Engine [95] Idles High At Start, But Comes Back Down?

rlee94

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• Apr 16, 2015

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I have a 95 that will rev up to about 1800 RPM on start, stay there for about 10 seconds, then come back to about 600. If the engine is warm, the idle is lower (1400 RPM) on start.

After calling around and looking through the forum, I've eliminated the IACV. The only other part to check is the TPS. Where is it located and how do I check it?

Thank you in advance!

 

addermk2

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Apr 17, 2015

#2

• Apr 17, 2015

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High idle at start is completely normal. Leave your car alone.

 

Noobz347

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What he said.

 

rlee94

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Apr 18, 2015

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Thanks guys. Is it just a common problem or by design?

 

Noobz347

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rlee94 said:

Thanks guys. Is it just a common problem or by design?

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It's by design. You'll probably notice it less as weather gets warmer. If it hangs and doesn't come down, THEN you would have an issue.

 

rlee94

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Apr 18, 2015

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Noobz347 said:

It's by design

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Care to elaborate why? Or is there a reference where I could read about it? Just seems like an odd design.

 

addermk2

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Apr 19, 2015

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• Apr 19, 2015

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rlee94 said:

Care to elaborate why? Or is there a reference where I could read about it? Just seems like an odd design.

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Would you like us to cover 30+ years of EFI functionality for you?

It's mostly for emissions. A rich engine doesn't like to run at low RPM. A cold engine doesn't like to run stoich... etc etc

 

jozsefsz

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I've never owned or driven any make or model of car that doesn't rev up at a cold start. Is this your first car?

 

Noobz347

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jozsefsz said:

I've never owned or driven any make or model of car that doesn't rev up at a cold start. Is this your first car?

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True story, carbureted or fuel injected.

To the OP: There is a better explanation and I don't have it right here on my fingertips. In a nut shell, cold starting engines are enrich at start-up to get enough heat into the combustion chambers so that the flame in the cylinders perpetuates with just one smack of the spark plug at each revolution. Without the enriching the mixture, there's a good chance that cylinder ignition will not last through the entire cycle (it's pretty violent in there).

If you think about it, it's the same thing with forced induction. You enrich the fuel and reduce the heat of the ignition source. It just so happens that the enriched fuel mixture causes your engine to idle higher for spell because of the additional energy introduced.

 

doobismaximus

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• Apr 21, 2015

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Learn something new everyday

 

jozsefsz

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Noobz347 said:

It just so happens that the enriched fuel mixture causes your engine to idle higher for spell because of the additional energy introduced.

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I can only add to this that in a carbureted car you'll have a high idle cam which mechanically opens the throttle a little bit. Until the choke is opened you'll be riding on that cam with higher RPM's. This is in addition to any effect the enriched mixture would have.

Similarly, in the EEC-IV program, there's both a startup enrichment (until temperature is reached), and a fast-idle period (for a set amount of time at a given temperature).

The goal of both is to allow for an easy start and to avoid stalling as the car warms.

 

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rlee94

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jozsefsz said:

I've never owned or driven any make or model of car that doesn't rev up at a cold start. Is this your first car?

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No, but I've never noticed it this much on other vehicles. My '00 Explorer only revs to about 1,000.

 

jozsefsz

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#13

• Apr 23, 2015

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rlee94 said:

No, but I've never noticed it this much on other vehicles. My '00 Explorer only revs to about 1,000.

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The 3.8 revs up quite a bit at start. That was one of Ford's first OBD-II EEC-V engines (the '95 5.0's actually have the OBD-II plug and harness but don't actually use it) so I'm sure they refined the tunes many times by 2000. I don't know exactly how high it's supposed to rev honestly, I haven't driven a 3.8 in quite a few years, but 1800 doesn't sound out of line. If in doubt checking your TPS and IAC are pretty much all I can suggest, though I never had a problem with those on a 3.8. And since it revs down after a bit, and you don't mention stalling or driveability issues, I think you're good to go.

 

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