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Stalls when Pressing Brake When Engine is Cold

S

Stuka87

NAXJA Forum User
Location
Twain Harte, CA
Hey all,

So my Wife's '89 XJ (4.0 with AW4 and converted to vacuum brakes from ABS) is having a strange issue that I have not been able to pinpoint.

When the engine is cold, you apply the brakes, the idle drops down, and sometimes kills the engine. However, once the engine warms up, this doesn't happen.

When I did the brake conversion, I connected to the vacuum line going to the booster to the larger nipple that comes out of the intake at the base of the throttle body. And I used a brand new booster and master cylinder.

Is this maybe a CTO valve going bad someplace, or some other sensor? Being that it only happens when the engine is cold, I don't think its a vacuum leak, but I am going to check for that when I get a chance.

This issue at first only happened now and then, but now does it almost every day. I put the new booster in over a year ago, so I don't think its failing already.

Thanks!
 
definitely sounds like a vacuum leak, particularly the one leading from the TB/intake manifold to the MAP sensor. This would explain the cold idle issue.

Also in play is the intake manifold to engine block connection. Check that all the bolts are torqued to spec. - you may need a new gasket.
 
Sounds a lot like what I'm getting from the huge vac leak in my WJ booster right now. When I step on the brake the engine RPMs drop significantly till the IAC opens up and fixes it, I get an idle surge when I let off the brakes too.
 
Ok, so I pulled out the IAC, and it really was not very dirty. The brass tip was actually spotless. Although it does have a fairly deep wear grove in it. I also used some carb cleaner on the hole that it went in.

The idle still fluctuates around, but will have to wait until its cold again to check the cold running issue.

I don't think the IAC is letting in too much air, as if I pull a vacuum line, the idle shoots up when this issue is happening. So if anything, it isn't opening enough.

If I get time, I am just going to measure the IAC, the MAP, and the TPS with a multi-meter. Hopefully I can find the issue. I don't mind cars breaking down on me, but it sucks when it happens to my Wife :/
 
I wasn't saying your IAC was bad (though it's good you checked it) - I was saying my brake booster causes the same thing you are seeing, and the IAC just lags in compensating for it.
 
Oh, and the exact reason it stalls (for mine anyways, not sure about yours but it sounds like it) - when the pedal isn't being touched, the leak is active. The IAC closes all the way to bring the idle back down, since the engine is getting air through the brake booster. Then you stomp on the pedal and suddenly it isn't getting air from the brake booster OR the IAC, and the ECU can't open the IAC fast enough to keep the engine from dying.
 
That's the regular way they operate - the engine computer can't change it too fast, because it will get ahead of the engine and end up making it surge, then nearly stall, then surge, then nearly stall before it gets the idle set right. It has to go slow enough that the motor has time to react and it has time to see the engine go faster or slower before making another change. If you want to read up on it, some good internet search terms would be overdamped, underdamped, critically damped, and control systems theory.
 
Figured I would drop everybody who may come across this an update.

So after I cleaned the IAC with carb cleaner, and the hole that it goes into, the stalling issue has completely stopped happening. Even in the 30 degree temps we have been having. Before cleaning it it would stall 1-3 times when my Wife was on her way to work. Now, it runs perfectly.

So now I get my JK back now that her XJ is back up and going :)
 
Glad you figured it out! Guess I was wrong on the booster part :looney:
 
sweet
 
kastein said:
Glad you figured it out! Guess I was wrong on the booster part :looney:
Click to expand...


Well no, you are right there kind of. When I press on the brakes, the idle goes up a bit, but not down. If I pump them, then it starts to climb about 250 rpm. So the idle does change, but it doesn't drop.
 
My idle speed will drop when pressing the brake pedal, now Im starting to wonder if the booster is bad like Kastein mentioned. I tried cleaning the IAC but still seems to drop rpm when on the brakes. Might be a small manifold leak... looks like I have some work to do this weeked. Thanks for follow up Stuka87!
 
If you don't get a hissing noise when you step on the brakes (or when you let up), and they work fine, your booster is likely fine.
 
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