Anyone have any info on the hot start relay?

[Chris 16409]

Safe to say that Dave's relay allows for deleting the accumulator, too? That's a spectacular advantage, if so.

I don't know if I'd go that far. I believe the accumulator also has a function while the car is running to keep fuel pressures balanced or something like that. You still need to maintain some kind of rest pressure.

[Drive Stainless]

No

If you have ever had a hot start problem, even firing the CSV may get you started but your engine takes about 60 seconds for all the injector lines to clear of bubbles.

So that must be a problem related to the design of the CIS injectors, then. An EFI fuel system purges the fuel rails of air within a second or two when starting; no accumulator.

[Bitsyncmaster]

I don't know if I'd go that far. I believe the accumulator also has a function while the car is running to keep fuel pressures balanced or something like that. You still need to maintain some kind of rest pressure.

The book says it also evens out the pulses from the fuel pump. Don't know if that's required or not and I guess it would depend on what pump you are using.

[Bitsyncmaster]

So that must be a problem related to the design of the CIS injectors, then. An EFI fuel system purges the fuel rails of air within a second or two when starting; no accumulator.

EFI holds pressure also but EFI injectors open even with no fuel pressure. CIS injectors open around 40 PSI and I guess that makes it hard to purge any bubbles in the lines.

[David T]

My whole point is why mess with a thermotime switch when there is a modern replacement available.

DMC Thermotime Switch:
https://store.delorean.com/p-6265-sw...hermotime.aspx

These switches are now well over $150 so, it's more cost effective to get one of Dave M's relays. I'm not arguing against replacement of a fuel accumulator. Everyone with hart start issues should replace their accumulator. I'd just rather use a modern digital relay, then rely on a thermally activated switch designed sometime in the 1970's.

The whole CAR was designed in the 70's and many of the parts earlier than that! If you are going to use that as a reason to modify systems then you have a LOT of things to change. IMHO it is most often better to just replace what you need to when it breaks with an original or reproduction part. As expensive as parts are if you are doing your own work you are saving a lot of money on the labor so you can afford to buy the parts you need. All too often I see a car all hacked up and modified because the PO or the shop they took it to McGivered it when there really is no reason to have to do that (unless you don't know what you are doing or where to get the correct parts).

[mluder]

If you have all of your cold start systems operating properly there is no need to add a "Hot Start Relay". Adding one is just treating the symptom, not the underlying problem. Typically it is most often a wiring problem or a bad TTS. Sometimes it is because someone plugged something INTO the hot start relay plug! When I troubleshoot a hard cold start one of the first things I check for is if anything is plugged into that plug. A lot of people think if there is a plug then something MUST plug into it! People also wire switches into that plug to deal with a hard HOT starting problem. That way you don't have to do the plug swap thing to get going when you have a bad fuel accumulator. In that case I say just replace the accumulator, not add a switch.

I had a starting problem and found a relay plugged in to the hot start socket... I removed it and the problem went away... Just my two cents.

Cheers
Steve

[FABombjoy]

To borrow from electronics, the accumulator is a fuel capacitor.

In K-Jet (and KE), fuel itself is part of the regulatory mechanism. When you have a quick load transition, fuel is suddenly consumed which also briefly affects the base and control pressures. The accumulator is able to provide a near instant volume of fuel during these transitions to maintain operation of the system within expected parameters. Owners of other K-jet cars (Volkswagen) have done accumulator deletes with varying degrees of success.

In EFI the regulatory processes are electrical. All you need is an adequate supply of fuel and you can delete the accumulator if desired. As noted throughout this thread, the injectors can be opened regardless of the fuel situation making vapor purge a non-issue.

[David T]

In an EFI system the injectors are operated electronically. In K-jet the injectors are opened by fuel pressure. Completely different method of operation, the K-jet is very pressure dependent. There is no discreet Lambda system, the primary injectors can be individually controlled and each can be opened and closed just enough to control the amount of fuel necessary at any given time. An EFI system is as big an improvement as the K-Jet was over carburetors.

[DMCVegas]

So that must be a problem related to the design of the CIS injectors, then. An EFI fuel system purges the fuel rails of air within a second or two when starting; no accumulator.

CIS = Constant Idle Speed (the type of motor we use instead of the Auxillary Air Valve from the B27F).
CI = Constant Injection.

In the mechanical Constant Injection system that we use, the fuel itself is actually used in a hydraulic function to drive the system. Increased pressures on the fuel injectors cause the valve springs inside to spread open farther, and thus deliver more fuel. Same with the countering force on the air metering plate and plunger, as well as the Warm Up Regulator and Control Pressure Regulator which also work in tandem to control that line pressure; They all run off of the hydraulic function of the fuel itself.

