The RenX Files: High idle problems

[joshv98xj]

There should be two on the rear firewall, the one old_man describes below is on the drivers side at the rear of the head on the engine block. The other is a continuation of the battery ground from the battery to the engine block near the dipstick, it should continue up and over to the firewall from near the dip stick on the engine block, passenger side. Is it missing???? Are they missing????

Okay, here's what I got. First off, here's the battery cable I replaced!

Secondly, I don't really know if I'm missing anything, so I'll tell you what I've got. At the ground by the dipstick, there is the HD wire coming from the battery, and there are two small wires coming from the harness, I didn't pull the harness apart to see where they go. The wire from the battery dead ends at the ground bolt. I didn't know the big braided wire on the driver's side was a ground, I guess because it doesn't go anywhere- just from the head bolt to the firewall, nothing else. I found one small ground under the coolant resivoir, and that's about it.

I didn't get a chance to clean that ground or the braided one just yet. Here's a few pics of what I've got, let me know if you see something wrong or missing.

How do I begin to diagnose or trace a vacuum leak problem? Is there any method to it?

 

[89xj]

most obviuos way to check for a vacuum leak is that you can hear it. other way is to trace your lines and look for crackes and spray wd40 on the lines and listen for increase in rpm.

also, when you found loose manifold bolts and tightened them, did you look for missing ones. i know on my 89 i had a few loose and was completely missing one.

oh, where are you located near philly? im in delaware county. could lend a hand if your close.

 

[falcon556]

Posted in eror

 

[falcon556]

most obviuos way to check for a vacuum leak is that you can hear it. other way is to trace your lines and look for crackes and spray wd40 on the lines and listen for increase in rpm.

also, when you found loose manifold bolts and tightened them, did you look for missing ones. i know on my 89 i had a few loose and was completely missing one.

oh, where are you located near philly? im in delaware county. could lend a hand if your close.

Those manifold gaskets don't last for ever.
They are cheap and easy to replace. I wonder how much difference a new gasket will make.

 

[joshv98xj]

When I sprayed the carb cleaner around the manifolds, I didn't notice much of an rpm change, so I think the manifolds are good. I'll have to double check the bolts when the engine is cold, lately I've been trying to work on it while it is hot. I'm in Montgomery county, just off of 276, near 611 (turns into Broad St. in the city). I'd love a hand, but right now my time is limited. I'm leaving in a week to go to upstate NY to work at a camp for the summer. I'm trying to iron out some of these idle issues before I drive it up there, but my wife and I also have to pack up and move out of our apartment, so there is a lot going on right now. I've got free time during the day on Thurs and Friday, as my last day of work was today. If any of that works for you, let me know-

 

[Ecomike]

Two problems I see. One you still need to clean both ends of that braided wire, the flat surfaces on the ends of the braided wire, and the flat surfaces on the block and firewall that they mate to with sandpaper or scotch brite.

Second, you missing half of the battery ground wire. The OEM wire runs from the battery to the engine (like yours) but then contunes on with the same huge wire to the fire wall. So you are missing that huge ground wire from the battery to the firewall that runs across the engine block first.

Those may be your only 2 problems! Somebody took a short cut (before you) in replacing the battery to engine to firewall main ground cable.

Lastly, as I recall there were 3 or 4 extra small ground wire connections on mine next to the dip stick beside the one huge one from the battery. Don't know where they all went as they disapear into the wiring harness bundle on mine. But there was more than just 2 more there.

 

[8Mud]

I read back through your post again. It sounds like what my 88 was doing. Turned out to be the TPS. When I unplugged the TPS, the idle went down. Replaced the TPS and the idle stayed down. My high idle problems started after an engine wash.
I added another ground from the battery ground to the front clip and another from the dipstick to the firewall. You really can't have too many grounds. Way not have a little redundancy.
A quick way to test the ground circuit for your TPS, is to leave the connector connected, cut a small slit in the insulation on the ground wire, TPS side of the connector. Turn the key to run and measure with a volt meter from the TPS ground to chassis ground. It's called standing voltage and tells you roughly how much resistance there is in the circuit and how much affect it is having on the voltage your trying to push. Disconnect the plug and measure the ground wire to chassis ground with an ohm meter, there will typically be a small resistance built into the circuit.
I've tested every wire from every sensor, from the sensor to the ECU or TCU and then to ground in my XJ. I've found numerous choke points.
Sometimes it's a faulty sensor, sometimes poor contact in a connector, sometimes a little of both.
I've actually found wires fatigued all the way through, under the insulation where you can't see it, at the firewall end of the injector harness. The O2 sensor harness rubbing on the exhaust manifold, pins pushed partially out of connectors, corroded connectors and corroded ground rings. Some of the ground rings aren't soldered to the wire and can corrode and cause resistance. CPS wires cooking on the exhaust manifold. I usually swap out a sensor after checking the circuit, in my experience about half the time they circuit is faulty or has enough resistance to affect sensor values anyway.
Another little trick, cut a little slit into the insulation on your TPS wires near the connector, run the wires over the top of the TB to air box rubber elbow. You have them up high where they are easy to check. I don't even bother sealing the slit up anymore, never causes me any problems. Sure makes testing easy.

