90 Renix stumbles during low speed acceleration

Been reading threads on stumbling Renixes for days. None seem exactly the same but have been trying other's solutions to no avail.

I have a 90 Renix with 140k that runs great other than an irritating tendency to stumble and even die when punching the accelerator from a start or slow roll. An example would be slowing down for a speed bump and then hitting the gas to move on. She stumbles for about 2 seconds and then finds new life and accelerates normally. It does not do this through the other gears. Only when slow or from a stop. It's especially bad when in reverse and you're tapping the gas to achieve a short burst of power. It will often die in that scenario. Very lightly pressing the accelerator will not do it. Only when giving normal or excessive acceleration.

So far I've:
checked all the vacuum lines
new air filter and cleaned air box
cleaned throttle body
cleaned Idle Air Control
replaced Map Sensor
replaced O2 sensor

None of these solutions have had an affect. My next thought was to replace the fuel filter and possibly the pump.

There is an air hose from the air box to the block that looks bad to me. I notice that it deflates under acceleration. Can anyone tell me what that is for and if it could be contributing to my problem?

Check the large orange wire to the O2 sensor. It needs to have 12 volts when running. If it is good, then I would really suspect the TPS is bad, or has a bad ground!!!

Are you sure the vacuum line to the MAP sensor is sealed tight, no leak.

I doubt it is the fuel pump....It is bad data, slow data to the ECU, and thus late fuel injection additions. Usually the TPS, O2 or MAP data, so the sensors, wires, relays (O2 heater relay), or the ECU. I doubt yours is the ECU.


That hose is for cold morning warm up I think. It should NOT be drawing air there (or collapsing) except at a very cold first start up.

RENIX TPS ADJUSTMENT
Before attempting to adjust your TPS be sure the throttle body has been recently cleaned. It's especially important that the edges of the throttle butterfly are free of any carbon build-up.
IMPORTANT NOTE: With the Key OFF, and using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector of the TPS. The letters are embossed on the connector itself. Touch the black lead of your meter to the negative battery post. Wiggle the wiring harness where it parallels the valve cover and also over near the MAP sensor on the firewall. If you see more than 1 ohm of resistance, or fluctuation in your ohms reading, some modifications to the sensor ground harness will be necessary. The harness repair must be performed before proceeding. I can provide an instruction sheet for that if needed.
MANUAL TRANSMISSION:
RENIX manual transmission equipped XJs have only a flat three-wire TPS mounted on the throttle body and it provides data input to the ECU. It has three wires in the connector and they're clearly embossed with the letters A, B, and C. Wire "A" is positive. Wire "B" is ground. Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors. Note the voltage reading--this is your REFERENCE voltage. Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your OUTPUT voltage. Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For example: 4.82 volts X .17=.82 volts. Loosen both T-20 Torx screws attaching the TPS to the throttle body and rotate the TPS until you
have achieved your desired output voltage. Tighten the screws carefully while watching to see that your output voltage remains where it is supposed to be. If you can't achieve the correct output voltage, replace the TPS and start over.
Sometimes, after adjusting your TPS the way outlined above, you may experience a high idle upon starting. If that happens, shut the engine off and reconnect your probes to B and C. Start the engine and while watching your meter, turn the TPS clockwise until the idle drops to normal and then rotate it back counterclockwise to your desired output voltage.
AUTOMATIC TRANSMISSION: RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a flat three-wire connector, same as the manual transmission vehicles have, and it is tested the same as the manual transmission equipped vehicles, as outlined above—FOR ALL ENGINE MANAGEMENT RELATED ISSUES.
However, the automatic TPS also has a square four-wire connector, clearly embossed with the letters A,B,C, and D. It only uses three wires and provides information to the Transmission Control Module. THIS SQUARE FOUR WIRE CONNECTOR IS USED FOR TRANSMISSION/SHIFTING RELATED ISSUES ONLY. Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your REFERENCE voltage. 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. For example 4.8 volts X .83=3.98 volts. 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.
FOR AUTOMATIC TRANSMISSION RELATED ISSUES: Check the square four-wire connector side of the TPS.
If you have ENGINE ISSUES check the flat 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.
Revised 09-22-2012

rmorton: Here's a link to 25 mostly REnix write-ups I've done.

#s 1, 3, 4, 5, and 11 would be good for you to do along with the previously mentioned TPS adjustment. Maybe do them before the TPS.
http://www.cherokeeforum.com/f51/cruiser54s-mostly-renix-tips-153657/

rmorton said:

Been reading threads on stumbling Renixes for days. None seem exactly the same but have been trying other's solutions to no avail.

I have a 90 Renix with 140k that runs great other than an irritating tendency to stumble and even die when punching the accelerator from a start or slow roll. An example would be slowing down for a speed bump and then hitting the gas to move on. She stumbles for about 2 seconds and then finds new life and accelerates normally. It does not do this through the other gears. Only when slow or from a stop. It's especially bad when in reverse and you're tapping the gas to achieve a short burst of power. It will often die in that scenario. Very lightly pressing the accelerator will not do it. Only when giving normal or excessive acceleration.

