I'm having an odd problem that I've already troubleshooted a bit. Whenever i'm going fast down a road and have to come to a stop my engine's RPMs fluctuate from 1250 Rpms and 1000 rpms. It only does this when i push in the brakes. The harder i push the brake pedal the more it fluctuates. If i let off the brakes it stops. I checked pressure hoses and vacuum hoses and to my knowledge they are all setup correctly.

 

See the 2nd Q&A on the web page below:
- http://www.4crawler.com/4x4/TLCA_Tru...ech.04.09.html

In short, your idle speed is set too high, turn it down a little bit.

 

Could also be your brake booster leaking. Hows your brake pedal? Stiff?

 

Brakes seem fine.Not stiff, but not too loose.

I adjusted the idler screw on the throttle body. It is now as tight as it can go and it has helped a bit. It doesnt fluctuate as much, but it still does it.

I also did some reading in the truck's manual and it says that on fuel injected vehicles (which this is) the intake idle is not set by the screw, but by the computer.

I recently redid the head and all the valves. Did some replacing of old hoses and tubes as well. It never did this until I did the head job. Could it be the newer hoses and tubes? I don't see how going from old and cracked to new and pristine can cause this type of problem.

 

Idle speed is mainly set by the air bypass screw on the throttle body. Their is some influence from the TPS on the throttle body that tells the ECU the throttle is closed and that works in conjunction with the timing set jumper to make sure the base timing is correct:
http://www.4crawler.com/4x4/CheapTricks/TPS/index.shtml

Without that all working, the base timing can be off and iff too advanced, can lead to too fast an idle speed, even with the screw all the way in. Make sure all the vacuum lines are correctly routed, as incorrect they could cause a vacuum leak and that can affect idle speed as well.

 

Tightening that screw did help. But it occasionally surges still.

I've been reading other posts that it could be a vacuum leak. Could be, maybe. We did a head job and replaced ALL of the robber hoses in the engine. Vaccum lines and coolant lines.

But, while we had the head/intake/exhaust/throttle body apart, we also cleaned EVERYTHING. we had everything hot tanked and had the carbon buildups removed. I'm thinking when we removed a carbon chunk, we created a leak. The carbon buildup was bad. I mean entire pipes were clogged.

so what's the problem?

 

Read the first page I posted for some other fixes. Also try adjusting the RPM knob on the A/C amplifier (behind the glove box). It sets the lowest RPM A/C can operate at and the idle up valve sets the fast idle speed w/ A/C on. The fast idle should be just a little faster than normal idle (basic idea is to keep the engine from bogging down). Low idle speed is set a little below normal idle speed, basic idea is that is the engine does start to bog down, kick the A/C compressor off to let the engine recover. If those settings are messed up, it can cycle back and forth. And there is the idle speed set with the screw on the throttle body and the fast idle screw on the idle up valve. So the normal idle should be someplace around 800, fast idle in the 900 RPM range or so and all under the ~1100 RPM point where the ECU starts cutting fuel to the engine when you hit the brakes.

 

not to hijack, but my truck does this too, but only when the a/c is on. im assuming its the same problem though. a/c turns on and bumps the idle up and its too high causing it to bounce. Right?

 

Bet you got the vac lines mixed up goin to the vsv on the ps pump, or the vsv is goin bad.

 

Yes, turn the idle and fast idle speed down. On my '85 I can exactly duplicate this surging idle with the brakes on with a simple turn of the idle up valve adjuster. If I exceed about 1100 RPM it surges with the brakes applied and if I am below that RPM it does not.

What you are seeing is a built in feature of the ECU. If it sees an engine RPM over a certain value and the brakes are applied, it assumes you are braking to a stop or to slow down. So to help, it cuts the fuel to the injectors to help slow down faster and probably to cut emissions.

 

I don't have an A/C.

 

A/C makes no difference, although it can make the problem more likely to show up, since you have a normal and a faster idle speed when the A/C is running.

If the idle speed is higher than the ECU's threshold and you hit the brakes the ECU cuts fuel to the injectors until the RPM drops below that threshold. Repeat that cycle and you have a surging idle:
- http://www.4crawler.com/4x4/TLCA_Tru...ech.04.09.html

 

So I should just cut the wires? That seems a bit drastic.

That other link in the post http://www.geocities.com/toyotashawn/bizzareidle.html says it could be a timing problem.

I did the head job a few weeks ago. I also replaced all the hoses, got a new distributor cap, distributor wires, new (platinum) plugs, and new gaskets, along with new fluids (oil and antifreeze). We have the timing set so it sounds right, but we never had the timing light available to time it accurately. Could this be it?

Everything was fine BEFORE we did the head job, so something in between is causing this idle problem.

 

Yes, improper timing can cause the idle speed to change. I find on my '85 if I go more than 5 BTDC with the timing check jumper installed, the engine runs fine, but I can't get the idle down to the proper range. For example if I go to 7 BTDC, the idle is up around 1000 with the idle screw all the way in.

 

Ah. When we set the timing, we made sure everything was TDC. Then we just adjusted it by rotating the distributor cap. Is that how it's done with the 89 toyotas?

 

Process is noted at the bottom of the web page below:
- http://www.4crawler.com/4x4/CheapTri...ingChain.shtml