Just like when you have air in the lines on a brake system, air is compressible and doesn't deliver the proper hydraulic pressure to the ancillaries, so the system doesn't function as well. So while you're purging the K-Jetronic system of air, it will not run correctly. Rough idling and whatnot as the injectors fluctuate pressures and deliver inconsistent amounts of fuel. EFI is a bit different for a couple of reasons. First is that it has fewer lines to purge out. So that's a bonus. Second is that EFI injectors are electronically controlled instead. CI injectors won't crack open until the line pressures have been built up enough to force them open, but EFI units will open immediately and at least deliver *something* to the injectors, as well as have a longer duty cycle if cold to spray additional fuel which helps.

As per the Workshop Manual, here is what it says about the Hot Start Relay on page D:01:15

The car is wired for a hot start relay. This relay can be installed if a hot starting problem is experienced in excessively hot regions of the country. When this relay is used, the cold start valve will inject intermittently during cranking when engine is hot.

There is a LOT of misinformation about these device floating around the Internet. Not just for DeLoreans, but Mercedes, Porsches, Volvos, and all other vehicles that use K-Jetronic. Used in the 70's and very early 80's, these at least seemed to fall out of favor once the B28F engine came on the scene, and most service shops have never even heard of these devices. But apparently they did exist. According to Volvo, if you crank a hot engine and the CSV fires, then you have a defective Thermo Time Switch, and it must be replaced. Which makes sense because the CSV doesn't pulse, it stays fully on. And just like it can run the entire engine through crude carburetion if the injectors are off, it will actually choke and stall out the engine if it runs too long. Meaning that a defective TTS can actually cause hard starting on a properly functioning engine! So if you're having hard starting issues, that is something for you to check as well.

One thing to keep in mind is that lots of shade trees, or mechanics not familiar with K-Jetronic will try and blame a hot-start problem on "vapor lock", or "heat soak". If you hear this, run away as fast as you can! The problem was have is with leak-down pressure in the system that comes from the following places:

1. Fuel Injectors (6).
2. Cold Start Valve.
3. Primary Pressure Regulator.
4. Accumulator.
5. Fuel Pump Check Valve.

Those are TEN different spots where leaks in the system can occur to bleed-off pressure. And once you start cranking, you have to return pressure back to get the hydraulic functions of the injectors to work again. But in the mean time, that CSV will dump a TON of gasoline into the engine! It will crudely carburete the engine to get it going, and when it's cold enough, will do just that. So you'll have hard cranking, but the engine will start because it's running off the CSV until the injectors come back online. That is NOT vapor lock! Vapor Lock occurs when you have hot gasoline in a fuel line connected to a mechanical fuel pump on an engine, with no electric pump. Electric fuel pumps PUSH gasoline to the engine, and compress it. Mechanical pumps PULL gasoline out of the tank and pull it towards the engine. If the fuel is hot enough and subjected to enough of a vacuum, it will boil off in the fuel line and become a gas. Just like when you pull a vacuum on an Air Conditioning line to purge it of water. Except in this case the mechanical pump on the engine is designed for liquid. If there is no liquid in the line, it can't pull fresh, cool gasoline to feed the engine, thus the engine doesn't start. Ever since manufacturers started installing electric fuel pumps, this problem went away. So no, it isn't "vapor lock", and if a mechanic doesn't know that, it isn't a mechanic you should trust.

But getting back to the Hot Start Relay, read that wording from the Workshop Manual again about the HSR. It clearly says that it is for use in "...excessively hot regions of the country.". Meaning of course, the desert. The reason for that is probably simple: Everywhere else the cars would cool down fast enough to re-trigger the CSV. Which is supposed to fire anytime the coolant is below 95°F. Everywhere else it would be reasonable to assume that in the time it takes to bleed down the system, the water pump & thermotime switch would both have cooled down enough to fire the CSV next time it cranked. But in hot climates where the external temperature doesn't even drop below 95°F, you'd never even get the CSV to fire again. Thus it cannot step in to run the engine long enough to purge the air bubbles. Although if you turn the key a few times to trigger the fuel pump relay, you should be able to prime the system before cranking.

Personally, and this is just my opinion here, given the price of the Thermotime Switch, and the ability to potentially eliminate two separate types of starting problems, yes, it may very well be more beneficial to use the Hot Start Relay instead of the TTS to always deliver a pulse blast of fuel.

[Bitsyncmaster]

You can prime the system all you want and it does not get the engine with the hot start problem started. Yes priming will run fuel to the FD and back through the return line. But nothing get pushed through the injectors.