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I have an 88 horizon 5 speed idi with 81,000 miles on it. Runs very good and in great shape. I have all the service records for the car since 55,000 miles. The previous owners took great care of it. When I got it it was overheating a bit so I replaced the thermostat and all is well. Replaced the air filter oil filter, oil, coolant, rear struts for hatch, all new stereo system, fixed all the rust painted with exact matching paint, undercoated the whole car to keep it in the shape its in (previous owner undercoated it and its immaculate from that and starting to fade out so I re did it).

The only problem doesn't keep it from being drivable. It has the well known erratic and high idle. What do I need to check for common areas to fix this?

In addition if Im coasting in gear it will sort of lundge forward consistently nd also wen I go to tap the gas it hop forward a few times. also when I accelerate it sometimes stumbles a bit. not the engine skiing time of stumble. Im going to run seafoam through the tank to see if that helps.

Any suggestions? Thank you
 

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Check for cracks and splits in the plastic vacuum tubing behind the valve cover. Also, there are 1 or 2 plastic tees that sometimes split and break. Generally on a car that old, much of the vacuum tubing is shot if it is original.

Moving beyond that may require cleaning the throttle body and AIS motor and seat. If the the throttle body is removed, you need to make sure it is sealed well to the intake manifold. Any vacuum leaks to the intake or bottom of the throttle body will cause fast idle.

Once you resolve all the problems, that 88 Horizon will be a nice economical car.
 

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Agree. An intermittent fast idle may also be from a failing TPS (throttle position sensor) or as mentioned, the AIS (automatic idle speed) motor and its related circuitry.
When it is racing would be the time to go in with a meter or diagnostic tool and see what the TPS signal voltage is. Normal idle voltage should be about 0.65 volts. If it is around 0.80 to 1.00 volt, it may have a poor ground or might be failing internally. Wiggle the connector and listen for a reaction.
The AIS would best be diagnosed with a scan tool to see how the 'target' idle speed compares with the 'actual' idle speed. and how many 'steps' open the valve is open. Also the valve should react to changes in load, like turning on the A/C, turning the steering wheel or when letting out the clutch to drop engine speed.
 

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ok thanks for the advice I will do all that when I get a chance.

Last night I was driving and my temp guage climbed all the way to red hot, but the check engine light never came on. I pressed the brake and the needle instantly shot down to the normal temperature spot, so Im assuming it wasnt actually overheating...what advice can you give on that? bad temperature sensor, maybe waterpump? I also dont have very hot heat in the car...

btw yes all the fluids are normal, and the reason I dont think it was actually over heating is because anytime in the past the check engine light would go on too

any advice is welcome thank you
 
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