400 rpm idle in Drive? It may need some time to readjust itself... Are you on original ECU? What mileage are you at? Fan speed goes down at 400 rpm idle because of reduced alternator output, which is normal.

[Crimbo]

Sorry, do these photos work? The hose on the right in the second photo.

http://www.flickr.com/photos/7157914...n/photostream/
http://www.flickr.com/photos/7157914...n/photostream/

The original water by-pass hose had failed and leaked coolant everywhere.

Yes, I am on original ECU. And I am at 163,000 miles.

Thanks.

[silver04rollas]

Yeah, 400 rpm is way too low for idle. Are you sure there is no valve float in your engine?

[johnoh]

there is a small vacuum hose that attaches to the rear side of the intake manifold that may have come loose. If that is the problem, you would notice a loss of brake power though

[invader]

Photo shows elusive brake booster vacuum hose sitting on intake manifold fitting, which is hidden once throttle body is installed.

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[Crimbo]

Thanks for the tip, but I'm pretty certain that hose was re-attached when I put things back together. I will double check. Could that cause the low idle?

[invader]

You reconnected your idle air assist plug-in on throttle body? CEL is not on?

[Crimbo]

CEL is not on.
And yes the idle air assist plug in was re-attached.

Thanks.

[invader]

Did you reset ECU and erase trouble code when you replaced your intake manifold gasket, by disconnecting battery or by removing the blue 15 amp EFI fuse in fuse box behind battery for a while?

[Crimbo]

No I did not do that. Would that cause a problem?

[invader]

It might just take longer for ECU to readjust... Try resetting ECU (and double check vacuum hoses and connections) and see what happens.

[mikered30]

What color was the replacement intake manifold gasket?

[Crimbo]

invader: Thanks, I will try after work today.

mikered30: The replacement gasket is orange.

[Crimbo]

So all of the vacuum tubes seem to be in place as they should be and I removed the EFI fuse for several minutes and replaced it. Same problem.

Interestingly, on startup the engine is fine, but the RPM's are at about 1000. After ~3 minutes when the engine starts to warm up the RPM's drop to about 400 and it gets really rough. Normally, the engine starts up to about 1500 RPM and then drops down to 1000 after warming up. Could this be some kind of thermostat/ coolant issue?