Chasing my tail

My 1990 4.0 is losing power about 1/3 into the pedel. I posted it up and was told to check my TPS. Did the Neg D terminal to Positive A terminal had 4.73 volts. Hooked up positive to B had 4.18 volts. Pretty close to specs. 5.0 and 4.2 repectively are what is called for. Checked for Vacuum leaks (BTW I am not a mechanic.) couldn't find any. After I hooked everything back up the XJ seemed to idle a little better. Was wandering between 450-750 before. But my unexplained power loss is still there. So from 1/3 to right before WOT real low power. When you put it to the floor tranny kicks down and away she goes.

ECOMIKE I checked 02 sensor and TPS sensor They are hooked up correctly.
So could it still be my TPS sensor. I was an idiot and did not mess with the throttle to see if the volts changed with it. Suggestions??????

First off, the side of the TPS your checking is for the tranny. The TPS is usually adjusted to the tranny half of the sensor in the book. The inside (the half of the TPS nearest the TB) is for the ECU (motor) and is tested just like a stick shift TPS. If I remember correctly it starts out at 0.8 volts (or 16% of input voltage) and the volts go up as the throttle is opened (just the opposite of the tranny/TCU side of the TPS). In my experience, the TPS can get a flat spot, like power dies and then comes back. Though it seems plausible the values could be off through the whole range or the top two thirds. Clean the connector up good. You may have to do an ohm test on the wiring between the TPS and the ECU, the harness goes through numerous connectors any of which could have excess resistance. I wouldn't attempt it without a good wiring diagram, the wire colors may change somewhere in the harness (at the connectors)
Sounds like what you may have going on is a fuel problem or timing (advance) problem. Fuel is controlled mostly by the 02 sensor. Or maybe low fuel pressure. Timing advance is mostly controlled by the TPS.
How is th idle? Ignition problems, like weak spark, will often give you a crappy idle and hurt you through the mid and high RPM ranges.
Check your air filter and keep the Cat in the back of your mind. Both can work OK at lower RPM's but can plug up the whole system at higher RPM's if they are partially restricted. Oil soaked air filters are pretty common.

brandon_750 said:

My 1990 4.0 is losing power about 1/3 into the pedel. I posted it up and was told to check my TPS. Did the Neg D terminal to Positive A terminal had 4.73 volts. Hooked up positive to B had 4.18 volts. Pretty close to specs. 5.0 and 4.2 repectively are what is called for. Checked for Vacuum leaks (BTW I am not a mechanic.) couldn't find any. After I hooked everything back up the XJ seemed to idle a little better. Was wandering between 450-750 before. But my unexplained power loss is still there. So from 1/3 to right before WOT real low power. When you put it to the floor tranny kicks down and away she goes.

ECOMIKE I checked 02 sensor and TPS sensor They are hooked up correctly.
So could it still be my TPS sensor. I was an idiot and did not mess with the throttle to see if the volts changed with it. Suggestions??????

Click to expand...

Brandon_750, although your problem sounds similar to mine there are some differences. I assume from your comments that you have an automatic tranny. You need to check the TPS volatages at a idle and at WOT, with ignition on, but with out the engine running (Do not start the engine, simply turn the ignition power to on but do not start).

Disconnect the ECU/TPS connector (3 pin connector) and you should have 5.0 volts from pin A to ground (pin B), if OK, then reconnect the TPS/ECU three pin connector, then test the voltage from pin B to C through the back side (push a needle probe in from the on the back side) of the 3 pin connector at the TPS connection to the ECU (not the 4 pin tranny connection). You should have 0.8 volts at idle (power on, engine off-not started).

Disconnect the ECU/TPS connector and you should have 5.0 volts from pin A to ground (pin B). Then if that is OK run the TPS slowly from idle to WOT while still testing the B to C connector (an analog ohm meter works better on this next step) and see if the voltage changes smoothly or if it jumps around. If it jumps around the TPS is bad. Your problem sounds like it may be at least partly a bad TPS. Also check clean and tighten any dirty or loose connections to the sensors (all of them) and check the firewall and engine ground to the battery negative terminal. They should all read less than 1 ohm. If higher you need to clean the ground wire connections to the battery too.

Mike McGinness
South Houston, TX

Remember that the TPS is just a variable resistor. It could read correctly at both ends of its range and still have a flaw in the middle. The only way to be completely sure it's doing its thing correctly is to hook up an analog voltmeter (or a digital with a quick-responding analog bar) and move it throughout its range with ignition on, or to disconnect it and use an analog ohmmeter to see if its resistance changes smoothly throughout its range.