By: Ron Lee
Engine surging normally occurs at idle, especially on modified engines, with the engine speed dropping to about 500 RPM and then speeding up to 1200 RPM or so. Normally, the longer you stay at idle the lower limit will keep going lower (and the upper limit higher) until the engine finally dies (unless you get to drive away before it does). It is highly frustrating. Most of us have been there. The normal cause is that the computer is not getting the correct signals from the sensors. Assuming your tune up is in decent shape, one of three solutions should fix the problem of surging. Check your tune up first (plugs, cap, rotor, and plug wires) and correct any problems with that first. If your surging continues, read on.
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First off, is your check engine light on, or has it come on recently (especially if your engine surging just developed)? Your check engine light should not come on unless something has failed (usually a sensor or the connection thereto). Normally when a problem first starts to develop, the check engine light will come on and then go back out after a few seconds because the problem is intermittent at first. Either way, if your check engine light has come on, you need to plug into your computer diagnostic port and let your computer tell you what is wrong. If you don't know how, most of the better automotive service places have the proper diagnostic tool that will do this for you. Many of these tools are now handheld. The computer will tell you exactly which sensor (or sensors) needs replaced and then you can replace only the sensor that needs repaired. Before you replace the sensor, have them wiggle the wire leading up to the sensor in question with the diagnostic tool still attached. Sometimes it is just the wire connector rather than the sensor.
Second, if your car does not have a mass air flow sensor (86-88 5.0 cars) and you have modified your engine much at all, you may as well face up to the fact that you need to add a mass air flow sensor. Without a mass air flow sensor, you are limited to about 10 percent more power (about 20 horsepower) above factory stock before engine surging will rear it's ugly head. I would take a mass air flow sensor from a wrecked car or buy the mass air conversion kit from Ford. Remember that you need the mass air sensor, the air inlet hoses, the computer, and the wiring harness that goes from the computer through the fire wall to make it all work. The conversion kit will come with it all; from the junkyard you are on your own.
Lastly, highly modified cars (especially ones with very large mass air flow sensors) the air flow is not even enough for the mass air flow sensor to correctly read the volume of air passing through the engine. This is one reason the factory mass air flow sensor is only 55mm in diameter. In this case, the easiest (and cheapest) way to correct it is to simply bump the idle up a little bit. You can adjust the idle speed with a 10mm wrench near where the throttle cable attaches to the throttle body. On my car, after I changed out my heads, I had to bump up my idle to about 900 RPM to get the engine surging under control. Once in while the engine still surges a little bit, but not enough to make the engine stall (which means I don't care).
One more idea to try is cleaning the idle bypass valve. It is located on the front side of the throttle body. The idle bypass valve adjusts the amount of air entering your engine at idle for any irregularities (which happen all the time). Sometimes the idle bypass valve will get dirty and that will positively cause you engine to surge. I haven't tried this one yet, but I will when I get the chance. Carbeurator cleaner is probably the best thing to use to clean it with.
One more last thing I will offer is that the multiple spark ignition systems have also been known to help eliminate surging in some cases. If all else fails, you can try this, but no guarantees, and this is not cheap (about $150-$200).
One comment that only applies to people with long tube headers. For the oxygen sensors to correctly operate, they need to be up to about 600 degrees. With long tube headers, especially in the winter, it will take a few seconds (maybe more) for them to reach that temperature. You will just have to tolerate the surging until the oxygen sensors warm up to operating temperature. One thing that helps (but does not eliminate) this problem is to get the headers ceramic (jet hot) coated. This helps retain the heat in the header which will help warm up the oxygen sensors quicker. I still love my long tube headers, and yes they cause my car to surge most of the winter until the car is almost completely warmed up.
©Copyright 1998 Ron Lee