Vapor Lock...a different view?

We know a fuel pump cannot pump a gas vapor. That was back in the carb days when the fuel pumps were within the carb. Engine heat evaporated the fuel, the pump couldn’t pump the vapor. Hence vapor lock.

Move on to the V12s and hard starting…many cases which we called “vapor lock”. EXCEPT…the fuel pump is in the tank…submerged in fuel. The fuel in the tank is liquid. It SHOULD be able to be pumped to the rail, thru the injectors and into the engine. So, why are these engines prone ? to hard starting after being run and heated?

I think it’s PRESSURE in the fuel rail from fuel vapors. The fuel vaporizes in the rail from engine heat, the vapors expand, the expansion increases internal rail pressure, and that rail pressure is greater than pump pressure…or A bank regulator. I think the ID of the fuel hose sizes ( on the injectors) and the rail ID size is so small that it doesn’t take much vaporization to create a large pressure increase. A pressure that is greater than not dead head pump pressure but the A bank pressure regulator…3 Bar.

Hence a long cranking time. The opening/closing times for a combination of 12 injectors…operating at milliseconds .starts the decline in rail vapor pressure as each injector opens, until the vapor pressure is reduced (purged) to less that the A bank regulator and liquid fuel begins to flow into the rail and injectors.

Maybe a member schooled in fluid mechanics could chime in here.

SD Faircloth

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I did not attend fluid mechanic school, but do have some thoughts to add.
The pressure in the fuel rail is controlled by the “B” bank regulator. The “A” bank regulator doesn’t count.
The ultimate pressure in the rail caused by heat related expansion will always be relieved at the pressure setting of the “B” regulator, whether the fuel is a liquid or a vapor. That is what I see on my permanently engine mounted gauge.
When the fuel in the rail boils and vaporizes, it does so at a certain pressure. When the fuel (now a gas) is injected into the engine it is too lean to fire properly. If the thermal valve , which senses the heat of the rail is working properly, then vacuum is reduced in the “B” bank regulator, which then raises the fuel pressure, and consequently the boiling point of the fuel, which helps the fuel liquify and become a spray of liquid fuel again, rather than a gas.

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I think it’s the opposite. When I didn’t have my check valve in line after the fuel pump (has no check valve built in) to maintain pressure in the rail, I had very difficult hot starts with a good 30 seconds of very rough idle.

I tested the fuel rail temp right before a difficult hot start, and it was 175F. Without my check valve, I assume there would be little to no pressure in the fuel rail after shutdown, so that the fuel has a lower boiling point. So it vaporizes much easier.

I simply put my check valve back in (right after fuel pump). Now with pressure in the rail maintained after a shutdown, I assume that 175F is not enough to vaporize the fuel, so I get a much better hot start, and the rough idle is noticeably improved and only lasts 10 seconds.

I should have tested actual fuel rail pressure in all of this, but sorry I did not.

Isn’t this the reason we have a pressurized cooling system? To raise boiling point of the coolant?

Re…B bank regulator. If it is controlling pressure at 2.5 Bar…(+/- 36 psi), (which I agree with) why is there a hard start condition anyway ? Presumably, the vaporization would not occur…said another way…the B bank regulator would purge out any pressure caused by ANY reason…if it is greater that 2.5 Bar. Your point that the fuel is too lean to burn may some merit, and may be a “better” answer than a pressure related cause. SD

You might like to call it vapour lock, but all you actually know is that the wrong pulsewidth of fuel is being fed into the cylinders for the air and water temperatures measured by the sensors. The pressure devices are dumb - all they know is to open against a spring at a certain pressure. Whether this is to let through liquid fuel or gaseous fuel is unknown to them. Certainly gaseous petrol will contain fewer fuel molecules per unit of volume than liquid petrol so a lean mixture would result until the rail is purged and filled with liquid fuel only. So we know fuel rail pressure must be at or below the target pressure and that the fuel flow is sufficient to be running at ~6000rpm, so most fuel return to the tank (or swirl pot) at low revs - the pump doesn’t run slower at low revs.

But this may not be the problem. Aftermarket ECUs work by having a separate fuel map for cranking based upon coolant temperature, distinct from the main map which once the car is above cranking rpm. This second (main) map has adjustments applied to it based on coolant temperature, air temperature plus other factors. I suspect that the cranking map probably may not have these, so starting a heatsoaked engine may mean the air temperature assumed in the basic calculation may be woefully out, since the air temperature may be ~60’c rather than 15-20’c. Cranking the engine will eventually draw air in at a recorded temperature close enough for combustion to occur. Also, the water pump will start to move water past a heatsoaked coolant sensor, so better inform the ecu what pulsewidth it should be commanding. Relocating the air temperature sensor may substantially improve hot starting times. OEM ecus from the 1980s are unlikely to be more sophisticated than the current range of aftermarket items so it is likely a case that duff information from the sensors is part of the problem.

kind regards

You do realize there’s another thread going on right now that points to poor alternator performance as the root cause of hot start issues?

Might be my other thread outlining the alternator issue…didn’t think that both issues were related.

The engine turns over at 120-140rpm when cranking.

Alternator performance and hot start issues are thus unlikely to be connected. The alternator will produce no useful output voltage below idle speed. Poor running in the 120 seconds or so after starting may have a contributing factor in the form of a lack of electricity, but opening the throttle ought to excite the alternator.

All of the vapour lock talk is characterised as a tuning issue on the Megasquirt forums. It is usually solved by retuning to take into account coolant and air temperature sensor readings together with the amount of ignition advance over the first thirty seconds of running. Because no one with a 16CU has access to altering the injector pulsewidth or even looking at the datalogs of rough running just after the engine starts, they describe the problem in terms of only what they can see.

