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It's a 1999 Mazda Familia.

When I start driving and the engine's cold the car often stutters and fluctuates between 500 and 750 revs. Then when I accelerate the revs go up but the car goes very slowly for a few seconds, then produces a loud thunk and launches fine (real bloody hassle at a busy intersection). Because the revs were increasing as normal I thought it might have been a transmission issue but the car still stutters and shakes like it's about to stall even when it's in neutral. Also when it's driving and it dips below about 1500 revs, the accelerator will skip for a few seconds before righting itself. This usually happens when the engine is cold and about 90% of the time it goes away after a few minutes when it's warmed up.

I've booked it in for a service this weekend and I'm thinking it might be a spark plug or fuel line issue but I wanted some ideas from someone who knows more than me about cars (~99.9% of all people)

Update: The car now always shudders, so it's getting worse. I've booked it in for a mechanic on Monday.

Update: I've taken it in to a mechanic and he says it's probably the idle air control valve but the bigger issue is one of the coils is buggered so the car's running on three cylinders. It should hopefully be sorted tomorrow.

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  • Any codes? I would start with cleaning the throttle body (focus on the automatic idles speed valve), and test the coolant temperature sensor to make sure it is reading correctly when the engine is cold.
    – rpmerf
    Commented Jul 15, 2016 at 13:00
  • If its carbureted, I would look at the choke. By 1999, I would think the car would be fuel injection.
    – rpmerf
    Commented Jul 15, 2016 at 13:51
  • It's an FI car but there's no check engine light so no codes to pull.
    – Richard Myburgh
    Commented Jul 16, 2016 at 0:39

2 Answers 2

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Bad spark plugs would make an engine shake almost always, but they won't make the revs go up and down. I would blame: choke and/or idle valve, probably stuck if by manual wire, or clogged with dirt/oil if electronic. Abnormal (low, because it won't be higher) gas line pressure will make the engine stall or go almost to stall, but won't raise the revs neither. So check choke and idle valve.

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  • The wire isn't stuck. I took out the air filter housing to get into the throttle to see if the idle air control valve was gunked up but it's got a weird shaped screw so I can't take out the gasket at home. It also reeks of gasoline so I'm thinking it could be a fuel line issue?
    – Richard Myburgh
    Commented Jul 16, 2016 at 2:24
  • I still think it is the idle control. Manual or electronic, nevermind, something is bad there...gummed up, too dirty, stuck, some have a very small hole in the needle which gets stuck with debris. Strong gas smell = too little intake air, too much gas intake, confirming that perhaps the idle valve or the choke are getting stuck.
    – Aram Alvarez
    Commented Jul 16, 2016 at 20:34
  • Yeah I'm leaning towards it definitely being idle air control, the rich condition giving it the gas smell. Unfortunately I can't take out the gasket on the idle air control valve at home because it has a weird shaped screw, so I'll take it to my mechanic friend on Monday and get him to pop it open to clean it out and check if it runs better after that
    – Richard Myburgh
    Commented Jul 17, 2016 at 0:51
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Maybe try adding spme Heet to the has tank. Sometimes water builds up and causes reving in engines. This helped our rev issue.

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