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Hey everyone,
I have an 07 Crv with 170,000 miles on it. When I start the car it starts at a high rpm. Once I put it into another gear the rpm's drop down. I also am hearing a rattling noise which I believe is coming from the exhaust but I could be wrong (had an accord which rattled too but that was the engine mount). On top of that I am getting poor gas mileage. Could it be the cat converter? Would that account for all of the issues? I am not getting a check engine light at all which is odd. Some have said the O2 sensor but would that account for the rattle? Could it be both O2 sensor and the CAT?

Thanks!
 

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The cat will rattle if the inside disintegrates. Had that happen with a Maxima.
If you say it idles high on startup - it's supposed to do that until the engine reaches operating temp, then it will drop down to around 700rpm.
If it still does it when the engine is warm then that's a problem.
The rattling could also be the valves, were they ever adjusted for clearance?
Your best bet would be to have a competent mechanic diagnose the problem, then you can go from there.
 

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At 170k miles I'd say the cat is gone inside. That could also account for the poor mileage as the passages aren't what they're supposed to be if the baffles are all loose in there. The cat may be plugged up to a certain extent as well.
I'd start with the valve adjustment though, when looking at the mileage problem. Get that done first as then the baseline for performance will be known. If it's never been done, or not recently (within 40k miles) then that is the most likely cause of poor performance.
 

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I think you may have a couple of different issues.

The most common source of rattling is the heat shield on the catalytic converter. Pull/cut it off and see if that quiets things down.

A cold idle of 1500/2000 RPM in neutral is in spec. What is yours?

A dirty throttle body or intake air temp (IAT) sensor can cause the ECU to idle the engine at the wrong speed.

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I agree with the folks above, get a valve adjustment, maybe new plugs (if they are old or the wrong type, the gaps could be very wide). Check/clean the items I mentioned.

Then do a test to verify whether poor MPG is because you are doing short trips or has another cause:


Fill up your gas tank, note the mileage.

Go on a nice long drive (90 minutes at least).

Before you get home, top off the tank again.

Divide miles traveled by gallons used, post back.
 
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