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My 4.9 Seville idles high on start up, around 2,000rpm. It stays that way for about 3-4 min before settling down to around 800-1,000 rpm. Once the car is warmed up, I can shut it off and restart it and the car idles normally. I replaced the ISC with a spare one and did the idle learn procedure, and it does not seem to make any difference. I am showing codes E030 and E055.

Some possible things I am thinking about checking is the timing and TPS. The problem is I don't know how to check if the TPS is OK or not. Is there anything else I am missing?

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By chance could the coolant temp sensor be faulty? If it thinks it is colder it will idle higher to warm it. Once warming up it will drop the RPMs. 

And you mention restart is normal. 

Welcome To CaddyInfo.

I am not highly familiar with the 4.9l so this is just a suggestion

GM FAN FOREVER

Nice, clean, luxury= fine automobile

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In the service manual.....when you have eo55 and eo30 together.....you chase the eo30. 

E030 and P030 are the same thing. P030 is 4 pages in service manual. 

po30(1)2.jpg

po30(2)2.jpg

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