Parksy Posted December 5, 2005 Share Posted December 5, 2005 Out tonight in the Supe, aminly to fill up on fuel. Just been turning the car over in the garage the past few weeks. Went for a half hour drive around just to get fluids and stuff moving round the engine and give the car a general run out as well. When I was stopping the revs on the car were jumping up from its idle point (about 800rpm) to about 12/1300. After 10 seconds or so it would drop back down to its normal 800. Had a bit of a play and realised it was jumping up when braking and coming to a stand still. Just wondering if this is just the manual choke kicking in due to low battery or something, or if this is normal behaviour after the car has sat for a little while? In the garage when I turn it over I normally leave it on for 15 mins or so and it always seems to drop to the 800rpm idle after 5 mins or so as per usual. Just noticed this behaviour when actually using the motor? Link to comment Share on other sites More sharing options...
TrickTT Posted December 5, 2005 Share Posted December 5, 2005 is the air con switched on, that can raise the tickover. Link to comment Share on other sites More sharing options...
Parksy Posted December 5, 2005 Author Share Posted December 5, 2005 Nope aircon is off... Is not even ticking over high all the time, just appears to do it for 10 seconds or so after I have braked... :S Link to comment Share on other sites More sharing options...
suprasport Posted December 5, 2005 Share Posted December 5, 2005 Nope aircon is off... Is not even ticking over high all the time, just appears to do it for 10 seconds or so after I have braked... :S did you run low on fuel, seems strange on breaking Link to comment Share on other sites More sharing options...
Ian C Posted December 5, 2005 Share Posted December 5, 2005 Can you make it happen when stationary by pressing the brake pedal? -Ian Link to comment Share on other sites More sharing options...
Parksy Posted December 5, 2005 Author Share Posted December 5, 2005 Yeah but just the first time I press the pedal when stationary. If I roll to a stop the revs are 800, hit the brake they go to the 12/1300 mark. Stay stationary and it drops back to 800. Hit the brake again and the revs stay at 800? Most peculiar. Like I said had no electrics but the lights on, no heaters/aircon, radio or anything? Thought I was catching my foot on the accelerator as well as the brake at first (as remote a possiblity as that could be ). Link to comment Share on other sites More sharing options...
Parksy Posted December 5, 2005 Author Share Posted December 5, 2005 did you run low on fuel, seems strange on breaking This was after I filled up, didn't notice it before. This is the lowest I've ever let the fuel run if that makes any difference. Light had just came on about 5 mile previous though. Link to comment Share on other sites More sharing options...
Ian C Posted December 5, 2005 Share Posted December 5, 2005 Next time it's sat idling at 800rpm see if you can replicate the problem by taking this hose off (circled in yellow in the pic) You only have to take it off the intake plenum, not the other end. -Ian Link to comment Share on other sites More sharing options...
suprasport Posted December 5, 2005 Share Posted December 5, 2005 stop teasing. Link to comment Share on other sites More sharing options...
Ian C Posted December 5, 2005 Share Posted December 5, 2005 Eh? Er, oh, OK, I'm not explaining stuff sorry That's the hose that goes to the brake vacuum booster. If there is a problem with the brake booster unit then there is a chance that the intake plenum can draw in extra air through it, via that hose. On a MAP based setup, any air leaks actually raise the idle because the extra air is successfully metered by the MAP sensor and fuelled for (unless it's past the plenum, e.g. at the plenum -> runners or runners -> cylinder head gaskets. Implausible as they are unstressed and metal, unless some idiot doesn't physically do up the bolts right). So, if you pull that hose, the engine can suck air in via it's hose barb, bypassing the idle control valve and the throttle plate. This will raise the idle for sure, but the size of the hose fitting controls how much extra air gets in. If it's similar in behaviour to the problem at hand, then the brake booster unit bears further investigation. If the engine runs faster than idle but slower than the problem it's definitely not that because that hose barb is the maximum size of any leak further up the chain IYSWIM. If the engine runs faster than idle *and* the problem it could still be the brake booster but the air leak in that is smaller in size than the one we artificially induce by taking the hose off. Next diagnostic after that would be to get the engine racing and then use (cloth-wrapped) pliers to clamp that hose completely closed. If the engine speed drops back to normal, test it again and once more and if it happens every time, your problem lies within the brake booster (or at least something along that chain). It's the only way I can think of braking affecting the idle speed. And perhaps it only happens until some sort of reservoir is full, hence it only happening once. You asked -Ian Link to comment Share on other sites More sharing options...
Parksy Posted December 6, 2005 Author Share Posted December 6, 2005 Excellent!! Couldn't have asked for a better explanation!! Thankyou very much Now to giv tia try after work tomorrow. Gonna give it another run first though just to check if it is just a glitch with it not moving for a few weeks! Link to comment Share on other sites More sharing options...
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