3.0 CIS difficult to start when engine is warm

[jbscheff]

I recently got my ‘73 911 put back together after a six-year, bare-metal rebuild. There are still some issues I’m working through as I did most of the disassembly/reassembly myself.

My immediate issue has to do with starting. The car starts up right away when the engine is cold. But after driving it for 20 min or more and switching the car off, it takes a lot to get it started again. It turns over just fine, but won’t fire up until it’s been cranking for at least a minute. It also doesn’t fire up smooth; I need to keep the starter engaged for another few seconds until it seems all cylinders are firing.

I’m thinking that maybe it doesn’t get fuel right away and then not all cylinders at first? I don’t smell gas in or around the car so I don’t think there’s a fuel leak. Do the CIS motors have a warm up regulator or something that might be causing this? Could it be a fuel pump issue? Any ideas?

Other items I’m dealing with:
-Small transmission leak from gear selector shaft seal
-Mechanical speedo sticks at first- pull it out of dash and shake it and it works again.
-Improperly installed rear window heater wiring (not by me)

All in all it’s not too bad and car looks and drives great - when it starts.

[Litle brother]

You probably need to change your fuel pressure accumulator.
If it is defect it will give you air bubbles in the fuel lines when engine is half, fully warmed up and thus give you starting problems.

Cheers

Eskild

[ramonesfreak]

Had same problem and it was fuel accumulator. Replaced and problem resolved

[john walker's workshop]

Get the large late SC accumulator. That may fix it, but there is still the fuel pump check valve and another in the fuel distributor that can leak down. You can lift the sensor plate arm manually after it sits hot for a few minutes and if it feels like there's about a pound of pressure on it, instead of nothing, then it's likely fixed.

[jbscheff]

Thanks for the quick replies. I’ll order a new accumulator and see if that does it

[Targalid]

Do you have a fuel pressure gauge? Checking your fuel pressure, cold,warm and residual is the place to start. Don't replace parts without a good diagnosis.

[93nav]

If it starts right up cold, would that not indicate that the fuel accumulator was doing it's job?

[john walker's workshop]

Not related to cold start. That's what the cold start valve is for.

[proporsche]

before you spend any bucks..start with the simple thing..replace your fuel pump check valve...of course the fuel press gauges would help.That would tell you exactly what is up.
When you shut the engine off all the fuel is going back to the fuel tank .The check valve is not holding the fuel press.in the system....

Ivan

something like this

[boyt911sc]

Test the residual fuel pressure. If the residual fuel pressure is below spec, there are several most likely culprits and identify which of the following is causing the pressure loss. These are the 3 most common culprits for residual fuel loss:
a). Fuel accumulator.
b). Fuel pump check valve.
c). Fuel distributor primary valve seal.

Don’t replace parts on the fly. Test and verify that the subject culprit is actually causing the problem. People have the habit of blaming the fuel accumulator without even testing it. Do not replace parts unless confirmed defective.

Tony

[DonNewton]

Had this issue on my '83. Replaced the ground strap about 14 years ago and haven't had a problem since. Just throwing it out there.

[jbscheff]

Ground strap eh? I think I’m good on that one.

All, I appreciate all the admonishments to test the fuel pressure before replacing any parts. Good advice if you’ve already got the equipment, which I don’t. So I’ve ordered a new accumulator and I’m looking for a test kit. Clearly I’m putting my money on the accumulator. I will also pull out that check valve and test it as well.

[mhackney]

Fuel pressure gauge set is not expensive and a requirement for a CIS owner. Certainly a lot less expensive and a lot more effective than randomly replacing parts. boy911sc knows his stuff as do others. I started down the path of "replacing this, replacing that" and never found the root cause. It is easy to fall into the trap. CIS is not rocket science and now there is a great body of knowledge to demystify it.