RPMs fluctuate as brakes pushed
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.
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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?
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Originally Posted by Adam F
(Post 50913634)
Could also be your brake booster leaking. Hows your brake pedal? Stiff?
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? |
Originally Posted by DupermanDave
(Post 50923187)
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? |
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?
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Originally Posted by DupermanDave
(Post 50923187)
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? |
Originally Posted by sixt5
(Post 50923280)
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?
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.
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Originally Posted by DupermanDave
(Post 50923440)
I don't have an A/C.
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 |
Originally Posted by 4Crawler
(Post 50923442)
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 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.
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Originally Posted by 4Crawler
(Post 50923602)
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.
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Process is noted at the bottom of the web page below:
- http://www.4crawler.com/4x4/CheapTri...ingChain.shtml |
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