Not your average high idle

Hello all, I have a 87 4.0L with 240k miles and it has a high idle issue. The engine starts just fine but will usually idle a little rough at low rpm but once the engine is at normal operating temp the engine will idle around 1000rpm. If I continue to let it run it will increase to 1300rpm but usually never exceeds. I have recently replaced the TPS and coolant temp sensor and within the last 2 years I replaced the IAC valve and the crank position sensor (which resolved the no-starting problem). Could I have installed faulty equipment or just wore out this quick? This all started on a hot day during a jeeping trip, as soon as I drive through a deep creek the engine increases to 1100rpm shortly after. Could a cracked header cause high idle? When the engine warms up the crack expands and causes unregulated air movement? All the bolts are present and tight on the intake/exhaust gasket. I also ran the vacuum lines with carb cleaner and they checked out. Like most other peoples high idle problems i read about, mine doesnt start as soon as i start the engine, mine appears once the engine is warm. Any ideas what could be causing this? Thanks…
-Pete

Read through this, try a few things, post up results: http://www.naxja.org/forum/showthread.php?t=905849

Check the O2 sensor voltages, and the MAT (IAT) sensor voltages. One of them may be off. Search "Ecomike High Idle" and "RenX Files" to find some good threads and details on testing those sensors. I also wrote a thread here on "Renix O2 sensor testing". The water hit might have affected the O2 sensor, or cracked the header, or both. Cracked header can affect O2 sensor and idle!

ok thanks guys, good to know, i'll check it all out

Well i tested the voltage and resistance of the IAC and the O2 sensor, and they all seem to be kosher... hmm...

Ecomike said:

Check the O2 sensor voltages, and the MAT (IAT) sensor voltages. One of them may be off. Search "Ecomike High Idle" and "RenX Files" to find some good threads and details on testing those sensors. I also wrote a thread here on "Renix O2 sensor testing". The water hit might have affected the O2 sensor, or cracked the header, or both. Cracked header can affect O2 sensor and idle!

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I haven't fixed mine yet...but I believe the cracked header to be my problem. reaper1972 has one of those "camera wands" and I have a "nice" crack at the collector. My O2 sensor is new by the way.............
Mine idles nicely till it gets warm then to 1200 to 1300 like yours .

If it is the crack affecting the O2 sensor, disconnect the O2 sensor to see if the high idle stops, if it does you have isolated the problem.

Well yesterday when i tested the O2 sensor and unplugged it while it was running it had no effect on the idle. I cant get a good view of the header but there seems to be no crack around the O2 bung. I'm kinda at a loss, i also went ahead and recalibrated the TPS and checked the Coolant temp sensor and the vaccuum lines. I just cant seem shake this high idle. I had this high idle problem a year ago, i changed the cheapest part (the coolant temp sensor) and the high idle went away. I replace it just a few weeks ago but this time it still remains.

well i worked looked at it a little bit before work. got the engine running up around 200degrees, i noticed today that with the engine that warm and running at 1100rpm it didnt change the idle when i unplugged the IAC, O2 or Coolant temp sensor. I just thought it was strange that nothing happened when i unplugged all the sensors, only messing with the vaccum lines changed the idle, but sadly it only increased it even more... around 1300rpm. Also the coolant temp sensor was hovering around 280-290ohms, is that too high?

CTS may be seeing cooler coolant than the gauge sensor. Here is a link to temp versus resistance.

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

185 ohms is at 212 F, so it is not off by much, if off any at all. The coolant from the radiator returning to the block can be 10 to 35 F cooler than the coolant at the thermostat.

Check the air temp sensor in the intake manifold, same resistance curve as the CTS. You need to measure the intake manifold temp at the same time. I will not be the same as the coolant temp!!! My air intake runs about 160 F at hot idle in summer with ambient at 90 F. Check it again at cold start up.

My air temp sensor was bad and was causing hard starts cold, and high idles hot! Mine is an 87 Renix.

well with the engine running warm and its about 22degrees in my garage i was reading around 26,000ohms does that seem high? shouldnt it be closer to 13,000ohms?

Peter's87XJ said:

well with the engine running warm and its about 22 F degrees in my garage i was reading around 26,000ohms does that seem high? shouldnt it be closer to 13,000ohms?

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If the intake manifold is hot too, it should be even lower than 13,000 ohms. More like 5,000 to 2,500 ohms at least! Time to toss it!

