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Questions and Answers

Hesitating Jimmy

Q. I have a 1990 full size 4WD Jimmy with a 350, TBI, auto transmission and mileage is 180,000. The problem I'm experiencing is that the motor is hesitating mainly in the lower range (at least that is where it is most noticeable) but seems to run fine on the high.

It is also is requiring that I have to make two attempts to start the vehicle. The hesitation is quite dangerous when trying to pull into traffic. I have replaced the plugs, cap, rotor, fuel filter, air filter is in good condition and I have had a flow test done on the catalytic converter, which showed no blockage.

I have not had the fuel pressure checked yet but will and am suspecting the fuel pump, fuel pressure regulator or possibly the injectors. The only modifications I have made are a TBI riser plate, which was to improve performance with quicker response, better gas mileage and a significant torque increase.

I also changed the thermostat to a 160 degree, which cut the normal operating temperature in half (makes the motor froggy all the time which the richer fuel mixture) and makes the motor run cooler when pulling my boat. I have no improvement on the low end but have had a noticeable improvement on the high end. Feels like the motor is starving for fuel on the low end. Any help would be appreciated.

Robert

A. There's one basic law when you do a modification to improve engine performance. If you improve high end performance, you lose low end performance. If you improve low end, you lose high end. So we should keep that in mind here. Also, the only thing that will significantly improve torque is the crankshaft. Torque is a function of physics and geometry.

As for the thermostat, the engine was designed to operate at a certain temperature. If you lower the operating temperature, you change the engine operating parameters and there will be driveability issues. If you do modifications like this you should upgrade the PROM in the computer to a performance PROM which is better able to handle any changes.

I would definitely have the fuel pressure checked, especially to see if it is holding when the engine is off. This would make it harder to start since the pump has to build up pressure. An engine with that kind of milage is getting tired. Tolerances and specifications change due to wear and it will simply not perform like new. And with 180,000 miles, I would not be surprised that the injectors need replacing instead of just cleaning.

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