High RPMs on Startup?

[YW84U]

I've tried searching, but didn't come across anything relevant yet

91 XLT OHV with ~60K on a rebuilt longblock. Lately when first starting it up from cold, the RPM's shoot way up around 2000 or more for approximately one full second, then drastically plummet to the regular idle of ~850. When starting, I've always waited the two or so seconds for the fuel pump to prime, then turned it over - It's never done this in the past 10+ years....should I be concerned? Could it be vacuum-related?

If anyone can offer me some places to start troubleshooting, it would be much appreciated!

Thanks In Advance,

 

[WizardOfOdds]

Mine does the same... I don't think its anything to worry about, I think its normal

 

[Masterd]

Mines always done it as well. As far as I know, it revs up to heat things up a little faster.

 

[Willard]

yea, mine normally revs to like 1500 and slowly drops down to idle. I wouldn't worry about it, especially if it's just for a second. It's an old truck, it will do things like that.

 

[YW84U]

Thanks for the responses! I guess I was just worried because it drops so 'sharply' now compared to the usual.

I'll maybe poke around anyway and see if there's anything obvious just to be safe

 

[Willard]

sounds good, sounds like a good chance to clean your MAF, IAC, and replace anything else that is getting old

 

[manaen]

You know I've noticed that on some of the older 80 to mid 90's engines. It just seems like a bad design since revving the engine on startup is the worst thing you can do since the oil pump is not fully primed for a second of so. Especially if you've just changed the oil. Bad things can happen, I had to replace an engine becuase of a freak chain of events related to a similar situation. It turned out to be a 2000 dollar incident, needless to say I wasn't happy (you can read about it here)

But I guess it is normal during those years for the engine to do that.

 

[thehillman2000]

mine dose the same thing and this morning it was high for a little longer, was like 0 out BERRRRRRRRR

 

[Charlie]

Mine always does that when cold.

 

[Masterd]

It's purely by design.

Blip the throttle while its revved up and it'll drop right down to idle.

 

[Rattosh51]

Your fast idle is supposed to kick on when first started cold, then gradually level off.
Usually any idle problems...too low or too fast are directly related to the IAC valve.
Cleaning it works, replacing it is better...it's a $60-75 part.

 

[Anime]

It's usually the IAC, a cleaning or replacement usually fixes it. It's not such a big deal if it just revs up to 2K RPM on startup, but it's gotta get fixed once the engine idle at normal temp is higher than normal..like over 1K.

 

[YW84U]

Thanks for the guidance! Methinks its time to clean the IAC and the MAF while I'm under there