Yet another hard starting thread

RockAble · 01-15-2012, 12:16 PM

I have a 74 Carrera with an RS spec MFI engine. It cranks right up most of the time when it is cold BUT if I drive it a while and let it sit for a few minutes it sometimes seems like it will never start. Sometimes, it start on a couple of cylinders just as I'm releasing the key from the start position.

I know the pump is timed correctly and the engine runs great when it is running. it acts like it is not firing. Sometimes, when it does fire, it will backfire through the exhaust. This leads me to believe there is raw fuel available but insignificant spark. Oh yeah, I've got a manual cold start switch and my rpm module works when I close the micro switch on the throttle linkage.

This weekend I replaced the ignition switch (argh!), cleaned the points, cleaned and checked all of the electrical connections, replaced a weak battery, bench tested my Permatune and probably something else I forgot. No change.

I've been working on 911s since 1983 and have done it all but this is kicking my butt. Any suggestions are appreciated.

RockAble · 01-15-2012, 04:40 PM

Could leaking injectors cause this problem?

ossiblue · 01-15-2012, 04:49 PM

When you have the no-start condition, can you smell fuel in the engine bay?

Next time it happens, be ready, and do a quick check for spark using a spare plug. You may have an ignition problem that appears when components/wiring gets warm.

RockAble · 01-15-2012, 05:10 PM

I've tried that. Problem is finding an extra person every time I need to check it.

. I like the way you think, however. We're on the same page. Yes, often I smell fuel.

timmy2 · timmy2's Garage

Fuel accumulator
The fuel accumulator maintains the
pressure in the fuel system for a certain
time after the engine has been switched
off in order to facilitate restarting, particularly
when the engine is hot. The special
design of the accumulator housing
(Figure 5) deadens the sound of the fuel
pump when the engine is running.
The interior of the fuel accumulator is
divided into two chambers by means of a
diaphragm. One chamber serves as the
accumulator for the fuel whilst the other
represents the compensation volume
and is connected to the atmosphere or to
the fuel tank by means of a vent fitting.
During operation, the accumulator
chamber is filled with fuel and the diaphragm
is caused to bend back against
the force of the spring until it is halted by
the stops in the spring chamber. The
diaphragm remains in this position, which
corresponds to the maximum accumulator
volume, as long as the engine is
running.
K-Jetronic
15

RockAble · 01-16-2012, 03:12 AM

There is no fuel accumulator on a MFI car. Only CIS and later.

rwest · rwest's Garage

I had the same symptoms on my CIS car and it was the prematune module, seemed to fail with heat. Took forever to actually verify that was the problem, I started keeping a timing light and automotive scope ready for quick hook up and was then able to prove it bad. I installed a MSD this time.

Good luck.
Rutager

timmy2 · timmy2's Garage

Quote:

Originally Posted by RockAble

There is no fuel accumulator on a MFI car. Only CIS and later.

Oops didn't see it was MFI. Oh well. Maybe the fuel is boiling off when it's hot. Is there a fuel line check valve to test?

tobluforu · tobluforu's Garage

Do you have the three fuse block in the engine bay, if so make sure the middle fuse is not blown and that you are getting power? This powers the fuel enrichment solinoid which is located on top of the fuel filter if you have one inside the engine bay. Do you have this anymore, or did you take it out?

RockAble · 01-16-2012, 05:08 PM

The fuel enrichment solenoid works. I bench tested the permatune and all values, including capacitance, are spot on. It runs great after it starts.

Im going to make sure it is not the tach or the module that shoves the fuel rack to zero output under deceleration. I'm also going to make sure the timing is ok at. rest.

rwest · rwest's Garage

My Permatune would bench test great too- do the tests when you are in a no start issue and see what you get. I had a timing light with a removable wiring harness and I left the harness hooked up and would hook the light up to verify no spark during no start; after I realized my problem was spark related I then isolated the permatune box and tested it during no start events and finally verified it as the culprit. It took some time and a great deal of frustration since it would cool down with the engine lid open and start working again before I was able to complete the test.

-Rutager

RockAble · 03:02 PM

Quote:

Originally Posted by rwest

My Permatune would bench test great too- do the tests when you are in a no start issue and see what you get. I had a timing light with a removable wiring harness and I left the harness hooked up and would hook the light up to verify no spark during no start; after I realized my problem was spark related I then isolated the permatune box and tested it during no start events and finally verified it as the culprit. It took some time and a great deal of frustration since it would cool down with the engine lid open and start working again before I was able to complete the test.

-Rutager

Explain how you used the timing light. I have one with a removable harness. Did you just turn the engine over and see if it would trigger the light?

Would yours eventually crank and run fine until you shut it off?

If this sucker is bad, I'm going to put a MSD 6A on it. No more Permatune for me, if that's the case.

RockAble · 01-18-2012, 09:52 AM

btt

RockAble · 01-22-2012, 11:34 AM

Well, I used the timing light to check for spark which is a great idea. The good news is that my Permatune seems ok, so I guess I don't have an ignition problem.

I have a manual cold start switch, so I checked to make sure the solenoid valve is opening. I hooked up a hose and ran it into a bottle. It is working fine. The car runs great but is just hard to start after its been driven. I am stumped.