 

[joshv98xj]

Second, you missing half of the battery ground wire. The OEM wire runs from the battery to the engine (like yours) but then contunes on with the same huge wire to the fire wall. So you are missing that huge ground wire from the battery to the firewall that runs across the engine block first.

Does anyone have a picture of what this is supposed to look like? If not, could someone snap one and post it for me? I don't have access to any that show me how it's supposed to be- all I know is that mine is incorrect.....

 

[Ecomike]

Does anyone have a picture of what this is supposed to look like? If not, could someone snap one and post it for me? I don't have access to any that show me how it's supposed to be- all I know is that mine is incorrect.....

Well I just went and took photos of mine and discovered I was wrong (at least about mine having a continuous very large dia. ground cable running from the battery ground, to the block and then to the firewall. Mine stops at the engine block near the dipstick and only runs from there to the battery negative post. Now I am wondering where I got the idea it ran all the way to the firewall on the passenger side. I may have seen it in a photo or diagram somewhere else. Where is 5-90 when I need him, Help?

 

[Ecomike]

joshv98xj,

I was wrong (as I listed below) looks like all you still need to do is clean the flat contact surfaces on both ends of the silver braided ground wire on the passenger side. It is also a critical sensor ground for the Renix controls.

 

[Flash]

joshv98xj,

I was wrong (as I listed below) looks like all you still need to do is clean the flat contact surfaces on both ends of the silver braided ground wire on the passenger side. It is also a critical sensor ground for the Renix controls.

I would bet that that extra wire your talking about is in 5-90 Custom ground kit!!


Flash.

 

[joshv98xj]

Well, I'm at a loss for what to do. I cleaned the braided wire, no change. I checked the calibration of the TPS afterwards, still at .85. I sprayed carb cleaner on the majority of my vacuum connections- didn't find anything. What are the chances that my idle is just set wrong? Should I take it to my mechanic (whom I trust completely) and let him play with it? I ran the tests of the map MAP and TPS sensors, which showed they are working. I'm really all out of ideas. :dunno: :dunno: :dunno: :dunno: :dunno:

 

[langer1]

No one said they checked the coolant and Air temperature sensors did they, if it don't sense a warm engine the idle will be high.

The Gage uses a different sensor.

 

[Ecomike]

.85 for the TPS idle setting is still too high, needs to be between .80 and .82

I think langer 1 is right, I do not see where joshv98xj has tested the two ECU temperature sensors, namely the air intake manifold temperature sensor which is toward the firewall on the top of the air intake manifold and the coolant temperature sensor hiding under the two manifolds in the drivers side of the block. I think I got confused between this thread and 2 others this week working on similar idle problems.

If the coolant sensors are OK (you need to test them cold and hot with the ohm meter while disconnected), and once the TPS is set to .82 Volts at idle (the difference between .82 and .85 volts is several hundred RPM!), and once you are convinced the grounds are good, if you still have a high idle I would bit the bullet and replace the manifold gaskets (intake and exhaust) and the throttle body gasket.

You said you cleaned the braided ground wire, but did not say you cleaned the contact on each end, and the surfaces they make contact with (sorry for being an#l, but trying to be thourough at this point.)

Use the ohm meter to test between the engine block and the battery ground, engine block and clean bare spot on the firewall, and the firewall and the battery ground all with the power turned off. They should all read less than 1 ohm. If one or more reads higher than 1 ohm you still have a ground problem. There are three temperature sensors, the one on the rear drivers side threaded into the engine head itself is just for the gauge. The other two report to the computer, ECU, they should read 3,400 ohhms at 70 F, 7,500 ohms at 40 F, 1,600 ohms at 100 F, 450 ohms at 160 F, 185 ohms at 212 F.

