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90 XJ coasting issue.

9

90xj06

NAXJA Forum User
Location
Peabody, MA
well i have a 90 XJ and it runs great.

when im on the highway and doing about 55-65 and i let off the gas it goes into (i think its deceleration mode but i cant find my renix papers at this time) and then it will come back out. so the rpms go down and back up alot. you can here it better. now i will put that some times when i start it it will rev up to 1500rpm then back down to normal. so i think this may be the tps sensor.
i have replaced it and i tried to set it to the voltage setting that it calls for. but that will leave it reving up at 2000-2500 rpms. so i set it so it runs good and shifts alright.

sorry for the long post. and also the ecu is it located above the drivers right knee?

thanks for the help.
 
My 90 does something similar, when I let off the gas it will coast but when I accelerate back, even light throttle, it will down shift then quickly shift back into OD. Adjusting the TPS has helped the situation but it will still occasionally do this.

I would say adjust the TPS
 
whats what i thought. im going to go and fix all the voltage drops also. so i just need to know where the ecu is. i haven't used my xj for about 7 months so forget every thing that i was doing to it.
 
Digging up this thread...

Ive struggled with this issue for a long time on my 90.. even with the Stroker i built. From what i can tell its caused by the transition between Decel and non-Decel modes.

Upon Decel, you will be lean because the computer cuts off the Injectors so as you decel or "coast" the injectors will stay off until you reach a certain RPM somewhere around 1000. Once you reach that RPM, the injectors are turned on again. This is where i am assuming the problem lies. Once the Injectors start to spit fuel unto the combustion chamber, the RPM's rise a bit thus making the RPM's higher. When the RPM's go higher the ECU turns the injectors off again until the RPM's drop down.

This cycle continues and thats what make the reving happen...

Does everyone agree with this conclusion on why it happens? Has anyone fixed the issue or has anyone seen similar results and just dealt with it?
 
1987 XJ 4.0, AW-4 NP 231


Mine was doing that: let off at 2500 RPM, it would go down some, back up,
down more. Up and down as the RPM dropped to 1000 or so.

I had broken the vacuum line from the MAP at the TB, and drilled out the grommet to take the full hose(the original hose had a small nipple on the endvgoing into the TB. I slathered RTV all around the hose.

This was ok for about six weeks then the first symptom was the fluctuating
RPM. I didn't really explore this until the JEEP started stalling and running
poorly.

My half-assed connection was leaking. I tapped into another vacuum source with better fittings, and today had no RPM fluctuations and perfect idle. So in my case, it was a vacuum leak in the MAP line. And it wasn't a big one, even when it started stalling.
 
Muad'Dib said:
Digging up this thread...

Ive struggled with this issue for a long time on my 90.. even with the Stroker i built. From what i can tell its caused by the transition between Decel and non-Decel modes.

Upon Decel, you will be lean because the computer cuts off the Injectors so as you decel or "coast" the injectors will stay off until you reach a certain RPM somewhere around 1000. Once you reach that RPM, the injectors are turned on again. This is where i am assuming the problem lies. Once the Injectors start to spit fuel unto the combustion chamber, the RPM's rise a bit thus making the RPM's higher. When the RPM's go higher the ECU turns the injectors off again until the RPM's drop down. I have an idiot light on the TC lock power and it is confirmed by engine rpms!

This cycle continues and that's what make the revving happen...

Does everyone agree with this conclusion on why it happens? Has anyone fixed the issue or has anyone seen similar results and just dealt with it?
Click to expand...

I don't have this problem on either the 87 or the 89. Also, the 87 now has a working locking Torque Converter, and correctly set TPS,....good grounds, vacuum lines..etc, and it stays locked until about 24 mph, then unlocks and the engine slows down steady to about 1100 rpm and sets there until I brake to a very low speed, maybe 5 mph (?), then drops to about 800 rpm. Also my TC stays locked even with the brake on until 24-25 mph!!!
 
Muad'Dib said:
Digging up this thread...

Ive struggled with this issue for a long time on my 90.. even with the Stroker i built. From what i can tell its caused by the transition between Decel and non-Decel modes.

Upon Decel, you will be lean because the computer cuts off the Injectors so as you decel or "coast" the injectors will stay off until you reach a certain RPM somewhere around 1000. Once you reach that RPM, the injectors are turned on again. This is where i am assuming the problem lies. Once the Injectors start to spit fuel unto the combustion chamber, the RPM's rise a bit thus making the RPM's higher. When the RPM's go higher the ECU turns the injectors off again until the RPM's drop down.

This cycle continues and thats what make the reving happen...

Does everyone agree with this conclusion on why it happens? Has anyone fixed the issue or has anyone seen similar results and just dealt with it?
Click to expand...

Both my 88 and 90 do this. I've been toying with the idea of plugging the TB vac source for the MAP and using the intake manifold as a source. GregGa may be on to something.
 
Maud, as far as your theory on ECU cycling the injectors, I'm not sure. I think I'll switch the vacuum source on the MAP for S&Gs.

OP-What method are you using to adjust your TPS? Don't use the square connector for driveability issues. That connector is for the trans. Use the flat 3 wire connector.


