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1980 L-jet system, rough idle problem with cold start warm-up. When the engine is cold it will initially start and idle fine for the first 15 seconds of the cycle. Then it will drop the idle from 1000 rpm to 500 - 700 rpm and not idle well. have to open throttle to keep it running. As the engine warms up to normal operating temperature it progressively improves. At normal operating temperature the engine idles well at 1000 rpm and runs well. The problem is only from cold to normal temp. Does the thermo time switch come into play for the warm-up enrichment cycle? Any thoughts on the problem is appreciated,,,,Thanks Don

Short answer: yes

You can check the FAQs for the L-Jet schematic and description

KenB

No, thermo time and cold start injector only works on cold starts, ie when it's cold enough and the engine is cranking. In no way is it a warm up device. The AAV (aux air) isn't working well to change airflow between cold and hot.

^^ that

So, now that I am recalling how things work better, the thermo-time switch merely determines the length of the initial cold-start injector pulse (if any), correct?

And any such pulse is over within a matter of few seconds, even in the coldest of situations, yes?

KenB

KenB wrote:So, now that I am recalling how things work better, the thermo-time switch merely determines the length of the initial cold-start injector pulse (if any), correct?

And any such pulse is over within a matter of few seconds, even in the coldest of situations, yes?

KenB

Correct, I believe it's a maximum of 8 seconds. And it won't do it again until it cools off. But it's only when cranking.

Maybe check fuel pressure regulator. (?)

Noticed the vacuum line came off. The engine had been starting fast in the summer heat. It would run at 1200 for thirty seconds then drop and barely stay lit. Rev to keep it cleaned out, as engine temp climbed it ran better. Finally normal at normal operating temp.

Put the vacuum line back on the fuel pressure regulator. No magic solution. It ran rough, different, but all I was doing was moving it to a different location in the garage so the saga continues.

Google test fuel pressure search said to test it off the fuel rail fitting that we don't have.

I found the aux air valve to be very finicky in the cold temperatures (below 40F). It would run too rich in the cold and not idle even though the setting seems perfect for warmer temperatures.

DonTulza wrote:1980 L-jet system, rough idle problem with cold start warm-up. When the engine is cold it will initially start and idle fine for the first 15 seconds of the cycle. Then it will drop the idle from 1000 rpm to 500 - 700 rpm and not idle well. have to open throttle to keep it running. As the engine warms up to normal operating temperature it progressively improves. At normal operating temperature the engine idles well at 1000 rpm and runs well. The problem is only from cold to normal temp. Does the thermo time switch come into play for the warm-up enrichment cycle? Any thoughts on the problem is appreciated,,,,Thanks Don

The aux air valve is likely a big player in this behavior. It's probably not opening enough and you're doing the rest with your foot.

I consider my car to be in very good working order and I still see the behavior you describe once in a while. A shot of lubricant in the AAV usually helps. I always allow ample time (min 5 minutes) to warm up and drive. My car isn't a daily driver.

Thanks for all the input. I pulled the Aux air valve, cleaned and spray lubed it and it now works well. The idle holds steady after the initial cold start and works thru the warm-up cycle. With perfect hindsight I should have seen this,, Big Thanks to Robert Bondi and everyone else who commented. This is what makes First Fives sooo Good.

DonTulza wrote:Thanks for all the input. I pulled the Aux air valve, cleaned and spray lubed it and it now works well. The idle holds steady after the initial cold start and works thru the warm-up cycle. With perfect hindsight I should have seen this,, Big Thanks to Robert Bondi and everyone else who commented. This is what makes First Fives sooo Good.

Interesting, I may have to do this as well!

I installed a new air slide valve for the final solution. After I cleaned and lubed the old valve it improved significantly, but it degraded and was still troublesome. With the new valve - Problem solved. Thanks again for all the input.

T.Hanson wrote:Maybe check fuel pressure regulator. (?)
Google test fuel pressure search said to test it off the fuel rail fitting that we don't have.

lol, thats great