Need help Diagnosing my Renix

So I have had this problem since my trip to the Fall Fling last October. The jeep has a rough and high idle and is stuttering bad ~2000 rpm. Running really rich also.

90 XJ, AW4, 4.7 stroker from Titan. 24# mustang injectors. Ran great for 30k

New plugs (gapped), new cap, new rotor, new fuel filter, new 02 sensor, new cat.

I bought a Snap-On scanner to help me out and this is the info it is giving me.

RPM 1000
o2 voltage .12 @2k RPM its .2-.33
INJ (ms) 6.4 @2k RPM its 12-14
Loop Status, open
exhaust, rich
ST fuel trim, 128
Lt Fuel Trim, 128
MAP Sensor Voltage 1.5
MAP (KPa) 39
Man Vac (KPa) 60
Baro Pres (KPa) 100
TPS (v) .78
Throttle % - 16
Throttle SW - closed
Fuel Sync - flutters between +/-
Spark ADV 14* and then at 2k its 30-44
EGR is usually off and then after its stutters it will come on and then go back off.

I haven't done a compression test.

:helpme: Any help is greatly appreciated and if any more info or data is needed I will get it as quickly as possible. This has really been driving me mad and I am sure it is something retardedly simple and easy.

It appears you are in OPEN loop. I'd concentrate on that. You should be in closed loop when engine is hot.

That would explain running rich as open loop uses a rich default fuel/air mixture. Time to start testing.....

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

Concur. If the system drops into "Limp Home" for any particular reason, it defaults to a rich AFR (something like 13.5:1, up around "best power")

Start looking for wiring faults.

oops! I though it was open on warm and closed when it was cold. Thats what I get for staying up late reading.

What kind of wiring faults am I looking for? anything specific? (I hate wires)

5-90 or anyone, can you please elaborate exactly what inputs constitute going into "closed loop"? I know heated oxygen sensor for sure, but what else?

I've always wanted to know EXACTLY what it takes to reach closed loop. Also, is it different for RENIX, then for OBD1, then for OBD2??

Please educate me......and the guy that posted.

We gotta get that guy into closed loop and life will be much better!

The MAT and CTS?

"Open Loop" and "Closed Loop" simply refer to when the ECU thinks it's getting a valid HEGO signal.

"Open Loop" simply means that all of the other sensor inputs are considered valid, but the ECU does not trim fuel delivery according to HEGO output, since the HEGO signal is not yet considered "valid."

"Closed Loop" means that the fuel is delivered according to the other sensors inputs, and then trimmed according to the HEGO output signal for fine-tuning. It really refers to feedback after combustion - "Open Loop" means that there's no feedback after-the-fact.

The reason for the Open Loop and Closed Loop modes is simple - the HEGO isn't able to generate a valid signal until the sensor tip gets up to around 700*C. The earliest "one-wire" EGO units lacked the heating element that has since been added, and simply generated an output signal used to trim fuel delivery (they were also used with carburettors, if you can get that! The only thing worse than rebuilding a Rochester Feedback was rebuilding one of those damned Ford Variable Venturis - both of those made a Honda Keihin 3V seem simple!)

The jury is still out on whether the coolant temperature is used as a timer to determine if the HEGO signal should be valid, or if there is a "hard timer" in the ECU proper. With the advent of HEGO, I'm more inclined to believe in the "hard timer" - since the characteristic heat curve of the thing should now be known.

Open Loop tends to run about as rich as "Limp Home" - which explains the fuel economy trouble (rich enough that you don't risk lean ping, but not so rich that you risk cylinder washdown.)

The MAT/IAT, CTS, MAP, TPS, CKP, CMP and other sensors report conditions, but aren't particularly dependent upon conditions to generate valid signals. Therefore, the HEGO is pretty much the only difference between OL and CL.

Glossary
EGO - Exhaust Gas Oxygen (sensor)
HEGO - Heated Exhaust Gas Oxygen (sensor)
MAT - Manifold Air Temperature (sensor)
IAT - Intake Air Temperature (sensor)
MAP - Manifold Absoute Pressure (sensor)
TPS - Throttle Position Sensor
CKP - CranKshaft Positon (sensor)
CMP - CaMshaft Position (sensor)
OL - Open Loop. No feedback for fuel trim after combustion
CL - Closed Loop. Feedback signal generated to monitor remaining oxygen after combustion, this signal is used to trim fuel delivery for maximum economy.
ECU - Engine Control Unit.

One of these days, I'm going to find out what the real difference is - then I'll make sure to pass it along!

