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Discussion Starter · #1 · (Edited)
It happened a couple of times. For a few seconds, the engine revved after starting up and then went back to the usual fast idle. No trouble codes. Is something about to fail?
 

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It happened a couple of times. For a few seconds, the engine revved after starting up and then went back to the usual fast idle. Is something about to fail?
We would need more details in order to give you an answer.

It could be a lot of things, but I would start by checking for obvious vacuum leaks (like the elbow on the passenger side of the intake).
 

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Discussion Starter · #3 ·
Thanks. I have nothing to add, unfortunately. Does not happen all the time.
Why would a vacuum leak go away in a few seconds? It's either there or not there.
My suspect is idle speed control. Anybody with similar experience?
 

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On cold start your call will be in open loop and once the O2 sensor kicks in it'll be in closed loop.

Example is if I have a vacuum leak and my car was running in open loop I can feel how choppy it is, but once closed loop kicks in it smooths it out by adding fuel and adjusting to hit the ideal air/fuel ratio.

If you have an obd1 cable with a laptop you can see these details, in the image below because of my vacuum leak it was making the car run lean so it had to adjust for it. I did fix it as it was an intake manifold leak and now it reads better.

Rectangle Font Parallel Slope Pattern
 

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Discussion Starter · #6 ·
On cold start your call will be in open loop and once the O2 sensor kicks in it'll be in closed loop.

Example is if I have a vacuum leak and my car was running in open loop I can feel how choppy it is, but once closed loop kicks in it smooths it out by adding fuel and adjusting to hit the ideal air/fuel ratio.

If you have an obd1 cable with a laptop you can see these details, in the image below because of my vacuum leak it was making the car run lean so it had to adjust for it. I did fix it as it was an intake manifold leak and now it reads better.

View attachment 201666
I'll repeat - it does it for a few seconds... Obviously has nothing to do with open loop closed loop.
You need to quantify "too fast". Is it supposed to idle faster for a bit when started cold? Yes.
It's like you press the gas pedal accidentally while starting the car.can
You need to quantify "too fast". Is it supposed to idle faster for a bit when started cold? Yes.
It's like depressing the gas pedal accidentally while starting the engine. Probably some 1200 - 1500 RPM.
 

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The title says "Engine is too fast at startup." Which is essentially where it would start in open loop, once the o2 sensors get to temp it triggers closed loop which happens fairly quickly.

Easiest way to tell is to get an obd1 cable, plug into a laptop and data log it.
 

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Thanks. I have nothing to add, unfortunately. Does not happen all the time.
Why would a vacuum leak go away in a few seconds? It's either there or not there.
My suspect is idle speed control. Anybody with similar experience?
If you have a vacuum leak it doesn't go away, it's just your PCM adjusting for it through fuel trims in CLOSED LOOP to smooth it out. So the few seconds your car is in open loop on start up you will feel erratic behavior if you have vacuum leaks (though I think high idle in cold start is always normal).

Case in point my car felt perfectly fine but found out it was running lean when I plugged in a data logger. I went through the hoses in the car found a bunch of cracked one's. Still didn't resolve the issue, eventually found the leak coming from the intake manifold.

And if you haven't serviced those rubber hoses I would make the assumption you do have vacuum leaks in the car, everyone will go through it considering it's 26 year old rubber
 

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Need more info. Does it have a tuned PCM? When was the last time the idle air passages and controller cleaned? Has the outside temps suddenly gotten much colder?

-Brian
 

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Discussion Starter · #11 ·
Need more info. Does it have a tuned PCM? When was the last time the idle air passages and controller cleaned? Has the outside temps suddenly gotten much colder?

-Brian
Brian,
I have never cleaned the controller. Never touched PCM either. It is getting colder. Could be a combination of contributing factors. My guess is some minor mechanical issue like sticky plunger or cable. Just wanted to see if someone have had a similar experience so I could address it faster.
 

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Brian,
I have never cleaned the controller. Never touched PCM either. It is getting colder. Could be a combination of contributing factors. My guess is some minor mechanical issue like sticky plunger or cable. Just wanted to see if someone have had a similar experience so I could address it faster.
That was my thought. Sticking throttle blades and or linkage.
 

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Thanks. I have nothing to add, unfortunately. Does not happen all the time.
Why would a vacuum leak go away in a few seconds? It's either there or not there.
My suspect is idle speed control. Anybody with similar experienc
It happened a couple of times. For a few seconds, the engine revved after starting up and then went back to the usual fast idle. No trouble codes. Is something about to fail?
Easy fixed, this is causing by the throttle plate or its shaft or its linkages wear, all you need is adjust the fast idle srew a tiny bit when it happened. ( Only adjust at the time it happens) .
 

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It happened a couple of times. For a few seconds, the engine revved after starting up and then went back to the usual fast idle. No trouble codes. Is something about to fai
It happened a couple of times. For a few seconds, the engine revved after starting up and then went back to the usual fast idle. No trouble codes. Is something about to fail?
Clean out carbon deposit in the IAC, EGR passages and check the IAC valve.
 

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Could be some "extra" air sneaking in through worn throttle shaft before IAC adjusts idle back down.
A thorough cleaning of throttle body ,idle passages may help. Check throttle blades for slight binding
in their bores which can/does place some pressure on throttle shaft. Adj. set screw accordingly.
 

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Besides air sneaking in because of a mechanical issue, idle is first controlled throttle air (IAC steps). It is a function of IAC steps vs engine coolant temp. The values vary a lot over the temp range. The PCM then adjusts the steps until the targetted RPM vs temp is achieved. If you're having issues with the ECT sensor, it could explain also.
 

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Hi, my car just started doing that and it was bad enough to throw a P0507 code. If you have an FSM read about it. yours may be in need to cleaning replacing.....
-ALF out...
 
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