That 22 F is probably about 100 F after passing through a warm to hot intake manifold. I get about a 70 F temp rise in my manifold, so 22 F plus 70 F (warm to hot engine) is 92 F! That is what the MAT (IAT) air temp sensor should be showing in ohms! So about 3000 ohms, not 26,000 ohms is what it should be showing! That would likely cause a high idle that ignored the TPS, O2 and CTS inputs to the ECU based on my prior experience with a bad MAT (IAT)

If you haven't done this yet, you should. Clean the oxidation off of the ground wires that get bolted to the dipstick tube. It solved my idle problems.

Lets start from scratch.

Pull the intake tube from the TB. Remove the IAC. Clean the TB and IAC port with a TB-safe aerosol cleaner, cotton swabs, old toothbrush. Clean the IAC pintle--be gentle, they can break.

Now, there is a 3/32 Allen screw that Uncle Bob thinks is the idle adjustment screw. Uncle Bob isn't retarded, but at 35 he is the oldest student in Kindergarten. That 3/32 Allen screw ISN'T for idle adjustment. It's sole purpose in life is to keep the STEEL throttle plate from slamming fully closed and damaging the ALLOY throttle body. Ok, back out the 3/32 Allen screw until it no longer effects the throttle plate position--back it out enough that the throttle plate fully closes. Now, while observing the throttle plate SLOWLY turn the 3/32 Allen screw in until you see the first perceptible movement of the throttle plate and STOP. Ok, put the IAC back on along with the intake tube. Now, readjust your TPS, it is likely out of adjustment now.

Ok--the correct idle speed is 700-750 RPM after 20 minutes of engine operation. That idle speed for a manual is in neutral; for an automatic transmission it is checked in DRIVE, parking brake set, and with a foot on the service brake pedal. There is NO SPECIFICATION for idle speed of an automatic equipped vehicle in Park or Neutral. Don't rely on the instrument panel tach, hook up an external tach.

Post up what you get after the above cleaning and adjustments.

In regards to the grounding wires they are as clean as a whistle

In regards to "uncle bob" lol:roflmao:

But yea i will give the throttle body another once over and keep you posted...

On an 87 with the infamous C101 connector behind and above the brake booster, I suggest unbolting the halves and cleaning out all the black tar crap out ot if. Many, close to all, important signals between your engine management sensors and the ECU travel through this crappy connector. Start with a small screwdriver and move on to carb or electronics cleaner with your little brother's toothbrush. Clean it numerous times, apply dielectric grease and bolt it back together. This is "Refreshing 101" and a normal needed maintenance procedure to be performed at least once in an early Renix Jeep's lifetime.

Why is everyone always so hung up on the "black tar crap" in the C-101 connectors? Mine still has it, and it has done a great job of keeping the connectors in mine from oxidizing for 24 years in my 87 Wagoneer XJ. I wish they had put that black crap in other places where I have had problems!

:wave1:

I have however tightened the female pins with jewelers needle nose pliers in mine! But when I did, It never solved any of the real problems I had. Replacing sensors and fixing ground connections, new starter switch and wiring at the starter switch and head light switch, and fuse box repairs were my main issues with my 85, 87 and 89.

I must have the only working C-101 in ? But then I don't like crawling around in mud or wadding in muddy water either? LOL

It was a recommended procedure from JeepTech in it's time and it did in fact solve many problems. Maybe not yours..... There's a good reason they eliminated the C101 after 2 years.

cruiser54 said:

It was a recommended procedure from JeepTech in it's time and it did in fact solve many problems. Maybe not yours..... There's a good reason they eliminated the C101 after 2 years.

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Eliminating the C-101 is one thing. Eliminating the "black gunk " that "Jeep engineers specified for a good reason" is another thing? LOL

I have done many things to my jeeps that APPEARED at the time to fix the problem, but in the long run were just hit and run gremlins that always came back, after letting me think this or that had fixed the real problem.

I had a 76 Dodge Charger (400 four barrel lean burn) that the best Chrysler dealership in Houston Texas fixed (rebuilt the transmission) 7 times in the first four years I owned it! They purportedly knew what they were doing too!

I recommended replacing the black junk with dielectric grease after cleaning the contacts.

It was Jeep engineering that recommended removing the black junk and they didn't even suggest replacing it with dielectric.