MFI cold start problems

[356RS]

Thanks Dick. OK, you have heat exchangers so you could go back to the original thermostat if you wanted to.
With your aluminum plate, when do you operate the lever that is mounted on the plate? I’m assuming you move the lever to richen up the pump for cold starts? After the engine warms up you then move the lever on the plate back to the warm running condition for the pump, thermostat rocker lever in the pump is pushed in all the way. Right.

[tobluforu]

I think he uses the button method to start the car, but also uses a screw as well to adjust afr from out side the pump? I don't think he has the same system that suptec sells.

[dicklague]

Originally Posted by 356RS

Thanks Dick. OK, you have heat exchangers so you could go back to the original thermostat if you wanted to.
With your aluminum plate, when do you operate the lever that is mounted on the plate? I’m assuming you move the lever to richen up the pump for cold starts? After the engine warms up you then move the lever on the plate back to the warm running condition for the pump, thermostat rocker lever in the pump is pushed in all the way. Right.

THE "LEVER" is just a handle on the 6mm threaded rod. I use the button hooked to the solenoid through a relay to cold start the car. I can make some changes in
AFR by turning the screw in or out. Of course most of the adjustment is done with the other adjustment screws.

[Jeff Higgins]

Maybe a picture is worth a thousand words...

Here is my MFI pump, minus the cold start solenoid and thermostat. The upper right trapezoidal shaped block off plate with the allen screw in it replaces the thermostat, the one with all of the discs in it that has the hose to the left heat exchanger. Those of us that remove this troublesome thermostat replace it with this simple adjustment screw, or with a lever and cable arrangement that can be actuated from the driver's seat.



Whichever one you have, the screw or plunger in its forward most position (screwed all the way in) represents the leanest, or the warmed up and running position. Backing the screw out, or actuating the cable to allow the plunger to move aft, will richen the mixture for cold starting. NEVER, I repeat NEVER use this adjustment as a general, overall mixture adjustment. That is not what it is for.

Even with this adjustment at full rich (backed all the way out), that is still not going to provide enough fuel delivery to start a cold motor. That is where the bypass solenoid comes in. This solenoid is not located on the MFI pump, but rather on the top of the filter console, at the forward left of the engine bay. It will have a wire connected to the top of it.

It is this "bypass" solenoid that is tripped by the thermal/time switch when you first turn the ignition key. With the electric fuel pump running, this "bypass" solenoid keeps the fuel from entering the filter console to be delivered to the MFI pump. It blocks that delivery and instead routs the fuel to the priming nozzles located in the air cleaner housing (early magnesium stacks) or the sides of the stacks themselves (later plastic stacks). When this solenoid is tripped, raw fuel simply sprays through these priming nozzles into the stacks. A lot like pumping the accelerator pumps on carbs - the motor is being primed by raw fuel.

This is all well and good when it works. It almost never does. The thermal/time switch receives current directly from the hot lead from the starter solenoid to the starter. This current heats it up. When the thermal/time switch is cold, it sends current to that bypass solenoid which makes it fire, directing fuel to the priming nozzles. When hot - either through warmed up engine heat or by the current it receives from the starter - the thermal/time switch shuts off current to that bypass solenoid, shutting it off in turn, and redirecting fuel to the MFI pump. If this happens too soon, there is not enough fuel to prime the motor for starting. If it goes on too long, it floods it.

Many of us get tired of that crap and simply wire the bypass solenoid to a momentary contact switch that we put somewhere on the dash. Now we have complete control over how much priming fuel gets delivered. That is what your car has. That's a good thing.

Most cars, when stone cold, will take about four to five seconds with your finger on that switch to fire the bypass solenoid. Turn the key to where you hear the electric pump running, but stop short of engaging the starter motor. Hold the bypass solenoid button down for four to five seconds. Pull the floor throttle all the way up and, for God's sake, keep your foot off the gas. Stepping on the gas pedal will often cause the cheap ass little plastic arm in the transmission tunnel that pushes on the throttle rod to slip off of the part swaged onto that throttle rod. Watch the gas pedal - make sure it moves downward a bit when you pull up on the floor throttle. If it doesn't, that cheap ass little plastic piece is broken. If that's the case, you are allowed to step on the gas pedal. No more than quarter throttle.

This will make your motor start reliably when cold. I promise. When warm, luke warm, hot, or whatever, you will have to determine, through practice, how long to hold down that button that fires the bypass solenoid, and how much floor throttle to pull up. Both decrease with engine temperature. When in doubt, more fuel and more floor throttle is usually the right approach.

