MFI cold start problems

[30westrob]

For my completely stock, street driven, 73S MFI system, has not been adjusted or touched for 15 years, other than routine filter replacement. The car starts well, and runs well, and I would not want to change a thing. Others may disagree.
Rob

[kltarga72]

Besides Pelican Parts when seeking advice check out Early 911 S Registry. Great source of information on the early 911 models.

Lash

[dicklague]

Originally Posted by Jeff Higgins

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.

Jeff, I agree 100%....... I am one of those guys that have invested lots of time learning how to get MFI to work. I, like you prefer to have direct control over the cold start and warmup, and DO NOT rely on some 50+ year old electrical design and a stack of bi-metallic washers that is quite crude and not very reliable.

I have been running with a manual push button for cold start solenoid manually operated and the warm up stack removed for over 10 years and over 100,000 miles and monitor AFR with a data gathering system that records RPM, MAP, and AFR. IT IS NOT INFERIOR to the warm up regulator setup.

[dicklague]

Originally Posted by 30westrob

For my completely stock, street driven, 73S MFI system, has not been adjusted or touched for 15 years, other than routine filter replacement. The car starts well, and runs well, and I would not want to change a thing. Others may disagree.
Rob

How do you know it is running as well as it could be? Maybe the mixture is wrong and maybe you have flat spots that you have gotten used to.

[cmcfaul]

Jeff and Dick,

How can your cars run optimally at all temperatures with ability change mixture according to temperature eliminated (by eliminating the thermostat to the MFI rack)? Its like saying putting a brick on the gas pedal works as well as cruise control. The screw jerry rig will set mixture at a fixed level. It will not allow the car to run richer when cold and leaner when hot automatically which is ideal for a street car and why the thermostat is there. So you either have to set your screw in the middle so it runs when cold and hot (neither optimally). Or set it lean or rich. These cars will run just fine rich but are leaving HP on the table. leaner is faster (but dont run well when lean and cold). Another example, Do you set your timing with just a screw and eliminate the centrifugal (automatic) advance?

Chris

[cmcfaul]

Clean enough
My 73 E when just installed but not completely hooked up.

[Jeff Higgins]

Originally Posted by cmcfaul

Jeff and Dick,

How can your cars run optimally at all temperatures with ability change mixture according to temperature eliminated (by eliminating the thermostat to the MFI rack)? Its like saying putting a brick on the gas pedal works as well as cruise control. The screw jerry rig will set mixture at a fixed level. It will not allow the car to run richer when cold and leaner when hot automatically which is ideal for a street car and why the thermostat is there. So you either have to set your screw in the middle so it runs when cold and hot (neither optimally). Or set it lean or rich. These cars will run just fine rich but are leaving HP on the table. leaner is faster (but dont run well when lean and cold). Another example, Do you set your timing with just a screw and eliminate the centrifugal (automatic) advance?

Chris

A fair question, but one that is posed in such a manner as to convince me that you never did actually try this yourself, or at least not for any length of time.

As I stated earlier in this thread, when I first installed the screw, I would religiously open the deck lid and back it out to its full rich position to start the car when cold. I would drive a block or two or three, pull in somewhere, get out and screw it all the way in to its full lean position. This was admittedly less than convenient.

After awhile, I just got lazy about it and left it screwed all the way in to the leanest position. All the time. That was over 15 years ago and I have never touched it since. In that light, calling it an "adjustment screw" is kind of a misnomer, I guess. All it serves to do now is to keep the lever on which the plunger in the thermostat acted pushed all the way forward to its full lean position, all the time.

This has resulted in no discernible affect on drivability whatsoever. None. One might think it would run so darn lean when cold that it would hesitate, stutter, pop and bang out the exhaust, and all that kind of stuff. It does none of that.

We drive these cars with respect and restraint while they are warming up anyways. By the time the oil temp is up to where we can start having some fun, the thermostat would have pushed its plunger all the way out anyway.

So, yeah - just keep the thing screwed all the way in, all the time. Any affect on cold running is trumped by the operational restrictions we self impose while the motor is warming up. You will quite honestly never even notice it.

[Jonny042]

Originally Posted by cmcfaul

Jeff and Dick,

How can your cars run optimally at all temperatures with ability change mixture according to temperature eliminated (by eliminating the thermostat to the MFI rack)? Its like saying putting a brick on the gas pedal works as well as cruise control. The screw jerry rig will set mixture at a fixed level. It will not allow the car to run richer when cold and leaner when hot automatically which is ideal for a street car and why the thermostat is there. So you either have to set your screw in the middle so it runs when cold and hot (neither optimally). Or set it lean or rich. These cars will run just fine rich but are leaving HP on the table. leaner is faster (but dont run well when lean and cold). Another example, Do you set your timing with just a screw and eliminate the centrifugal (automatic) advance?

