Tail pipe soot help please

I have a 1999 with a 5.7 engine in it and 150,000 miles. It starts right up on the first start of the day. Last week it got real hard to start when the engine was hot. You have to turn the engine over a long time before it cachets. When it dose cachet the RPM are about 400 and then slowly moves to 600. I replaced the plugs but that did not help. I noted that the tail pipe had a LOT of black soot in it. I also have 1996 5.7 and looked in that tail pipe and saw very little soot. Any ideas on how to fix the starting problem? Any help or ideas would be great. Thanks Dave

sounds like the engine coolant temp sensor is stuck. this will tell computer to send rich fuel mixture when the engine is cold. it will make the engine hard to start when it is warm and blow black out your tailpipe.

Happened to my old Honda... engine is trying to start "cold" after it's already hot.

Clean your throttle body too. Probably long over due.

Thanks for your help on my problem. The temp gage reads the same as when I bought it new. If the coolent valve is stuck would it read on the gage? Someone told me it might be the fuel pressure regulator. The way I understand it is I must take off the intake manifold to get to the fuel presser regulator. Is this true? I was at NAPA buying anti freeze and ask them about the soot and being loaded up when hot. They said that there is an air mixing valve part that might be bad, that is by the manifold somewhere. They said it was not a returnable part because it was electric and the cost is $89.00. I am not so sure they know too much about this, but they might. Do you know what they are talking about and might this be the problem? I have a 1997 2door Tahoe and a 1999 suburban so I could switch parts to test for the bad part if needed. Are there any other parts might be bad before I remove the manifold to replace the fuel pressure regulator? Thanks Dave

the coolant temp sensor has nothing to do with the gauge. it only sends a signal to the computer to tell it if the engine cold or hot. if the engine is cold , it tell the computer to give the engine a rich mixture of fuel. if the sensor sticks it might still be telling the computer the engine is cold when it is not.

pies, Could please tell me where the coolant temp sensor is located at and what it looks like. I have a 1997 2door Tahoe and a 1999 suburban so I could switch parts to test for the bad part if needed. Thanks very much for youe help. Dave

check the c/k forum they should be able to help you too, probably better than us.....if its not coolant temp sensor

and the sensor is usually on tstat housing or near it...little threaded brass plug with a electrical wire coming off of it