MFI Hot Start Problem (long)

sessa · 07:49 PM

I have a 1970 911S and been fighting this issue for about a year now its time for some help.
The background:
Since I bought the car if it sits for approx twenty minutes after running the car will not start. It starts cold fine and it starts after short stops IE) gas station with very little hesitation (within 2-5 seconds of cranking). At the twenty-minute mark I've followed the Porsche hot start procedures to no success. Last year if I let the pump run for a minute after I turned the car off and let it run for about a minute when I got back in it would start, this year the times have gotten longer and this method has been less effective. The symptom when the car fist cranks the engine catches sputters and stops, I've tried to push the pedal to give it some gas and there is no response from the gas peddle. To start I have to let the pump prime for about 5 minutes and then pump the gas peddle and after about 10-15 seconds of cranking it just catches.
What I've done:
Fist I pulled the plugs all of them looked good (brownish white) except for #3, which is not wet but is very black. I replaced them with a set of NKG no change.
Second: maybe the check valve on the fuel pump, I clamped the delivery fuel line after I shut off the car (act as my own check valve) and had a second set of hand unclamp attempted to start same as before. While the twenty minutes went by I listened under the hood to see if I could hear the injectors leaking and it was completely silent.
Conclusion:
Somewhere I'm losing fuel pressure and fuel out of the supply line could it be the injector on #3 is leaking how do I check? if not where to next?

sessa · 05-29-2003, 07:49 PM

I have also replaced the trans ground strap and I'm getting a good ground.

jeff m · 05-29-2003, 10:20 PM

As your cold starting is no prob then your thermo time switch is probably ok.
I don't think one injecter would make it difficult to start hot, more likely to be the thermostat not working propery. There are quite a few threads that can be found on cleaning. (search MFI thermostat)
You can check it it is operating by removing the small triangular plate top front of the pump you will see a pin. Check it hot and cold.
I assume you have the upper and lower 30mm hoses to op the thermostat.
Jeff

69911e · 05-30-2003, 03:57 AM

A few MFI people(inclucing me) have bypassed the thromotime switch, and installed a button on the dash. A press of the button will start the car when it's not fully warmed up. You can test this by connectiong the rear window defroster wire (in back) to the cold start solonoid(over the fuel filter); just do not leave the rear window defroster switch on while the engine is not being cranked.

jluetjen · jluetjen's Garage

Does the car smell at all when it's not starting. You comment about the car starting OK at a gas station - by chance is the gas station close to your house? To put it differently, was the car warm when you stopped at the gas station. By warm I mean had the oil temperature guage come off of the bottom peg at all?

sessa · 05-30-2003, 05:38 AM

Quote:

Originally posted by jeff m
As your cold starting is no prob then your thermo time switch is probably ok.
I don't think one injecter would make it difficult to start hot, more likely to be the thermostat not working propery. There are quite a few threads that can be found on cleaning. (search MFI thermostat)
You can check it it is operating by removing the small triangular plate top front of the pump you will see a pin. Check it hot and cold.
I assume you have the upper and lower 30mm hoses to op the thermostat.
Jeff

what is the function of the thermostat and how does it change under tempature conditions? secondly what should I be looking for the pin to be doing under both conditionds?

sessa · 05-30-2003, 05:43 AM

Quote:

Originally posted by jluetjen
Does the car smell at all when it's not starting. You comment about the car starting OK at a gas station - by chance is the gas station close to your house? To put it differently, was the car warm when you stopped at the gas station. By warm I mean had the oil temperature guage come off of the bottom peg at all?

Yes the car does have a smell not like gas but thereis a noticable smell. Yes the car is fully warm it can be after two hours of running or twenty minuites if the car is started within the five minute window it will start any longer and the problem starts.

Peterfrans · 05-30-2003, 06:10 AM

The function of the thermostat is to richen the mixture on a cold engine. Enrichment stops at a temp of approx 53deg. celcius. After the engine warms up the hot air which is coming from the left heat exchanger flows through the expansion elements in the thermostat. This causes the control shaft to push a lever in the pump, leaning out the mixture.

I wonder if this is the problem, why would the engine refuse to start if it is idling without problems before shutting it down. A non working thermostat would cause an overly rich mixture on a running engine too. (His plugs seem to be ok indicating a normal mixture).

Have you checked your cold start circuit? Have you tried starting the engine (after the 20 minutes) with this system disconnected?

I find the whole cold start system, dumping fuel into the stacks somewhat crude. I have installed a control lamp on the dash to check its operating.

power · 05-30-2003, 06:38 AM

I put a momentary contact push button to operate the cold start solenoid in the cab of my 72t. It cleared up my warm start problems.

john walker's workshop · 05-30-2003, 07:07 AM

on hot starting, do you hold the throttle open pretty wide as you crank? carbed engines start better this way also. no pumping, just open throttle.

Mr Beau · 05-30-2003, 09:16 AM

Have you checked the condition of the spark when you have these problems? Electrical components are notorious for not working after heat soaking for a little while...

jstobo · 05-30-2003, 10:53 AM

What pressure is the fuel pump putting out? On the MFI it should be(if I remember correctly) slightly more than 1 bar. If the fuel pump is not putting up enough pressure vapor lock could occur as the engine gets hot. Just a thought.

Paul Thomas · 05-31-2003, 11:53 AM

Oddly enough i have been having the exact same problems with my car. We have slightly different situations though, because my car is far from stock.

I have a push button for my cold start and for "fresh" starts, it works great. If the car has just been run it fires right up. If it has sat for 20+ minutes it is a b!tch. If i use the cold start to fire it up, it works, but i will foul one or more plugs. I can clear them pretty quickly when i get on the gas, but obviously this is not an optimal solution and it makes my car sound like a piece of crap. I am going to do this in my driveway and foul them on purpose, to see which ones it is and if it is repeatable.

I may have a new set of pump to injectors lines bent by Classic Tube if i can determine that the same plug are fouling. I know that #1 and #5 have seeped slightly in the past and that may be a contributor. Last resort is buy a new set of injectors, but i am not keen on spending $360 right now.

Paul

hbrand · hbrand's Garage

The button for "cold start" is still the best solution. Press longer when cold engine, just tap it with hot engine and pedal 1/2 to the floor.
Thanks, harold

sessa · 06-05-2003, 12:00 PM

It seems thanks to John Walker the problem was with the drivers technique. For the last week I've let the pump run for about fifteen seconds then held the peddle down 1/2 way and turn the key and it fires right away!! So it seems the problem is solved thanks for your help everyone I wish all of my fixes were this simple and cost effective.