my 88 bronco idles up n down
- Thread starter mbronco
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Rons beast
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That is usually an indication of a vacuum leak. Check all vacuum lines, and intake manifold gaskets. also the brake booster may be leaking.
BroncoJoe19
Active member
I agree 
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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 ... Don't forget to remind folks to check the PCV valve, hose and grommet ! PCV = ( Positive Crankcase Ventilation ) valve.
Courtesy our friend Ramnasal... 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.
/emoticons/[email protected] 2x" width="20" height="20" />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 ... Don't forget to remind folks to check the PCV valve, hose and grommet ! PCV = ( Positive Crankcase Ventilation ) valve.
Courtesy our friend Ramnasal... 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.
My guess would be that your throttle body is sooted up which lets too little air by. This causes such low idle that the computer gooses the idle with the IAC, which gives a high idle until the computer shuts off the IAC, and the cycle repeats. Try cleaning the soot away near the throttle body butterflies, so more air can pass by when they are nearly closed at idle. Ford sold a idle air bypass kit that fits under the base of the IAC to correct this problem without having to clean the soot out, but cleaning the soot out will tell you if that is the cause of the idle fluctuation.
yo,
Have you checked for Codes yet?
a Self Test for Diagnostic Trouble Codes (DTC)s by my pal, BroncoJoe19
The engine temperature must be greater than 50° F for the Key On Engine Off (KOEO) Self-Test and greater than 180° F for the Key On Engine Running (KOER) Self-Test.
Run it around to heat the engine up and shift thru all gears incl Reverse. Then turn off all accessories/lights, etc.
Make sure A/C is off and transmission is in Park (automatic) or in Neutral for a Manual & release clutch.
Do Key On Engine Off (KOEO) portion first.
Look Codes up in my broncolinks.com site using the new Search function.
And Post em here according to:
KOEO
&
KOER
For example
DTC 12 can't control engine RPM high RPM self test; vacuum leak, TB base idle off, idle air valve dirty or bad, EGR stuck open. Check the IAC valve port in TB for Sludge; Suspect throttle body coking.
Source: by miesk5
DTC 13 Cannot control RPM during ER Self-Test, low RPM & Possible Causes; "...vacuum leak, use garden hose section to listen for leak noise; TB base idle off- Idle Air Control (IAC) Sludge; Poor Idle TSB 91-25-07 for 85-92 Bronco & F Series & many others; air bypass valve dirty or bad..." READ MORE
Source: by miesk5
more in my site @ http://www.broncolinks.com/index.php?index=397
Idle Air Control (IAC) Sludge; Poor Idle TSB 91-25-07 for 85-92 Bronco & F Series & many others, Modification; "...I have spent some time this week trying to set up an air bypass around the IAC valve by cutting away some of the gasket between the IAC in and out air holes, rather than pay big bucks for Fords spacer kit. I cut away the center of the original IAC gasket from the outside of each hole to the outside of the other hole, giving about 1/2" gap for the air to pass through. I noticed an immediate improvement in the engines starting behavior, but it would still die occasionally. For most of my trucks life it has been dying when it was started hot. It would rev up and then drop the idle so low it would die unless I would catch the idle with the accelerator. It would never die when cold as the IAC would keep the idle speed up, and it never died at stop lights. Now it's worst situation seems to be when it has been shut down for a half hour or so, and restarted. I wonder if the heat soak is fooling the ETC into thinking the engine is warmer than it really is. I decided that more improvement could be had if I had a thicker gasket, because the original IAC gasket was very thin (0.018"). I had some 0.030" gasket material so I made my own with the same 1/2" cut out. This gave enough bypass to noticeably raise the idle speed and almost eliminate the dying. After maybe 50 starts in the last few days, it only died twice. After so many years of catching it with the accelerator it is darned hard to leave my foot off it. It is clear to me that by trial and error one can tune the air bypass without the need for the expensive Ford kit. I may combine the two gaskets for more bypass, but the idle is about as fast as I would want now, especially for driving in snow..."; miesk5 Note, Tomco makes an idle plate P/N 8491 = to Ford, P/N F2PZ9F939A; Like Ford’s service kit, Tomco’s spacer plat e (arrow) is installed between the air bypass valve and its mounting boss using two gaskets. The two holes in the plate are for bleeder screws, which provide a minimum air rate adjustment independent of the throttle opening.
