jeepfan1969

I've had a problem with my XJ since I bought it. It won't rev past 2500 rpm. It seems to stumble like it's hitting a rev limiter. If you let off the gas, it smooths back out and starts running well again. It's had all new plugs, wires, cap, rotor, fuel filter, air cleaner. The check engine light isn't on. The garage hooked it up to a code reader and it showed no codes.

Another issue it has is it runs very rich. I don't know if it's related or coincidence or what.

It doesnt matter if it's in gear or not. It does it in all weather conditions and all positions (leaning up, down or sideways).

Any good ideas on how to fix this durn thing. I've been fighting with it for years now and it's getting a little old

mcrilly09

manual right?

jeepfan1969

Oops...missed that part. Automatic with the dash selector switch...power and economy, or something like that.

mcrilly09

my 1990 xj is automatic and shifts at 2000-2500 unless i put it on 3 for inclines does it do the same on inclines or while you are driving i am thinking it you might want to check your spark plugs and distributer cap because if it is problems with your camshaft or cylinders then you are in for a big problem

jeepfan1969

Thanks for the input.

It doesn't seem to matter what gear I'm in or whether it's going uphill or downhill. It does it in park and neutral too.

It's had new plugs and distributor cap recently and it hasn't had any effect on it.

tjwalker

About the first thing a shop tech. would test is fuel pressure. With a gauge.

It must be ruled in or out early in the troubleshooting process for this symptom.

For a 1990, you should have 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to approx. 39 psi.

If that checks out, I would test the "throttle position sensor". Failure of them is pretty common, especially on 87-90. Here is more.
-----------------------------------

RENIX TPS
(Thanks to Joe Peters from NAXJA for this information)

MANUAL TRANSMISSION:

RENIX manual transmission equipped XJs have a three-wire TPS mounted on the throttle body. This manual transmission vehicle TPS provides data input to the ECU. The manual transmission TPS has three wires in the connector. Wire "A" is positive. Wire "D" is ground. Remove the three-wire connector from the TPS. Key ON, measure voltage from "A" positive to "D" ground. Note the voltage reading--this is your REFERENCE voltage. Now, reconnect the three-wire connector to the TPS. Key ON, back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT voltage. Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. Adjust the TPS until you have achieved this percentage, if you can't achieve the correct output voltage replace the TPS and start over.

AUTOMATIC TRANSMISSION:

RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a three-wire connector, same as the manual transmission vehicles have, and it is tested the same as the manual transmission equipped vehicles--FOR ENGINE MANAGEMENT RELATED ISSUES. However, the automatic TPS also has a four-wire connector. For the automatic transmission equipped vehicles the four-wire connector provides data to the TCU. Disconnect the four-wire connector. Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your REFERENCE voltage. Now, reconnect the four-wire connector to the TPS. Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage. Adjust the TPS until you have achieved this percentage, if you can't replace the TPS and start over.

So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and the other side feeds the TCU. If you have TRANSMISSION issues check the four-wire connector side of the TPS. If you have ENGINE issues check the three-wire connector side of the TPS.

For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.

jeepfan1969

Great info. Thanks TJ.

Next time I get it into the garage I'll check those items.

LastSonofKrypton

My XJ also usually shifts around 2100-2500 RPM. I used to think she just didn't like revving higher and then I floored her the other day and she went up to 4000 (and boy did she fly!). It's never been a problem she just revs "low". Does wonders for my gas mileage!