RENIX IAC question?

Well, I'm still fighting a rough idle on my newly rebuilt 90'. All of the ignition system is new, the O2 is new, the intake and TB is very clean, the TPS has been properly adjusted and is working good, there are no vacuum leaks(all new vacuum lines), the intake is tight, the fuel press. reg is working, the injectors are new, the EGR is blocked off and plugged, all the other various sensors have been tested and are working properly. It drives fine otherwise, which tells me its in the idle circuit.

The IAC is the only thing I can't test, but I can unplug the IAC and the idle does not change at all, cold engine, hot engine, it doesn't mater. Is this a sign that the IAC is not functioning. The rough idle seems to be from a rich running engine since I can smell gas through the exhaust. I've ordered a replacement from Holley(part # 543-105), but I just want to see if anyone else has fixed this problem with a IAC replacement?

Keep in mind that most of the work already done is due to the restoration of this XJ and not a direct result of my idle problem.

I'm grasping at anything at this point!

The IAC could cause a rough idle due to a too rich of mixture. You should be able to take the IAC out of the throttle body and check the pintle tip for carbon.

The IAT sensor could also cause this possibly? The IAT is located on the manifold just behind the throttle body. Probably not your problem but is worth a shot. I only mentioned this because it is one sensor that can be modified or moved to kinda "hack" the system into giving a more rich mixture.

There are many things that can cause a rough idle. Can you give more specific details into this? Is this rough idle only when cold start or any temp? Is it only on cooler days, or maybe hotter? Does it smooth out immediately upon off-idle throttle positions?

Note: I edited out a bad suggestion. Sorry, was under the impression that was a proper way to check an IAC.

The idle is smooth for a min. or so upon cold start up, but then gets rougher as the engine gets up to temp.

The idle fluctuates a 100 rpm or so from 750 to 850 in a up and down roughness.

It is most definatly running rich.

No smoke from the exhaust but it smells like gas.

DO NOT DO what Tally_XJ said. It can over travel the pintle and destroy the IAC. Never have the IAC connected when it is not in the throttle body. Never force the pintle in and out as well. You can carefully clean the pintle with some carb cleaner and clean the orifice into which it is seated, but don't get it into the stepper motor of the IAC.

Before you say there are no vacuum leaks, hook up a gauge and look at the readings. Even things like the brake booster can cause a leak.

Is the Powerlatch relay working, that controls the IAC.

Where can I find the power latch relay? I'd really like to see if it is working before I install the new IAC!

I'v disconnected the brake booster and plugged the vacuum fitting and no change to the Idle problem.

I'd like to check the vacuum but my guage is dead, I'm looking for another one now.

Thanks for everyone's help.

Jon

Heres a picture of the Power latch.BUT I don't think the IAC is the problem, it will not cause a rich/lean condition. The mixture is controlled by the ECU and injector on time.
The serging is cause by the ECU trying to correct the rich mixture and idle speed but can't.
You need to get you pressure gage fixed and test your vaccum and fuel pressure first.
I sounds more to me the mixture is rich because one or more of your temp sensors may not be working. I bad CTS or MAT sensor will tell the ECU the engine is cold when it's not. The ECU will then richen up the mixture, but the O2 sensor will detect a rich mix and the ECU will lean it out causing the rpm to go up. Then the engine speed sensor will tell the ECU to close the IAC so up and down things go.

The voltage = temp Ohm readings are in the renix manual.

The IAC is not going to cause rough idle. Its function is idle speed only. Its a electro/mechanical devise controlled by the computer to add or subtract air to the intake manifold--it does not affect fuel mixture. Normal failure for the AIC system is carbon build up in the AIC port of the throttle body, allowing more air (bypass of the throttle plate), resulting in high idle.
For rich fuel mixture, look to things like temp sensors, which, through the computer, adjust fuel mixture relative to engine temp and manifold air temp. Cold engine=rich mixture, leaning out as the engine warms up to operating temp. At operating temp, the O2 circuit senses O2 level and informs the computer regarding lean/rich, with the computer adjusting mixture accordingly; an incorrect O2 CIRCUIT, or an exhaust manifold leak could result in a rich running engine (leak results in O2 sensor telling computer the mixture is lean, so the that the computer compensates with more fuel). Or fuel pressure--pressure must be correct for the computer to properly control the amount of fuel injected. The computer assumes the pressure is correct. If it's not, the computer cannot control the mixture.

