2000 Mountaineer 5.0 stumbles badly in hot weather when accelerating.

[ibalearner]

In hot weather, 2000 Mountaineer 5.0 stumbles badly on acceleration.

[shorod]

Have you checked for diagnostic codes? If the Check Engine Light is not on, that doesn't necessarily mean there are not diagnostic codes.

When is the last time you performed a tune up on the truck (spark plugs, plug wires, fuel filter, and checked the air filter)?

Have you checked fuel pressure when hot?

-Rod

[ibalearner]

Rod, thanks for your reply. Check Engine light came on for the first time last weekend. Plugs and wires, and fuel filter fairly new. Probably time for air filter. Has been stumbling only in hot weather for about 5 years. Thinking vapor lock of some kind? Possibly EGR VALVE? Where is it located?

[shorod]

The EGR valve wouldn't be my first suspicion for a stumble on acceleration. While an EGR is that not closing fully could cause stalling or rough idle (it's a vacuum leak if not closed at idle), the engine should be able to accommodate a slightly open EGR valve on acceleration. If you are familiar with what an EGR valve looks like, it should be pretty easy to spot under the hood. It's been quite a few years since I had my 5.0L Mountaineer to recall what it looked like under the hood.

A weak fuel pump could cause the stumble and would be pretty consistent with acting up when it's hot. Do you have access to a scan tool to retrieve the diagnostic code? That code might prove quite helpful in determining where to focus your energy.

-Rod

[ibalearner]

Rod, thanks for your reply. Scan tool will be connected tomorrow morning at local repair shop. I will post results. Thanks again.