Go look at the MS forums and read up about hot start issues. The issues are the same but the MS crowd has access to diagnostics and can alter fuel and spark to suit.

kind regards

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Well, that’s kinda my thinking, too. I guess the first question is just exactly what is the problem with hot starts? It’s been a while, but my recollection was that the engine would start OK, it just wouldn’t run worth a rip. It’d idle really slow, sometimes dying, and couldn’t be revved without stalling. Once it had fumbled its way through running for a minute, it’d start running better – leading me and others to conclude it just had to work the hot fuel out of the rail and get some cold fuel in there. Jaguar’s solution of installing a sensor in the rail seemed to confirm that.

But poor alt performance as a root cause makes just enough sense that it calls for serious consideration. If the engine is idling slowly and won’t rev high enough to get the alt to put out a serious charge, there might be something to this. If it starts all right when cold, it might just be because when cold the injectors are being held open so long that there’s enough fuel for starting even though the voltage is down.

It’d be great to have more data. If someone out there with a hot start issue would do us a favor and connect up a VOM to the cigar lighter next time they’re trying a hot start, he could report on when the car started to run better and whether it correlated with voltage rising.

A better way would be to get a couple of potentiometers and connect those in place of the water and air sensors. Initially set them at the “current” (heatoaked) values. Once it is running roughly, dial the air temperature sensor up or down to see whether it runs instantly better. This way, you eliminate or confirm which are the contributing factors.

kind regards

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That isn’t too far from what Jaguar did with the first fix, an electrical switch in the fuel rail. It simply disconnected the air temp sensor in the LH air filter housing, which I guess makes the car think it’s cryogenic outside today and richens it up big time.

Just because it fixes the issue doesn’t necessarily mean it points to the cause, though. Running rich can cover lots of evils.

It is my belief that once the fuel in the rail gets hot enough to vaporize, the engine will run badly re-starting until the rail is purged. I have used a preventative approach on my car that has worked very well.
The trick is to stop the fuel absorbing heat, in the first place.
The incoming fuel line before the “A” bank FPR passes quite close to the exhaust manifold.
Mine is insulated.
Both FPR’s have been replaced, and hold pressure for hours.
My hood has louvers, which release a tremendous amount of trapped heat.
The small electric fan is controlled by two thermostats.
The OE device for coolant temperature, and a piggy-backed adjustable thermostat, which senses the air temperature close to the fuel rail.
A well-maintained cooling system is important, too.
All things considered, I think that once the fuel gets too hot, tinkering around using a curative approach is a band-aid, at best. My car does not have a hot-start problem!

Perhaps you’d like to just randomly solve the issue.

And an air temperature sensor which is incorrectly recording “hot” will lean out the mixture, not richen it.

So if you mimic colder air in place of the incorrectly reading heatsoaked air sensor and it solves the problem, then I’d say that’s pretty good science.

kind regards

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Hah! It’s the air temp sensor itself that’s been cooked by underhood heat! Good theory, I like it! I’d still wanna know why Jaguar didn’t stick with the electrical fuel temp sensor and instead traded it for a vacuum sensor that disables the vacuum to the LH FPR.

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Hmmm. What would happen if you just relocated that inlet air temp sensor to somewhere else? Like perhaps lower within the engine compartment?

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Interesting indeed.

It would be extremely easy to attach a NC thermostat to the rail that disconnects the Air Temp Sensor when the rail goes above a critical temperature.

Any idea what that critical temperature would be?

Actually one with frequent hot start issues could just disconnect the sensor, try to start and see if there is any improvement.
And I wonder if it would result in better starts altogether.

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Relocation of the air temperature sensor is indeed one fix. Mine has moved from the end of the inlet manifold (where it was mounted into alloy) to being on a plastic grommet inside the air cleaner. An even better location would be somewhere inside the bonnet air dam at the front of the car where it’d see a continuous flow of air only as opposed to slowly being influenced by the radiation from the engine block. This is a tuning issue, so some of the “wrongness” is factored into the fueling equation already. It is only when you get very wrong that rough running results.

A vacuum sensor will be harder to fool than a temperature sensor when considering heatsoak at startup.

Air temperature generally is below 30’c. The heatsoked sensor can go to ~60’c and the car still runs ok. The question is whether it is disproportionally “wrong” for the there and then ambient conditions.

I’ll pot some log files when I can, but my car generally works, so they don’t really highlight exactly what they need to.

kind regards

My understanding of the electrical temperature sensor on the fuel rail was that it was only on convertibles to turn on the a/c compressor to cool the fuel via the fuel cooler in the rail return to the tank. People driving with the top down and a/c system off, imagine that. Don’t see that helping on a hot soak.
I like the air temp sensor theory; seems like moving it would be an idea. Or devise a circuit to disable it for 45 seconds on start via the existing system…except I deleted my timer because I couldn’t get it to function despite a new Pektron module after replacing the electrolytic caps on the original. Neither module work on the bench, either.
Anyone know offhand what the temp sensor in front of the radiator is for? I thought it was the ambient sensor for the HVAC, but since read that is on the right blower inlet; don’t recall if there was one there when I fixed the blower. Seems like a good place for the EFI air temperature sensor…

Robert, believe it or not, I think it is for the heated windshield washers!

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I dug out my wiring diagram and you are correct! I assumed they were connected to the heated mirrors and rear window defogger.
Interesting…those Jaguar engineers were always thinking, logically you would want them heated when there is a risk of freezing whether the defogger was in use or not.