940 Weird fuel issue in extreme heat

I have a somewhat unusual issue that only happens on very hot days (95+ degrees). What happens is, after I've been driving for a while I begin to lose power intermittently, as if I was running out of fuel. This gets more and more frequent until I have to pull over. I then pull over, wait 15 - 30 minutes, and can then start the engine and drive off. It runs fine for maybe 20 minutes, and then I begin to lose power and so on. This continues until I get where I'm going or the temperature drops into the 80s, at which point it goes away and the engine runs perfectly. There have only been 3 days where it's gotten hot enough for this to happen, otherwise this car has zero issues.

Some context, the car is a 94 940 NA with 280k miles. As far as the fuel system goes I replaced the main fuel pump and filter (Bosch oem) about a year ago when this first happened. I replaced the in-tank pump about 4 years ago (AutoZone pump). This problem happens even with a completely full tank of gas. I know it has to be fuel related because when I start it up after it's died I have to crank it longer than usual, as if there were air in the lines. I've checked the current draw of both fuel pumps and they are normal.

I'm at a bit of a loss, the only other component in the fuel system I haven't replaced is the pressure regulator. A mechanic I was talking to suggested vapor lock which does sort of match the symptoms but I have never heard of happening with FI engines, so I don't know if that's a possibility. I can't diagnose it because it happens so rarely.

Does anyone have any ideas? Have you ever had problems related to very hot days?

The power stage could be dying.

Is that the ignition power stage, the little module mounted on the firewall? That seems like a plausible suspect.

If your car is LH2.4 you have a powerstage and it is mounted on the driver side inner fender.

If you have Regina- you have NO powerstage (ignition amp) and just that big ugly ignition module/coil.

You gotta do some diag.

I do have the bosch ignition system with a regular looking coil.

I would go after the fuel pump relay, the powerstage, and the crankshaft position sensor.

I have had two powerstages that started failing HOT and then eventually quit.

TacoSalad said:

What happens is, after I've been driving for a while I begin to lose power intermittently, as if I was running out of fuel.

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Lose power? As in a complete loss of power / engine shut down or is power output just declining? If it is a complete loss of power then it could be a thermal problem in the electronics, either a component going way out of tolerance or a thermal fracture in a solder connection.

Since it is intermittent, to help you diagnose the problem, when it occurs you could try spraying the components identified by ZVOLV with a cooling spray to see if that quickly resolves the problem. Cooling sprays are typically sold by Electronics Parts Vendors (not stereo stores). GC Freeze Mist is what I have on my shelf; but, there are others.

Spraying the crank position sensor will probably not work because the thermal mass of everything it is attached to won't allow the spray to cool the sensor down. However, always being in direct contact with a hot engine probably means it is less susceptible to changes in ambient air temperature so lower on the likely cause list. The ignition module would be high on my candidate list.

Looking through some older threads on the subject it looks like the powerstage was installed with thermal compound between it and the mounting bracket/surface, to help sink the heat into the surrounding metal? That stuff dries out and becomes less effective over the years.

I would suggest refreshing that stuff, whether you replace the powerstage or not - either with a tube of thermal compound or a slim "thermal pad" for easier installation. The cheap stuff off Amazon should be fine.