Starts/runs rough when it is hot outside and engine is hot

THESKYMARTY

MEMBER
1995 JEEP GRAND CHEROKEE
190,000 MILES

Runs fine when the weather is cool or just warm. Lately it has been around 100 degrees where I live. When I drive the car to the point where it's fully warmed up, and then shut it off for a few minutes, it will run very rough on start up until I clear it out with the throttle. If I open the hood and let the heat dissipate for a few minutes, then all is fine. Obviously my problem is heat related, since it doesn't do this on days where it's 75 degrees or less. It's acting like it has some type of vapor lock. Today it was around 100 degrees out, and for the first time it did this while it was running. I let it idle for about 10 minutes while I loaded things in the back. It started to shake, rattle and roll, and it was blowing blue smoke out the back. After I got in it and cleared the engine (drove it a few blocks), it was running fine again. Any ideas?

3 Replies

CARADIODOC

MECHANIC
33,871 POSTS

Problems like this are hard to find without a scanner to view live data while the engine is running. You can check for diagnostic fault codes but they're just going to tell you the result of the problem which you already know. If there are no stored fault codes, the next step is to watch the sensor readings. It sounds like it's running way too rich. We can rule out a leaking injector because that wouldn't cause a noticeable problem while running. Fuel pressure regulators leaking into their vacuum hoses are very uncommon on Chrysler products but even that wouldn't clear up at colder temperatures. The fuel supply system is not monitored by the Engine Computer so incorrect pressure will not set a fault code, but a code could be set related to running too rich.

One common suspect would be the MAP sensor. He has the biggest say in how much fuel enters the engine, and since there is electronic circuitry inside it, becoming heat-sensitive is common. There is a range of signal voltage it can send out. As long as it stays within that range no fault code will be set but if that voltage is wrong, the computer can interpret that as needing more fuel. There are many other things that can trick the computer into delivering too much fuel but none of them typically are heat-related.

Don't overlook a simple misfire either. If it's been a while since the spark plugs and wires were replaced that would be a good place to start.