Idle takes a long time to settle down on warm start

[Nioreh]

When starting the car, I know the idle is supposed to be a little high in the initial seconds just like any car. But on my car it takes maybe 10 seconds if I start it warm. Is this normal? I feel kind of embarassed about it sometimes when starting the car again after for example a fill-up at the gas station. I can get around it by turning the key to the run position and wait 10 seconds before cranking. There seems to be some timer that goes from turning the ignition on until it allows normal idle.

[Michael]

Tons if possibilities from an intake leak to PPR seals. Maybe take a look at the microswitch and make sure it's not only engaging but the idle circuit is functional.

[Bitsyncmaster]

What RPM is it when you see this problem?

[Nioreh]

Tons if possibilities from an intake leak to PPR seals. Maybe take a look at the microswitch and make sure it's not only engaging but the idle circuit is functional.

It works perfectly fine, it just takes 10 seconds to kick in after turning ignition on.

[Elvis]

When starting the car, I know the idle is supposed to be a little high in the initial seconds just like any car. But on my car it takes maybe 10 seconds if I start it warm. Is this normal? I feel kind of embarassed about it sometimes when starting the car again after for example a fill-up at the gas station. I can get around it by turning the key to the run position and wait 10 seconds before cranking. There seems to be some timer that goes from turning the ignition on until it allows normal idle.



no, not normal.

I can for sure fix it but I'm in Germany and most likely shipping is the problem ?

[Michael]

I can get around it by turning the key to the run position and wait 10 seconds before cranking. There seems to be some timer that goes from turning the ignition on until it allows normal idle.

I would look first at the idle speed motor and verify it is working correctly and there are no broken air lines in that area. Be sure to check the vacuum lines on the back of the valve cover as well.

[Nioreh]

I would look first at the idle speed motor and verify it is working correctly and there are no broken air lines in that area. Be sure to check the vacuum lines on the back of the valve cover as well.

Idle motor is new and works fine. Like I said, everything is working great. I even expect a flare up on dtart, since most cars do that. But it takes about 10 seconds before it returns to normal idle. And I can avoid the flare up by just waiting 10 seconds to start the car.

But I know now by your answers that this is not normal. So I assume my idle computer has some issue. Either that or some thermal sensor maybe thinks the engine is cold and takes a while to give correct readings.

[David T]

My guess is you have vacuum leaks, looseness in the quadrant link, and your CPR probably needs to be rebuilt. If it idles fine after 10 seconds and everything else seems fine, I would not do anything about it right away. On my car, if it hasn't been run in a while, it runs a little rough for about 10 seconds on a cold start. The way all of the systems work during the warm-up cycle, the motor won't run like a modern car with a full blown engine management system. It should be fine once you get into "closed cycle". The real test is your Miles Per Gallon (MPG). Should be at least 20 MPG.

[Nioreh]

My guess is you have vacuum leaks, looseness in the quadrant link, and your CPR probably needs to be rebuilt. If it idles fine after 10 seconds and everything else seems fine, I would not do anything about it right away. On my car, if it hasn't been run in a while, it runs a little rough for about 10 seconds on a cold start. The way all of the systems work during the warm-up cycle, the motor won't run like a modern car with a full blown engine management system. It should be fine once you get into "closed cycle". The real test is your Miles Per Gallon (MPG). Should be at least 20 MPG.

How would a vacuum leak do this? How could I possibly avoid the issue by waiting 10 seconds to crank it if it was a vacuum leak? That makes no sense. And it is a hot car, not "not been run for a while".

[Elvis]

so you're not interested in having it repaired ?

[David T]

How would a vacuum leak do this? How could I possibly avoid the issue by waiting 10 seconds to crank it if it was a vacuum leak? That makes no sense. And it is a hot car, not "not been run for a while".

There are several systems that are activated during start-up. If any of them are leaky it could affect the idle. Some activate during a cold start, some during a hot start and some during either. It will take troubleshooting and diagnosing each system to make sure it is working when and as it should. If the problem goes away in 10 seconds it isn't worth the effort to fix it unless you are a perfectionist and can do it yourself.

[Nioreh]

so you're not interested in having it repaired ?

No thank you, I will troubleshoot it myself. No problem. I just wanted to know if it was normal behavior or not. And now I know it isn't.

