Renix Idle and Oil Pressure questions

Ok, So I've been searching for the last hour, I just have a few simple questions.
1. What's everybody's idle speed? My idle does not hunt, but it seems very low. My 4.0 likes to idle at about 3-400 RPM. I've adjusted the TPS, cleaned the IAC and TB. It had a full tune up when I bought it a month ago, plugs, wires, dist, rotor, fuel and air filter.
2. Where does the tach get it's input signal? This relates to the first question. I'm not sure if it's showing a true number because I don't know how it works.
3. In my searching I've come across the EGR causing many troubles relating to low idel speed. How do I disable the EGR in my 87. Just pull and plug the Vac line on apparatus on the drivers side fender?
4. What's normal oil pressure for an older 4.0? I'm running regular 10w40 in this engine. Cold start is a shae above 60, cold idle is about 40. Warm cruising speed @ ~2K RPM is right at 40. Warm idle is right around 20. The oil pressure is very throttle responsive. A blip of the throttle send the guage skyrocketing. I imagine that getting my engine to the proper idle will get the oil pressure to 30 PSI, as soon as I ind out what the idle speed should be.
Ok, specs:
1987 MJ, 4.0, AX-15
Odo says 242K miles, but it's not the original tranny, and the engine block is orange, so it's probably not original. No lifter ticking, minimal valve train noise.
Sorry for the book, I'm just a paranoid newbie 4.0 owner.

87manche said:

Ok, So I've been searching for the last hour, I just have a few simple questions.
1. What's everybody's idle speed? My idle does not hunt, but it seems very low. My 4.0 likes to idle at about 3-400 RPM. I've adjusted the TPS, cleaned the IAC and TB. It had a full tune up when I bought it a month ago, plugs, wires, dist, rotor, fuel and air filter.
2. Where does the tach get it's input signal? This relates to the first question. I'm not sure if it's showing a true number because I don't know how it works.
3. In my searching I've come across the EGR causing many troubles relating to low idel speed. How do I disable the EGR in my 87. Just pull and plug the Vac line on apparatus on the drivers side fender?
4. What's normal oil pressure for an older 4.0? I'm running regular 10w40 in this engine. Cold start is a shae above 60, cold idle is about 40. Warm cruising speed @ ~2K RPM is right at 40. Warm idle is right around 20. The oil pressure is very throttle responsive. A blip of the throttle send the guage skyrocketing. I imagine that getting my engine to the proper idle will get the oil pressure to 30 PSI, as soon as I ind out what the idle speed should be.
Ok, specs:
1987 MJ, 4.0, AX-15
Odo says 242K miles, but it's not the original tranny, and the engine block is orange, so it's probably not original. No lifter ticking, minimal valve train noise.
Sorry for the book, I'm just a paranoid newbie 4.0 owner.

Click to expand...

The tach gets it's signal from the ignition module.

Oil pressure 13psi and above is spec at idle.

Low idle can be caused by a misadjusted TPS, or more likely, a clogged or inoperative IAC.

thanks for the answers.
What is my idle supposed to be at? Just so I when I get it right.

IIRC it is 600-800rpm

Well, you have your answer for the tach above.

There are also two intake adjustments on the throtle body itself. But before you tweak them, make sure the intake is clean (including the IAC air port, and the small one at the 3 o'clock position if you were looking down at the top of the TB with the TPS facing you), your TPS is set correctly and the IAC is working! Average idle, from what I've read here, seems to be from around 600-900. I have mind set at 500. If you need help with this, post again when all else fails.

Oil presures seem fine to me too.

Thanks AJ. I dind't know of the other hole at the 3 o'clock position. I'll see if I can't clean that. Unfortunately it's butt cold in ohio ATM. I'm gonna try and solve a few issues this wekend. Low engine temp and low idle.

what is the proper way to clean a IAC and throtle body?

Mine idles about 600

Oil pressure at idle, hot, is about 15....everything else is in line with what you are seeing.

