87 911 Warm/Hot Start Problem

[jduckworth]

I'm having a problem with warm/hot starts. The car starts and runs fine when cold but won't start after driving about 15 minutes. Once the car cools, it again starts and runs fine. Also, the idle does not surge at any time.

What should I check first?

Thanks in advance

[rusnak]

You can check the O2 sensor, and make sure the vacuum line to the fuel pressure regulator is connected. If you have a fuel pressure gauge, you can check fuel pressure. Maybe have the coil checked, but after eliminating the other items.

[rcaradimos]

1. Get a new DME relay save old as spare What typically goes out: DME or DME Relay (is there a diff)
2. Check CHT sensor; replace if is original single wire w/ updated 2 wire Head Temperature Sensor Replacement

Our host sells both

[kidrock]

great suggestions above, especially about the DME relay.

regarding the O2 sensor...I may be wrong, but you may be able to disconnect the O2 sensor at the connector underneath your fuel filter to isolate the problem. In theory, I guess, you will bypass the O2 sensor and the DME itself will run in "closed loop" (or something like that). Your car will run a bit rich, but should run nonetheless.

I know I've put this out there many times, but I had a similar problem where the motor started perfectly when cold, but took a crap when warmed. Ultimately, the problem was the fuel pump. Intermittent problems are the worst kind to analyse.

[86 911 Targa]

Originally Posted by rusnak

You can check the O2 sensor, and make sure the vacuum line to the fuel pressure regulator is connected. If you have a fuel pressure gauge, you can check fuel pressure. Maybe have the coil checked, but after eliminating the other items.

Here are the vaccum lines.

Good luck,

Gerry

[rusnak]

It's SOP to disconnect the O2 sensor when you're setting idle mixture and speed with an exhaust gas analyzer or AFM, but that's just to set the baseline so that the DME has a good staring point from which to adjust the mixture. Fuel trim is not the proper term because really it's just altering the fuel injection timing.

Jon, if you can rent or borrow a CO meter or AFM, then you're going to hone in on it pretty fast.

Gerry, nice diagrams. You're becoming the go-to guru for exploded parts diagrams.

[jduckworth]

Thanks for all the help guys, I'll definitely try your suggestions. My research is also pointing me toward the fuel check valve. I know what I'll be doing this weekend!

Jon

[Por_sha911]

Originally Posted by rcaradimos

1. Get a new DME relay save old as spare What typically goes out: DME or DME Relay (is there a diff)
2. Check CHT sensor; replace if is original single wire w/ updated 2 wire Head Temperature Sensor Replacement
Our host sells both

+1 Swap the relay first (you need a spare anyway). Next, you can do a search on how to check the CHT (a $5 Harbor Freight multimeter is all you need). Old coils will be affected by temp but I don't think that 15 minutes in the winter will affect it. If the first two don't fix it, find another car of similar year that you can swap with-that way it is no cost check.

[Jetjockey]

I would lean towards the fuel check valve not holding pressure and causing vapor lock. When the car cools, fuel turns back to liquid. A fuel pressure guage costs more than the check valve. I would just change the valve.