When i start up my '98 5.4L it immediately climbs to 1500rpms, then after a few seconds it drops down to 900, then goes back up to 1200, and then goes back down to 900. I doubt that this is really a problem to be worried about. Actually, i kinda like it cause it sounds cool But, what causes it? I know quite a few people here have posted about it also.

 

My 99 5.4 does the same. I never worried about it, it always settles down, before I get it in gear. Part of its personality, I guess.

 

Mine does the same thing. Not to worry...

 

How many times does the rpm's go up and down? If it's just a couple of times after the initial startup and then smooth, I think that's normal, but if it does it for awhile until it's warmed up then it could be that the computer needs to be reset to re-establish the correct cold idle speed.

After a computer reset and you start the engine up for the first time, you need to let it just sit and idle for a couple of minutes without touching anything. Make sure your heater controls are turned off too.

Hope that helps

 

I am glad to hear others experince this same thing. I was beginning to wonder about that.

 

My 02 150 does this as well, usually about 5-6 times before smoth ideal dosnt bother me, or the truck any.

 

For many years it has been mandated by federal laws for vehicles to do this. It burns any excess gases or fuel that settled when the engine was shut down. Every vehicle that I've driven since 1987 or so has done this. It should immediately run at 1500-2000 rpms on startup and continue to monitor itself based on fuel mixture, hence the second boost. Engines are extremely inefficent until they warm up. This keeps emissions to a minimum and helps the engine warm up.

 

I stared to reply and realized I had no idea what I was saying so I read what seacrow said and liked it.

So.....what seacrow said.

 

Mine never did that until I "cleaned out" the IAC valve?? It would have the normal high idle and then drop down soon afterwards. Now it has the high idle, drops down, goes back up, drops down, and so on. Never went up and down til I "fixed it". I guess it will return to normal if I replace the IAC with a new one.,,,,98

 

Hi Dan

I have an '01 S_crew and it does the same thing intermittently
now and then.It doesn't go away until after I start driving.