Engine startup

[skidordie6531]

I have 92 with a 302 EFI when i go out in the morning to start her up she will rev up to 1500-2000 rpm then drop to 500 then fly back up again and will do that for about 5 minutes or so if you do not gently step on the gas and hold it at about 1000 rpm then you can let off anf it idles at between 700-900 rpm. I need to fix this as the wife doesnt understand to do these steps and i have an up comming deployment and do not want an email that she blew the engine.

Any help is appreciated.

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[mikes86bronco]

I have 92 with a 302 EFI when i go out in the morning to start her up she will rev up to 1500-2000 rpm then drop to 500 then fly back up again and will do that for about 5 minutes or so if you do not gently step on the gas and hold it at about 1000 rpm then you can let off anf it idles at between 700-900 rpm. I need to fix this as the wife doesnt understand to do these steps and i have an up comming deployment and do not want an email that she blew the engine.Any help is appreciated.

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Check for vac leaks 1st, then check fuel rail pressure. could be clogged filters and/or fuel pressure regulator.

also check egr system & pcv system

 

[bobstrat]

my 90 behaved similarly when the gasket between the upper plenum and intake manifold was shot.

the 700-900 rpm idle, is that in neutral or drive?

 

[AdamDude04]

How cold is it where you live? I noticed here in Colorado, Hot summers, very cold winters (two weeks highs in the 20s!).. when its cold my truck does the same thing. You start it up and it'll idle around 1500rpm for a couple minutes. Then it'll idle around 700rpm in P or N. When in D it'll go down to 5-600.

Never goes back and forth tho for about 5 minutes.. I assume if it was a vacume issue, it'd be constant, hot or cold..?

 

[BroncoJoe19]

A surging idle is often a vacuum leak, or a dirty sticking IAC.

Check and clean your IAC

http://www.ford-trucks.com/article/idx/8/170/article/Ford_Explorer_IAC_Fix.html

TheOldWizard Has some issues with the video I'll link to below, and while I agree with him, I think that overall the video is helpful and worth looking at.

First' date=' the example Idle Air Control/Bypass (IAC/IAB) motor they showed during most of the video is very different from the style used on most (all?) Ford vehicles. They showed a Ford design briefly and described it as one with an internal pintle.
Second, they said specifically not to spray cleaner on IAC with internal pintles. BS ! Any carb cleaner will work. Most will dry quickly. Just make certain to not leave any gunk.

Third, I have seen IACs work properly when warm, but stick when cold so their suggested testing procedures may not be perfect. In other words, you could pass all of the tests and still have a sticking IAC when cold.

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youtube video

http://www.youtube.com/watch?v=406BTo4JE3A&feature=player_embedded

Tips for Checking for vacuum leaks.

Some of the sensors and some of the actuators either read the amount of vacuum, or use vacuum to move controls, therefore it is important to the proper functioning of the engine that your vacuum system is fully intact. Otherwise one may get codes falsely indicating that there is a problem with a controller/actuator, or a sensor when in reality it is caused by a lack of vacuum.

Look at your rubber vacuum hoses (they are about 1/4 - 3/8 inches in diameter, if they are dry rotted or cracked, replace them. IF they slip on/off of their connectors too easily, they may have stretched out a little bit, cut the ends off, and put them back on.

Courtesy Miesk5 BroncoZone.com. You may listen for a leak. Sometimes using a cut off piece of garden hose is a good aid to listen with. Sometimes a cardboard tube (like from gift wrapping paper) is helpful. Just remember that whenever you stick your head inside a running engine compartment, that you are not wearing any loose clothing or jewelry that can get caught.

Another suggestion is to use a spray can of carb cleaner and spray around the bottom of the intake manifold, and the base of the throttle body assembly. IF there is a leak, you should hear a change in your engine.

Keep a fire extinguisher handy, or use water instead of carb cleaner (it works, but not as well.) Be aware that if you use water and the wires get wet and it runs worse... your wires may be bad, and the water is allowing them to arc instead of jump the gap of the spark plug.

Another suggestion is: to take a propane torch, turn it on (but don't light it) and move it over the vacuum hoses and connections. The engine RPM's will increase if there is a leak and the propane gets sucked in.

Courtesy of TheOldWizard ford-forums... Don't forget to remind folks to check the PCV valve, hose and grommet !

Courtesy Ramnasal ford-forums... Clamp off the hose that runs to the vacuum booster (brake booster) or pull it and plug it. It is possible that the diaphram on the inside of the booster may go bad and cause a vacuum leak.

And one more place that is not always obvious is that the vacuum reserve canister in older vehicles sometimes rots out and leaks, but rarely gets checked. In older vehicles it looks like a metal juice can, in newer vehicles it is made of plastic.