VW hot start relay

[Bitsyncmaster]

One of my customers sent me a photo of a hot start relay he had plugged into his DeLorean. I had him send it to me with his core return so I could test it. It turns out this is a VW hot start relay. So my thought that DeLorean designers put in the hot start relay socket as an after thought was probably wrong.

The VW hot start relay (pn 813 906 064) delays firing the CSV for 1.5 seconds. It then fires the CSV for 250 ms., off for 720 ms., then repeats as long as it stays powered. So maybe the VW CSV was a higher flow rate then used in our DeLorean engines. Not sure if those short CSV pulses would be enough to help the DeLorean engine start.

[David T]

My guess for the hot start relay plug and wiring is as follows:
When they designed the electrical system they had to follow the engineering designs and standards of the Bosch K-Jet fuel system as designed by Bosch. One of them had to be to provision for a hot start relay. After building the car and doing the necessary testing they figured out it wasn't necessary so they never put the relay in but the wring harnesses were already designed and made so they left the plug. Kind of the same situation with the wire for the heated rear view mirrors and the defogger. In future iterations they would have added and deleted things if production continued and they had to reorder parts. The ballast bypass relay is another thing that was added.

[DavidProehl]

I've wondered what the part number was for that. Any reason not to have this on hand in the event of an accumulator failure? I supposed the hot swap method for the CSV plug would also work, but this would be the "proper" way to do it right?

[David T]

As far as I know a part # was never issued for the hot start relay by DMC. The vendors may have made one up for it. It should not be necessary to use the hot start relay plug for the plug swap. Since it is only for testing and emergency use it does not have to be convenient or easy.

[Bitsyncmaster]

I've wondered what the part number was for that. Any reason not to have this on hand in the event of an accumulator failure? I supposed the hot swap method for the CSV plug would also work, but this would be the "proper" way to do it right?

My Hot start relay does not pulse the CSV, it just turns it on after one second of cranking. Not sure if that 25% pulse from the VW relay would inject enough fuel with our CSV. I think the VW part is hard to find anyway.

[DMCVegas]

My guess for the hot start relay plug and wiring is as follows:
When they designed the electrical system they had to follow the engineering designs and standards of the Bosch K-Jet fuel system as designed by Bosch. One of them had to be to provision for a hot start relay. After building the car and doing the necessary testing they figured out it wasn't necessary so they never put the relay in but the wring harnesses were already designed and made so they left the plug. Kind of the same situation with the wire for the heated rear view mirrors and the defogger. In future iterations they would have added and deleted things if production continued and they had to reorder parts. The ballast bypass relay is another thing that was added.

That has always been my guess as well. K-Jetronic seems to always have a provision for a Hot Start Relay, and most likely due to BOSCH standards. But the DMC-12 simply happened to skip it. The documentation in the Workshop Manual clearly states that only in certain hotter climates would the relay be necessary (D:01:15). Those would always been your desert areas where the temperature never drops below 95°F during the day to trigger the Thermotime Switch with a low fuel pressure issue. But even then that is assuming a vehicle has a bad Fuel Pump Check Valve/Accumulator/Primary Pressure Regulator O-Ring, AND is in a hot climate.

Luckily the DeLorean has a much more well ventilated engine compartment thanks to its location. So sitting temperatures drop rapidly, meaning that the issue of Hot Starting is reduced when compared to other K-Jetronic vehicles. Better yet, the new pre-production cars with properly functioning systems would probably never have exhibited these symptoms on the Arizona proving grounds.

With a forecast production schedule of 30,000 cars per year, and a wholesale cost of $2 per relay (pure speculation based on current prices), that would have been a savings of $60,000 per year for DMC to omit that relay. Then if cars had problems later on, they could have dropped a relay in for a customer satisfaction issue.

But this is all before ethanol blended gasoline which may be causing a more rapid decay of internal rubber parts nowadays.

[FABombjoy]

The VW hot start relay (pn 813 906 064) delays firing the CSV for 1.5 seconds. It then fires the CSV for 250 ms., off for 720 ms., then repeats as long as it stays powered. So maybe the VW CSV was a higher flow rate then used in our DeLorean engines. Not sure if those short CSV pulses would be enough to help the DeLorean engine start.

It seems to be intended to help bleed a fuel system under vapor lock, adding just enough fuel to get things moving.

http://mastertechmag.com/pdf/1989/06...etHotStart.pdf

[Bitsyncmaster]

It seems to be intended to help bleed a fuel system under vapor lock, adding just enough fuel to get things moving.

http://mastertechmag.com/pdf/1989/06...etHotStart.pdf

They suggest the CSV speeds up cranking faster (I guess because you get some firing with it's added gas) and that pulls more vacuum which pulls the K-jet plate down a little. Sounds correct. The same thing happens with our K-jet.

