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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / February 2005Bad idle on '91 626
Here is the problem. '91 Mazda 626 2.2 without turbo (Yeah, I wish.) has a rolling miss at idle when in gear. It runs and idles smoothly at all other times, but if the car is fully warmed up and I stop at a traffic light, it will idle smoothly for about four or five seconds, then the idle will drop and surge, drop and surge, from almost stalled to normal. If I put it back into gear it will go right to normal (750 rpm) and stay there. I have looked for a vacuum leak, but there could still be one that I missed. (Zillions of hoses.) I know that it could be just about anything, but I wonder whether there's anything that jumps to mind as a likley suspect. The manual talks about troubleshooting the various controls using an "SST." Unless they mean the Concorde, I don't know what that is. Do I need to corner the Snap-on man? How much will he do me for? I realize that if you don't have all the diagnostic stuff, sometimes the only available approach is to replace something to find out whether that is the problem, but I don't really know where to start. If you say, "Try replacing the throttle sensor." and I do and it doesn't straighten it out, I am not going to be miffed. I just don't have a best guess. Thanks for any help you can offer. Hamish A 'searching idle' is commonly cured by a slight fuel/air idle mixture adjustment. Not familiar with this model as to adjustment screw or components that affect this. rick > Here is the problem. > [quoted text clipped - 23 lines] > > Hamish >Here is the problem. >'91 Mazda 626 2.2 without turbo (Yeah, I wish.) has a rolling miss at >idle when in gear. It runs and idles smoothly at all other times, but >if the car is fully warmed up and I stop at a traffic light, it will >idle smoothly for about four or five seconds, then the idle will drop >and surge, drop and surge, from almost stalled to normal. If I put it >back into gear it will go right to normal (750 rpm) and stay there. >I have looked for a vacuum leak, but there could still be one that I >missed. (Zillions of hoses.) I know that it could be just about There should NOT be zillions of vacuum lines unless somehow mazda slipped you an engine from the late 70's. There should be less than a half dozen actuall vacuum lines. Stuff like the brake power unit, the idle air manifold, and the cruise control actuator. Start at the intake manifold and look for stuff coming off it. Does the ECU report any errors? Is your ignition system perfect -- nice fat spark if you pull a wire and hold it 1/4-1/3" from a ground? Timing correct? Don't replace anything unless you've actually tested it bad. One mistake will easily cost you more than having a mechanic diagnose the problem. >There should NOT be zillions of vacuum lines unless somehow mazda slipped >you an engine from the late 70's. > >There should be less than a half dozen actuall vacuum lines. Stuff like the >brake power unit, the idle air manifold, and the cruise control actuator. >Start at the intake manifold and look for stuff coming off it. ================ Well, I'm kind of an old-timer. More than one going to the distributor (and in the really old days, one going to the wipers) kind of throws me. ================ >Does the ECU report any errors? ================ No errors show on the dash, but I don't think it is equipped to do that. Doesn't seem to have a "Check Engine" light or whatever. Is there a port to plug into with a relatively affordable unit to read out such codes? I could use a hint like that. =============== Is your ignition system perfect -- nice fat >spark if you pull a wire and hold it 1/4-1/3" from a ground? Timing correct? ============== Timing is dead-on. Plugs are new within three months. Cap and rotor new. Wires could be questionable. Filters are good. Haven't checked spark, but will on Saturday, if the weather is bearable. ============= >Don't replace anything unless you've actually tested it bad. One mistake will >easily cost you more than having a mechanic diagnose the problem. ============ I only meant that, if this is a textbook case of the symptoms of some particular failed control, part that I would be willing to roll the dice. No, I don't want to go replacing bushels of parts on a whim. PLEASE NOTE that the idle only goes silly when the car is in DRIVE. Not Park, not neutral, and not even when it is in REVERSE. This seems kind of odd to me. Thanks for any help you can offer, Hamish Original Post Below: >>Here is the problem. > [quoted text clipped - 7 lines] >>I have looked for a vacuum leak, but there could still be one that I >>missed. (Zillions of hoses.) I know that it could be just about >>There should NOT be zillions of vacuum lines unless somehow mazda slipped >>you an engine from the late 70's. >> >>There should be less than a half dozen actuall vacuum lines. Stuff like the >>brake power unit, the idle air manifold, and the cruise control actuator. >>Start at the intake manifold and look for stuff coming off it. >================ >Well, I'm kind of an old-timer. More than one going to the >distributor (and in the really old days, one going to the wipers) kind >of throws me. >================ There shouldn't be any going to the distributor. All that's done in the ECU nowadays. Did a '91 vehicle really have a vacuum advance? I'd be amazed if that was really true. >>Does the ECU report any errors? >================ >No errors show on the dash, but I don't think it is equipped to do >that. Doesn't seem to have a "Check Engine" light or whatever. An ECU will often have errors to report without lighting the check-engine light. There's usually a procedure like flipping a switch and then turning the ignition on, or doing something wierd like turning the ignition on then off 3 times within 10 seconds. >Is there a port to plug into with a relatively affordable unit to read >out such codes? I could use a hint like that. >=============== Many car parts stores will read the codes to you for free as it is good for their business to do so. Autozone for example. ... > Here is the problem. > [quoted text clipped - 23 lines] > > Hamish I very common failure with these (I own a 91 626 and I'm also a mechanic), is small cracks in the intake air hose. These are very hard to see without removing the hose. I fixed mine by wrapping the entire hose with 2 layers of electrical tape. Larry > > Here is the problem. > > [quoted text clipped - 30 lines] > > Larry I did mine with duct tape. Take the intake "boot" off and inspect by flexing it. Cracks do not appear in the "valleys" but do on the "peaks" on mine. Al |
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