Can anyone maybe tell me what a normal flare up is on a hot start? (If any)

[Nioreh]

There are several systems that are activated during start-up. If any of them are leaky it could affect the idle. Some activate during a cold start, some during a hot start and some during either. It will take troubleshooting and diagnosing each system to make sure it is working when and as it should. If the problem goes away in 10 seconds it isn't worth the effort to fix it unless you are a perfectionist and can do it yourself.

I can do it, I just needed to know if it was expected behavior, so I don't spend time on something that is expected.

[David T]

I can do it, I just needed to know if it was expected behavior, so I don't spend time on something that is expected.

If you are going to do this yourself have the Workshop Manual, a dwell meter, and a smoke machine. Familiarize yourself with all of the systems on the motor and check each one. The big thing to look for are small vacuum leaks. Each one is cumulative so the total becomes large. Old injector seals, the pipe in the bottom of the mixture unit, old, brittle, cracked vacuum hoses, missing hardware or gaskets, all of these contribute to vacuum leaks. Difficult to find without a smoke machine. BTW, a small initial surge is normal, the Cold Start Valve (CSV) during cranking on a cold motor squirts raw fuel into the intake manifold but that gets used up in a few seconds, certainly much less than 10 seconds. It should not operate on a warmed up or hot motor. Prepare to delve deeply into the theory and operation of the Bosch K-Jetronic system as it relates to the Delorean. By the time you are done you will be the local expert.

[82DMC12]

My guess is you have vacuum leaks, looseness in the quadrant link, and your CPR probably needs to be rebuilt. If it idles fine after 10 seconds and everything else seems fine, I would not do anything about it right away. On my car, if it hasn't been run in a while, it runs a little rough for about 10 seconds on a cold start. The way all of the systems work during the warm-up cycle, the motor won't run like a modern car with a full blown engine management system. It should be fine once you get into "closed cycle". The real test is your Miles Per Gallon (MPG). Should be at least 20 MPG.

My car used to run a little rough on cold start as well as warm start (like after an hour of shutting down), turns out the issue was a cracked PPR o-ring on the tip of the plunger. Replaced it and perfection ever since.

[Rich]

Can anyone maybe tell me what a normal flare up is on a hot start? (If any)

Here is some "normal" hot start test data.

Our car is a good example.
Engine RPM immediately rises past idle speed after hot engine start then just as quickly settles to idle speed. 2-3 seconds from ignition to normal idle.


- Stock engine configuration - correct idle linkage tuning, correct mixture (dwell) control, no vacuum leaks per smoke test
- Engine surface temp 160-180F near water pump during startup observation 5 minutes after the end of a drive
- Idle RPM >3sec after starting is 770-790RPM per dwell-tach - within factory spec (775 +/-50RPM)



Bottom line: 10 seconds of high idle after hot start isn't normal.

[Bitsyncmaster]

My idle ECU does a little longer high idle after any start. Do you have my idle ECU? It would have a blue wire coming out between the connectors.

[Rich]

My idle ECU does a little longer high idle after any start. Do you have my idle ECU? It would have a blue wire coming out between the connectors.

In case this question is for me the answer is our D runs the full-stock K-Jet setup.

[Nioreh]

Here is some "normal" hot start test data.

Our car is a good example.
Engine RPM immediately rises past idle speed after hot engine start then just as quickly settles to idle speed. 2-3 seconds from ignition to normal idle.


- Stock engine configuration - correct idle linkage tuning, correct mixture (dwell) control, no vacuum leaks per smoke test
- Engine surface temp 160-180F near water pump during startup observation 5 minutes after the end of a drive
- Idle RPM >3sec after starting is 770-790RPM per dwell-tach - within factory spec (775 +/-50RPM)



Bottom line: 10 seconds of high idle after hot start isn't normal.

Thank you, good info. So there is a flare on a hot engine even on a normally working one, but should be 2-3 seconds and not 10.


My idle ECU does a little longer high idle after any start. Do you have my idle ECU? It would have a blue wire coming out between the connectors.

This is on stock ECU. I have one of your ECUs, but not installed yet.

[The_J]

my car did not run when i bought it. After i got it running, i noticed hot or cold, my car would rev up to about 1500 and sit there for 5-6 seconds before coming down to idle. I ran through all the normal tests, could not find a fault. My problem turned out to be a failed idle ecu. I installed one of Dave's digital idle ecu's and it fixed me right up!