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Discussion Starter #1
I have an 88 horizon (IDI) 5 speed. When im driving it will be fine but when coasting down the road in gear just no gas the car will be bucking a little bit every few seconds. it depends on the gear so obviously 1st gear (usually you don't coast in it though lol) is a bigger lunge forward, then 5th gear is a slight buck comparable to going over a small bump in the road. it only does this if coasting in gear if im in neutral it wont buck...when coasting in neutral it idles at 2000-2500 rpm cause i need to check for vacuum leaks

what could be causing the bucking while coasting in gear? Ive driven only manual transmission cars in my life so I do know its abnormal

any input is appreciated
 

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Check the motor mounts to see if they are shot. While a friend observes, set the parking brake firmly and slowly let out the clutch while in first, and again in reverse. The engine should not rock either direction more than about 1/2" to 3/4". If it moves more than that, one or more motor mounts needs to be replaced - usually the front one by the radiator.

Coasting in neutral may produce an engine speed higher than normal idle. However, if the car drives itself on level ground for up to a mile without touching the gas, or if it races up to 2000-3000 RPM when you come to a stop and put the clutch in, then you have a vacuum leak at the throttle body or one of the hoses, or a stuck AIS.
 

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Agree. There is also a 'bobble strut' from the rear of the transaxle to the crossmember to control and firm-up the engine/transaxle movement on stick-shift cars. Looks like a little shock absorber. If the strut is wet with its own oil or the rubber end bushings are shot, replace it.
Any slight misfire can also cause bucking more in a stick-shift than an automatic whose torque converter can absorb slight variations in engine speed. Make sure that the EFI and ignition system is in good condition, particularly the cap, rotor, plugs and wires.
Use the recommended Champion N12YC plugs for best results.
 

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Discussion Starter #4
thank you for the replys, Im pretty sure the engine mounts are all good.

I know I have vacuum leaks that I need to get found and repaired (will be doing it soon) which is causing my high and erratic idle, is it possible that the vacuum leaks would cause the bucking while driving? I just find it wierd that it only happens when Im coasting in gear not giving it gas

thanks
 

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Vacuum leaks will cause improper air/fuel mix, so you need to get them fixed right away.

If you haven't done the engine mount test, you cannot know if they are good or not. Just looking at them does not tell.
 

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Discussion Starter #6
I havent done the engine mount test you stated but i have had the hood open and manually revved the engine and i dont think its moved all that much.
 

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It won't until you do it with the engine in gear. Doesn't show up just revving it in neutral - you have to load the mounts.
 

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Park the car on a hard level surface in 1st gear and the parking brake off. While looking under the hood, rock the car forwards and backwards.
The front mount under the radiator takes the most load in fore/aft movement as does the bobble strut at the rear.
Any soft or worn mount or bobble strut with looseness or play will magnify a misfire bumping into a harsh jerking/bucking.
If the hydraulic fluid has pumped out of the bobble strut, it will not dampen fore/aft engine movement well.
 
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