It's an 85 xcab efi. When I hit the brakes my idle drops off 500 rps or so then recovers, over and over again. It dosen't happen when It's warming up, just when hot. I disconected the vacuum line at the manafold for the booster, blocked it off and hit the brakes. Same surgeing idle. So i figure it's electrical. I pop out the brake light fuse hit the brakes and it's all good. So I put the fuse back in with the brake pedal pushed in.... a few sparks later the surge is back. I repeat the test 3 or 4 more times with the pedal pushed in, same results. So the question is why does the brake light circuit fuck up my idle? And how do i fix it? Thanks for the help.
Sounds like you have a short in the brake light system. Replace the brake light blubs and see if that helps. Sometimes the filament breaks and it drops and shorts out, causing a direct short. Had a problem that every time I turned on the left blinker I blew a fuse and it turned out to be a light shorting out. :usa:
I would bet you that your idle is set to high. Sometimes the screw works loose with vibration. If your idle is at like 1100, it's too high. With the engine warm, try lowering the idle at the intake manifold to about 900 or so. That will probably solve your problem. Mine was doing the same thing last week after the new header. Adjusted the idle and it's fine.
Post back if that solved the problem to help others.
SO.... OOPS, I popped out the brake light assemblys and disconnected them and repeated the test. no luck, just rough idle, all bulbs were just fine. So now I'll work my way down the brake wires to the fuse box? or even the pedal could be pinching together two wires? Fuck if I know for now. Thanks and keep the ideas flowin'.
SO.... OPP's, I popped out the brake light assemblys and disconnected them and repeated the test. no luck, just rough idle, all bulbs were just fine. So now I'll work my way down the brake wires to the fuse box? or even the pedal could be pinching together two wires? Fuck if I know for now. Thanks and keep the ideas flowin'.
I think i understand what you're talking about, although it was a little vague. just kidding. could i put a sock in to my intake to slow the idle? Thanks, I'll let you know how it worked.
I have an article related to this problem on my website. Thus far I have not been able to figure it out. Follow my link below and go to the tech section for more information. I'm almost wondering if this is a coolant issue. Two of my vehicles had either an engine replaced or coolant flushed and refilled just prior to the problem starting. I've had numerous emails from people saying this or that fixed the problem (ranging from everything from spark plugs and wires to an EGR valve). I've heard a few people mention idle speed too. However, both my vehicles were idling properly around 800 or whatever spec was as found on the label under the hood. One thing that was common with many of the people I heard from was recent engine work such as head gasket replacement, new head, or new engine.
This news of the brake signal to the computer is very interesting and I will have to check this out (thanks for the message Jasonmt!). How did you learn about this? I knew the ECU was somehow compensating for something with the brakes applied (seeing as the brake wire runs straight into the ECU). I haven't really paid much attention--but under normal circumstances do the RPM's drop significantly if the brakes are applied while cruising around? I don't mean the loping problem, but just normal RPM dropping not just caused by slowing down in speed as with an auto tranny or like with pushing in the clutch pedal with a manual tranny.
Fuck me runnin'! It worked! I had to turn the idle screw at least five times to get the idle to do any thing....whoa..... but so far it works. I didn't go to far but it seemed all good. I got a good day of driving tomorrow. So if it comes back I'll let ya'll know. Thanks again. And I get two warm feet.
Mine happened a couple weeks ago when I put on my Thorley header and Flowmaster. The idle was fine before, but the header must have allowed the gases to flow easier, somehow bumping up the idle with the less restrictive header. At first I though it was the AC idle up relay acting up, but that wasn't it. I just happened to turn down the idle to get it back to spec and that solved it.
Does this ever happen at random intervals? Mine seems to be a little rough at idle. If I'm sitting at a stop, brake on in D (automatic) the rmp will surge for a second and then go back to how it was - not a repeating cycle but more random? I'll check the idle when I drive it today. V6 should be around 750rmp right?
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