Chris

You are giving the cold start thermostat a lot more credit than it deserves. It's not a continuously variable air temperature compensation system. It's for cold start, and pretty crude at that. My understanding is that once the motor has warmed up it's pretty much pegged the mixture to full lean no matter what the ambient temperature is. Much like Jeff's adjustment screw.

There is, however, a barometric pressure compensation system as well, which would work much more like you are imagining. How much it helps maintain the afr during temperature swings I do not know, but it might be able to predict the weather.

[Jonny042]

Originally Posted by cmcfaul

Clean enough
My 73 E when just installed but not completely hooked up.

That's a sharp looking motor!

I believe there was a factory bulletin advising the removal of that hot air blend box thing. Not sure the details of that but you'll see quite a few cars simplified to run without it.

[Jonny042]

Originally Posted by 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.

Jeff, this is an excellent and very detailed post - thanks! I have two questions I'd like your view on, as I am building a 3.2SS MFI motor as we speak - have built an MFI test rig and am in the middle of calibrating my pump.

#1 - I have a supertech manual enrichment system - my observation on the test bench is that the range of enrichment is quite large with this. In your opinion, would it offer enough enrichment for cold start without the solenoid and spray nozzles? Assume ambient temp would be at least above freezing.

#2 - I see you also have eliminated the stop solenoid for fuel cut - I apologize if I missed or have forgotten this from your other threads, but can you comment on the before/after effect of that? Assume it's an enthusiast's car and fuel economy is not a primary concern

Also let me say your contributions to the MFI community (of which I am looking forward to being a card-carrying member of) appreciate your efforts to add to the knowledge base! I hope to do the same.

[Jeff Higgins]

Originally Posted by Jonny042

You are giving the cold start thermostat a lot more credit than it deserves. It's not a continuously variable air temperature compensation system. It's for cold start, and pretty crude at that. My understanding is that once the motor has warmed up it's pretty much pegged the mixture to full lean no matter what the ambient temperature is. Much like Jeff's adjustment screw.

Exactly. In theory, anyway, when it actually works as designed. It often does not - there can be a leak in the hose going to the left heat exchanger, the discs inside are gummed up and stuck, and a host of other potential problems. It most certainly does not continually adjust the mix. Additionally, the thermostat actually goes to full extension - the plunger is all the way out, putting the mixture to its full lean running state - far faster than the engine warms up.

Originally Posted by Jonny042

There is, however, a barometric pressure compensation system as well, which would work much more like you are imagining. How much it helps maintain the afr during temperature swings I do not know, but it might be able to predict the weather.

[Jeff Higgins]

Originally Posted by Jonny042

Jeff, this is an excellent and very detailed post - thanks! I have two questions I'd like your view on, as I am building a 3.2SS MFI motor as we speak - have built an MFI test rig and am in the middle of calibrating my pump.

#1 - I have a supertech manual enrichment system - my observation on the test bench is that the range of enrichment is quite large with this. In your opinion, would it offer enough enrichment for cold start without the solenoid and spray nozzles? Assume ambient temp would be at least above freezing.

#2 - I see you also have eliminated the stop solenoid for fuel cut - I apologize if I missed or have forgotten this from your other threads, but can you comment on the before/after effect of that? Assume it's an enthusiast's car and fuel economy is not a primary concern

Also let me say your contributions to the MFI community (of which I am looking forward to being a card-carrying member of) appreciate your efforts to add to the knowledge base! I hope to do the same.

Thank you for the kind words.

#1) In my experience, no. We have tried a number of methods for starting both my car and my son's '68 912 (with my old '72 2.4) when cold. We have tried it without hitting our priming buttons, with the screw that replaces the thermostat backed all the way out to "full rich". They do eventually get enough fuel in them to start, but it takes an awfully long time, even in warm summer weather. I wouldn't run a street car without the priming nozzles. Maybe a race car, where you would manually prime down the stacks.

#2) I have seen no real affect on drivability or reduction in gas mileage. There is such a minimal amount of fuel being delivered when the car is decelerating and the pump is at its idle delivery, and this happens for such a short period of time, that it just does not add up to much fuel.

