Hot start relay - 74/75 D-Jets. Bill Adkins.

[Graham]

Bill Adkins mentioned this relay in an earlier post. He recalled (and I confirmed from diagrams) that the relay has something to do with the air sensor.

But, I cannot tell from wiring diagram just what the relay does.

Does anyone know?

If it provides or changes an input to the ECU, does that mean that it would only work with later ECUs and it could not be adapted to the earlier D-jets?

[Bill Adkins]

You like to post questions...LOL I see you didn't get a taker so I'll tell ya. The hot start relays function was to help with vapor lock after a hot soak. When the engine is cranked the relay shorts the water temp sensor to cause maximum enrichment duration for an instant allowing a flushing effect at the fuel rail. That vented unusable fuel vapors to allow liquid injection to happen. To be honest it would only apply to this style on fuel management as later types had much higher pressure and volume systems. I actually did it my own way with pretty good results. There was another method where the water temp. sensor was moved to a lower location in the water jacket, it didn't work all that well either.
Bill

[Graham]

Originally Posted by Bill Adkins

You like to post questions...LOL I see you didn't get a taker so I'll tell ya. The hot start relays function was to help with vapor lock after a hot soak. When the engine is cranked the relay shorts the water temp sensor to cause maximum enrichment duration for an instant allowing a flushing effect at the fuel rail. That vented unusable fuel vapors to allow liquid injection to happen. To be honest it would only apply to this style on fuel management as later types had much higher pressure and volume systems. I actually did it my own way with pretty good results. There was another method where the water temp. sensor was moved to a lower location in the water jacket, it didn't work all that well either.
Bill

Hi Bill - Yes I HAVE been asking a lot of questions I have had this car for almost 20 years. Still keeps me interested! Cosmetics and engine are good - now just tracking down some niggling issues.

The D-Jets almost all have the start problem after a hot-soak. MB tried various "fixes". There are also many "fixes" to be found on the net. These include:

1. Adding check valve to exit from fuel pump.
2. installing a new fuel pump!
3. installing a manual switch to run the fuel pump for longer before starting or just turn the ignition on/off a few times to run fuel pump longer
4. installing a bypass with small orifice around fuel pressure regulator to bleed down pressure.
5. wrapping fuel rail with insulation
6. raising fuel pressure
7. installing switch to disable cold-start injector

What was your way?

When you mentioned the HOT start relay, I started looking for it in the manuals. It shows on the '74s and '75s! The diagrams show that the coolant sensor (not the air sensor) is involved, but I couldn't figure out from diagrams, exactly how. It "looked" like it might open the coolant sensor circuit during starting. From on line D-Jet "bible", open cct enrichens, shorted cct leans.

IF this fix actually worked, I guess we could install a relay or manual switch on the earlier cars (71-73) to do the same thing.

PS: What I would really like to try, is to install lower capacity yellow injectors and raise pressure to bring capacity back to equivalent of blue/green OE ones. This would get fuel pressure up to 36 -38psi range and suppress boiling. But still looking for that set of used yellows

[Tomguy]

To be honest, here's how I solved my hard hot-start issues:
Cracked the AAV screw open another 1/2 turn.

I know you get less manifold vac but it never affected my engine's performance at all that I noticed aside from starting easier hot.

FWIW (and FYI) the yellow injectors are rated the same flow as the blue injectors at every reference site I could ever find. You don't want to raise the pressure @ the rails anymore.

I've thought of swapping out the old injectors for more modern lower-flow injectors (since D-Jet's pulse width is actually quite long for such high-flowing injectors) for better misting, and to run higher pressure, but I've never had a problem with the fuel in the rails boiling from heat soak until I put the 3.5 heads on, and raising the AAV screw 1/2 turn fixed that.

[Graham]

Originally Posted by Tomguy

To be honest, here's how I solved my hard hot-start issues:
Cracked the AAV screw open another 1/2 turn.

You mean you just changed idle screw to increase idle rpm?

FWIW (and FYI) the yellow injectors are rated the same flow as the blue injectors at every reference site I could ever find. You don't want to raise the pressure @ the rails anymore.

The ratings vary from site to site. But, this one confirms that the yellow 034 is quite a bit less than the blue 036 that our car has. These are ratings at 3bar. At 2 bar, flows are of course correspondingly less. Hoses and pump should be no problem at slightly higher pressure. And pressure will stop vapor-locking.

I've thought of swapping out the old injectors for more modern lower-flow injectors

What type of injectors were you considering?

[Bill Adkins]

I actually drilled a small hole, maybe .5 mm in the fuel pressure regulator through one of the fuel rail hose connection nipples. Imagine what you want to do is create a small venting orifice within the pressure regulator so the vapor bubbles will quickly pass leaving solid fuel to run the engine. The downside is a slight lag in time starting cold because any pressures will be lost so it needs to build up for maybe 2 seconds cranking. Then set the fuel pressure to 28.5 lbs, the lesser setting as it was 28-32 lbs. factory. You will then adjust the pressure sensor as I previously explained to correct any lean condition created with the procedure. Keep adjustments of the pressure sensor to a minimum and in response to other posts I have read the pressure sensor is the weakest link so to speak. I was a foreman at a dealership when these cars were every where and ended up running the dyno during that era.
Bill

[Tomguy]

I was thinking something like these, except ones that'd fit the 4.5 heads (with the elongated pintles):

http://cgi.ebay.com/ebaymotors/36LB-MUSTANG-LT1-LS1-LS6-BMW-BOSCH-FUEL-INJECTORS-TURBO_W0QQcmdZViewItemQQ_trksidZp3286Q2ec0Q2em14QQhashZitem1e5857d05aQQitemZ130331168858QQptZMotorsQ 5fCarQ5fTruckQ5fPartsQ5fAccessories

I know these specific ones won't fit, just along the lines I was thinking. I'd need to custom fab a fuel rail but if I restore my 4.5, I'm converting to Megasquirt so it wouldnt be a big issue at that point, since everything will be redone.