So far I've:
checked all the vacuum lines
new air filter and cleaned air box
cleaned throttle body
cleaned Idle Air Control
replaced Map Sensor
replaced O2 sensor

None of these solutions have had an affect. My next thought was to replace the fuel filter and possibly the pump.

There is an air hose from the air box to the block that looks bad to me. I notice that it deflates under acceleration. Can anyone tell me what that is for and if it could be contributing to my problem?

Click to expand...

That hose is hooked to a diverter (valve) that selects either cold air coming from the grill area or warmed air from around the exhause manifold (to prevent icing). If it is warm weather, then you have a problem. You should only be getting warmed air if it is around 32 degrees. I can't remember if it is vacuum controlled or electrical. I have been running a snorkle for 10 years and that removes that valve. Just pull off the hose until you can get a new one. If the problem goes away (probably not) then you found your problem. It will run just fine without the hose.

That hose is in poor shape. That wire coil is there to provide support which yours looks shot. I recently pulled a backup from the local PnP.

Given the symptoms, I'd check the fuel pressure regulator. You can connect a low-side A/C gauge to the fitting on the fuel rail. The hose must have a depressor in the center of the swivel fitting, to depress the Schrader valve in the fuel rail fitting. Run the engine and check the fuel pressure. Normal pressure at idle is 31 psi. Pull the vacuum line off the fuel pressure regulator and check the pressure. It should be around 39 psi. If either pressure is low, pinch the return line and check the pressure. It must rise above 39 psi. Less than that, change the pump and filters. If the max pressure with the return line blocked is over 39 psi and the regulated pressures are less than 31 and 39 respectively, try a new fuel pressure regulator.

Thanks for all the input here guys. I've got my list to run down tomorrow. I'll post what I find.

Since you've got a Renix and they've got some quirky issues anyway, I would suggest doing #1 through #5 of the 25 write-ups in this link.

http://www.cherokeeforum.com/f51/cruiser54s-mostly-renix-tips-153657/

Ok, so far I have:

Disconnected warm air bypass hose
cleaned all the grounds
cleaned throttlebody again
cleaned IAC

No dice so far.

Now I'm in the process of adjusting TPS. Mine is currently at 15.5% ( 4.92 reference voltage / .75 Output voltage). Is that out enough to try to tweak? If so, I don't see how the TPS will rotate once the mounting screws are loosened. They appear to pass through holes in the TPS and into the throttlebody.

Any clarification on that?

TPS just rotates with the screws loose. It has slots under those washers on the bolts.

So 15.5% is far enough out to justify moving it to achieve 17%? Does this sound like it could be contributing to my problem?

rmorton said:

So 15.5% is far enough out to justify moving it to achieve 17%? Does this sound like it could be contributing to my problem?

Click to expand...

One way to find out. Adjust it properly and see what the results are.

rmorton said:

So 15.5% is far enough out to justify moving it to achieve 17%? Does this sound like it could be contributing to my problem?

Click to expand...

Yes. But the the guts of the TPS wear, the contacts get worn, noisy, and that can also cause the problem.

Ecomike said:

Yes. But the the guts of the TPS wear, the contacts get worn, noisy, and that can also cause the problem.

Click to expand...

Correct. Try adjusting it.

I got the TPS adjusted to EXACTLY 17%. (4.92 / .836). Wasn't that hard to adjust. However there is no noticeable difference. Still runs great down the road but stumbles upon acceleration from a stop.

At this point, should I start throwing parts at it? The IAC looked ok, but is there any way to effectively test it without replacing? The only thing I haven't checked yet is the fuel pressure. I don't have a gauge...

rmorton said:

I got the TPS adjusted to EXACTLY 17%. (4.92 / .836). Wasn't that hard to adjust. However there is no noticeable difference. Still runs great down the road but stumbles upon acceleration from a stop.

At this point, should I start throwing parts at it? The IAC looked ok, but is there any way to effectively test it without replacing? The only thing I haven't checked yet is the fuel pressure. I don't have a gauge...

Click to expand...

IAC has nothing to do with a stumble upon acceleration. All it ever does is set the idle RPM.

It is far more likely to be a bad TPS. The ECU needs instant accurate data on acceleration from the TPS, MAP sensor and the O2 sensor only to avoid the stumble. It needs good working fuel injectors and a good working fuel pressure regulator that delivers the right amount of fuel on acceleration.

The most common source of your problem is the first the TPS (Bad), second a vacuum line leak on the line to the MAP sensor, third a bad heater or loss of power to the heater (bad relay or wires) inside the O2 sensor.

1 and 3 can be verified with a volt meter!!! #1 requires a good analog volt meter!!! #2 requires a good visual inspection!!!

Read this!!

http://www.naxja.org/forum/showthread.php?t=1012701&highlight=oxygen+sensor+test

and this:

http://www.naxja.org/forum/showthread.php?t=905849