You can eliminate the IAC as a major issue by disconnecting it at cold start up at a lower idle. With the IAC disconnected the idle should only increase by 200 to 300 rpm as the engine warms up under normal circumstances (based on tests I ran with a working, normal system). With IAC disconnected the ECU can not adjust it, so this helps isolate the cause of the increasing idle. If it runs up by 800 rpm from say 700 to 1500 rpm as it warms up that leaves ground issues, vacuum leaks that are changing rates with temperature increase, or a bad temperature sensor in the air intake manifold ot the coolant temp sensor in the block.

 

[falcon556]

vacuum leaks that are changing rates with temperature increase, or a bad temperature sensor in the air intake manifold ot the coolant temp sensor in the block.

I still think that the vacuum issue which is easy to check has not been completely explored. Spraying carb cleaner tells a lot but not eveything. If the leak is further down the line, you will never find it.
I think it is time to plug at the sourse. Remove the line at the manifold and plug it.
For example a leaking reservoir cannot be detected by spraying. An internal leak at the cruise servo, motors for the heater etc.

 

[joshv98xj]

Question- is the TPS setting different for a manual than it is for an auto? I Chilton's doesn't differentiate, but I read another post that says it's .82 for auto, and .17 for manual?

 

[8Mud]

Question- is the TPS setting different for a manual than it is for an auto? I Chilton's doesn't differentiate, but I read another post that says it's .82 for auto, and .17 for manual?

The TPS is two parts for the auto, actually two TPS's sandwhiched together. The tranny half (front/outer) starts at 82% of input voltage and drops as the throttle is applied. The ECU (computer half) starts at 18% of input voltage and rises as the throttle is applied. For the Renix.
There is a little flexibility in the adjustments, a little plus and minus doesn't seem to hurt things much.
But to make things interesting the base voltage for the TCU half of the TPS originates at the TCU and the ECU half of the TPS base voltage originates at the ECU and they can sometimes be fairly different. Making it highly unlikely to ever get a true 82% 18% ratio on either and have your motor run correctly or the tranny shift properly.
Unequal grounds can also cause much the same problem. There are two ground rings on the dipstick holder. One for the TCU and one for the ECU. I've noticed more than a few XJ's, where the ground ring for the TCU, is only crimped and not soldered and fairly corroded.

 

[joshv98xj]

I've been following this post, trying to diagnose the idle problem on my MJ, an '88, 4.0, 5spd. Have I set the TPS wrong if I've set it to approx .82 with my multimeter? Should I instead be at around .18 since it's a 5spd?

 

[Ecomike]

Question- is the TPS setting different for a manual than it is for an auto? I Chilton's doesn't differentiate, but I read another post that says it's .82 for auto, and .17 for manual?

I would say the posted .17 is wrong, maybe a typo. Where is it posted?

My Renix, 87 FSM does not list a different TPS voltage setting for Manual transmissions, but since there is no second half of the TPS on manual transmission models the front half is the only part to calibrate. I have seen a variation from 0.80 to 0.82 in different year FSMs, varies over the years, for the TPS Idle setting on the front half regardless of the transmission, for 4.0 engines.

Maybe the .17 was for a different engine?

 

[Ecomike]

The TPS is two parts for the auto, actually two TPS's sandwhiched together. The tranny half (front/outer) starts at 82% of input voltage and drops as the throttle is applied. The ECU (computer half) starts at 18% of input voltage and rises as the throttle is applied. For the Renix.
There is a little flexibility in the adjustments, a little plus and minus doesn't seem to hurt things much.
But to make things interesting the base voltage for the TCU half of the TPS originates at the TCU and the ECU half of the TPS base voltage originates at the ECU and they can sometimes be fairly different. Making it highly unlikely to ever get a true 82% 18% ratio on either and have your motor run correctly or the tranny shift properly.
Unequal grounds can also cause much the same problem. There are two ground rings on the dipstick holder. One for the TCU and one for the ECU. I've noticed more than a few XJ's, where the ground ring for the TCU, is only crimped and not soldered and fairly corroded.

I must agree, "Very Interesting" indeed!

I wonder if a precision resistor in one side to correct the imbalance would be worth investigating?

I have always just set the ECU side and pretty much ignored the TCU side. I am starting to wonder if that was a mistake. I am still getting less mileage than I think I should (10-12 local, maybe 15 to 16 highway), and have to get my speed up to 42 to 43 mph to get the last shift (4th or clutch lock up, not sure which it is yet, I am thinking 4th). If I go from zero to 35 and cruise at 35 mph I am still at 1800 rpm, if I go to 42 mph and drop back to 35 mph cruising the RPM is at 1100 to 1200 rpm.