Edit: Most of us simply leave the adjustment screw that replaces the thermostat screwed all the way in, all the time. Even up here in the Pacific Northwest, on our coldest days, my car will start easily without any need to fiddle with this screw. When I first installed it, I would religiously back it out to richen the mix for cold starts, let it run for awhile, and screw it in to the full lean running position. I eventually gave that up and now never use it. It just stays screwed all the way in. The floor throttle and bypass button on the dash are more than enough to start it in the coldest weather.

[steve meltzer]

Jeff, thanx so much for your explanation and to everyone else, a big thanx for all of your help. steve

[cmcfaul]

if the car would run better with just a screw, Porsche would have designed it that way. The further off the Porsche engineering path you go the less performance you will ultimately get.

The screw makes the car run well when hot OR cold. The purpose of the thermostat is that it richens the mixture when the car is cold, making it run better. Then leans it out when hot. Watching an AFR confirms this and every increment in between.

Porsche engineers are pretty smart.

Chris
73 E

[Jeff Higgins]

Originally Posted by cmcfaul

if the car would run better with just a screw, Porsche would have designed it that way. The further off the Porsche engineering path you go the less performance you will ultimately get.

The screw makes the car run well when hot OR cold. The purpose of the thermostat is that it richens the mixture when the car is cold, making it run better. Then leans it out when hot. Watching an AFR confirms this and every increment in between.

Porsche engineers are pretty smart.

Chris
73 E

Porsche actually did "design it that way" - the only MFI systems that used this hoopty ass thermostat were on their street going 911 engines.

Porsche only employed this less than optimum cold start system for the sake of the convenience it afforded their customers. Porsche understood full well that the customers buying their cars would never put up with opening the deck lid to fiddle with a screw. Maybe, at best, an extra lever to fiddle with next to the floor throttle, but even that would have been a stretch for too many customers.

Not everything on road going Porsches was (or even still is) optimized for best performance - much is compromised for customer convenience. This is one of those cases. If you want to see what Porsche considers to be optimized for performance, take a look at their race cars. Not a one used this Rupe Goldberg thermostat setup.

These cars have long since been relegated to being "enthusiasts' cars" or "hobby cars". As such, our expectations of convenience and daily usability have changed dramatically. Many modern owners are now closer to the race car owners' mindsets when these cars were new than they are to the gentleman sports car owners' mindsets. As such, many owners don't see fiddling with a screw as that big of a deal. Anyone who is willing to do this is far better off than those who stubbornly insist on trying to keep the thermostat working.

[steve meltzer]

thanx again for the info, Chris...this has been very, very interesting to me. I assume once you've held the button down for 5 seconds, you continue to hold it while the hand throttle is up and your cranking the starter. right? thanx s

[Jeff Higgins]

Originally Posted by steve meltzer

thanx again for the info, Chris...this has been very, very interesting to me. I assume once you've held the button down for 5 seconds, you continue to hold it while the hand throttle is up and your cranking the starter. right? thanx s

No, do NOT continue to hold your priming button down once you start cranking the starter. Here is what I do:

Turn the key to its running position to get the electric fuel pump running and leave it in that position.

Press and hold down on the "priming button", the one I have wired to the bypass solenoid. Hold down for 4-5 seconds and release.

Pull floor throttle all the way up.

Turn key all the way to engage the starter.

If it fails to start after a few seconds of cranking over on the starter, turn the key to the full "off" position, return it to the "running" position, and prime it again. Release the button and crank the starter again. Repeat as necessary, but it should not be necessary... If it fails to fire up after a couple of tries like this, there may be something else amiss.

[cmcfaul]

For a track only car the screw is fine. Who cares how it runs when cold or warm.

For cars run in the winter or street they run horrible without the thermostat or hose from HE connected until hot

Chris

73 E

[dicklague]

Originally Posted by cmcfaul

For a track only car the screw is fine. Who cares how it runs when cold or warm.

For cars run in the winter or street they run horrible without the thermostat or hose from HE connected until hot

Chris

73 E

Really? Not my experience in over 150,000 street miles. Including winter.

[Jeff Higgins]

Originally Posted by cmcfaul

For a track only car the screw is fine. Who cares how it runs when cold or warm.

For cars run in the winter or street they run horrible without the thermostat or hose from HE connected until hot

Chris

73 E

That is simply untrue. I'm beginning to think you might not have any actual first hand experience with cars set up in this manner.

