Engine rev flucuates
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Senior Member
Joined: Oct 2011
Posts: 698
Likes: 6
From: Gilbert, AZ
When my truck in at idle, the engine reves up slightly then back down and repeats...
Any suggestions why?
I do have a check engine light on as of yesterday and will get it coded.
Any suggestions why?
I do have a check engine light on as of yesterday and will get it coded.
Last edited by Big Pete; Nov 20, 2011 at 12:08 AM.
Dodge+Ford = always ready
Joined: Apr 2011
Posts: 8,376
Likes: 112
From: Montana
sounds exactly like IAC went out. tell us what you get when the codes are read. if its anything related to IAC 1 way to test it is to unhook the IAC, if it gets worse your IAC is good, if it stays the same, your IAC is out.
Senior Member
Joined: Jul 2008
Posts: 58,557
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From: Long Beach Calimexifornia
Alway's remove, clean it, reinstall, then test the IAC by disconnecting.
The symptom your describing is more indicative of a dirty, or fouled IAC. Also, there is no error code for the IAC failing itself (ie: IAC out of range) because it is a electro-magnetic solenoid, not a sensor.
However, it's possible (but I would not bet the farm on it) it could produce a RPM at Idle code.
It should crash your "at normal operating temp" idle, when disconnected while idling (after you clean it).
A good possibility is a loose fitting or cracked Vac Hose/ tube. Also a loose fitting PCV mount grommet or hose.
The symptom your describing is more indicative of a dirty, or fouled IAC. Also, there is no error code for the IAC failing itself (ie: IAC out of range) because it is a electro-magnetic solenoid, not a sensor.
However, it's possible (but I would not bet the farm on it) it could produce a RPM at Idle code.
It should crash your "at normal operating temp" idle, when disconnected while idling (after you clean it).
A good possibility is a loose fitting or cracked Vac Hose/ tube. Also a loose fitting PCV mount grommet or hose.
Last edited by ymeski56; Nov 20, 2011 at 11:40 AM.
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Joined: Nov 2011
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I'm tracking an idle problem also...
This one is pretty severe ('87 4.9L C6...owned it for about three weeks). The truck was running and shifting great but I noticed it shifting a bit weird a few days back. Yesterday afternoon a rather quick onset of idling high and sometimes revving WAY high set in...turning off the key and restarting the engine calmed idle down and sometimes...most times...returned everything to normal but only for a short period. The problem always came back.
I did notice that the previously perfectly shifting transmission was shifting way high...like 30mph to second and 45mph to third and hitting hard on shifts which was obviously no bueno and led me to believe I have a vacuum issue of some sort as the transmission is controlled by engine vacuum. Another symptom was the engine oil which turned black in maybe 200 miles after an oil change and while I haven't checked compression the motor runs really smooth and makes good power, doesn't smoke on start or while running, the tailpipe is 'clean' and no blow-by is evident with the oil filler cap removed.
A few minutes ago I slid under the truck and pulled the vacuum hose from the vacuum modulator on the tranny and was greeted with tranny fluid pouring out of the hose...almost a sixteenth of a pint so a modulator valve is going in today if I can find the right one.
I disconnected the vacuum hose from the EGR valve just to see and there was no change at all when disconnecting or reconnecting.
I'm pretty sure the vacuum modulator will take care of my issues and will post results when I get a new one put in.
BOL...
I did notice that the previously perfectly shifting transmission was shifting way high...like 30mph to second and 45mph to third and hitting hard on shifts which was obviously no bueno and led me to believe I have a vacuum issue of some sort as the transmission is controlled by engine vacuum. Another symptom was the engine oil which turned black in maybe 200 miles after an oil change and while I haven't checked compression the motor runs really smooth and makes good power, doesn't smoke on start or while running, the tailpipe is 'clean' and no blow-by is evident with the oil filler cap removed.
A few minutes ago I slid under the truck and pulled the vacuum hose from the vacuum modulator on the tranny and was greeted with tranny fluid pouring out of the hose...almost a sixteenth of a pint so a modulator valve is going in today if I can find the right one.
I disconnected the vacuum hose from the EGR valve just to see and there was no change at all when disconnecting or reconnecting.
I'm pretty sure the vacuum modulator will take care of my issues and will post results when I get a new one put in.
BOL...
Last edited by 22hornet; Nov 20, 2011 at 12:04 PM. Reason: spelling
Senior Member
Joined: Jul 2008
Posts: 58,557
Likes: 1,165
From: Long Beach Calimexifornia
This one is pretty severe ('87 4.9L C6...owned it for about three weeks). The truck was running and shifting great but I noticed it shifting a bit weird a few days back. Yesterday afternoon a rather quick onset of idling high and sometimes revving WAY high set in...turning off the key and restarting the engine calmed idle down and sometimes...most times...returned everything to normal but only for a short period. The problem always came back.
I did notice that the previously perfectly shifting transmission was shifting way high...like 30mph to second and 45mph to third and hitting hard on shifts which was obviously no bueno and led me to believe I have a vacuum issue of some sort as the transmission is controlled by engine vacuum. Another symptom was the engine oil which turned black in maybe 200 miles after an oil change and while I haven't checked compression the motor runs really smooth and makes good power, doesn't smoke on start or while running, the tailpipe is 'clean' and no blow-by is evident with the oil filler cap removed.
A few minutes ago I slid under the truck and pulled the vacuum hose from the vacuum modulator on the tranny and was greeted with tranny fluid pouring out of the hose...almost a sixteenth of a pint so a modulator valve is going in today if I can find the right one.
I disconnected the vacuum hose from the EGR valve just to see and there was no change at all when disconnecting or reconnecting.
I'm pretty sure the vacuum modulator will take care of my issues and will post results when I get a new one put in.
BOL...
I did notice that the previously perfectly shifting transmission was shifting way high...like 30mph to second and 45mph to third and hitting hard on shifts which was obviously no bueno and led me to believe I have a vacuum issue of some sort as the transmission is controlled by engine vacuum. Another symptom was the engine oil which turned black in maybe 200 miles after an oil change and while I haven't checked compression the motor runs really smooth and makes good power, doesn't smoke on start or while running, the tailpipe is 'clean' and no blow-by is evident with the oil filler cap removed.
A few minutes ago I slid under the truck and pulled the vacuum hose from the vacuum modulator on the tranny and was greeted with tranny fluid pouring out of the hose...almost a sixteenth of a pint so a modulator valve is going in today if I can find the right one.
I disconnected the vacuum hose from the EGR valve just to see and there was no change at all when disconnecting or reconnecting.
I'm pretty sure the vacuum modulator will take care of my issues and will post results when I get a new one put in.
BOL...
Clean your IAC regardless, should be part of regular maintenance anyway.
There should be no effect by removal of EGR Vac hose at idle, as the EGR is not involved at idle.
Check PVC for proper function.
Sound's like a good time to do a Seafoam treatment.
1/3 can through booster brake Vac hose at warm idle (Combustion chambers).
1/3 added to oil (Desludge).
1/3 added to gas Injection).
See link: https://www.f150forum.com/f33/how-se...-1991-a-31505/
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It's a Canadian thing eh!
Joined: Nov 2010
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From: Ontario, living across a hay field
Right on top of the throttle body, about the size of a C type battery with two wires out the back. Try a parts cleaner to get it loosen up but try not to soak the coil part with it. Squirt into the air inlets
