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Poor running Toyota Tacoma

Hi there. I have a 1999 Toyota Tacoma 4X4 pickup that is running terrible at any RPM under 1800. It is also bad under acceleration at any RPM, or when the a/c is on. The truck is showing no problem codes at all. I have been throwing parts at it with no success. I have replaced in order - the air cleaner, fuel filter, fuel pressure regulator, fuel pump, and even the timing belt. Can any one help me with this?

How many miles are on it? How old are the spark plugs & wires? If it has a distributor, how about cap & rotor?

You referred to air & fuel filters as “throwing parts.” Those aren’t parts that you replace to “repair” a problem. They’re basic, routine maintenance items that you replace before you ever have problems. So some more info about your approach to maintenance would help.

New fuel system parts are fine, but did you ever put an actual fuel pressure gauge on it?

Check your coolant temp sensor.

Cigroller’s philosophy of actually checking the fuel pressure is the philosophy I’d recommend. Before changing any other parts, try to find out what’s malfunctioning. Get a Haynes repair manual at the local bookstore and begin testing the fuel pump and regulator as well as the temp sensor Cig mentioned and the signals from the other sensors, as well as checking to see if you have reliable spark from each cylinder. A shop would, if the cause were not readily determinable, probably put the ignition system on an oscilloscope to see erratic or unreliable traces in the spark pulses. If the spark seemed good, they’d know that the ignition components (igniter, crank sensor, coil(s), etc.) were probably good. They’d probably want to test the cylinder compression as well. Lastly, they’d probably check the vacuum and check to see that there was no induction system leaks and that the exhaust system wasn’t restricted.

In short, they check the spark, check the fuel delivery, check for compression, check for vacuum leaks, and check to see that the engine can breath out. And the vacuum tester can check for problems with valves as well.

You can if you wish pay a hundred or two, let a good shop diagnose it, and do the repairs yourself to save money. Between the cost of replacing unnecessary parts and of test equipment, you’ll probably end up spending far more than that before you stumble on the correct part. It’ll likely be worth the few hundred bucks in the end.