Rev

87manche said:

Ok, So I've been searching for the last hour, I just have a few simple questions.
1. What's everybody's idle speed? My idle does not hunt, but it seems very low. My 4.0 likes to idle at about 3-400 RPM. I've adjusted the TPS, cleaned the IAC and TB. It had a full tune up when I bought it a month ago, plugs, wires, dist, rotor, fuel and air filter.
2. Where does the tach get it's input signal? This relates to the first question. I'm not sure if it's showing a true number because I don't know how it works.
3. In my searching I've come across the EGR causing many troubles relating to low idel speed. How do I disable the EGR in my 87. Just pull and plug the Vac line on apparatus on the drivers side fender?
4. What's normal oil pressure for an older 4.0? I'm running regular 10w40 in this engine. Cold start is a shae above 60, cold idle is about 40. Warm cruising speed @ ~2K RPM is right at 40. Warm idle is right around 20. The oil pressure is very throttle responsive. A blip of the throttle send the guage skyrocketing. I imagine that getting my engine to the proper idle will get the oil pressure to 30 PSI, as soon as I ind out what the idle speed should be.
Ok, specs:
1987 MJ, 4.0, AX-15
Odo says 242K miles, but it's not the original tranny, and the engine block is orange, so it's probably not original. No lifter ticking, minimal valve train noise.
Sorry for the book, I'm just a paranoid newbie 4.0 owner.

Click to expand...

My idle speed has been all over the place while debugging my 87. Everything is working right now and I have 750 to 800 rpm at idle in park and 600 rpm at idle in drive.

I beg to differ with Langer1 on this, The tach signal comes from the ECU (I believe) which calculates the engine speed from the CPS / speed sensor on the transmission bell housing near the flywheel. Haynes manual and my Renix FSM for 1987 electrical drawing calls the CPS the "speed sensor" but its signal goes straight to the ECU only! Note that it lists the sync sensor in the distributor separately which also talks only to the Renix ECU! The CPS/Speed sensor senses TDC and engine speed! I would trust the engine speed gauge!!!!

Unfortunately non of the available drawings show the rpm gauge in a wiring diagram!

If the EGR is bad I would replace it, as it can casue other problems even if disabled (it can stick open on its own).

Your oil pressure sounds very typical and normal for the Renix engine. I am running just a bit lower on the hot idle oil pressure. If you get your idle up to 750 (FSM Spec) in park and 600 in drive, your 20 psi should come up a few more pounds to maybe 25 psi. Jeep manual says 13 psi minimum at idle is still OK. I am running 20W50 with Lucas to hold 22 to 25 psi hot idle in mine (depending on just how hot it gets).

If you are getting those pressures with 10W40 and 3-400 rpm you are in good shape engine wise.

Although the IAC is a low idle suspect, since this is a Renix I would clean each and every ground contact on the engine, firewall and battery first, then recheck and readjust the TPS. I fought my idle problems for 2 years, replaced every part under the hood, only to find a 13 ohm ground problem (battery to engine) and poor (loose and dirty) ECU/Sensor ground contacts at the oil dip stick - engine ground contact point were also part (and in the end, the only remaining problem left). Ground signal from the engine to the battery should be less than 1 ohm. Also the sensor ground to the battery should be less than 1 ohm. My loose ground was so bad it would come and go, so I had a variable ground casuing a variable idle speed problem. High one day, low the next. Nearly drove me nuts!

