Pontiac Repair: black smoke when car is started, upper intake manifold, coolant temp

Question
QUESTION: I have an 86 2.8 Firebird. It will start fine then after it is driven a bit and shut off, upon restart it will be hard to fire and belch black smoke out the tailpipe. I do notice it takes a long time to warm up. I know the MAF has been replaced recently and im going to replace the O2 sensor. This problem only occurs after the car has been driven a while. Could the coolant temp sensor be the culprit or maybe a leaky cold start injector? I know a good bit about EFI but im stumped on this one.

ANSWER: Hi steve:

well all of the parts you menntioned except the O2 sensor can cause this issue. It may not be a bad idea to replace the o2 sensor anyways being that the car is 24 years old!  but my guess here would be either a leaking injector(s) or cold start injector leaking. the only real way to find it for sure is to get the engine hot then take off the upper intake manifold and leave the fuel line connected and put a piece of paer towel under the injectors including the cold start injetor and then cycle the key a few times to prime the fuel system and let the car sit and if you have leaking injector the paper towel will be wet under the one(s) that are leaking. If you have a fuel gauge put that on too and see if the pressure bleeds off rapidly You could also have a vapor lock condition here if the pressure bleeds of back into the tank rather then into the engine. What happens is if the fuel is not under pressure and on a hot engine the fuel that is left in the rail will vaporize and when you try to start the car your getting fuel vapor at first instead of liquid fuel.. that is where I would start looking. As for the coolant temp sensor you can find a tempature to resistance chart on the net via a googlec search and check the sensor when it's been sitting out all night and comapir the resistance reading to what the air temp is and see if it's close.. Good luck :)


---------- FOLLOW-UP ----------

QUESTION: Thanks for the timely reply!! I want to add that i have checked for codes and there were none. Replaced all vacuum lines. At first i had suspected that the O2 sensor was cooling off enough for the ECM to drop out of closed loop then i remembered that it has no bearing on how the car starts. If theres a vaopr lock problem, how would i solve this? Should i try to replace the coolant temp sensor first due to the slow warm up? ( i have a sensor scanner.) Like i said im stumped here. Ill put a fuel pressure gauge on the rail and check the pressure.

Answer
No do not replace the coolant temp sensor unless like I said it doesn't read what the outside temp is on a cold start and watch it as the engine warms up you will be able to see it working and that car more then likel has a 195 degree thermostat and you can see the that the thermostat is working then as well by watching the senor take a dip once the thermostat opens for the first time.  As for the vapor lok That is a second most likely cause the injectors is wher I would start looking first but if you put a gauge on it and turn the key off and the pressure drops off very qouick with in a minute or so your leaking it back into the tank either past the regulator backto the tank or the check valve in the pump is stuck.