Looks like I'm heading in the right direction...but I'm on the wrong road!

I'll recheck my MAT nd CTS again. I've sprayed carb cleaner over every vacuum line and around the man. gasket with no leaks detected.

Is it possible as XJBUBBA has stated that I have a possible O2 sensor circuit problem?

The sensor is new and was a direct plug in, no wires have been hacked on the Jeep anywhere, its all stock/original.

I'll double check the plug in and make sure it is correct.

Give me some more ideas while I check these out!

Thanks for all your help.

Jon

Ok, lets get back to shade tree mechanics 101. If you have a miss at an idle, pull one spark plug wire at a time. The cylinder that has the problem will cause the least drop in rpms. If you locate one cylinder that drops less, swap spark plug and wires with the adjacent cylinder and try again. If it is the same you have just eliminated the ignition system with the possible exception of a distributor cap (rare).

I just thought of a possible culprit. You said it was a new engine. Check that you have the right injector wires on the correct cylinders. They are real easy to swap.

#1 LT Blue
#2 LT Green
#3 Tan
#4 Yellow
#5 White
#6 Brown.

If that doesn't fix it, beg borrow or steal a set of noid lights and make sure the injector on the offending cylinder is getting pulses.

I have also seen a distributor that was indexed off by one tooth and it would run at high rpms but puked at an idle.

Thanks OLD MAN, I've done the ignition test, they are all dropping the idle off. I've been a diesel tech for 20 years, so the basic stuff I know well, its the electronics that I normally don't deal with that seem to stump me!

I just got done testing the CTS and MAT, here are the numbers;

CTS, with the temp guage reading 205 I've got 142 ohms

MAT, same circumstances, I've got 390 ohms.

The CTS seems low and the MAT seems high.

What are your opinions?

Do you have a copy of the Renix Fuel Injection Manual?

old_man said:

Do you have a copy of the Renix Fuel Injection Manual?

Click to expand...

If not PM me with your email address and I will send you a copy.

PM sent,

Thanks

Jon

Thanks for handling that John, I was away from the computer for a while. You saw where I was going.

Thanks guys...I'll educate myself a bunch tomorrow!

100 pages!

O.K., I had to bring this back up because the after checking each sensor and "properly" adjusting the TPS, I've still got an awful idle, poor fuel mileage(10-12 mpg on a stock XJ?) and a tranny that at steady throttle will hunt between OD, lock-up to unlock and then back through them again, it otherwise runs very good.

This all seems like the TPS right?

But the TPS has been adjusted properly and checks out good.

The MAP is a little high, at stabilized it is reading 1.7 versus .5-1.5 range.

The CTS and MAT are in range.

The IAC is new and the power latch relay is working. Unplugging the IAC has no effect on the idle.

I've checked the spark at each cylinder, I'm getting a blue spark from the wire almost a foot away from any ground and I'm tired of getting shocked! The idle drops on each wire pulled so I'm rulling out ignition.

I've pulled all the vacuum lines one at at a time and checked with no luck.

The engine is all newly rebuilt. It has been bored 40 over and the head shaved and the block was decked a total of 20k. Could the change in compression ratio have an effect on the idle. I'm running a stock grind cam.

Could it be the fuel pressure regulator? Pulling the vacuum line has no effect on the idle.

I'm at the end of my rope, short of just becomming a parts changer, I don't know where to go next.

Go back and test the MAP, also check vacuum, you may be running lean at idle with the Mods and my need to build an adjustable map sensor.
You can also do a propane test to see if more fuel will smooth out the idle, just hold a unlit propane torch over the throttle body.

Did you check the wiring colors on the injectors. Every 4.0L Renix I have seen has the wire lengths screwed up on the #3 and #4. So if you put them on based on length, you will have 3 & 4 swapped.

Check for arc over on your sparkplug wires. Also check the inside of the distributor cap for arc tracks. You said the idle speed drops. You should expect that. The key is to watch for how much it drops. The offending cylinder will drop less.

XJ Trail,

If you are still having troubles, i would suggest bringing it to a friend of mine. He is 30 mins from Roanoke and he'll get it fixed for you.

noah