There is a sequence of events and certain conditions that lead up to hot soak. It's hard to accurately duplicate these conditions for testing, so chasing down and proving the problem can be difficult.
I will attempt to explain the issue as I understand it. Since the van is a mid-engine vehicle the engine compartment cannot be vented on top (smoke and noxious fumes in the passenger compartment isn't a good thing). So as a result the engine is sitting under a sealed metal dome. When you shut off the engine, since the cooling fan is run by the water pump, the coolant circulation and the air ventilation go away. Since heat rises it gets trapped there in the dome and cannot escape. If it's a hot day and/or if you've been driving it hard (like going up a steep hill or freeway driving) then there's even more heat getting trapped there. To complicate things, the fuel rail, injectors, pulsation damper, and FPR (Fuel Pressure Regulator) sit right above the exhaust manifold and are covered by the upper half of the intake manifold.
The extreme heat that's trapped there soaks into the fuel rail and depending on the quality of gasoline and the pressure in the rail, the gasoline may boil. The result is expansion and vaporization.......otherwise known as a vapor lock condition. Since there is still pressure in the fuel rail, the regulator will not allow more fuel into the rail until pressure is reduced. The only way vapor/pressure can escape is through the injectors (when you are trying to start). Of course vapor isn't dense like liquid, so there's not enough energy being delivered for your engine to start or run correctly. As the vapor/pressure escapes into the manifold new fuel trickles into the hot rail and immediately vaporizes. When vaporization occurs, volume increases exponentially (expansion) and that keeps the pressure up. This in turn prevents sufficient amounts of fuel from entering the rail. Once started, keeping the engine RPM's up helps because the injectors will vent vapor/pressure faster and allow more new gas to enter the rail. The faster the cool gas flows into the rail, the quicker the rail cools. Once the rail is cool enough to hold liquid gasoline the problem disappears and the van runs normal again.
Because this involves boiling and vaporization, there is a direct correlation between the quality of gasoline and this problem. Back in the 80s this wasn't such an issue because gasoline had a higher boiling temp. Todays gasoline has lots of additives (such as ethanol) and this can significantly lower the boiling point. You can minimize this problem by staying away from the gas that says it has a high percentage of alcohol/ethanol but this option may not be available. It's my belief that gasoline is becoming standardized and brand name doesn't matter much anymore.
So this leaves you 2 other options:
1. Increase fuel pressure in the fuel rail (higher pressure = higher boiling temp)
2. Provide ventilation to your engine compartment after shut-down
I'm not crazy about option #1 because increased pressure = more stress on injectors and other fuel system components (IMO this is more of a safety issue). If and when a fuel system leak occurs, more fuel will leak and your fire danger goes up. That being said, there is an easy way to automatically increase your fuel pressure for only a couple of minutes after start-up. IMO, the benefit of this is questionable because you will only be increasing pressure AFTER the fuel has been boiled. In other words this will minimize the effects of hot soak, but will not prevent it from happening.
There is also a way to permanently increase pressure in the fuel rail, and this makes more sense to me because it's more likely to prevent the problem from occurring. Of course this benefit is off-set by decreased safety.......and that's not to be taken lightly. Fuel leaks in the fuel rail area are not uncommon and several vans have gone up in smoke (sometimes taking other property with them).
Before changes are made, one should understand how the system works. Our FPRs (Fuel Pressure Regulators) have 2 output pressures. When vacuum is applied to the FPR diaphragm it will put out around 30 psi. When no vacuum is present, it will put out around 40 psi. There is a temperature switch on the engine coolant filler neck (vans that came with factory AC). This is a simple on/off switch that's triggered by coolant temp. The switch is normally open but when coolant temp hits 230 deg F it closes and grounds the wire going to it. That wire is attached to the ECU (Electronic Controlled Unit). Each time the van is started, the ECU monitors that circuit and if it detects a ground condition it activates the EFI VSV (Electronic Fuel Injection Vacuum Switching Valve) for a period of about 2 minutes. When the EFI VSV is activated, it shuts off vacuum to the FPR and vents the FPR diaphragm to atmosphere. With no vacuum, the FPR holds pressure at around 40 psi. After about 2 minutes, the ECU turns off the EFI VSV, vacuum returns to the FPR, and pressure returns to around 30 psi.
The existence of this system tells me that Toyota realized the potential for heat soak, but miscalculated the activation temp (of course the changing fuel market is partly to blame). Later on when heat soak became a bigger issue Toyota issued a service bulletin (TSB #E020). Part of that bulletin requires mechanics to replace the 230 deg F switch with a much much lower temp 140 deg F switch. The lower temp switches were only available for this service bulletin campaign (see PDF file attached to this post) and are now NLA (No Longer Available).
If you think about it, the normal operating temp of the engine is about 185° - 210° F deg F, so this means the new switch will activate the "pressure up" mode of the fuel injection system almost every time the van is started (unless the van has had over an hr to cool). Because of this, I think it's reasonable to assume it wouldn't hurt for the "pressure up" mode to be activated on every start (including cold ones). This can be easily achieved by simply cutting the wire off of this sensor and attaching it to ground.
To permanently increase pressure, there are several ways this can be done, but probably the simplest method would be to disconnect the vacuum line from the FPR and plug it to prevent a vacuum leak. I wouldn't put a cap on the FPR port because you'd want the diaphragm in there to breath. Again, I don't recommend this, but I'm providing the info to help create a better understanding of these issues.
Personally, the remedy I like best is the "ventilation after shut-down" method. I'm using this method (and this method only) on my van and I've been happy with it. A side benefit to this ventilation is increased life span of rubber seals, rubber hoses, and other sensitive parts (due to reduced temps in the engine compartment). I've heard other guys talk about opening their hoods each time they stop or using computer fans and ducts to ventilate small amounts of cool air into this area, and I don't disagree with the success of such methods, but who wants to have a super heated and/or smelly cabin on a hot day? As for the small fans and ducts, there's already limited space here and access to key areas is already tough enough. That's why I chose to install electric cooling fans on my radiator. I built a shroud and installed (2) 10" high output electric fans on the back-side (engine side) of my radiator. I rigged them up to be thermostatically controlled and wired direct to the battery (so they have power all the time). I got an adjustable t-stat with a submersible probe, put the probe into the extra 16mm port near the front of the head, and set the t-stat to come on at 200 deg F. Now whenever the coolant temp is 200 deg F or above, the fans will run (doesn't matter if the ignition is on or off). This has worked out great for me and completely solved the heat soak issue.
The fans do come on and run (on and off for about 20 - 30 min after shut down), but they eventually stop and I've always been able to restart the van easily. Tim
PS: Installing an 89 FPR will help, but only slightly. I probably woudn't replace an earlier version just for this benefit, but if it needed replacement anyhow, I'd definitely go with the 89 FPR. FWIW, normal operating pressure of the earlier FPR's is 27-31 psi and the "pressure-up" mode is 33-38 psi. 89 FPR's normal operating pressure is 30-33 psi & "pressure-up" mode is 38-44 psi.
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