Fluctuating idle with AC on and brake pressed - DEFINITIVE FIX!!!!
#1
Registered User
Thread Starter
Fluctuating idle with AC on and brake pressed - DEFINITIVE FIX!!!!
Well, looks like I finally nailed it down last night.
I've been dealing with this problem for 11 years, or just about a year after I bought the truck new. I never had any clue as to what was causing it, nor where to start diagnosing the problem. I tried various adjustments, and changing all the vacuum hoses, and never had real success. I could adjust it out with the idle screw and the AC idle up valve, but as soon as the weather changed the problem would either come back or the idle would drop so low that I had to turn it back up. I had begun to accept that I was just going to have to live with this...
That is, until I saw a link to this at another forum recently:
Hi Roger;
I have a 94 4wd p/u w/ 22re. My engine runs great, except for when it is idling and I depress the brake pedal. The engine then idles really weird, kind of a strong then weak then strong then weak, so on and so forth. It never cuts out, though, and continues to idle normally when I take my foot off of the brake. I?ve only noticed this after driving, like at a stoplight. Thanks; Jason McDannold
Hi Jason;
Assuming there are no vacuum leaks, such as in the brake booster, it?s likely because your idle speed is set too high. I can exactly duplicate this ?problem? in my ?85 by bumping up the idle speed to a bit over 1000 RPM and it?ll act as described. Turn the idle speed back to normal, and it works fine. It?s really not a problem, its supposed to work this way and if everything is set properly, and you won?t have the ?problem?.
Why is this the case? Because the ECU has logic built in that says if you hit the brakes and the engine is turning over 1000-1100 RPM, it?ll cut the fuel to the injectors in an attempt to help you stop faster. After all, no sense pumping gas into the engine when you are braking to a stop. So idle rises, fuel is cut, idle falls, fuel restored, idle rises, etc. Turn the idle back to 850-900 RPM where it?s supposed to be and it?ll get back to normal. Idle speed is adjusted with the large slotted screw atop the throttle body. Turn it in to lower the amount of air passing through the throttle body at idle (and thus lower the idle speed).
If you don?t want to do that or it doesn?t fix the problem, here?s a more drastic fix:
http://www.geocities.com/toyotashawn/bizzareidle.html
Which basically involves cutting the wire that feeds the brake light signal to the ECU. Without that input, the ECU will never know you are braking, so it?ll not be able to cut off the fuel injectors.
The geocity link describes a wire running to the ECM that tells it that you have the brake pressed which causes it to cut the flow to the fuel injectors when the idle is too high and you're trying to stop. That's what causes the up-down-up-down of the idle.
The guy at geocities says there is a wire from the pedal to the ecm, then the signal goes back out of the ecm to the brake lights. He wouldn't give colors or locations because according to him the wires are different on different years. His fix was to find the two wires, cut them both before the ecm, and splice them together bypassing the computer all together.
So I got my meter out and started testing wires. I was able to find a wire that I thought was the output from the ecm to the brake lights but I couldn't figure out which was the input wire. To make matters worse, at some point in my truck's life someone removed one of the plugs from it ( I have no friggin' idea who, I bought my truck new and have never fooled with it... )and broke the little release pin off so i couldn't get the plug out. I ended the first night with frustration because I was only testing the plugs from the back and couldn't find the other wire...
Last night I went back out and started by wrestling the broken plug out first so I could undo all three plugs and test them right. What I figured out was than the pin I thought was an output was in fact the input, and, I noticed that with all three plugs out of the ecm that my brake lights were still on which indicated to me that there weren't two wires to the ecm after all - AT LEAST IN 1995 MODEL TRUCKS!!! Your mileage may vary if you have a different year.
So, I snipped off the wire, wire nutted both sides and buttoned everything back up. And held my breath that I didn't screw something up.
This morning on the way to work it was the perfect weather when this problem would be really bad and at every single stop I had a nice, smooth idle that was just a tad high, but no hint of a fluctuation at all.
Problem solved.
Before you go cutting wires on your own ecm, make sure of a few things.
Man, it was so nice on the way in today to not have to take my foot off the brake or turn off the AC when I stopped. I have a feeling this is one habit that will be very hard to break.
That said, here's the pic of the plugs and the wires I cut. It's the green one with a white stripe that I put the two red arrows on in this pic.
and if you need a larger version of the pic, click here:
http://www.ovrrdrive.made2own.com/up...brakelight.jpg
I've been dealing with this problem for 11 years, or just about a year after I bought the truck new. I never had any clue as to what was causing it, nor where to start diagnosing the problem. I tried various adjustments, and changing all the vacuum hoses, and never had real success. I could adjust it out with the idle screw and the AC idle up valve, but as soon as the weather changed the problem would either come back or the idle would drop so low that I had to turn it back up. I had begun to accept that I was just going to have to live with this...
That is, until I saw a link to this at another forum recently:
Hi Roger;
I have a 94 4wd p/u w/ 22re. My engine runs great, except for when it is idling and I depress the brake pedal. The engine then idles really weird, kind of a strong then weak then strong then weak, so on and so forth. It never cuts out, though, and continues to idle normally when I take my foot off of the brake. I?ve only noticed this after driving, like at a stoplight. Thanks; Jason McDannold
Hi Jason;
Assuming there are no vacuum leaks, such as in the brake booster, it?s likely because your idle speed is set too high. I can exactly duplicate this ?problem? in my ?85 by bumping up the idle speed to a bit over 1000 RPM and it?ll act as described. Turn the idle speed back to normal, and it works fine. It?s really not a problem, its supposed to work this way and if everything is set properly, and you won?t have the ?problem?.
