0 RPM Dip

[buckeyeguy87]

I have an interesting problem. I just bought my 99 Civic EX coupe a couple weeks ago. However, I am guessing the reason I got such a good deal because there is a problem, although there was no way to detect it when I test drove it. The problem is very intermittent, and very puzzling. I will explain: The car starts perfect, normal idle @ 1500-1700 at startup before driving, around 750ish idling after driving. However, occasionally when I am not giving it gas such as approaching a red light without braking but more or less letting the car decelerate by itself, the battery light comes on and I look at the tachometer and it shows the needle all the way down to between 0 and 500, (sometimes at 0) and it stalls or is about to unless I give it gas/rev the engine. However, it also does this sometimes at a red light when I am pressing the brake and not giving gas/rev. I also ran fuel injector cleaner twice in case that was the problem, which apparently wasn't.... The problem doesn't happen in a specific situation such as on a hill, during a wet/rainy day, etc....it is very intermittent, so spotty that it could do it before I run into the grocery store, and once I start it up again and leave, it won't do it on the way home at all. I have just changed the spark plugs and wires and am considering doing the distributor & cap. I also heard to possibly check the fuel filter as the source of the problem. I know the timing belt and water pump haven't yet been changed as it only has 94000 miles on it and I asked the owner if they had ever been changed to which he said no....I am planning on changing those in 2 weeks. Supposedly the timing belt could also be causing the problem (it is a little loose, but I am replacing it and the water pump anyways). Finally, I also heard it could possibly be the ECU not giving enough power, but I am hoping that isn't the case since that would be pricey. Notes: The car has a cold air intake and the cam gear pulleys were replaced to give it more torque BEFORE I GOT IT...I would never do either, personally I would rather have it stock....but I will also replace the air filter soon as it is a different one since it has the stupid cold air intake on it.....the Civic is a manual transmission. So far I haven't noticed any electrical problems, and I had the alternator and battery tested and they put out 13.9V and 510 amperes respectively....PLEASE HELP IF YOU HAVE ANY SUGGESTIONS AT ALL! If I am missing any important info that would be helpful, let me know...THANK YOU.

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[gearbox]

have you tried cleaning the idle valves and intake system parts (throttle body, etc)? that could cause the rpm to drop when you are not pressing the gas.

[buckeyeguy87]

I have no idea how to do those things, I am not very in depth on mechanical things....would it involve taking things apart? I am not skilled to do advanced things...sadly.

[gearbox]

yeah but its pretty basic stuff. taking out bolts and screws, spraying carb cleaner inside to get all the black stuff out, then bolting back together. if you dont think you can do it, take it to a shop and tell them you want all the intake parts cleaned like throttle body, idle air valve, and fast idle thermo valve. might as well replace the pcv valve since it only costs a few $$.

[marek91]

vacume leak possibly ?