73S MFI/ Electric problems (Im clueless)

[herman maire]

Ive already checked the archives so I wouldnt have to bore you guys with another trouble shooting problem but found nothing so here it is.Its pretty long.
I will do my best to explain the prob.

Car is a 73S, has a MSD 6al ign. It starts great, Runs a little rough until it gets a little warm then idles fine. Engine is extremly smooth( very surprised for MFI)
from idle to 7300rpm amazing for 73. You can drive the car as hard as you like when its up to running temps( 180).
But when it gets up to 200 just below the 210 mark it starts missing.
If youre coasting down a hill on compression it back fires.
It will do this until it dies.

when I got the car.
1.new spark plugs NGK B8ES recomended here and yesterday new NGK BP7ES.
( not problem)
2.changed fuel filter assembly and cold start valve off my 72E wich runs great.
(thought it was getting blocked not problem)

3. Beru spark plugs wires off the 72(not prob)

4.points , cap , rotor( not prob)

5. swaped IGN with the permatune off my 72( ran great, no difference)

6. fuel pump is pretty new.

After car dies it still has some spark its like the MFI pump when hot leans out to a point where it not injecting any fuel, or the distributer gets messed up.
something is getting messed up with heat.
Im thinking wire harness or MFI pump thermostat

Would really apreciate any info
thanks

Heres a pic of the trouble maker

[Zeke]

I believe the MFI thermostat benefits from the heat build up in the engine compartment and the overall heat sink effect. Things usually have to cool down significantly to have the t-stat discs return to their cold status. So, for now, I would eliminate that.

You say wiring harness. A long shot, but when was the last time you cleaned all the ground wires including the one on the trans to body? If those are all in order, maybe it is in the wiring harness. That would be a toughy.

[Early_S_Man]

Herman,

Check your points for proper gap and dwell.

To determine if it could be fuel supply related, I suggest two tests:

1. Test the fuel pump output directly into a clear glass or plastic container of 1.5 liter or greater capacity. In exactly 30 seconds it should pump 900 - 1000 ml. If fuel in the clear container is cloudy or has rust particle settle out ... fuel tank strainer or fuel line rust issues exist that need to be addressed.

2. Connect a fuel pressure gauge in the loop between fuel filter console and injection pump. Check the pressure when engine is operating normally... and when it is dying.

[herman maire]

thanks guys
zeke, early_S_man you guys are always out to the rescue. Thanks

I messed around with it last night heres what I got. The fuel flow was waaaay down.
ran it up to 190 or 200 when it died checked fuel flow (400ml)
let it cool then I got (500ml) . I never suspected it because the pump sound perfect and the car runs incredible up to below the 210 mark. pretty wierd when its only pumping less than half the volume.

I compered this with my 72 wich runs great and all I got was around ( 800ml)
still a ways off from 1000ml.

As for the gounds Im going to look at this today. The ones in the engine bay beside the fuel filter are perfect.
zeke-How would a ground effect the fuel system?

the fuel guage is an awsome idea. what do you think about leaving it permantly inline
between filter and pump?

Do you guys think its the fuel pump or something is blocking fuel suply. Gas wasnt rusty or cloudy.
this is far fetched but Im thinking maybe after it burns off the fuel that gets recirculated the suply from the tank is not quick enough and maybe it gets air in the system. But even thought 400ml is enough to keep it running

oh and the points are new and gaped. just to be sure what is the gap?
Is the dwell on the spark plugs?
thanks

[Andy Ring]

I had what seems like a similar problem with my 72T. Could start the car and drive it for a while, but then it would start missing and then finally die completely. If you left if for 5/10 minutes it would start up fine and then die once it ran for awhile. Tried all sorts of fixes and finally tracked it down to crud in the tank. Best guess is that is got sucked into the filter and stopped flow, but then cleared out when the car sat. About a year ago someone else posted same symptoms here and they too tracked it down to crud in their tank.

I'm just relating my experience--the others giving you advice here have way more technical expertise than I do. Good luck.

[Grady Clay]

I agree. The next step is to drain the fuel from the tank and remove
the screen filter for cleaning. Thoroughly clean the tank also.

The problem isn’t engine temperature related. It happens to get
to temperature when the screen gets plugged. The circulation of
fuel picks up debris from the bottom of the tank and “sucks” it
against the screen. When you turn off the fuel pump the debris
falls off sufficient for the engine to run again.

A “quick fix” to get you home is to blow (by mouth) into the
center hose at the fuel filter console. “Return to tank” below.
You should hear bubbling in the tank. Do not use shop air pressure.
IMAGE MFIfilterConsole.jpg


This is only a rescue fix. The filter screen needs to come out and
be mechanically cleaned with a tooth brush and carb cleaner.
The cause (rust in the tank) needs to be addressed.

Best,
Grady

[Zeke]

Originally posted by herman maire
2.changed fuel filter assembly and cold start valve off my 72E wich runs great.
(thought it was getting blocked not problem)

zeke-How would a ground effect the fuel system?

I don't know. You lost me somewhere. I'll learn to leave this stuff to Grady and Warren. Good luck with it.

[Clay Perrine]

Grady,
I wish I had see that picture before last weekend! I spend most of last Saturday trying to figure out why my MFI equipped 914/6 would not start. I learned the hard way that the diagrams in the factory manual show the lines going into the fuel filter console reversed. If you follow the factory diagram, the injection pump never gets fuel pressure. I finally ripped the filter console out and replumbed it. Now I have fuel pressure and the car runs!!!!


Now I have a broken MFI timing belt to contend with. ( I should have put on a NEW one before installing the engine).


Thanks for your info. You are a valuable MFI resource.

Here's a picture of my 2.4L MFI six powered 914.

[Early_S_Man]

Herman,

I believe you are headed down the correct [fuel] path to resolve the dying issue! The fuel tank needs to come out as Grady suggested. Disassemble the fuel outlet ... gland nut and fine bronze strainer screen need to come off, then the tank can be steam cleaned and sealed. A small, local aircraft maintenance shop can probably clean and seal -- tell them it is for a car and doesn't need FAA paperwork! The usual fee will probably be less than $80. Rerun the pump output test after tank is clean and sealed ... if still low, re-test with new rubber pump supply hose direct from tank outlet! The steel fuel line in the tunnel may need to be replaced if pump output is normal 900 - 1000 ml with new rubber supply line for test.

The reason I suggested checking the points was ... in case they were used/pitted the gap can be difficult to check accurately, so dwell is checked to see if it within specs. New points gapped at 0.014" will have the correct dwell of around 38°. I have seen points closed to the extent [0.004"] that at around 220°F ... the gap closed up completely, the engine would die, and couldn't be restarted until it had cooled down for 1 - 1.5 hours! I didn't check the dwell angle in that particular case because I was in far east Texas, but it would have been very low, say 20% or less, yet the CDI started the engine fine when cold and the engine showed no odd running characteristics -- while running! The culprit was a loose tensioning spring and ball bearing for the point plate inside the 169-series distributor!

Regarding Milt's suggestion to check wiring and grounds ... if existing pump was original and wiring undisturbed ... corrosion at supply or ground terminal could reduce the Voltage supplied to the pump and cause high current drain and overheating.

Good luck!