911 SC cold start issue

[Lar911sc]

Hello! I have a cold start issue that I’d love some help diagnosing.

The car:

• 1979 911 SC
• 3.0
• 72,000 Miles
• Brand new tune up – wires, plugs, rotor, distro
• Clean fresh gas

When I start the car with a cold engine it fires right up, but immediately drops RPMs and behaves as if it will stall – but so far it hasn’t stalled out. After a few seconds the RPMs rise to ~900rpm, then up to ~2000rpm a few seconds later. The car stays at this RPM range for about minute before it drops to ~1000rpm and is ready to go. Once the car is warm this problem does not occur on restart.

Video of a cold start: https://youtu.be/t4ccCql7o-8

I suspect the WUR but was hoping that maybe someone here had experienced and diagnosed a similar problem before I throw a $400 part at it.

Thanks!

[ossiblue]

Originally Posted by Lar911sc

Hello! I have a cold start issue that I’d love some help diagnosing.

The car:

• 1979 911 SC
• 3.0
• 72,000 Miles
• Brand new tune up – wires, plugs, rotor, distro
• Clean fresh gas

When I start the car with a cold engine it fires right up, but immediately drops RPMs and behaves as if it will stall – but so far it hasn’t stalled out. After a few seconds the RPMs rise to ~900rpm, then up to ~2000rpm a few seconds later. The car stays at this RPM range for about minute before it drops to ~1000rpm and is ready to go. Once the car is warm this problem does not occur on restart.

Video of a cold start: https://youtu.be/t4ccCql7o-8

I suspect the WUR but was hoping that maybe someone here had experienced and diagnosed a similar problem before I throw a $400 part at it.

Thanks!

The issue could very well be the WUR, but you do not necessarily have to replace it. You do need to invest in a fuel gauge pressue set, however. You need to establish if your system, cold control, warm control, and residual pressures are in spec--assuming all else, like timing, ignition, and valve clearances are good. Your symptoms are consistent with a too high cold control pressure, but you need fuel gauges to check. If so, it is easily remedied without buying a whole new WUR.

The key to your issue is to eliminate possibilities, of which the WUR is but one. You need a fuel pressure gauge to verify its proper function.

[RSBob]

Ossi speaks the truth.

[boyt911sc]

Where are you located? Thanks.

Tony

[Bob Kontak]

My 81 has lagged longer than that with WUR within pressure specifications. At least it's livable before remedied. Just an annoyance.

Here is a cheater procedure you can do for fun. It may start better but does not fix the root cause(s). Plug the distributor retard hose with a golf tee. Don't worry about plugging the metal tube on the distributor. It advances your timing 5-10 degrees but only at idle. It compensates for little tired things in the CIS. It may also be a tiny bit zippier off idle.

That said, every part on the CIS is testable. If you are new to the car, I cannot think of a better learning curve to go through than to read up, buy a few tools and perform due diligence work on your existing components.

I would start with a pressure test as ossi noted. Get a decent set of gauges.

[pmead]

I just had a similar issue with a 83 SC. Went through the CIS pressure testing and found my rebuilt WUR was a little out of cold spec. (to rich) But the big problem was actually my base timing. I had engine work done and the car was returned to me with the base timing set to 5 ATDC instead of 5 BTDC. Reset timing and now have perfect cold starts. Your issue may not be timing, but it is a quick thing to check and remedy. Tony and Bob will not steer you wrong with CIS issues.