Toyota Repair: Toyota Truck Problems, engine rpms, valve clearances

Question

Thanks so much for your reply...I just have one further question..
Considering the possibility of the timing chain, why do you think the idle would go screwy only when the brake pedal is depressed?  Also, do you think there is a chance that the problem is deeper than the timing chain? Maybe something like worn pistion pins, bearings..ect?

-=Dax=-
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Followup To
Question -
Hi Ted,
Im sure you get a bunch of questions, and I hope you have time to answer a few for me.

Firstly, the vehicle in question here is a 1991 Toyota Pickup. It's a 2.4L 22RE engine, 5 speed. Power steering, 2WD, no a/c, 391,000 Kms

The first problem is an idle problem. Occassionaly, and seemingly intermittently, when I am stopped for a moment, lets say at a red light or tim hortons for coffee, the idle will go from 1500rpm to 500rpm and jump up and down like that until I release the brake. The idle stays around 1000 - 1500 RPM during our normal winter driving.  when the brake is depressed, it will rise and fall every few seconds. Even if the brake pedal is depressed extremely lightly, it still affects the engine.  If the brake is not depressed, the idle stays normal.  Im not really sure where to start looking.

There is also a metallic rattling sound that seems to be coming from the valve cover area.  It's fairly loud when the engine starts, and then gets quieter after a few moments.  I've noticed that at specific engine RPMs, the noise is louder.  I started with cheking the valve clearances, and adjusted them to spec. This did not help any.  My dad suggested adjusting them a few thousandths below spec to allow for engine wear. I tried this and there were no noticeable changes.  Obviously, the engine has a lot of Kms on it, so I'd imagine it to be hard to diagnose a problem like this. I was wondering what a toyota pro had to say about this.  

Thanks... hope to hear from you soon.
Answer -
Taking care of the second part of your question may just fix the first part also.
The noise is coming from the timing chain, it is worn and making contact with the timing cover, the chain stretches to the point where the tensioner can no longer take up the slack of the chain and that is what is causing the noise, this can also affect the basic engine timing and cause idling problems.
The solution is to replace the timing chain along with the timing gears and the tensioner.

Answer
The idling problem may be related to a vacuum leak on the intake system, the brake booster receives vacuum from the intake manifold to provide power braking, there may be a leak from the vacuum hose to the power booster or there could be an internal leak at the booster, check for any possible vacuum leaks on the intake side of the EFI system.