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 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 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. 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 flat 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 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. 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 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.
Revised 05-28-2012
 
Confirm by testing both sides, as one side of the TPS, either side, can be defective or out of balance with nthe other side, such that either side alone can be calibrated properly and unset the other side in the process. BOTH sides must be up to OEM spec!!!!! So once one side is calibrated, the other side must be tested to confirm that it is also calibrated. If it is not, a new TPS is needed!!!

cruiser54 said:
Maud, as far as your theory on ECU cycling the injectors, I'm not sure. I think I'll switch the vacuum source on the MAP for S&Gs.

OP-What method are you using to adjust your TPS? Don't use the square connector for driveability issues. That connector is for the trans. Use the flat 3 wire connector.


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 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 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. 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 flat 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 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. 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 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.
Revised 05-28-2012
Click to expand...
 
87 XJ 4.0 AW-4 NP-231

Cruiser54: I already had a "T" in a large line off the intake manifold. I used this tee to connect to an Autometer vacuum gauge I installed back in '92 or so. The line from the gauge went into a rubber boot that connected to the tee. No here's the interesting part(maybe:laugh3:): the orifice of the tee is TINY. I mean TINY. way smaller than the orifice on the vacuum line tubing.

Have you broken the nipple off the line to the TB, and using the full size tube in the grommet? I know when the line broke, I had to drill out the
broken piece, so it was also smaller than the diameter of the main tube.

I don't have a clue what effect this tiny little hole vs. standard diameter tube has. I don't ever recall the fluctuating RPMS prior to a couple of days ago when the leak developed. All I know it idles great now, until the next gremlin:).

Ecomike: pardon my ignorance, but could you describe how I know if I have "torque converter lockup." Heard the term, but never explored any
farther since the trans does everything it's supposed to do!

Thanks guys.

Greg
 
When I bought my 87 wagoneer in about 2004, it had a lot of gremlin issues :laugh3:

One of the last ones to get fixed was the AW4 shifting and TC (Torque Converter) lock up. I was having trouble decided what was TC lock up and what was 3rd or 4th gear shifts, because my TC lockup was only partial. I would get a drop in RPMs, but never the real total lock where the RPM locks up between the engine and rear tires. So I installed a red LED on the dash and pulled the 12 volts from the wire that runs to the #3 solenoid that locks up the TC. Last year the AW4 finally needed a rebuild at 273,000 miles. I used another TC, and this one locks up noticeably. The red LED is just confirmation of when it locks and unlocks now. I can see it visually in a split second as the light flashes off then on, unlock, shift and relock for about 1 second tops when it shifts from 3-4 and 4-3 now!!!!!:woohoo:
 
I originally thought this might be a TPS or vacuum issue also.. but i have confirmed 1000x's that its not either of those in my case. I even went as far as ordering a $150 Mopar (OEM) TPS from the dealer (No Change). The only other thing i can think of besides my ECU theory is TC lockup.. but i wouldnt even know where to begin to start tracking that one down.
FWIW, my vacuum line to my MAP Sensor is to my Intake Manifold (99+) and i dont have a stock Throttle body anymore. Interestingly enough, the problem in my case was present before and after a complete Stroker build. The Stroker build included a 99+ Intake manifold and a Dodge Dakota TB. So in my case it really seems to be ECU programming or something with the Transmission.
 
Let us know what you discover. These jeeps are an enigma. I fixed my vacuum leak, now my hot start idle flare has reappeared after a LONG time.
Go figure:helpme:
 
Muad'Dib said:
I originally thought this might be a TPS or vacuum issue also.. but i have confirmed 1000x's that its not either of those in my case. I even went as far as ordering a $150 Mopar (OEM) TPS from the dealer (No Change). The only other thing i can think of besides my ECU theory is TC lockup.. but i wouldnt even know where to begin to start tracking that one down.
FWIW, my vacuum line to my MAP Sensor is to my Intake Manifold (99+) and i dont have a stock Throttle body anymore. Interestingly enough, the problem in my case was present before and after a complete Stroker build. The Stroker build included a 99+ Intake manifold and a Dodge Dakota TB. So in my case it really seems to be ECU programming or something with the Transmission.
Click to expand...

AW4 valve body issue, seal leaks in the piston-valve body, oscillating or Solenoid #3, some combo doing an od switching cycle? My AW4 had no reverse on cold morning for 4 years, but worked once the fluid warmed up to 70 F, then year 5 I lost all gears. Also I had the odd TC behavior I mentioned below all those years. We think the seals on the near 300,000 mile AW4 just wore out in the pistons-valve body area, and could not long longer hold full hydraulic pump pressure. The pan never had any metal or solids, or fibers all those years!!!
 
Note from DJ
A note of caution to cruiser54 or anybody else that is considering drilling and tapping there intake manifold. Somebody had a post on hear some time in the past regarding doing this procedure. I believe they even removed the throttle body and stuffed some rags in the intake manifold past the point of the drill bit entry, then stuck a shop vacuum hose in this cavity to suck up any shavings. In other words they took what could be considered normal precautions for doing this with the manifold still attached to the head. But at some point later on they were doing some other work on their engine and found aluminum shavings where they didn’t belong. So their advices was never do this unless you have the manifold off where you can clean it out thoroughly after you have drill and tap it.
 
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