Racing Idle Woes Topic Again (Renix 4.0)

Guys:

I have been haunted forever and a day with the high idle renix woes... Cleand the intake and all that... Now in the past, a simple TPS adjustment would fix the racing at start up... (engine starts at about 2000 rpm and drops to like 1300 in gear) Auto tranny here...

My cure has always been to disconect the IAC and Latch Relay and this has fixed it but i like having the IAC cuzz it does kind of help starting out and holding a nice idle..

Anyways any ideas here... Correct me if i am wrong but this is just an idea:


when you test /adjust the TPS you are checking and adjusting it on the tranny side of the connector right(????)

so the signal from the tps on the ECU side could be bad RIGHT? or there could be a short/break in the wire from the TPS to the ecu right.... (i do have the dreaded C-101 bulkhead connector)

Am i on the right track here? My plan was to either 1st replace the TPS or take the meter to the TPS to ECU wires... anything else that would cause a surge... Even thought of replacing the HEGO sensor and MAT sensors for hah-hahs.... I just moved and all my renix stuff is packed...

When the renix ECU calls for startup what sensors does it look at

Any help would be apprecated guys

Before you replace anything, check for a vacuum leak. That is the most common reason for high idle issues.

You can pull and plug every vacuum line you got, but the MAP, for a test.
I tested just about every sensor wire for resistance between the sensor and the ECU. I'd jump the pins at the sensor and do a resistance test at the ECU connectors. Some I did just the opposite and jumped the pins at the ECU connectors. My eyes aren't that good, a magnifying glass and flashlight might come in handy, to read the numbers on the ECU connector.
Most of my troubles were the grounds and the TPS connector. On a hunch I tested voltage for the TPS on both sides of the connector and got two radically different values. The connector pins had a coating of something resistive on them.
My TPS (for the ECU), the three pin plug, had a dead spot in it. The motor just went blah at about 1/3 rd pedal, I could accelerate through the dead spot though.
When I scrubbed out all the connectors on the lower left of my block, with a small brush (used for cleaning between the teeth) and a good contact spray, 90% of my hot starts disappeared.
I also periodically let a drop or two of penetrating oil run behind the piston on my IAC, I took an old one apart and the grease on the rod driving the piston, was pretty darned thick, almost putty.
Besides the normal vacuum leak areas, I've had trouble with the TB gasket before. Had one XJ, where the gasket had shrunk and would high idle sometimes, depending on motor temps and bumps. Had another gasket that I reused and didn't get it exactly back in the same depressions, the ridges and valleys didn't line up exactly. I still reuse the gaskets, but put a light coat of copper grease on them and make sure the alignment is good.

old_man said:

Before you replace anything, check for a vacuum leak. That is the most common reason for high idle issues.

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Yeah - what he said. With my four RENIX XJ's, I've found a lot of vacuum leaks.

Once you get done checking all the hoses, lines, and such, take an hour and retorque all the manifol bolts. I retorque mine once a year, and usually catch one or two just a little loose...

5-90

Thanks guys... I had my ear all over the engine listening for leaks but did not hear any (im going to investigate the leaks some more) This morning with the Latch Relay removed, the engine started out fine but after a mile of driving it was idling in gear slightly high, and when put in park or neutral, shot up to about 1300 rpms... Something tells me it is either a ground or a sensor issue...

What would cause the ECU to call for the IAC to open too far after driving ?

'88 XJ with 253,000+ miles. This is an IAC issue. It's dirty. Every time I have had the fast idle problem, I have filled up with fresh gasoline, dumped two bottles of injector cleaner into the tank, and within two days the problem has disappeared. Clean your throttle body, and run injector cleaner for the next couple of tanks of gas.

wilcharl said:

Thanks guys... I had my ear all over the engine listening for leaks but did not hear any (im going to investigate the leaks some more) This morning with the Latch Relay removed, the engine started out fine but after a mile of driving it was idling in gear slightly high, and when put in park or neutral, shot up to about 1300 rpms... Something tells me it is either a ground or a sensor issue...
What would cause the ECU to call for the IAC to open too far after driving?

Click to expand...

There are a lot of inputs for the idle, easy for something to get out of whack. The O2 sensor is gonna try and keep an ideal, fuel air mix. But there are conflicts. If you have a vacuum leak, the ECU is gonna add fuel so the motor doesn't run too lean. If the cat (O2) isn't getting enough oxygen, it's gonna make adjustments, add air (IAC).
A couple if things that can screw up the process, are a vacuum leak. The vacuum leak is likely closer to some cylinders than others, so some are running lean, the O2/ECU compensates, then some are running rich. This also changes at higher RPM's, when a small vacuum leak becomes less relevant.
The MAT and the engine temp sensors both have a fairly large influence on idle. As does the O2 sensor (and O2 sensor temperature). At extended idle the O2 sensor does get covered in soot.
I got my idle to steady down a lot, by cleaning the grounds. Then added a new O2 sensor. I adjusted my TPS, which also had a minor influence. Changed plug wires (I did have a miss, every miss is extra unburnt fuel down the exhaust). I constantly (every couple of months) service the IAC. Cleaning the connectors to the *engine temp. sensor* ( What would cause the ECU to call for the IAC to open too far after driving ?) and the O2 sensor inputs (What would cause the ECU to call for the IAC to open too far after driving?) had a fairly profound effect on mine. But it will still vary
200 RPM depending on motor temp. I think my fan clutch is getting weak and the temp swings are influencing my idle and/or I have a sticky thermostat.
As a side note, my vacuum has always been a little on the low side. I also used to have a fairly large variation in cylinder pressure, during a compression test.
I was also able to tune some of the idle RPM swings out, by adjusting the idle bypass adjustment (or minimum idle adjustment). By the trial and error method.
If I remember correctly there are 12-14 inputs for closed loop idle.
And like Eagle said, periodic doses of injector cleaner also help, every miss messes up the O2 sensor inputs some.
One poster mentioned the throttle plate, not seating properly. I adjusted my throttle stop so the plate stops just before completely seating, then readjusted my TPS and minimum idle screw.
I also noticed the idle circuit seems to work best set somewhat low, around 400-500 RPM.
Sorry about the book, but the idle circuit, isn't one of the simpler parts of the Renix system.