As AJPulley says there are adjustments (3 actually) that can be made on the throttle body, but they should be done last, once everthing else is up to factory spec. The 3rd adjustment is to bend the ear on the throttle linkage :gag: (not recomended, but your prior owner may have already solved a high idle by bending it to lower the idle speed like I did once on mine). Once I got everthing on mine working right I had to bend mine back where it belonged!:doh:

Someone has already posted several times in various threads the "how to" properly clean and lube the IAC and clean the throttle body in this forum. Just search the three words "clean Idle Control"

haha old thread. i use throttle body cleaner

87manche said:

Ok, So I've been searching for the last hour, I just have a few simple questions.
1. What's everybody's idle speed? My idle does not hunt, but it seems very low. My 4.0 likes to idle at about 3-400 RPM. I've adjusted the TPS, cleaned the IAC and TB. It had a full tune up when I bought it a month ago, plugs, wires, dist, rotor, fuel and air filter.
2. Where does the tach get it's input signal? This relates to the first question. I'm not sure if it's showing a true number because I don't know how it works.
3. In my searching I've come across the EGR causing many troubles relating to low idel speed. How do I disable the EGR in my 87. Just pull and plug the Vac line on apparatus on the drivers side fender?
4. What's normal oil pressure for an older 4.0? I'm running regular 10w40 in this engine. Cold start is a shae above 60, cold idle is about 40. Warm cruising speed @ ~2K RPM is right at 40. Warm idle is right around 20. The oil pressure is very throttle responsive. A blip of the throttle send the guage skyrocketing. I imagine that getting my engine to the proper idle will get the oil pressure to 30 PSI, as soon as I ind out what the idle speed should be.
Ok, specs:
1987 MJ, 4.0, AX-15
Odo says 242K miles, but it's not the original tranny, and the engine block is orange, so it's probably not original. No lifter ticking, minimal valve train noise.
Sorry for the book, I'm just a paranoid newbie 4.0 owner.

Click to expand...

My idle on my newly rebuilt 90'(only thing old was the block,crank,head,valves)is the same as yours 3-400 rpm, all of my sensors, injectors, dizzy,module, if you can think of it it is new. Tried a hand held tach and the same results.
I've tried a blanking plate on the EGR and plugged the vacuum circuit and no difference, in fact I have less or no detonation with the EGR so its new and hooked up.
Oil pressure with 10w-30 is 45 at idle and 75 at cruise.

It runs too dang good for me to worry about the idle. Also the replacement of all of the parts is due to the fact that this is a restoration project and I wanted everything to be as new. I have 4 XJ's and this RENIX will out run any of the HO's in my driveway.

I've found on these 4.0's that no 2 will run similar oil pressure, these engines were not built to that close of a tolerance, but they last amazingly well.

BTW, the tach signal comes off the coil...

the distributor sensor is used to determine piston 1 and 6 at set points, and the 'speed sensor' is engine speed. The RENIX has no need for vehicle speed. The TCU handles that.

In addition, there are 2 things you could consider for idle speed:

Hold onto the IAC connector, ready to pull it. If you want a lower idle, pull a vacuum line, if a higher one, short a plug wire then unshort it. In each case, the ECU will try to compensate for the wrong mixture, closing the IAC for the vacuum leak or openning it for the rich condition (the injector doesn't turn all the way off on the 'dead' cylinder). After replacing the vacuum hose, or unshorting, leave the IAC connector off. You now have manually 'set' the idle. It may take a couple of tries to freeze it at the right speed. I've done this before and it stays very steady, AC on or off. Normally, they tend to race at startup. I don't like that, even for a second. With this mod, it comes on, and if cold, at a couple hundred rpm lower than hot idle. Rock solid start though. You can just bump it up with your foot a minute and it comes up and stays as it warms a little. The 'soft start' is much nicer on the engine! And oil pressure comes right up even at low rpm and cold.

Next up would be to add in some vacuum line to a small aquarium air needle valve, then to the air filter or a scintered metal filter. Set the idle to its lowest value, the use the needle valve to introduce some vacuum leak and the idle increases. Good for cold starts if you're lazy, or if you need it to fast idle at times.

You can also 'turn' the IAC stepper with a 4 pos multipole switch hooked to the stepper, effectively making an idle speed knob and take control of it that way.

The IAC is overrated, can be a problem area, and with these tricks you can get a little more versatility out of it...

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