No hot start, but cold starts are fine

[cowtown]

Engine '77 930 3.0
Chassis: 75 911S

Hi,
I've got a CIS fuel and/or a timing problem, I think.

I can start the car when cold (around 70F) very easily. I pull the hand throttle up and crank it, then manually bring down the engine speed. It will idle at 1000RPM.

But it wont start up again once it's warmed up! You can warm it up, kill it, then try instantly to restart, and it just cranks with no sign of combustion at all.

System and hot/cold control pressures are all in spec, except hot control pressure is about 5psi too high. This is suspicious, but as far as I can tell there's no way to adjust hot control pressure beyond setting system pressure, right? Maybe the WUR has a blockage? I have pushed down the fuel metering plate when cranking to manually give it some gas and air, and this does nothing.

Fuel pressure retention is fine, filter is new, timing is set to spec at 7 deg. BTDC@1000RPM with vacuum line on the distributor. Advance curve is fine when revved. I have spark.

If I manually advance the timing to around 20ATDC, it will reluctantly start up when hot, and I can then retard back to spec, and it will keep running. Vacuum advance maybe? But why is this a problem only when hot?

Details:

-no AAV (removed)
-no Thermotime (removed)
-no coldstart injector (plugged)
-no decel valve (not equipped)
-no lambda or frequency valve (not equipped)
-nothing really there at all, except the fuel distributor, metering plate, and WUR (heated).

Thank you for any clues. I'm open to putting back some of this stuff, but it seems like most of it is to help with cold starts, when the problem is hot starts.

Colin

[nostatic]

does it have a fuel accumulator? That is often a culprit for SC hot start problems.

[sammyg2]

Usually a problem with the accumulator or check valve takes a little while to show up. The fuel in the lines has to vaporize, and the pressure has to drop so it takes some time, like 10 or 15 minutes. I've never heard of one going into vapor lock immediately like this, I'd say it's something else.
You said that you raised the sensor plate by hand, have you tried raising it until you hear the injectors fire for a few seconds while not cranking it and then try to start it?
Basically you would be priming the engine. if you do this and it starts, that would tell you that you may not be getting enough fuel to start the engine. If it doesn't start, it tells you that you have an overly rich condition and you have to figure out why. have you verified the sensor plate rest position is set correctly?

Of course this is all assuming it isn't ignition related. The distributors in these cars are getting very old and while they are very reliable we are seeing more and more problems with the advance/retard mechanisms inside the dizzy failing. Some folks have had luck lubricating and freeing up sticky advance mechanisms, not sure if this helps but good luck.

[ruf-porsche]

Before replacing parts, you should determine the problem.

No start when the engine is hot.

Do you have a spark? If not check the ignition system. Could be a bad CDI that fail when the engine is hot.

If you have spark, do you have gas?
Check the fuel system.

In order to fix the problem, you have to know what is causing the problem.

[cowtown]

Thanks for the help guys. RUF, I do have spark, and pressures are OK. CDI is new and good. I pull plugs regularly to verify spark. Nostatic, I think the accumulator is OK because the system pressure is retained per spec....

SammyG, you might be on to something:

Originally posted by sammyg2
You said that you raised the sensor plate by hand, have you tried raising it until you hear the injectors fire for a few seconds while not cranking it and then try to start it?
Basically you would be priming the engine. if you do this and it starts, that would tell you that you may not be getting enough fuel to start the engine. If it doesn't start, it tells you that you have an overly rich condition and you have to figure out why. have you verified the sensor plate rest position is set correctly?

This was my thinking when I pushed down the plate (downdraft plate on Turbos). I did open the injectors and prime the cylinders this way, with no change. Rest position is in spec, and the plate is centered correctly. Maybe it is actually too rich for hot start?

...OR (and I'm starting to like the theory below)...


Of course this is all assuming it isn't ignition related. The distributors in these cars are getting very old and while they are very reliable we are seeing more and more problems with the advance/retard mechanisms inside the dizzy failing. Some folks have had luck lubricating and freeing up sticky advance mechanisms

Can you help me in testing the advance mechanism? It's supposed to be at its fully advanced position when the engine is not running, correct? If it is stuck in a retarded position, this could explain why I have to advance to get it started when hot...i think. Is there any way to test this mechanism with a compressor? I don't have a vacuum tool.

[ruf-porsche]

Most dizzy are either centrifugal advance or vacuum advance.

Could it be that the dizzy is a teeth off on the gear? Did you ever remove the dizzy and replace it exactly as before? When did you start having this problem?

A quick check would be to set the crank pulley at Z1 and see where the rotor is pointed. It should be pointed at number one cylinder if not rotate the crank 360 degree and look again is it at number one cylinder?

If you answer no it's not at number one cylinder for both situation, your dizzy is off by a teeth or maybe a couple of teeth on the timing gear.

[ruf-porsche]

One more thing

Does you dizzy have point? If yes, when was the last time they were changed or gapped?

[cowtown]

Update: I got it sorted out after Sammy turned me on to the distributor. I don't fully understand WHY this works now, but it does start and run great, hot and cold:

I found that there was no vacuum beinge generated at the port the distributor hose was hooked up to. Since it's retard-on vacuum by around -10 degrees, I guess the base idle timing was too advanced when I set it at idle with the hose on.

I hooked the vac hose to the unused brake booster vac connection on the throttle body and immediately got good vacuum and more retarded timing. Reset timing, and now it starts hot just fine.

Strangely, the same distributor vac port that screwed up the distributor IS giving enough vacuum to run my Autometer mechanical vacuum/boost gauge. Maybe it's fouled up?

So now I'm getting my full advance curve... but how this fixed the hot start is beyond me. It's the only thing I changed though, so...it works.

Thanks for the help!

PS-Ruf, this Turbo distributor has both centrifugal advance and vacuum retard, and it is without points. It's not off a tooth - timing is correct.