william50

This message is about my 1990 Jeep 4.0 XJ


I have an intermittent throttle hesitation when I accelerate from a complete stop.

The hesitation sometimes can be severe enough to stall the engine.

It never happens when the engine is cold.

It is most severe when I am stopped for at least a minute before trying accelerate.

I've tested the fuel pump and it shows steady pressure while revving the motor.

I've also cleaned the ground connections at the dipstick.

The throttle body is spotless.


I've replaced:

TPS (this improved the shifting in comfort mode)
MAP sensor
O2 sensor (NTK)
Fuel injectors (4 hole style)
Spark Plugs (NGK copper core)
Ignition wires
Distributor cap/rotor

I'm stumped and looking for suggestions, tnx.

TrailerTrash

After you replaced the injectors did you reset the ECU?

william50

The 1990 Jeep XJ has the Renix ECU, it does not store codes so there is nothing to reset, tnx.

Nolan771

I am having a similar issue with my 1988 4.0 XJ. I have narrowed it down to an issue with the communication between the TPS sensor and the ECU, but don't know where to go from here. I've replaced the TPS twice with no luck, and the problem totally goes away when the engine side of the TPS is unplugged. I have resistance tested the wires leading from the TPS to the ECU and there are no issues there. Short of swapping out the ECU, I am at a loss. Let me know if you find anything else with your 1990!

cruiser54

Go to www.cruiser54.com and complete Tips 1 through 5.

cruiser54

Have you done Tip 5?

How did you adjust your TPS?

william50

Yes I did Tip 5. I am getting 0.3 ohms.

I adjusted the TPS per your instructions @ 17% of the reference voltage.

BTW, I also indexed the distributor, no change in the hesitation.

tjwalker

You mentioned that the fuel pressure was steady. What was the exact reading? Details matter with fuel pressure.

The fuel pressure for your 1990 Renix 4.0 should be 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to approx. 39 psi. Be sure that the vacuum line to the FPR is pulling GOOD vacuum and is not wet with fuel. If that line has gas in it, you have a ruptured fuel pressure regulator.

william50

Those are exactly the readings that I got. 31 @ idle and 39 with the regulator disconnected. No fuel smell from the vacuum line.

I did watch the pressure gauge when I revved the motor. Fuel pressure did blip down momentarily to around 29 just as I brought it off of idle. The motor had its usual dead spot and then quickly gained RPMs and fuel pressure went back to 31 and remained there.

jordan96xj

Because it does not happen when it is cold, you should do a quick test by disconnecting the upstream O2 sensor and see if that keeps it from happening. Your symptoms are consistent with a motor that is running lean/rich due to improper air intake or exhaust oxygen measurements.

william50

Fixed!

Even though I put in a new o2 sensor, jordan96xj put me on the right track.

I ended up checking voltages at the o2 sensor and determined that I had a bad 02 sensor heater relay.

Replaced the relay and all is good.

I must have been running in open loop condition?

cruiser54

That Jordan guy is good to have around.......