Questions and Answers

Ford Bronco II Stumbles And Hesitates Q. I have a 1984 Ford Bronco II, V-6, standard transmission with 181,000 miles. My problem is when I slightly hold my foot on the accelerator pedal, like I was taking off from a stop light, it will surge or race for a second and then idle back down for a second and then up and down for as long as I hold slight pressure on the gas pedal.

Once I force the acceleration past this point and give it plenty of gas, then it runs fine. As you can imagine, this causes jerky starts, and dying, from every stop I make. Oh yes, it does seem to get worse as the engine warms from a cold start. Any suggestions would be appreciated.

Thanks,
Jon

A. First thing I would do is check the PCM for any DTCs that may be stored. If there are DTCs present, they should be addressed as required. For information on how to access the DTCs and what they are, see Diagnostic Trouble Codes.

If there aren't any DTCs stored in the PCM there are a few things to check for. I would check the Coolant Temperature Sensor and make sure it is reading accurately. If it's reading higher than the actual engine temperature, the PCM will lean out the fuel mixture and cause the problem you describe. A scan tool is best for this. You can compare the temperature it is reading to the ambient air temperature and they should be within 3° or 4° of each other.

Another thing to check is the Throttle Position Sensor (TPS). This can be tested with an analog, not digital, ohmmeter. There should be a smooth rise in the reading, any jerks or drops indicates the TPS is bad and needs to be replaced. The TPS is adjustable. Here is how to check and adjust the TPS.

• Engines with EEC III Turn ignition to RUN position and remove vacuum hose from throttle kicker, as equipped. With engine not running, compare voltage reading against specifications and adjust as needed.
• Engines with EEC IV Connect positive probe of DVOM along terminal C (lower) of TPS and connect negative probe along terminal A (upper). Turn ignition on but do not start vehicle. Adjust TPS to specified value.

A fuel pressure and volume test should be done to be sure the fuel pump is delivering the right amount of fuel and pressure to the engine. An injector cleaning wouldn't be a bad idea either.

Cleaning the throttle body has helped a number of problems like this as well. DIY: Cleaning Throttle Bodies

Another thing to keep in mind is this engine has a lot of miles on it. A wet/dry compression test will tell you what kind of shape the engine is in. Low compression will cause the engine to stumble and hesitate as well. Also I would check the timing chain for stretch and if it's tight, try bumping the timing up a couple of degrees. This will compensate a bit for the older engine.