Anyone have any info on the hot start relay?

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Recently had an issue with starting my Delorean and during troubleshooting I ran across a line or two concerning the fact that the car was wired for a hot start relay in the tech info manual. From what I gathered, a relay can be installed if a hot starting problem is experienced in excessively hot regions of the country. Problem is, the relay socket provided isn't wired properly to allow a typical relay to active anything in the circuit so I'm assuming that there was a special relay provided to owners when they purchased the car new. Is this true or am I smoking too much wackyweed? Found my starting problem, the connector to the thermo-time switch was loose but I'm still curious about that hot start relay.

[Bitsyncmaster]

DMC never produced a hot start relay. Yes they wired in the socket. A normal relay will not work. I do sell my own designed hot start relay.

[Christian Dietrich]

And they work AWSOME!!!!!

Sent from my SM-N920P using Tapatalk

[Chris 16409]

The nice thing about Dave's relay is you can "delete" the ThermoTime switch from the fuel system. You can get a plug for the water pump and remove the ThermoTime switch completely. In effect it becomes a "cold start relay". But it will allow the cold start valve to fire regardless of engine temperature.

[Drive Stainless]

The nice thing about Dave's relay is you can "delete" the ThermoTime switch from the fuel system. You can get a plug for the water pump and remove the ThermoTime switch completely. In effect it becomes a "cold start relay". But it will allow the cold start valve to fire regardless of engine temperature.

How does Dave's relay activate the cold start valve if you delete the thermotime switch?

[Bitsyncmaster]

How does Dave's relay activate the cold start valve if you delete the thermotime switch?

My hot start relay activates the cold start valve after one second of the starter cranking. It does that with a hot or cold engine. It will only activate the CSV for four seconds so cranking longer than 5 seconds will not help getting the engine started. If you stop cranking then you can repeat the cycle.

I found my engine will start faster in the summer without the thermal time switch connected. So I just don't run the TTS at all. You can keep the TTS working if you want the instant CSV to fire with a cold engine.

So yes, I do consider the hot start relay to be a cold start relay also. I always recommend fixing any rest pressure problems but with my hot start relay, you should not have to do the "plug swap" if you have a hot start problem.

[David T]

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.

[Chris 16409]

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-switch-thermotime.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.

[Drive Stainless]

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-switch-thermotime.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.

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

[Bitsyncmaster]

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

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.

[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-switch-thermotime.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:

Fuel Injectors (6).
Cold Start Valve.
Primary Pressure Regulator.
Accumulator.
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.

[David T]

If you look in the Technical Information Manual on page 56 it says over the plug for the relay, "Wiring for Hot Start Relay (Not Used)". My guess is when the wiring harness was designed they used the info from Bosch and made provision for it but decided during production it wasn't necessary. There are other wiring "quirks". The bypass relay for the ignition ballast resistor was an "add-on" to the wiring harness. The documentation never caught up with it. Wires were provided for heater mirrors but they were never installed (maybe due to price?). Noise suppression was installed for the radio. I could make a list. Point is the wiring was a work in progress and they made running changes. The wiring system is probably the one system most in need up a total upgrade.

[DMCVegas]

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.

That is correct, and a piece that needs some clarification:

If you have air in one or more of your injectors, priming the system by tripping the RPM relay without cranking will not get the air out of those lines, no. The reason for that is because since the engine isn't cranking, there is no air flow to press against the metering plate to trigger the distributor to increase flow to the injectors. So the full pressure will not be made available to purge the air out of those lines, no.

NOTE: Keeping this in mind, that is precisely why Page 14 of the Owner's Manual clearly states that you are supposed to press the accelerator pedal down ¼ of the way before and during cranking. The additional airflow being delivered to the engine increases fuel pressure and flow to the injectors which aids in starting the engine. It also of course aids in purging the fuel system of air to get the car going that much faster.

Getting back to priming the fuel system, if the supply feed from the tank back to the engine is drained (which it probably is), priming the system by triggering the RPM relay will at least get fuel and some pressure back into that line to shorten cranking time. The Fuel Pump would generate enough pressure to pop the Primary Pressure Regulator to at least purge the feed line. Doesn't get *all* of the air out of the system, but definitely a good chunk.

Of course if your system is in proper functioning condition, this isn't a problem though.



If you look in the Technical Information Manual on page 56 it says over the plug for the relay, "Wiring for Hot Start Relay (Not Used)". My guess is when the wiring harness was designed they used the info from Bosch and made provision for it but decided during production it wasn't necessary.

Quite possibly, yes. This add-on was created something like 45 years ago for the preceding K-Jetronic systems before ours, and as stated, isn't needed in every area. Which is why it wasn't mandatory. And the DeLorean isn't the only car to have this plug/relay. Likewise the online consensus from other marques is that while it was a BOSCH design that car manufacturers kept providing a socket for, it really wasn't used very much going forward in the 1980's.

