Vacuum leak from brake booster. If you hear a sucking noise when you press the brake pedal replace it. Another way to check it is to place truck in gear set e-brake press brake pedal repeatedly and see if idle goes up and down. DO NOT PRESS GAS PEDAL DURING TEST.
Thanks i'll have to test it out tomorrow
No sucking noise or idle going up and down from either test
by any chance has the distributor been removed or replaced. There is a procedure to set the cam variance. basically you run the engine up to full throttle while the computer learns the timing range from idle to full throttle. The problem is that only the high end scanners can do the test. so that does complicate things unless you have access to a scanner.
No luck no scanner
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SOURCE: high idle 1997 Madza 626 4 cylinder auto transamission
The PCM controls idle speed to ensure good driveability. The PCM will only attempt to maintain the desired idle speed when it “knows” the throttle is closed. A throttle switch or the TPS input is used to determine when the throttle is in the idle position. Motors and solenoids are used by most PCM’s to control idle speed. Many PCM’s are programmed to attempt to control the idle speed by adjusting spark timing and then by controlling the idle speed actuator. Some computers of the early 80's did not control idle speed.
SOURCE: high revs
being a fuel injected engine , there are many reasons this could be happening.
from the top , lets check the Manifold Absolute Pressure (M.A.P.) sensor , as it is easiest.
Find the MAP sensor , it has a couple a wires and a vacuum tube going to it and is on the firewall or fender support under the bonnet.
Disconnect the wire harness from it (small plug with wires).
If the engines RPMs drop to normal , replace the MAP sensor.
SOURCE: no acceleration
I had the same problem with one exception. It would rev higher than 3500 but the higher it revved the more it bogged. I had the catalytic converter replaced. I thought I bought a new car. Acceleration is just like new. It was easy enough to test. My mechanic cut a hole in the exhaust pipe in front of the catalytic converter. It sounded like a truck but the acceleration difference was immediate. Easy diagnosis.
The car is hard to shift because the more you press on the accelerator the more the computer for the transmission won't let it shift. Which in turn causes the car to bog more sending more unburned fuel into the catalytic converter. Which in turn causes more carbon build up and more back pressure. Vicious cycle. It will only get worse.
My thermostat caused the entire problem. Thermostat was stuck open, engine never got to operating temp most days. Car burned rich. Bad gas mileage. A lot of unspent fuel being dumped into the converter. Probably have some underlying cause that needs to be addressed. Bad plugs, wires, coil, fuel filter.
SOURCE: 94 L reg clio 16v 1.8 tickover irregularities
Sounds like idle air control valve sticking. If i remember correctly, the idle air control valve is built into the throttle body. This is the part that connects between the air filter and inlet manifold. take this off and clean thoroughly with carb cleaner. if this does not work, then renew part.
TL
SOURCE: 04 silverado rough idle wants to stall when stopping Help please
Check o2 sensors, and throttle position sensor as well as any vacuum leaks. Make sure you are getting plenty of spark and fuel pressure.
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