Question: My 1986 GMC S-15 pickup with the 2.8-liter V-6 has a crate motor with 10,000 miles on it. It runs fine when I first start it at the beginning of the day. Once I get somewhere and shut it off, when I restart it, it seems like it wants to die and has a rough idle for approximately two minutes, then smoothes out when I give it gas. It does this every time. I've had the truck in the shop numerous times with mechanics just chasing parts, and the problem still exists. To date, they have replaced the air idle mixture valve, cold temperature water sensor, fuel pump/filter, spark plug wires, distributor module, catalytic converter, and coil, and they checked the vacuum lines and intake manifold, and there were no leaks in either. When I bought the truck last year, it had 6000 miles on the crate motor, but it had been sitting (not driven daily) for seven years.
Answer: There are lots of possible causes to rough-running on hot restart. First and foremost, diagnostics should begin with a scan tool to view all diagnostic trouble codes, and a good look for improper data received by the Electronic Control Module. Any problems found should be repaired. Here are a couple things to look out for: Remove the air cleaner, turn the ignition to On, and check for fuel dripping from either of the two fuel injectors mounted on top of the throttle body assembly. Leaking injectors will drip excessive fuel into the combustion chamber while sitting, and cause a rough startup.
There's also a Technical Service Bulletin on vehicles stored for long periods of time in reference to deterioration of the rubber hose coupler attached to the top of the fuel pump. (The coupler may not have been replaced when the new pump was.) Cracks in this hose can do the opposite of leaky injectors, but produce similar driveability symptoms. In this case, fuel is bleeding into the tank and reducing pressure delivered to the injectors. Either one of these fuel-delivery problems can be diagnosed visually, or with a fuel pressure gauge.
I haven't heard of this one on your particular truck, but a possible cause is coming to surface as we speak: ethanol. In the winter, petroleum producers distribute more volatile gas, meaning it burns better in cold temperatures. This is done by changing several components in the gasoline formula. A big one is the increase in ethanol (alcohol) content. The winter blend increases the efficiency of cold engine combustion, but also lowers the boiling point of the gas.
In some cases this lower boiling point will produce what we call a "hot soak" condition (in the old days referred to as vapor lock). The engine runs fine on a cold start, and continues to run great until the engine is shut down. But with the engine off, and the vehicle stopped, underhood temperature increases. This is when the temperature of the gas inside the throttle body and fuel rail surpasses the boiling point.
If you restart the engine within 10 to 30 minutes of shutdown, it'll misfire big time attempting to burn boiling fuel. Those minute boiling bubbles produce a super lean condition -- too much oxygen. Try draining the tank and refilling with premium fuel (typically not as drastically affected by the winter blend as regular). Also be sure the throttle plate is clean of carbon deposits.
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