Mine has been set up like this for over 15 years. My son's '68 912/911 with a 2.4 on MFI has been running like this for twelve years. They both run absolutely fantastic, all the time, with no warmup or cold running issues whatsoever. In my circle of early car hot rodders, the majority (that still run MFI) have set their cars up in this manner, and all agree it is superior to that troublesome thermostat. Every one of their cars that I have seen (and driven in many cases) run flawlessly with this setup.

[steve meltzer]

ah ooops! I'm sure I just insulted Jeff, who was trying to help me get the car started when I thanked Chris. Now that I acknowledge that I meant to say thanx to Jeff, i've insulted Chris. OK, now i'm going back to my beloved zinfandel for the night. Thanx to everyone who has tried to help out. Here...

Jeff, can i be reasonably sure the button i have is set up like yours? (The car is new to me) thanx steve

[cmcfaul]

Are you talking about cold start solenoid that sprays gas down the stacks? Yes replace thermal time switch with a manual switch and its just like a choke . No problem.

If you are talking about replacing the thermostat that enrichens when cold and leans when warm the mfi rack with a screw. You are sadly mistaken if you believe a screw will give you the same performance. You just dont know what you are missing. Probably running rich all the time.

.

[Jeff Higgins]

I'm sorry, I should have been using the term "thermostat" rather than "cold start solenoid". It's clear from your responses over the course of this discussion, however, that you were following along in spite of this ("For a track only car the screw is fine. Who cares how it runs when cold or warm.

For cars run in the winter or street they run horrible without the thermostat or hose from HE connected until hot").

That point of clarification out of the way, I am anything but "sadly mistaken". Like I mentioned above, I have employed this system - a screw in place of the thermostat for 15 years on my own car, my son has used it for 12 years on his, and I have personal experience with several other cars using it as well, all with excellent results.

So again I ask (you avoided answering this question once already) - do you have any personal experience with this? It's become quite apparent, from your erroneous assessment of the performance of this setup, that you do not.

[Jeff Higgins]

Originally Posted by steve meltzer

ah ooops! I'm sure I just insulted Jeff, who was trying to help me get the car started when I thanked Chris. Now that I acknowledge that I meant to say thanx to Jeff, i've insulted Chris. OK, now i'm going back to my beloved zinfandel for the night. Thanx to everyone who has tried to help out. Here...

Jeff, can i be reasonably sure the button i have is set up like yours? (The car is new to me) thanx steve

It's pretty easy to check if it is. Have someone sit in the car and turn the key on to the running position, so you can hear the electric fuel pump running. Put your finger on the bypass solenoid on top of the fuel filter console and have them hit what you think is the "priming button". You should both hear and feel that bypass solenoid click.

[cmcfaul]

Owned a 73 E for 20 years. Rebuilt the engine my self twice. Tracked it a few times a year. Car was freakishly fast and competitive in all but black group. Eventually increased compression and dc 30 cams to an otherwise stock car. Added an afr meter and am an expert on adjusting mfi. First hand experience of results of disconnecting thermostat to mfi rack. A screw is a jerry rig at best. Fine for a race car. A compromise at best for a street car. Will never run as designed missing a major component to a complicated system. It will work but far from ideal

[Jeff Higgins]

Originally Posted by cmcfaul

A screw is a jerry rig at best. Fine for a race car. A compromise at best for a street car. Will never run as designed missing a major component to a complicated system. It will work but far from ideal

You will find widespread disagreement with your assertions within the MFI community. A great many of us see replacing the thermostat with a simple screw to be superior in every way to that thermostat, and certainly in no way as a "compromise at best for a street car". I'm sorry that you had poor results, but there are an awful lot of us who have had no trouble whatsoever in making it work far better than the thermostat.

[steve meltzer]

Jeff, i'll check this tomorrow. thanx again...enjoying this discussion, as i learn from the back and forth. s

[steve meltzer]

Originally Posted by Jeff Higgins

It's pretty easy to check if it is. Have someone sit in the car and turn the key on to the running position, so you can hear the electric fuel pump running. Put your finger on the bypass solenoid on top of the fuel filter console and have them hit what you think is the "priming button". You should both hear and feel that bypass solenoid click.

Whatda know...there was no click! i immediately saw a broken wire....repaired and now the solenoid behaves as it should. didn't have time to see if it starts, better, but the repair "can't hoit". thanx, Jeff. steve