Idle (Keep Reading) With Foot on Brake
10-11-2011, 11:10 AM
#1
Idle (Keep Reading) With Foot on Brake
I am flummoxed.
My 3vze 1990 4Runner 4x4 5sp. cold idles fine (1100-1300rpms) and warm idles fine; perfectly actually at 850rpm. Except...
When the truck has just reached its warmed-up state (needle hits a certain spot on the temperature gauge and the truck goes into closed loop mode) and generally when warm with my foot on the brake.
To be perfectly clear, my foot being on the brake is the only thing that causes the idle to go wonky.
When the truck has just reached operating temperature, and my foot is firmly on the brake, for a minute or so, idle will bounce from cold idle-ish range of 1100 or so to warm idle-ish range of 700 or so. It happens very quickly back and forth for 5-6 cycles and then it's fine. Sort of.
The 400rpm variation becomes a 50rpm or so variation that I can hear, but that doesn't necessarily register on my old and worn out tach.
In the last several weeks, I've changed the plugs and confirmed the gaps on the new ones, replaced the distributor with a brand new one, replaced the TPS with a brand new one and calibrated it with the TB assembly off the truck, and it has a brand new cap and rotor as well. Timing is at 12BTDC jumpered in, and it drops to 8-9 jumpered out at warm idle.
I'm getting 16mpg or so in city driving, and there are no performance complaints, codes, funny smells or gremlins running around on the hood.
Could I have a failed/failing brake booster?
I pulled the vacuum line off the booster about 20 minutes after I shut down the truck and it had a strong hiss, and the brakes seem to work mostly relatively well (not much to compare it to; my wife's Civic will introduce your teeth to the dash if you stand on the big pedal) in that the pedal moves with reasonable pressure and the brakes will go to lock fairly easily.
What bugs me is that the the symptoms are more pronounced right around when the truck is transitioning from open loop to closed loop.
Any ideas?
My 3vze 1990 4Runner 4x4 5sp. cold idles fine (1100-1300rpms) and warm idles fine; perfectly actually at 850rpm. Except...
When the truck has just reached its warmed-up state (needle hits a certain spot on the temperature gauge and the truck goes into closed loop mode) and generally when warm with my foot on the brake.
To be perfectly clear, my foot being on the brake is the only thing that causes the idle to go wonky.
When the truck has just reached operating temperature, and my foot is firmly on the brake, for a minute or so, idle will bounce from cold idle-ish range of 1100 or so to warm idle-ish range of 700 or so. It happens very quickly back and forth for 5-6 cycles and then it's fine. Sort of.
The 400rpm variation becomes a 50rpm or so variation that I can hear, but that doesn't necessarily register on my old and worn out tach.
In the last several weeks, I've changed the plugs and confirmed the gaps on the new ones, replaced the distributor with a brand new one, replaced the TPS with a brand new one and calibrated it with the TB assembly off the truck, and it has a brand new cap and rotor as well. Timing is at 12BTDC jumpered in, and it drops to 8-9 jumpered out at warm idle.
I'm getting 16mpg or so in city driving, and there are no performance complaints, codes, funny smells or gremlins running around on the hood.
Could I have a failed/failing brake booster?
I pulled the vacuum line off the booster about 20 minutes after I shut down the truck and it had a strong hiss, and the brakes seem to work mostly relatively well (not much to compare it to; my wife's Civic will introduce your teeth to the dash if you stand on the big pedal) in that the pedal moves with reasonable pressure and the brakes will go to lock fairly easily.
What bugs me is that the the symptoms are more pronounced right around when the truck is transitioning from open loop to closed loop.
Any ideas?
10-11-2011, 01:57 PM
#3
I believe it does have a dashpot attached to the throttle body where the throttle armature meets the stop.
It does not appear to be a serviceable item, though. It moves... The FSM is a little unclear as to how to test it. Yes, there's a section for it, but it doesn't make sense in this head.
In any case, and please correct me if I'm wrong, but the dashpot is a vacuum load-balancer of sorts, right? So when system vacuum drops because I put the brake pedal all the way to the floor at idle, it should spin up the engine to increase vacuum, no?
What makes you think it's the dashpot? If my theory on its operation is correct, your idea makes sense to me; but what would make it a significant problem only around the time when the computer is moving into closed loop mode?
I mean, while I can hear engine speed change ever-so-slightly once it's all the way warmed up and the brake is floored, it's nothing like the 300-400rpm hit it takes at transition.
It does not appear to be a serviceable item, though. It moves... The FSM is a little unclear as to how to test it. Yes, there's a section for it, but it doesn't make sense in this head.
In any case, and please correct me if I'm wrong, but the dashpot is a vacuum load-balancer of sorts, right? So when system vacuum drops because I put the brake pedal all the way to the floor at idle, it should spin up the engine to increase vacuum, no?
What makes you think it's the dashpot? If my theory on its operation is correct, your idea makes sense to me; but what would make it a significant problem only around the time when the computer is moving into closed loop mode?
I mean, while I can hear engine speed change ever-so-slightly once it's all the way warmed up and the brake is floored, it's nothing like the 300-400rpm hit it takes at transition.
10-11-2011, 08:11 PM
#4
Registered User
If it only happened with your foot on the brakes, I would think it was your brake booster. They will develop a leak and they will lean out the mix. Since it is by-passing the air flow meter, it takes a bit for the 02 senders and the ECU to pick it up and stabilize it again. Check to see if it still does it with the booster disconnected and the hose plugged REALLY well. That will at least eliminate one thing and it is basically free to do.
10-12-2011, 11:59 AM
#5
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If you've confirmed all of the vacuum lines, my guess is that your booster is has a leak (tear in the diaphragm) to atmosphere in the cabin. You draw in atmospheric pressure from the front of the booster (the part that sticks through into the cabin by the pedal box) through the filter just behind where the booster pushrod clevis connects to the brake pedal. If there is a leak there, you will have a direct path for intake vacuum to leak to atmosphere when the brakes are applied. Depending on the size of the leak, it shouldn't cause a hard pedal either (the normal booster failure symptom).
If your truck starts easily with no brakes and difficult with the brakes applied (try cold starting your truck with the brakes applied), the above is likely true. If no difference, than probably not.
Good luck.
Last edited by angrybob; 10-12-2011 at 12:01 PM.
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