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1995 Dodge Caravan, Hard to start after sitting all day in the sun

I have a 1995 Dodge Caravan, equipped with the 2.5L 4-cylinder engine and 3-speed automatic. I have owned this van now for about two and a half years, and it currently has 123,xxx miles. I drive it every day to work.

The problem that I have is that after sitting all day in the hot sun, it is often hard to start, and the computer doesn’t set the initial idle speed properly. Sometimes, the idle drops as low as 250-300 RPM (the needle bounces around a lot) and the engine is shaking a lot, and I smell gasoline. Sometimes the idle jumps up to about 1500 RPM for a few seconds, then drops down to about 800 RPM, but I smell gasoline and I feel a misfire. When I start it up in the morning, the computer sets the idle at about 1000 RPM, and it runs smoothly and there is no fuel smell.

If I rev up the engine to at least 1000 RPM for a few seconds before pulling out of my parking space, when this problem happens, then it usually runs mostly fine. However, sometimes when idling in traffic, I feel a slight misfire, and the tachometer needle bounces around, and when this happens, the oil pressure and charging voltage gauges bounce around as well.

I took the van to a supposedly well-respected shop last summer, to have this diagnosed, and they claimed that there was no problem, as throttle-body fuel injection is inaccurate, and this type of engine runs this way. Since I have never owned a car with this type of engine before, I decided not to pursue the matter further. I had an identical van previously, but with the 3.0L V-6 and it ran very smoothly. It had over 200,000 miles on it, and still ran like new when it was totalled in an accident.

Anyways, today I had the opportunity to test-drive a Dodge Shadow with the same exact engine, and it ran very well, and idled very smoothly, at startup and in traffic. Sadly, no tachometer, so I could not see what the engine RPMs were.

I would like my van to run as well. I have already replaced so many parts on it within the past 2 years, including (but not limited to) the fuel pump, fuel filter, fuel injector, fuel pressure regulator, idle air control motor, throttle position sensor, MAP sensor, throttle body rebuild kit, both coolant temperature sensors, all belts and coolant hoses, thermostat, water pump, spark plugs, ignition wires, distributor cap and rotor, several vacuum lines, and various seals and gaskets, etc. Also, last winter, due to overheating and exhaust gas leakage into the coolant, I replaced the cylinder head (from a junkyard, rebuilt by a professional machine shop before I installed it), head gasket, bolts, intake and exhaust manifold gaskets, valve cover gasket, and oxygen sensor.

I assume the problem is due to unmetered air entering the engine somehow, but I am not sure. I replaced a lot of the old brittle vacuum lines, and vacuum fittings which seemed loose. Any ideas?