recently got this low mileage zx2 for my wife, what a great car! only problem is some midrange “hesitation”. replaced plugs, wires, air filter and fuel filter. problem still there. Could it be the coil-pack? passed smog with flying colors and no check engine light. car has 59,000 miles and was bought from original owner who says it has never been changed. just dont want to replace something that doesn’t need to be. thanks
Is this a cruise control equipped vehicle? If so, a common trait is that the Throttle Position Sensor will get a worn spot near midrange as it also happens to be the same range as cruise throttle. The Cruise Control servo is constantly moving the throttle back and forth to maintain set speed. This constant back and forth of the contact inside the TPS is what causes the wear and subsequent signal dropout/hesitation for this particular scenario. Another TPS related malady is plain old corrosion, especially if somebody was a little overzealous with the throttle cleaner and let it penetrate the TPS through the throttle shaft seal. Another possible problem is the Mass Air Flow sensor. Let us not forget a slipping torque convertor lock up clutch if it is automatic transmission. Does it have rough, unsteady idle? Rotten PCV elbows are also common.
thanks for the reply! no cruise control and it’s a 5-speed. wouldn’t the TPS and MAF throw a code? I would have never thought of the PCV. I will check both the valve and elbows. thanks for the tip!
There is such a thing as “in range failure”. Unless the failure meets the flag critieria, no trouble code and no check engine light will be set. Depending on how it is driven, you might be just bumping up against the fail threshold without crossing the line to a flag condition. One indicator of MAF trouble is to have somebody with a scanner look at the barometric value. Most scanners will display a Kpa (Kilo-pascal) value. It should be equal to the barometric pressure for your area/altitude. If it seems to be off by 5 or more Kpa then disconnect the battery negative cable for at least twenty minutes to dump keep alive memory and reset the fuel trims to zero. Then drive the car making sure that you do a few full throttle runs in the upper gears. The trick is to achieve better than 90% engine load for up to ten seconds at a time. After a couple of drive cycles have the baro reading checked again. If it is still off, then cleaning or replacement of the MAF may be in order.
Wow…that’s some deep info! My buddy has a shop and I’ll get together with him to look at the barometric value. Would I just do as well to clean the MAF, and if so, what’s the best method? I plan on doing the resonator removal and box chop (I noticed you did the same on your zx) so I will have it apart anyway. Once again, thank you for the valuable info.
Sometimes attempting to clean the MAF can cause more problems than it cures. If you do, use actual MAF cleaner or electronic parts cleaner NOT carb. cleaner. Allow the sensor to dry completely before running the vehicle. I also recommend against oil type air filters for MAF cars. There are dry type performance air filters out there. Personally I use a WIX brand OE replacement. They filter well and breath just fine for normally aspirated engines.
thanks once again for the reply. I will give it a try. Also, what is UDP and corn squeezins? Is the super unleaded for a knock sensor mod that I’ve read about?
UDP=Under Drive Pulley, I have the Eslinger. It was probably the single biggest improvement in low and midrange performance for a stock engine and really reduced the vibes and slow down from AC compressor engagement. Corn squeezin’s is my vernacular for the ethanol in gasoline. Using super unleaded allows the system to use maximum ignition timing and less VCT (EGR control for our cars) by keeping the detonation away. I tried the knock sensor mod and it was very bleh, no noticeable improvement. We also get better fuel economy with super.
Thank you so much for the info. planning the MAF clean hopefully this weekend and will post the results. Felt like a total newb when I found out the UDP meant underdrive pulley. Familiar with 5.0’s and the results of the use of a UDP (wow, look how I just used that like I’ve done it forever). Very familiar with esslinger as I have a turbo 2.3 waiting to go into something. Pintos were very popular in sandrails here in So. California and I used to work in the industry. You have been a great help and a ZX2 mentor for this green-horn!
Did the MAF clean…problem still there. Did the box chop and res removal, and I can really feel the difference. The problem is more like a miss than a hesitation which I described before. You can feel the power wanting to get thru. I notice the tires chirp even with just a little throttle and clutch release. The bottom-end is really there now. I can best describe the way the car is running as a dog straining on a leash…with hicups
What spark plugs are you using? Is there any carbon tracking on the plugs like from a dookie plug wire? Also, you might consider sending the injectors to FuelInjectorMan for service as well as replacing your fuel filter upon reinstallation. I can’t tell you how many times a restricted injector inlet screen drove me crazy. Does it idle smooth? (Well, as smooth as a zx2 can idle anyway 
lol. On rare occasion I have seen a bad coil. Sometimes the valve cover will leak into the plug wells until the oil level shorts out the spark. It happened to me.
plugs are motorcraft and proper heat range. no oil in the plug well(had the same problem on another car). considering that there was old fuel in the car when I got it, I’m thinking that your injector cleaning suggestion is a good idea. the local napa does a test/cleaning, they did the injectors on my son’s 240 with great results. I really don’t want to buy a coil just to find out it wasn’t the problem. Cleaning the MAF and injectors is rarely a waste so definetly doing those type of things first. thanks again