Fast engine idle

[kevin v]

FAST IDLE PROBLEM


My 1978 SC has developed a fast engine idle problem.
It has always idled at 900rpm but now has an erratic idle. The scenario is as follows:
1. On cold start up, it will idle around 1300-1400rpm for a few minutes, slowly reducing to 900rpm as it warms up.
2. After a few more minutes driving in the 2000 -3000rpm range (and thus a warmer engine), the idle speed only reduces to 1900rpm when the transmission is disengaged from the engine via the clutch. It will then remain at 1900rpm continuously with no reduction at all.
3. If the car is slowed by braking, thus reducing the engine speed below 1900rpm, disconnection of the transmission via the clutch results in a slow decrease (say 3-6 seconds) in engine speed to 1150rpm which is where it remains with no further reduction.
4. A later restart will repeat the above cycle
Another problem I’ve noted (and perhaps related) is that a cold start requires repeated attempts (say 4-6) and fast acceleration in the 3000-4000rpm range is very “lumpy”.
Any suggestions?

[86 911 Targa]

Originally Posted by kevin v

FAST IDLE PROBLEM


My 1978 SC has developed a fast engine idle problem.
It has always idled at 900rpm but now has an erratic idle. The scenario is as follows:
1. On cold start up, it will idle around 1300-1400rpm for a few minutes, slowly reducing to 900rpm as it warms up.
2. After a few more minutes driving in the 2000 -3000rpm range (and thus a warmer engine), the idle speed only reduces to 1900rpm when the transmission is disengaged from the engine via the clutch. It will then remain at 1900rpm continuously with no reduction at all.
3. If the car is slowed by braking, thus reducing the engine speed below 1900rpm, disconnection of the transmission via the clutch results in a slow decrease (say 3-6 seconds) in engine speed to 1150rpm which is where it remains with no further reduction.
4. A later restart will repeat the above cycle
Another problem I’ve noted (and perhaps related) is that a cold start requires repeated attempts (say 4-6) and fast acceleration in the 3000-4000rpm range is very “lumpy”.
Any suggestions?

I'm not familiar with your year/model however,

Vaccum leak?

Our '86 had nearly the same issue when I transposed the
throttle assy vaccum lines during some top end work.

Try removing the engine oil filler cap.

Good luck,

Gerry

[VaSteve]

#1 describes normal operation.
#2 - 4 sounds like you may have a dying battery/alternator. When my alternator died, the car had a high idle as you describe until the battery died and then it had no idle.

[Aurel]

WUR going bad?

[ossiblue]

Sounds like a classic air leak/lean condition. If so, there are many threads to help you track it down. Try Gerry's suggestion and remove the oil filler cap when the car is warm and see if you get a decrease in rpm. If not, you have an air leak.

Check the pop off valve in the air box (if you have one). Could be unseated or loose.

Next, you might try to adjust the fast idle by turning the idle screw, remembering its initial position. If turning the screw clockwise does not reduce fast idle, you have an air leak.

You may also have a faulty decel valve. Try removing the hose from the top of the valve and plugging the hose and capping the valve opening. "Blip" the accelerator and see if the rpm rapidly drops to normal idle. If so, it's your valve which may be adjustable on your car. If not, the leak is somewhere else.

It's always good to check your fuel pressures as the cold start symptom you mention could be related to too high cold pressure. However, the consistent high idle at warm points more toward an air leak.

[boyt911sc]

Originally Posted by kevin v

FAST IDLE PROBLEM


My 1978 SC has developed a fast engine idle problem.
It has always idled at 900rpm but now has an erratic idle. The scenario is as follows:
1. On cold start up, it will idle around 1300-1400rpm for a few minutes, slowly reducing to 900rpm as it warms up.
2. After a few more minutes driving in the 2000 -3000rpm range (and thus a warmer engine), the idle speed only reduces to 1900rpm when the transmission is disengaged from the engine via the clutch. It will then remain at 1900rpm continuously with no reduction at all.
3. If the car is slowed by braking, thus reducing the engine speed below 1900rpm, disconnection of the transmission via the clutch results in a slow decrease (say 3-6 seconds) in engine speed to 1150rpm which is where it remains with no further reduction.
4. A later restart will repeat the above cycle
Another problem I’ve noted (and perhaps related) is that a cold start requires repeated attempts (say 4-6) and fast acceleration in the 3000-4000rpm range is very “lumpy”.
Any suggestions?

Kevin,

Before you do any further diagnostic CIS troubleshooting, check your control and system pressures. This is the first step in any fuel injection troubleshooting. Start from the basic test. Without knowing the control and system fuel pressures, you'll be just doing guess-work investigation which could be very expensive and time consuming. The next step is a vacuum test of the system. Keep us posted.

Tony

[Richard C2S]

Tony,

I'm in Philadelphia, also, and have a few questions relative to my '81 SC's fast idle. Can I get in touch with you by phone? Please e-mail me at kessler993@msn.com .

Thank you.