[DMCVegas]

It seems to be intended to help bleed a fuel system under vapor lock, adding just enough fuel to get things moving.

http://mastertechmag.com/pdf/1989/06...etHotStart.pdf

Well, it certainly is indeed to get things moving. But I don't believe that the write-up cited is 100% correct. It's theories are flawed.

The big thing to understand is that it isn't Vapor Lock. It's never Vapor Lock. Vapor Lock happens on a fuel delivery system that is designed to pull instead of push. Like on a carbureted engine with a mechanical fuel pump on the block that is designed to pull gasoline all the way from the fuel tank on the opposite end of the vehicle. When you get heat soak on that, the fuel inside of the lines gets hot, and very volatile, as gasoline is want to do. The pulling action on the fuel causes pressure to drop, of course. But because of the nature of gasoline, instead of creating a hydraulic suction that pulls more fuel from the tank, the gasoline will rapidly boil inside of the line and turn to vapor (just like when you pull vacuum on an A/C system to purge moisture out). Thus the engine will not start, and you have to wait for it to cool down to condensate back into liquid. The easiest way to avoid this situation is to install an electric fuel pump in the tank to push the fuel up to the engine to ensure the gasoline in the line is always under pressure, and never suction.

But let me make clear: This is NOT what happens with the DeLorean! Nor does it happen with EFI, or even any carbed engine which has an electric fuel pump.

Now as we know, K-Jet of course being mechanical, runs on fuel pressure. It's been covered many times. Where EFI will open injectors immediately regardless of line pressures, K-Jetronic needs to prime itself up to the minimum fuel pressure to force the spring-loaded injectors open.

That said, K-Jetronic doesn't magically loose pressure just because the injectors are hot. The injectors are meant to hold pressure steady regardless of temp. The issue of dropped fuel pressure occurs when a check valve in the system fails. This could be the 7 injectors, fuel pump check valve, the accumulator, or the Primary Pressure Regulator. There are procedures to test each of these things in the Workshop Manual, or the BOSCH official guides. But a loss of fuel line pressure and air in the lines is a component failure, and not nearly as prohibitive to system function as you would see with a diesel system. Which is why unlike diesel, we don't see any places to manually bleed out air.

Now perhaps the cracking of the CSV helps with bleeding the system somewhat, but it certainly isn't the Hot Start Relay's main function. I would find that somewhat hard to believe given that the CSV is fed directly off of the supply line inside of the distributor, and wouldn't suck out any air from the Upper Chambers/Injector feeds as the tech article here surmises. It would help with popping the return line faster, sure, but not the injectors directly.

However, as has been covered before, the CSV dumps such a high amount of fuel into the engine that it can crudely feed the engine by itself by acting as a (albeit inefficient) Throttle Body Injector. Inefficient as on a continuous spray it will actually flood the engine. Which is why the Hot Start Relay was intended to work on a pulse to avoid that scenario! Now one thing the article absolutely gets correct (although for the wrong reason) is that increased engine vacuum will result in higher line pressure applied to the Fuel Injectors to help purge them of air faster. That isn't caused by the CSV or altering fuel mixtures. Engine vacuum is increased by pressing down on the accelerator pedal to allow more air to reach the cylinders by opening the throttle plates. That causes increased airflow, and higher pressure being applied to the injectors due to the function of the Metering Plate & Fuel Distributor. Which is why your Owner's Manual clearly tells you to press down ¼ of the way on the pedal while cranking. Cranking the engine with no fuel will still result in increased air flow while holding down the pedal. It just means the starter motor is responsible for the engine's rotation rather than internal combustion. Letting the CSV crudely feed the engine allows the starter to break contact and lets the engine turn under it's own power. The continual rotation keeps the RPM Relay tripped, thus the fuel pump running, and continues to the fuel feed & purge uninterrupted. As opposed to cranking the engine multiple times of course to prime the system.

Also, "Warm Engine" here is vary vague. The manual clearly states that the Thermotime Switch activates at temperatures below 95°F (for our application at least). Then of course it the cooler the engine is, the longer the firing of the CSV to help with cold cranking & Hot Starting Issues. With parallel circuits feeding the CSV, cold/warm temps will offer continuous flow from the injector. But hot temps will then switch to the intermittent firing pulse of the relay.

[Josh]

TLDR

Long winded reply, arguing for no specific reason

TLDR

Luke, how does it feel to be wrong?!