warm start problem 2.7L MFI
 

I'm having trouble starting when my engine is switched off for between 1-3hrs.
I have to crank it for few minutes to get the engine to fire up.

Cold and Hot - no issues

What could be the cause?

Any help appreciated

A faulty thermal time switch may be over or under priming it. This switch routes current to the bypass solenoid on top of the fuel filter console. When that solenoid fires, it does just that - it allows fuel going through the filter console to bypass it, and go directly to the priming ports (little rubber hoses connect them) on the stacks themselves. It's like pumping the gas on a car equiped with carbs that have accelerator pumps. When the engine is cold, the switch provides current and, therefore, a priming charge until the current going to the starter solenoid heats it and it shuts off. You will see a hot wire from the thermal time switch going to the starter, and the other one going to the filter console. When the motor is hot, it will never turn on. It's the in between times, when the motor is kinda sorta warm, that it gets "confused".

One simple, temporary "fix" is to pull the lead to the bypass solenoid when the car is luke warm. Not ideal, but it keeps it from flooding. Sometimes a simpler solution is to crank it a couple of times, turn the ignition off, then floor it and crank it until it fires. The best thing to do is to replace that darn switch, but they are kind of spendy.

I eliminated that switch entirely on my car. I wired a push button switch to the bypass solenoid and mounted it on the dash just right of the steering wheel. There is an unused hole, just the right size and location, punched through the steel of the dash face. It's hiding under the vinyl cover. Seems tailor made for that switch. This way, I have full control of the when and how much of the priming charge.

Hi Jeff,

My car is fitted with a little switch on the dash. It allows me to prime the engine with fuel manually. Is this what you're refering to in you last paragraph?

I only depress this switch when the engine is bone cold (overnight) and crank the engine whilst depressing it. Once the engine fires up, I release this switch. Usually takes only 2-3 secs to fire up when the engine is bone cold.

When it's hot (ie after filling up at gas station or stopping at convenience store 5-15mins), I don't need to depress this switch. Engine will fire up within 1-2secs of cranking

But when is warm (sitting for 1-3hrs after normal usage), I've tried both depressing the switch and cranking, and just cranking. Both can take an embarrassingly long time to start. In fact, on one occasion, I think i must have flooded the engine depressing the switch for too long and the engine gave out a mighty BANG. Sounded like a gunshot. On other occassions, I have to keep cranking for 2-3mins until it 'feels' (perhaps psychologically) like battery is almost dead.

Yup, that is exactly what I'm talking about. So scratch the thermal time switch as a culprit.

Do you use the floor throttle for these luke warm starts? You will probably need to have it about halfway up. A quick shot of gas with your switch, and it should fire right up.

Do you still have the thermostat hooked up, and does it work? If it is malfunctioning, it way be creating a too lean or too rich condition for those intermediate starts. I have removed mine and have replaced it with a screw, giving me more direct control over that function as well. I notice when I let the car sit as you describe, and I get too lazy to pop the engine cover and set it for a luke warm start, it can take awhile to get it started.

It may not even be MFI related. A weak coil will sometimes lead to the problems you describe. What is the general condition of the motor, its state of tune, and all of that?

Hi Jeff,

Thanks for the quick reply. I don't use the floor throttle for warm starts, but my hand throttle is at the halfway mark. When you say have the floor throttle 'halfway up', are you suggesting i push the gas pedal halfway down and crank? And by doing so, won't I flood the engine? I thought a warm or hot engine doesn't require any more fuel?

My thermostat is removed. Engine fully rebuilt with only 5000km on ODO, was running rich previously, but my mechanic leaned it out a bit recently. I doubt it's a coil issue because this my car was COMPLETELY rebuilt (took 1year) and was done by a 'reputable' shop.

I'm just trying to ustand the starting characteristics of these older 911, since my experience are with the newer generations (993 and newer), therefore not sure if my car is normal or not and if there is something I should be aware of or is there a starting procedure with a warm engine

Regards

Sorry about the confusion. I guess I use "floor throttle" and "hand throttle" interchangably. They are both the same thing to me. I just call the gas pedal the gas pedal. Anyway, the hand throttle should be about halfway up for these luke warm starts. Keep your foot off of the gas pedal. Give it just a quick shot of fuel with your bypass button, and it should fire.

What is in place of the thermostat? There has to be something to adjust the mixture for a cold vs. hot motor. Is there a screw in its place, or a second floor lever? Removing the thermostat will cause the pump to go permanently to the full rich, cold start position. There has to be something to push the rack forward to lean it out as the motor warms up. It makes sense that it was running rich in the absence of the thermostat, if nothing was put in place as a substitute for it. What did your mechanic do to lean it out? If he adjusted the rack via the part load adjustment, he may have screwed it all up, leading to your starting issues. It is very important to have either the thermostat or something functioning in its place.