If your system is in tip-top shape, you shouldn't need the hot start relay, no. Although it might not be a bad piece of insurance to have just in case. In particular for hot areas where the external temperature exceeds that 95°F threshold.

[dmc4087]

DMC never produced a hot start relay. Yes they wired in the socket. A normal relay will not work. I do sell my own designed hot start relay.

Are these still available? I don't see a price on the website. Thanks!

[Chris 16409]

If your system is in tip-top shape, you shouldn't need the hot start relay, no. Although it might not be a bad piece of insurance to have just in case. In particular for hot areas where the external temperature exceeds that 95°F threshold.

In the central valley it gets well over 100 degrees in the summer, so I like having this "insurance". During the summer there is British Car Show every year. One of my DeLorean friends was getting ready to leave and was having an issue starting his car. After sitting in the sun all day, the coolant was still too hot for the cold start valve to fire. I let him borrow my hot start relay and his car fired right up. He was so delighted, he bought one of Dave's relays too.

[DMCVegas]

In the central valley it gets well over 100 degrees in the summer, so I like having this "insurance". During the summer there is British Car Show every year. One of my DeLorean friends was getting ready to leave and was having an issue starting his car. After sitting in the sun all day, the coolant was still too hot for the cold start valve to fire. I let him borrow my hot start relay and his car fired right up. He was so delighted, he bought one of Dave's relays too.

Yup. We aren't the only car that has this relay socket as a provision. This is something that BOSCH had manufacturers install on their cars, but the most I could find on it is that it really only happened back in the 70's. So someone obviously thought that this was important enough to include. At the time anyway. As K-Jetronic evolved and we saw the additions of LAMBDA and CIS, who knows what other changes were implemented that were supposed to eradicate this issue. The schematic for it really isn't that complicated, and was actually designed to force the Cold Start Valve to behave in two separate ways.

When the coolant is cold, the Thermotime Switch will provide a constant path to ground, meaning that the CSV has a constant on condition where it just dumps fuel into the intake manifold to create the choke condition. BUT, when the engine is warmed and the TTS is off, the Hot Start Relay is then activated. And it provides the ground instead to the CSV to fire it. However, it is supposed to function as a pulsing relay (unknown what duty cycle) and intermittently grounds out in order to intermittently fire the CSV.

Now, I actually have a discrete CSV switch tucked away down next to my parking brake. With the experience I have using that I can definitely tell you that during hot starting issues I had (not all the time, but sometimes) if I held the button down during I'd stall the engine. So I had to keep flicking it on and off to get the engine to start.

As I said, I didn't need it all the time, and yes, a couple of years after when it got really bad, I had to replace split O-rings in the Primary Pressure Regulator. So yes, the HSR *can* mask a problem with a faulty system. So you should always strive to first resolve any other issues on your own. By the same token, I can easily see why BOSCH and everyone decided against including this thing.

Installing and even stocking this part would cost too much money.
Having to add an additional part after the fact looks bad.
Such a part could mask an underlying problem, and potentially keep owners from bringing in their car for proper service.
Aside from fuel economy issues, if cars don't come in for repairs, the Service Deparment isn't making money.


This clearly isn't something needed by everyone, and I would be very hesitant to just throw the use of an HSR out there for anyone who has a starting problem. Because even a bad TTS could fully ground out and cause poor hot starting conditions as well. But for those of us who need this part, I think it would be a good idea.

The interesting thing here is that both LAMBDA as well as the Warm-Up Regulator both are set to achieve an actual enriched AFR for cold starting conditions with the engine around 36°F colder. Why on earth did BOSCH decide to keep the TTS @ 95°F? That is a whole lot higher. I can't say for certain, but it almost makes me wonder if BOSCH knew rest pressure problems were going to be an issue, and decided to make that thing fire at as high of a temperature as possible without the additional cost of extra parts. But still gave themselves an emergency backdoor with that relay socket.

As we have seen with VW's Diesel Emissions Cheat, something like that wouldn't exactly be unheard of...

[Bitsyncmaster]

Are these still available? I don't see a price on the website. Thanks!

Yes I still sell them. I have to add additional shipping charges for non US locations.

Electronic fused Fan Fail, $114, strongly suggest you also get my Front Relay Ground buss with this unit.
Dome unit, $88
RPM relay, $83 with your old unit
Fan relay, $77
Blower relay, $72
Head Light relay, $72
Hot Start relay, $66
Duel side marker flasher, $30
Pair of window switch LED boards, $20, $25 installed, requires the new DMCH switches.
Idle ECU, $138 with your old unit. I have 11 pin units also for $150 with your 9 pin unit returned.
LED AC panel, $110. Your choice of blue or green backlighting.
Front Relay Bank Ground Buss, $15. $10 if combined with another product.
Round pin extractor, $40. One end for male pins other end for female. No replacement if you drop it.

Prices include shipping to lower 48 states