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The RenX Files: High idle problems

ok so i checked the manifold bolts. all are tight. i unplugged the 02 sensor, same problem. my xj has the extra plug for the second connector so i didn't know if i should plug it in. i plugged in the flat 3 wire job. i understand what the other connector is for. so basically i'm still in the same place. i had a perfectly normal, good running truck, then i cleaned the tb and it is going crazy
 
mntllycrzy said:
ok so i checked the manifold bolts. all are tight. i unplugged the 02 sensor, same problem. my xj has the extra plug for the second connector so i didn't know if i should plug it in. i plugged in the flat 3 wire job. i understand what the other connector is for. so basically i'm still in the same place. i had a perfectly normal, good running truck, then i cleaned the tb and it is going crazy
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I have heard this story before, it was solved by replacing the throttle body with one from a junk yard. Cause was never really found, just some phantom gremlin hidden in the throttle body.

Could still be an intake manifold gasket leak.

Mine is running high again this week due to a bad brake Vacuum booster. :rattle:
 
i'm really starting to hate this truck. i don't have time to get to a junkyard and it ran perfect before hand. it's embarassing sitting at red lights with this thing runnin at 2k rpms
 
ok so here's a new development in the high idle issue. yesterday my daughters and i were driving down the highway. it was pretty warm here and they were complaining about it being hot in the heep. so when we came to the next redlight the idle was around 1800. i turned on my a/c and rolled up the windows and noticed that my idle was down to around 800. so i decided to test this. i turned the ac on and off several times while at a stop and the idle went up and down with the ac like something was telling it to go up or down. then this morning i stopped to get coffee on my way to work and it was idling high again when i stopped. i flipped the ac on just to see if it would come down and nothing, the idle stayed high.
 
mntllycrzy said:
ok so here's a new development in the high idle issue. yesterday my daughters and i were driving down the highway. it was pretty warm here and they were complaining about it being hot in the heep. so when we came to the next redlight the idle was around 1800. i turned on my a/c and rolled up the windows and noticed that my idle was down to around 800. so i decided to test this. i turned the ac on and off several times while at a stop and the idle went up and down with the ac like something was telling it to go up or down. then this morning i stopped to get coffee on my way to work and it was idling high again when i stopped. i flipped the ac on just to see if it would come down and nothing, the idle stayed high.
Click to expand...

That sounds like a ground, ground wire problem. Have you tested the back voltage on the ground wire at the TPS on the ECU side, engine running, between that ground wire and the battery negative post? Also check the ECU grounds at the ECU.

Loose connections, moisture, heat and vibration can make the ground quality change which the ECU sees as a high TPS voltage making the ECU thing you have your foot on the gas.
 
mntllycrzy said:
so i need to check from the tps connector to ground? and from the ecu to ground? should i make auxilary ground wires for them?
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Yes. I have run several new grounds on mine, so have most others here with Renix jeeps. Any ground wire that has more than 1 ohm from the test point to the battery negative post is a problem.
 
new here but old problem bought my mj in 1988 can anyone tell me how much vac should be at the map when i unplug the vac line from my map and let the idle come down and then plug it back in it will idle fine until i shut it down. it will also idle fine when i first start it cold but only until i shut it down.
 
We had this exact same problem with our '90 4.0 5sp. Replaced everything twice TPS, CPS, IAC, etc. etc. - still the same high idle.. 2000 RPM's+

The solution ended up being to replace the EGR valve and EGR valve selenoid. (sp?) There was a small tear in the EGR valve causing a vacum leak and as soon as that was replaced the high idle problem went away and never came back.

Hope this helps..

Jeepie
 
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Jeepie said:
We had this exact same problem with our '90 4.0 5sp. Replaced everything twice TPS, CPS, IAC, etc. etc. - still the same high idle.. 2000 RPM's+

The solution ended up being to replace the EGR valve and EGR valve selenoid. (sp?) There was a small tear in the EGR valve causing a vacum leak and as soon as that was replaced the high idle problem went away and never came back.

Hope this helps..

Jeepie
Click to expand...

That is one high idle cause (vacuum leak) most of us have failed to check on or even think of. Good point!!!!! It is also one that is easy to diagnose and test!

:clap:
 
Maybe someone pointed this out on some page previous to this post, but I found my high idle problem finally n my 88 XJ. The fuel pressure regulator on the front of the fuel rail to the injectors had began leaking fuel accross the internal diaphram and into the vacuum system. You can pull the vacuum plug on the fuel pressure regulator while it is running and see wet fuel (easy test). This was causing intermittent hard start, high idle when cold that flipping with the heater/ac would correct occationally, poor or erratic warm idle and a lack of overall performance. I swappedit out, all of those problems went away. Easy to test, $44 neihoff replacement from Kragen auto Parts and fairly easy to replace (torx bolts a little hard to get to underneath). I hope this helps others too, it really drove me crazy looking for it!!!
 