And yes, turning the screw may raise idle a little bit (it raised mine about 25RPMs at that setting) but that's about it.

[Graham]

Originally Posted by Bill Adkins

I actually drilled a small hole, maybe .5 mm in the fuel pressure regulator through one of the fuel rail hose connection nipples. Imagine what you want to do is create a small venting orifice within the pressure regulator so the vapor bubbles will quickly pass leaving solid fuel to run the engine. The downside is a slight lag in time starting cold because any pressures will be lost so it needs to build up for maybe 2 seconds cranking. Then set the fuel pressure to 28.5 lbs, the lesser setting as it was 28-32 lbs. factory. You will then adjust the pressure sensor as I previously explained to correct any lean condition created with the procedure. Keep adjustments of the pressure sensor to a minimum and in response to other posts I have read the pressure sensor is the weakest link so to speak. I was a foreman at a dealership when these cars were every where and ended up running the dyno during that era.
Bill

I have heard something like that before but could not envisage just where that hole would be and where it would vent to. It's a bit like #4 in my list in earlier post above, but that one was hard piped with a small orifice in the bypass tubing.

[Graham]

Originally Posted by Tomguy

I know these specific ones won't fit, just along the lines I was thinking. I'd need to custom fab a fuel rail but if I restore my 4.5, I'm converting to Megasquirt so it wouldnt be a big issue at that point, since everything will be redone.

I just noticed that I forgot to add the link to the Witchhunter site that has the injector capacities. I added to my original post.

[Graham]

Originally Posted by Bill Adkins

. There was another method where the water temp. sensor was moved to a lower location in the water jacket, it didn't work all that well either.
Bill

Sounds like they thought the coolant sensor was messing up the MPS on hot starts.

If coolant flow stops and engine is hot, the AAV and the coolant sensor would see a high temperature on restart. This would give a false reading to MPS for a short while until coolant flow gets things back to normal? AAV 100% closed causes slow idle which adds to problem.

Maybe the electric fan should actuate while parked to help prevent hot soaking?

[Bill Adkins]

I forgot to mention it, the yellow injectors are for the 3.5
Bill

[Graham]

Originally Posted by Bill Adkins

I forgot to mention it, the yellow injectors are for the 3.5
Bill

Yes, I realized that - And I often wondered why they had lower capacity injectors than the 4.5L M117 engines for more HP.

[Bill Adkins]

An engine is an air pump, the larger the lung the more air it takes to fill it. In this case it's air fuel mix. The efficiency in which it's burned is what creates the power. Cylinder head as well as piston crown design determines how effectively a fuel charge is consumed and the thermal yield realized. The 3.5 was a way dirtier motor and that's what happened there. Quench area is the area at the edges of the combustion chamber as well as down the sides of the piston and that's another story.
Bill

[pchbreeze]

So do I understand correctly, that on hot start, we have vapor in the fuel lines?

Today is the second time I noticed that on a hot (not overheating though) engine, when I start the engine it idles very rough. If this were the case, wouldn't a second on the accelerator flush out the vapors and fill it with liquid?

Originally Posted by Graham

Sounds like they thought the coolant sensor was messing up the MPS on hot starts.

If coolant flow stops and engine is hot, the AAV and the coolant sensor would see a high temperature on restart. This would give a false reading to MPS for a short while until coolant flow gets things back to normal? AAV 100% closed causes slow idle which adds to problem.

Maybe the electric fan should actuate while parked to help prevent hot soaking?

An electric fan cycling on is an excellent idea, and from what I've read, seems the best option. I have one on my car but I guess someone put it in aftermarket? I plan to put in a "delay on break" relay so it runs a few minutes after each shut off. A quick google shows they do make them in 12v but we'll see how easy they'll be to find. I know that Honda uses a delay on some of their cars, hopefully its not through the ecu.

[Graham]

Originally Posted by pchbreeze

So do I understand correctly, that on hot start, we have vapor in the fuel lines?

Today is the second time I noticed that on a hot (not overheating though) engine, when I start the engine it idles very rough. If this were the case, wouldn't a second on the accelerator flush out the vapors and fill it with liquid?

The never ending saga of hot starts Seems MB tried various things with limited success.

If we have vapour in the lines, or even in the injector connections, getting rid of it is not easy. A gas/liquid mixture will flow much slower through an orifice than straight liquid. We need it to either pass through the injectors as they open or through the fuel pressure regulator.

Best solution may be some type of cooling like the electric fan. I read somewhere that in Australia, some cars had aux water pumps installed so coolant was recycled through rad for a short while after shutdown.

I only find it a mild annoyance, so not working on it at present. But will get to it once other Spring stuff is out of the way. Intend to first install my new pump, then lag fuel rails, then a couple of other ideas.