The real intent of the shutoff solenoid is to keep it from popping and banging through the exhaust on deceleration. I've found that few of the many muffler configurations I have tried will do this anyway. Even if they did, many of us now think it's pretty cool to fart a little flame out the pipes... So, yeah, another one of those, to me anyway, "needless complications" on MFI that can be removed on our now "enthusiasts' cars".

[Arne2]

Originally Posted by Jeff Higgins

Exactly. In theory, anyway, when it actually works as designed. It often does not - there can be a leak in the hose going to the left heat exchanger, the discs inside are gummed up and stuck, and a host of other potential problems. It most certainly does not continually adjust the mix. Additionally, the thermostat actually goes to full extension - the plunger is all the way out, putting the mixture to its full lean running state - far faster than the engine warms up.

This is an interesting discussion for me, as a noob on MFI. I have a '72 T project with a fresh rebuilt stock 2.4 coming soon. Still running original MFI. Pictures indicate that the paper tube/hose going to the thermostat is missing, as is the warm air blend box thing, and the original heat exchangers are in sad shape.

Rather than get a new hose, it sounds I could put the plate and screw in place of the thermostat, especially since I live in a rather mild climate. I'll never need that extra enrichment.

Much cleaner look back there without the hose and air blender.

Plus, I then have no need for the special MFI left exchanger, if I have no hose to the thermostat and no warm air blender. Am I thinking correctly?

[Jeff Higgins]

Originally Posted by Arne2

Am I thinking correctly?

Yes. The car will run much better and be a bit less troublesome with these well vetted modifications. As far as I'm concerned, the only reason to fuss with restoring these original systems to an operational state would be if you are looking for a faithful restoration, like for concours events or for resale value. If you just want a driver, you are better off simplifying by eliminating these features.

[Arne2]

Originally Posted by Jeff Higgins

Yes. The car will run much better and be a bit less troublesome with these well vetted modifications. As far as I'm concerned, the only reason to fuss with restoring these original systems to an operational state would be if you are looking for a faithful restoration, like for concours events or for resale value. If you just want a driver, you are better off simplifying by eliminating these features.

Since my metal fab skills are strictly on the Fred Flintstone level, are those block-off plates with screw available pre-made anywhere?

[Jonny042]

Yes...

https://goo.gl/images/BTqccV

[stownsen914]

I think I mentioned it earlier in the thread, but I have a 914/6 racecar with a 2.7L MFI motor. All the solenoids removed and the warmup circuit deleted from the MFI pump. I start my engine easily with no priming. I just put the accelerator to the floor and it starts every time, though granted I don't run it in cold weather. When I first built the car, I made a priming circuit using the cold start solenoid on the filter stand and actuated by a button on the dash. It worked well, but deleted that setup once I realized that flooring the throttle pedal while cranking worked just as well. One less thing to break or leak gasoline and start a fire.

Scott

[Jeff Higgins]

Originally Posted by Jonny042

Yes...

https://goo.gl/images/BTqccV

That's pretty darn cool.

I just made mine from the spacer, or "shim" in the diagram below, that sits between the thermostat and the pump body. I just tack welded a nut to the inside of it.

[Jeff Higgins]

Originally Posted by stownsen914

I think I mentioned it earlier in the thread, but I have a 914/6 racecar with a 2.7L MFI motor. All the solenoids removed and the warmup circuit deleted from the MFI pump. I start my engine easily with no priming. I just put the accelerator to the floor and it starts every time, though granted I don't run it in cold weather. When I first built the car, I made a priming circuit using the cold start solenoid on the filter stand and actuated by a button on the dash. It worked well, but deleted that setup once I realized that flooring the throttle pedal while cranking worked just as well. One less thing to break or leak gasoline and start a fire.

Scott

I have seen plenty of guys with theirs set up like this. I could just never get it to work myself. I do see guys with race cars with no bypass solenoid using a little squirt bottle of gasoline to prime them right down the stacks, sorta like a Top Fuel car.

I guess we all just have to play with it and learn our respective cars. The salient point is that all of that factory cold start stuff can be removed, and these things start and run just fine. It sure saves a lot of headaches trying to keep it all more or less "working".

[dicklague]

Originally Posted by Jeff Higgins

I have seen plenty of guys with theirs set up like this. I could just never get it to work myself. I do see guys with race cars with no bypass solenoid using a little squirt bottle of gasoline to prime them right down the stacks, sorta like a Top Fuel car.

I guess we all just have to play with it and learn our respective cars. The salient point is that all of that factory cold start stuff can be removed, and these things start and run just fine. It sure saves a lot of headaches trying to keep it all more or less "working".

Good point. Simplification and having control over the warm up routine just adds to the fun of these cars.