having issues with engine starting, then rough idle, and lurging during drive

[XJ Stryker]

We now ya have at least heard of it!


I don't need ya thinking i am nukin futs or anything. Just take my word for it. Hell maybe in a few months the High Idle will come back and prove me wrong but for the mean time I am happy that it is gone.

 

[Ecomike]

We now ya have at least heard of it!


I don't need ya thinking i am nukin futs or anything. Just take my word for it. Hell maybe in a few months the High Idle will come back and prove me wrong but for the mean time I am happy that it is gone.

How long, miles, and days, has it been since replacing the CPS fixed (or seems to have fixed) your high idle?

 

[XJ Stryker]

Hmmmm been since about June of last year. I stopped driving it after Nov. I just got it back out of the garage again today. But anyway this thread isn't bout my Heep.

So lets not over analyze the solution for my old Jeep. Yall have a tremendous amount of knowledge on Jeeps I was merely pointing out a solution that helped mine.

 

[Ecomike]

It's odd that I've been around these Jeeps since new as a service manager at a Jeep dealership for 13 years, currently own five of them, work on them, and have NEVER seen the CPS cause a high idle.

I have been around long enough to know that some weird sh*t happens in this world, so for now I will add it to my single case history file, the one with just 2 case histories we have on a failed injector firing transistor on the only 2 renix ECU failures 5-90 and I have seen or heard of, LOL.

 

[cruiser54]

Perhaps he's on to something Mike. Maybe the CPS generated too strong a signal and the ECU triggered the ICU to give out so much spark that the idle increased.

Maybe the ECU turned that extra strong CPS signal into super high fuel pressure along with a WOT signal from the TPS. He might be on to something. I'm tellin' ya!!!!

 

[Ecomike]

Perhaps he's on to something Mike. Maybe the CPS generated too strong a signal and the ECU triggered the ICU to give out so much spark that the idle increased.

Maybe the ECU turned that extra strong CPS signal into super high fuel pressure along with a WOT signal from the TPS. He might be on to something. I'm tellin' ya!!!!

LOL, I was thinking more along the lines of miss fires and O2 sensor seeing too much fuel as a result, then adding more air, then not getting missfires ( as good CPS signal returns) and then idle goes up as a result (as CPS signal strength to the ECU comes and goes in late stages of failure), thus causing a higher oscillation amplitude in the idle speed, thus a wandering idle from say 1000 to 1200 rpm, higher peak idle, which I have seen all too often in Renix jeeps. Another possibility is the timing of the firing periodically could be off due to poor CPS signal strength, resulting in ignition firing being late and or early, which could affect the completeness of a burn, late timing perhaps, incomplete combustion, setting off an O2 sensor cycle seeing rich and responding with more air or less fuel, where the O2 sensor and CPS set up a vicious cycle of idle speed swings and O2 rich to lean to rich swings, and over reaction leads to higher average idle.

Process control is a funny thing how it can wander up and down when you disturb the equilibrium with a little bad data. When it gets bad enough the entire control scheme collapses and your engine dies, floods, and or fails to restart.

Rereading what he said:

"Granted it did eventually lead to NO Start. But it started out with sputtering.

Uneven RPM

High Start RPM."

His symptoms do sound like a dying CPS. Mine sputtered, had a wandering idle just before it died for good.

So back our OP? he started with the same symptoms, but is now just fighting a high idle IIRC. High idle is most likely (50% prob) a vacuum leak, 35% Prob a bad sensor (MAT, CTS) or too much fuel, from a leaky FPR of injector, and lastly a bad MAP (<5% prob) based on frequency of those parts failing and causing the problem.

 

[Ecomike]

just drove it today for about 45 minutes, ran fine when warmed up no lurging when I let of gas petal abruptly, but still had a high idle as usual about 1100 rpms

What did you do that fixed the lurching & hard starting? New plug wires?

 

[steiny]

put new IAC in yesterday and drove to store, it turns over and fires but doesnt want to stay running sometimes. also, the idle shoots way to about 2000 rpms if I even touch the gas and let off (touch it, let off, rpms should settle but instead they shoot up to 2000 for couple seconds, then settle back down)

 

[Ecomike]

Starting to sound like a bad TPS, did you ever test it? Did you ever test any of the parts? Or are you just throwing parts on it?

 

[cruiser54]

Starting to sound like a bad TPS, did you ever test it? Did you ever test any of the parts? Or are you just throwing parts on it?

Sure does sound like a TPS. Test it.

 

[birchlakeXJ]

Test your TPS. You need to adopt a "testing based" strategy to find the culprit here.

-------------------------------------------------


The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

The TPS is sensitive to heat, moisture and vibration leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following: -

NOTE: The throttle position sensor is also DIRECTLY involved with transmission shifting characteristics! It should be verified early in the troubleshooting process, when a transmission issue is suspected!

 Poor idle control: The TPS is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".
 High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.
 Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.
 Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted acordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.
 Engine Misfire: A fault TPS can report values outside the deined acceptable range causing the ECU to incorrectly fuel the engine. This is noticable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

 

[Ecomike]

Sure does sound like a TPS. Test it.

I think some people think testing it means swapping the part for a new one!

:spin1:

LOL

:cheers:

 

[cruiser54]

I think some people think testing it means swapping the part for a new one!

:spin1:

LOL

:cheers:

Usually without adjusting the new one..........

 

[steiny]

Sure does sound like a TPS. Test it.

throttle position sensor? I put a new one in and nothing changed, so I took it back and kept using my old one. I know new sensors take time to adjust, but nothing new happened. Since the people who owned this xj before me, may years ago didn't do very good tune ups/maintenance, its nice to test, and check these sensors for problems - a lot of them need complete overhaul anyway. I don't like to just throw parts and money at it though, doesn't solve much.
when I put the new IAC in, my idle changed a bit, so i let it idle and shut it off to let the pintle reset. I never did a new cap and rotor since I bought it, so I'm putting new wires/cap/rotor in it tomorrow. The cap and rotor definitely need changing-been meaning to for awhile. I'll check CPS while I'm at it.
plugs were recently changed last summer, but I'll check them to make sure they're ok after all this weirdness.
-chris

 

[birchlakeXJ]

throttle position sensor? I put a new one in and nothing changed, so I took it back and kept using my old one. I know new sensors take time to adjust, but nothing new happened.
-chris

Chris, this is the first you've mentioned about replacing the TPS. You really need to tell us everything you've already done as far as replacing components or testing for these symptoms, so that you get accurate replies and that we can eliminate those things.

And there is no adjustment on a TPS for a 93. 91-01 require no adjustment. 87-90 do require adjustment.

Vacuum leak is still a possibility here.

 

[steiny]

Sorry about that, I was posting this in another forum and mentioned the TPS change with no result and thought I cross-referenced it on this one. I apologize. I must have just crossed out TPS in my head and ruled it out on here without typing it.
also, I sprayed starter fluid on vacuum lines and there was no rise in idle.

UPDATE on CAP/ROTOR. just installed new cap/rotor/wires today to no avail -but she looks nice

 

[cruiser54]

Did you spray where the intake manifold meets the head?

 

[steiny]

yes indeed. still no rise in idle.

 

[steiny]

scanned it. found some voltage issues. swapped the aux. fan out with a spare-fixed the issue, cleared scanner codes and re-tested. Found no codes. think it might be tps, or bad fuel pump. o2 sensor was in rough shape-replaced.
any thoughts?

 

[cruiser54]

Test the TPS or just replace it.