Why is this the case? Because the ECU has logic built in that says if you hit the brakes and the engine is turning over 1000-1100 RPM, it?ll cut the fuel to the injectors in an attempt to help you stop faster. After all, no sense pumping gas into the engine when you are braking to a stop. So idle rises, fuel is cut, idle falls, fuel restored, idle rises, etc. Turn the idle back to 850-900 RPM where it?s supposed to be and it?ll get back to normal. Idle speed is adjusted with the large slotted screw atop the throttle body. Turn it in to lower the amount of air passing through the throttle body at idle (and thus lower the idle speed).
If you don?t want to do that or it doesn?t fix the problem, here?s a more drastic fix:
http://www.geocities.com/toyotashawn/bizzareidle.html
Which basically involves cutting the wire that feeds the brake light signal to the ECU. Without that input, the ECU will never know you are braking, so it?ll not be able to cut off the fuel injectors.
The geocity link describes a wire running to the ECM that tells it that you have the brake pressed which causes it to cut the flow to the fuel injectors when the idle is too high and you're trying to stop. That's what causes the up-down-up-down of the idle.
The guy at geocities says there is a wire from the pedal to the ecm, then the signal goes back out of the ecm to the brake lights. He wouldn't give colors or locations because according to him the wires are different on different years. His fix was to find the two wires, cut them both before the ecm, and splice them together bypassing the computer all together.
So I got my meter out and started testing wires. I was able to find a wire that I thought was the output from the ecm to the brake lights but I couldn't figure out which was the input wire. To make matters worse, at some point in my truck's life someone removed one of the plugs from it ( I have no friggin' idea who, I bought my truck new and have never fooled with it... )and broke the little release pin off so i couldn't get the plug out. I ended the first night with frustration because I was only testing the plugs from the back and couldn't find the other wire...
Last night I went back out and started by wrestling the broken plug out first so I could undo all three plugs and test them right. What I figured out was than the pin I thought was an output was in fact the input, and, I noticed that with all three plugs out of the ecm that my brake lights were still on which indicated to me that there weren't two wires to the ecm after all - AT LEAST IN 1995 MODEL TRUCKS!!! Your mileage may vary if you have a different year.
So, I snipped off the wire, wire nutted both sides and buttoned everything back up. And held my breath that I didn't screw something up.
This morning on the way to work it was the perfect weather when this problem would be really bad and at every single stop I had a nice, smooth idle that was just a tad high, but no hint of a fluctuation at all.
Problem solved.
Before you go cutting wires on your own ecm, make sure of a few things.
- Make sure your ecm isn't as that other guy described with two wires.
- Make sure you test the wire and are confident that it is the brake lead on your truck.
- When you do cut it, be sure to leave enough wire on the plug side that you can splice it back together if it isn't the correct wire.
Man, it was so nice on the way in today to not have to take my foot off the brake or turn off the AC when I stopped. I have a feeling this is one habit that will be very hard to break.
That said, here's the pic of the plugs and the wires I cut. It's the green one with a white stripe that I put the two red arrows on in this pic.
and if you need a larger version of the pic, click here:
http://www.ovrrdrive.made2own.com/up...brakelight.jpg
Last edited by ovrrdrive; 01-12-2007 at 01:01 PM.
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hurricanes (06-29-2024)
#4
Registered User
Thread Starter
Or just turn down the idle speed:
- http://www.4crawler.com/4x4/TLCA_Tru...ech.04.09.html
Works for me.
- http://www.4crawler.com/4x4/TLCA_Tru...ech.04.09.html
Works for me.
If you read my post you would have seen that it worked for me too... For a week then the idle would be too low so I would turn it up and a week later have fluctuating. And so on and so on...
This is a sure fix. The ecm doesn't know when you have the brake on after this.
I say do whatever works for you.
#5
Registered User
Or just turn down the idle speed:
- http://www.4crawler.com/4x4/TLCA_Tru...ech.04.09.html
Works for me.
- http://www.4crawler.com/4x4/TLCA_Tru...ech.04.09.html
Works for me.
#6
Contributing Member
iTrader: (3)
It is a matter of getting the normal and fast-A/C idle speed set right. I can exactly duplicate this problem by turning the fast A/C idle speed up and exactly fix it by turning it down. But the idea of cutting fuel to the injectors is that the ECU "thinks" you are trying to stop when you put the brakes on and is attempting to help by cutting fuel to the engine. Probably will work OK with the wires cut, though. That is whey I list both fixes on that web page.
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#9
Contributing Member
It's not wrong, if you own the truck, do what you want with it. This mod doesn't make the truck unsafe, it probably only uses a little more gas with a higher idle.
Cutting the wire disables a feature of the truck, if you don't like an extra feature because it's too hard to adjust it correctly, then by all means, get rid of it!
#11
Registered User
Old thread, but what the hey....
This is interesting. How did you manage this? Without an ECU, my vehicles will not run....at all.
This is interesting. How did you manage this? Without an ECU, my vehicles will not run....at all.
#12
Registered User
Thread Starter
I think he was being facetious... AFAIK the engine won't run without the ECM unless you remove all of the FI and sensors from it and put a carb back on.
I do wonder what he was searching for to find this old thread if he went back to carbed...
I do wonder what he was searching for to find this old thread if he went back to carbed...
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