Oh yeah, I'm bringin this thread back.

So I've had some no-start and stall-out problems lately with my 87 4.0 and I've read about having to meticulously clean the grounds, but my engine bay is a jumble of wiring, so if someone has the spare time, can you take pictures of the ground locations in a renix, or describe them very specifically?

'Preciate it.

-Kev
 
Last edited by a moderator:
k.smith904 said:
Oh yeah, I'm bringin this thread back.

So I've had some no-start and stall-out problems lately with my 87 4.0 and I've read about having to meticulously clean the grounds, but my engine bay is a jumble of wiring, so if someone has the spare time, can you take pictures of the ground locations in a renix, or describe them very specifically?

'Preciate it.

-Kev
Click to expand...

no start and stall out is a sign of a bad cps. if not cps, fuel delivery(fuel pump, fuel filter...) or bad coil.

as for the grounds:
negative battery terminal to side of block
ground strap from head to fire wall
ground from main wiring harnes to drivers side fenderwall. on my 89 its right in front of windsheild washer fluid resivoir.
there are also grounds to the tps and map sensors.
 
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Today I went to check my tps and found no voltage in C and B terminals. I have 4.85v in A and B terminals. I hooked it up to a scan tool and it said for tps 0 volts. Could it be a bad tps?
 
88xjchief said:
Today I went to check my tps and found no voltage in C and B terminals. I have 4.85v in A and B terminals. I hooked it up to a scan tool and it said for tps 0 volts. Could it be a bad tps?
Click to expand...

Disconnect the TPS, retest the wires in the wiring harness to see if the short is in the wiring harness.

Sounds like A & B wires are shorted, but your statement is conflicted concerning the B terminal.
 
I SUSPECT SOME OF THE AFTERMARKET TPS UNITS HAVE THE WRONG VARISTER RESISTANCE VALUE. IF YOU TEST THEM USING AN OHM METER, YOU CAN SEE THE RESISTANCE CHANGE AS YOU MOVE THE LITTLE ARM, BUT IF THE TOTAL RESISTANCE IS WRONG ACROSS THE POT, THEN THE VOLTAGE THE COMPUTER SEES COULD BE HARD TO SET.
IN OTHER WORDS IT IS EASIER TO DIAL IN THE RIGHT SETTING ON A 100 OHM VARISTER THAN ON A 1000 OHM VARISTER ESPECIALLY IF THE FACTORY SPEC IS FOR A 100 OHM UNIT. OLD MAN RENIX IS SO PARTICULAR.

JUST AS LIKELY THOUGH IS THAT THE WIRING GOING TO THE CPU FROM THE TPS IS CORRODED SOMEWHERE. THIS WILL ALSO MAKE CALIBRATION NEAR TO IMPOSSIBLE.

IN THE FINAL ANALYSIS PITY THE POOR FOO WHO DRIVES A RENIX. AX ME HOW I KNOW...:D
 
I was plagued with the High Idle Curse again... and now i have a few questions.

This particular time, i couldnt get the idle to come down until i unpluged the TPS at the flat plug which i believe is to the ECU. Once unplugged, everything was back to normal. When i pluged it back in, the high idle was back.

This brought up some questions.. First of all, WTF is the ECU side of the TPS for then anyway? When unplugged i saw no difference in drivability of the Jeep compared to a normal working TPS adjusted correctly.
The reason i ask, is because i for a long time failed to believe that my TPS was the cause of a high idle. Reason being is because i swaped it out with about 5 different TPS's from the Junkyard. I figured there was no way that 5 TPS's caused the same problem in different ways. I am now about 98% convinced that all the TPS's that i have tried were either bad or failing. I have yet to try a new one. Is it possible that there is something wrong with my ECU?? Any Suggustions here would be appreciated.


On another note:

I have always read and heard that when adjusting the TPS, you want the output voltage to be approx 82-83 percent (it varies source to source) of the input voltage.
So if your input voltage is 4.65 volts, your output voltage should be adjusted to ~3.85 volts.
I have always followed this rule until i was reading my Factory Service Manual. In the FSM for a 90, it says to check the input voltage and it should be approx. 5volts. If it is, then adjust the output voltage to be 4.15 volts.
As you can see it doesnt say anything about relating your output voltage to your input voltage. It only says to make sure your input voltage is close to 5, and adjust your output to 4.15 period.
So last night when i changed out my TPS (yeah with another one of those 5 TPS's that i have laying around) i went ahead and adjusted it to 4.15 volts per my 90 FSM.
So far, i notice absoloutly no difference than before when i have adjusted the TPS to be 83% of the input.
 
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