Source: by Nelbur at Ford Bronco Zone Forums http://broncozone.com/topic/22150-fast-idle-problem/page__pid__115710#entry115710
Also, If you can borrow or buy a relatively inexpensive Vacuum Gauge, it helps a lot and is one of best DIY repair tools and alos a helpful way to improve MPG while driving if you get the so-called MPG type that has colored bands on the gauge that show by accelerating in a way that keeps the gauge pressure as high & as steady as possible.
Diagnosis - BEST!Source: by Craig U at http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm
Have you checked for Codes yet?
a Self Test for Diagnostic Trouble Codes (DTC)s by my pal, BroncoJoe19
The engine temperature must be greater than 50° F for the Key On Engine Off (KOEO) Self-Test and greater than 180° F for the Key On Engine Running (KOER) Self-Test.
Run it around to heat the engine up and shift thru all gears incl Reverse. Then turn off all accessories/lights, etc.
Make sure A/C is off and transmission is in Park (automatic) or in Neutral for a Manual & release clutch.
Do Key On Engine Off (KOEO) portion first.
Look Codes up in my broncolinks.com site using the new Search function.
And Post em here according to:
KOEO
&
KOER
For example
DTC 12 can't control engine RPM high RPM self test; vacuum leak, TB base idle off, idle air valve dirty or bad, EGR stuck open. Check the IAC valve port in TB for Sludge; Suspect throttle body coking.
Source: by miesk5
DTC 13 Cannot control RPM during ER Self-Test, low RPM & Possible Causes; "...vacuum leak, use garden hose section to listen for leak noise; TB base idle off- Idle Air Control (IAC) Sludge; Poor Idle TSB 91-25-07 for 85-92 Bronco & F Series & many others; air bypass valve dirty or bad..." READ MORE
Source: by miesk5
more in my site @ http://www.broncolinks.com/index.php?index=397
Idle Air Control (IAC) Sludge; Poor Idle TSB 91-25-07 for 85-92 Bronco & F Series & many others, Modification; "...I have spent some time this week trying to set up an air bypass around the IAC valve by cutting away some of the gasket between the IAC in and out air holes, rather than pay big bucks for Fords spacer kit. I cut away the center of the original IAC gasket from the outside of each hole to the outside of the other hole, giving about 1/2" gap for the air to pass through. I noticed an immediate improvement in the engines starting behavior, but it would still die occasionally. For most of my trucks life it has been dying when it was started hot. It would rev up and then drop the idle so low it would die unless I would catch the idle with the accelerator. It would never die when cold as the IAC would keep the idle speed up, and it never died at stop lights. Now it's worst situation seems to be when it has been shut down for a half hour or so, and restarted. I wonder if the heat soak is fooling the ETC into thinking the engine is warmer than it really is. I decided that more improvement could be had if I had a thicker gasket, because the original IAC gasket was very thin (0.018"). I had some 0.030" gasket material so I made my own with the same 1/2" cut out. This gave enough bypass to noticeably raise the idle speed and almost eliminate the dying. After maybe 50 starts in the last few days, it only died twice. After so many years of catching it with the accelerator it is darned hard to leave my foot off it. It is clear to me that by trial and error one can tune the air bypass without the need for the expensive Ford kit. I may combine the two gaskets for more bypass, but the idle is about as fast as I would want now, especially for driving in snow..."; miesk5 Note, Tomco makes an idle plate P/N 8491 = to Ford, P/N F2PZ9F939A; Like Ford’s service kit, Tomco’s spacer plat e (arrow) is installed between the air bypass valve and its mounting boss using two gaskets. The two holes in the plate are for bleeder screws, which provide a minimum air rate adjustment independent of the throttle opening.
Source: by Nelbur at Ford Bronco Zone Forums http://broncozone.com/topic/22150-fast-idle-problem/page__pid__115710#entry115710
Also, If you can borrow or buy a relatively inexpensive Vacuum Gauge, it helps a lot and is one of best DIY repair tools and alos a helpful way to improve MPG while driving if you get the so-called MPG type that has colored bands on the gauge that show by accelerating in a way that keeps the gauge pressure as high & as steady as possible.
Diagnosis - BEST!Source: by Craig U at http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm
Last edited by a moderator:
Hey check out my thread titled fast idle problem... At first I thought my idle problem was related to the idle control vavle. Further troubleshooting revealed my throttle position sensor was out of the specified voltage range. Pulling the engine computer fault codes can save a lot of time trying to find the source of the problem.
