Please bear with me as I try to describe my predicament with my 1991 Volvo 240. I purchased my brick in January of 2011. Ran like a champ and I never had any issues until that summer. I'm driving 70 mph on the highway in July in Oklahoma City (it was HOT out), and without warning she decelerates. I push the gas pedal to the floor without any result. I'm on the side of the road, I kill the motor. Turn the key, she reluctantly starts up, but she's not getting fuel and the car won't move. I shut her off and wait. 15 minutes later I turn the key. The car immediately starts, and goes as if nothing ever happened. I drove to a gas station thinking that maybe I had put bad gas in it, so I fill up and throw a bottle of additive in for good measure. Everything's great until I have the exact scenario a month later (i.e. 90+ degrees, highway driving, etc.); she quits, we wait, we go. I do some research, and replace the fuel pump relay and all seems well until August and we go through the same song and dance. In the same month I moved from Oklahoma to northern Michigan (trailered the crotchety 'ole b*****) and once in MI, decide to do some poking around. I check the inline fuel pump- looks new but one of the ring terminals is barely hanging on, so I replaced it and for 2 years the car ran great UNTIL last week when I drove it downstate when it was 88 degrees out. I made it almost 200 miles before experiencing this issue (mid-day), and it happened twice on the way to my destination (total trip oneway was 250 miles), but I drove all the way back in the evening and it was fine. I've spent a ton of time researching this problem and I don't really know where to start. I change my oil and can fix little things but I'm no mechanic, (I'm just a girl :-)) and it's hard to diagnose when obviously whatever part is causing this isn't actually broken! It just malfunctions when it reaches a certain temperature? Hopefully there is someone out there who knows what in the hell is going on so I don't have to reinvent the wheel. Thanks for your time.

The others are quite right about their diagnosis. I used to have a couple of Volvos (164's) from the early 1970's that didn't have the in-tank pump and your symptoms were exactly what I regularly experienced in the summer (in N.J.).

Vapor lock occurs when the there's a negative pressure somewhere in a hot area of the fuel line (such as near the exhaust?), allowing the fuel to vaporize into bubbles -- these are obviously not dense enough to provide adequate fuel density -- causing fuel starvation. Without a working in-tank pump, that section of fuel line between the gas tank and the main fuel pump is negative pressure because the main pump is "sucking" the fuel from the tank. With a working in-tank pump, the fuel is pressurized along this line as well, preventing the vaporization/bubbling.

Also, without a working in-tank pump, the main fuel pump is working harder, and may wear out sooner. It wasn't designed to bear the entire burden of sucking fuel from the gas tank.

Finally, when you replace the in-tank pump, be sure to also replace the short rubber hose between that pump and the metal fuel line (on the gauge sender) - be sure to use the Volvo-brand specific for this, despite it's price, because it's formulated/designed for full continuous immersion in gasoline and its additives. This hose is notorious for deteriorating and eventually splitting (especially when exposed to E10 and previously MBTE fuels). A sign of this split is when the car is fuel starved whenever the tank level drops below 1/3 (or higher on uphill roadways).

I agree with Art (and why not considering his frequent good advice)

The in tank pump pushes the fuel out of the tank - a much easier task than trying to suction fuel as the other pump must do.

Volvo would not use two pumps unless it is necessary.

If the fuel is under suction, hot weather makes it more likely to vaporize in the line, causing something like vapor lock - cooling the fuel in the line makes it a liquid again which can be pumped.

You guys are awesome! After reading about how the in tank pump functions and it's purpose, it makes sense. The car is at my mechanics getting the exhaust repaired (I'm a crappy welder) but I read over the article suggested by Art at cleanflametrap, and it *seems* (fingers crossed) relatively straight forward so we'll see how she goes. Again, thanks so much!

Art got to it first, but could you clarify: when you say "I check the inline fuel pump- looks new but one of the ring terminals is barely hanging on, so I replaced it," do you mean you replaced the terminal, or replaced the pump?

Wondering if you replaced a main pump weakened by the dead in-tank pump, and after two years of being over-worked, that pump, too, is failing.

Make sure your tank pump is working. Details in here:

In The Tank
--
Art Benstein near Baltimore

If you pick up a starving dog and make him prosperous, he will not bite you; that is the principal difference between a dog and a man.
-Mark Twain

Hello,

After spending the last three hours reading about symptoms of faulty check valves, fuel pressure regulators, sending units, etc. I'm thinking you're right about the prepump. That's where I'm looking next. Thanks so much!