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I have a Mazda 626 (sedan) -97 and lately its been running uneven and jerky.

Have had in in for repairs and changed battery, oil+filter and spark plugs. Still had problems and tried to use a engine and fuel injector cleaner (brands STP and Red Line).

This solved the problems temporarily. Until I had to refueled with gas again. I have done this process twice now with same results, and while the problem is gotten less it is still uneven and jerky with a new tank of gas.

Problem seems to worse at low speeds. And if you let go of the gas the speed will will go down until the car stalls.

Any suggestion or ideas?

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2 Answers

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I had a Ford Probe with a 2.2L Mazda engine with nearly the same issue. I did everything you said, multiple spark plug changes, fuel cleaners, sea foam, etc. etc.

The problem was the spark plug wires. They looked perfectly fine, but when I replaced them as a last resort the whole issue cleared right up. I later noted a small crack in one where the wire was only intermittently making contact. If you haven't got it fixed yet, try replacing your plug wires (and cap & rotor).

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Thank you! And no we have not done this. definitely worth a try. And just as you say; the spark plug wires looks fine, but perhaps they are still damaged. Can't cost that much after all to try. – jamietelin Apr 17 '12 at 9:24
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Nope, was $40 for the set for my Probe. I can't comment on the answer below, but releasing some vacuum when you open your filler cap is a good sign: it lets you know you don't have any evap/filler cap leaks, and your system is staying sealed. This tends not to happen as cars get really old. – Ehryk Apr 17 '12 at 9:30
I will get back to this question once I have tested and replaced the spark plug wires. Thanks you again for the suggestion. ;) – jamietelin Apr 17 '12 at 9:33
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Have you checked the fuel filter and the fuel tank breather? If either of these are clogged, it will reduce the amount of fuel getting to the engine, which can often lead to poor running at low revs. It is unusual to see problem occurring more with a full fuel tank, the opposite is more common, but that would definitely lead me to suspect the breather - this is a valve in the filler cap that allows air to flow in to the tank to replace the fuel as the level drops.

As a test, park the car outside in an area well away from any potential sparks. Switch on the engine, allowing it to idle - this presumably will cause it to stall? If so, open the fuel filler cap, and switch on the engine again. If the problem does not occur, that's your culprit! Obviously you must be VERY careful running the car with the cap removed, as petrol vapour is highly flammable.

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Modern gas caps do not have a vent to allow air in.This done by the evap emission system.Is the check engine light on? – mikes Mar 23 '12 at 1:35
@mikes No, no "warning lights" on. No indications that anything is wrong except what I have stated. Problem occurring whether tank is full or not. But always gets better if I add a bottle of engine and fuel injector cleaner towards the end of the tank. But gets back to its jerky and uneven when I fuel up again. – jamietelin Mar 23 '12 at 7:36
@Nick C I have however noticed sometimes when I remove the filler cap to refuel the car that it gives of a sound, like when opening a bottle of soda. Like air does get sucked in or something. But it is not always and I have noticed this even before I started having problems. – jamietelin Mar 23 '12 at 7:39

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