The ECM has circuitry to determine the pulse width of the injector (how much time the injector is ON, when designated to "squirt"). The designed neutral on time ASSUMES all sensors are within designed specifications; ie: the temp sensor resistance is in tolerance; MAP sensor variations are appropriate for the vacuum sensed, and, all vacuum hoses, electrical wires and connectors are in good condition and properly attached. These are a lot of assumptions! And the first step in solving an electronic fuel injected engine issue is to insure these assumptions are correct (there are others, as well, such as proper fuel pressure, correct minimum air/tps adjustment, etc.). As far as fuel ratio fine tuning goes--or "closed loop" operation, the ECM has a scheme whereby it uses a counter (sometimes called "fuel trim") that contains a possible 256 different "selections" for adjustments to the injector on time. The center number, 128, is the designed base injector pulse on-time, and in a perfect world indicates 14.7 to 1 fuel mixture. When the engine is first turned on, it's in "open Loop", as described by 5-90, above. After a prescribed time (internal ECM timer), and once the ECM sees a voltage from the temp sensors (engine coolant and the manifold air temp) that it interprets (assumes) to mean a predetermined engine operation temp has been achieved, the ECM looks at the voltage coming from the O2 sensor, looking for variation above /below .45 volts. Until it sees this variation, the ECM continues to use the 128 number in the counter to set BASE injector pulse on-time, modified by engine temp sensors, TPS and MAP variations. In other words, no fine tuning of the injector pulse occurs until the above close loop conditions are seen by the ECM. Assuming all this is correct, a 14.7-to-1 fuel mixture (perfection during warm idle and normal cruise), now exists. So now, since the O2 sensor is active (again, if the O2 sensors voltage is within a very broad range, and crossing back and forth across .45v often enough, within a prescribed time frame (in seconds), the ECM ASSUMES the O2 circuit is correct, and starts to modify injector pulse on-time via the 0-256 counter to adjust for perceived variations in fuel mixture ratios above and below the 14.7 "sweet spot". Scan tools can look at the "trim" table, and see the "128" setting solid Pre-closed loop, and can see the trim number vary after closed loop begins. Looking at this number can tell you if the system is running rich or lean. Numbers below 128 indicate the engine is running rich, and the closed loop between the ECM and the O2 sensor has driven the counter down (reduce base injector on-time). If the engine is running lean, you would see a number higher than 128, implying the injector base-time has been increased; a properly running engine would stay somewhere around 126 to 132; it would also continue to change, as the catalytic converter requires the exhaust gas to constantly vary between lean and rich to properly operate.
The Renix, as far as I know, does not have "long term trim", like say the GM ECM's, or the non-Renix (OBD I )ECM's. The OBD I schemes "learn" fuel trim by using two 0 to 256 counters. The first counter in both operate similarly, but the second counter "learns" from the first, with its number stored, and used to instantly modify injection base pulse on-time anytime the engine is in close loop. Using a scanner on an OBD I ECM instantly tells you if your engine has been running rich/lean on an ongoing basis. As an example, once the engine is in closed loop, the "Short term trim" counter may be regularly changing between 127/128 indicating a perfectly trimmed fuel ratio. A look at the "long term trim" may equal 132, indicating the long term trim is increasing the base pulse on-time to account for an ongoing "perceived" lean condition.
The ECM must assume a great deal. It's up to the engine "tech" to make sure the ECM isn't made an ass of by making the required assumptions regarding fuel pressure, and engine sensor accuracy and proper operation.

Open and closed mode is pretty simple.

At idle or cruise, normal operating temperature, the system should be in closed loop.

All other times it is open loop.

I've never checked throttle position on a scanner, but 16% seems a little high for idle. Have you re-adjusted the TPS since the problem started?

I wouldn't see where that would cause you to stay in open loop, but the extra fuel may be cooling the O2 sensor enough for it to be sending false readings.

joe_peters said:

Open and closed mode is pretty simple.

At idle or cruise, normal operating temperature, the system should be in closed loop.

All other times it is open loop.

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The operative word is "should be". Having an understanding of what the computer "thinks" is required to solve the more elusive issues. The system may be in closed loop, but you don't know for sure because there is no "check engine" light in a Renix system. Even if you knew it was in closed loop, that doesn't mean the fuel ratio is correct such that the engine would pass a SMOG test.
And a question? Is a '90 Renix, or OBD I?

87-90 Renix 91-95 OBD I 96+ OBD II

Yeah, I recalibrated it (TPS) and then replaced it to be safe because I really thought that was the problem initially. It says it is currently reading .78 (supposed to be .8?) volts but I can check it with an voltmeter to be sure.

I swapped out the MAT sensor with a buddies and now it would intermittently goes to closed loop and then go right back to open. It did idle lower and smoother though. Maybe his is faulty? I need to go get another ohmeter and find out.

Okay, I am seeing some conflicting info here, or maybe my reading comprehension sucks? I was under the impression that if the CTS indicates a cold engine temp then the engine will stay in open loop. What I am reading here is that it is actually on a timer (ECU controlled). Can somebody help clear this up for me?

From this thread: http://www.naxja.org/forum/showthread.php?t=101112

You may also enhance the injection some with a switchable 1000 ohm resister in the temp sensor wiring (to throw it into rich warm up mode) or add an adjustable pot MAP bias bridge (to bias the A/F ratio for more power in both open and closed loop modes -- works good with the other mods, but is not much good on it's own). These MPI modifications are simple but not really needed unless you want every little bit of power (and have a G-Tech or other performance measuring tool to tune the gains).

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Wouldn't really work if it was on a timer right? Not that it matters because at WOT you are running open loop right?

Maybe there are multiple problems here, like the distributor not being indexed properly which wouldn't cause the engine to stay in open loop but might cause the shuttering, and another problem causing the open loop symptom?

I tried the distributor indexing because the timing gets off the higher the RPMs are. It basically wouldn't run in any other index.

Exxon Valdez Jeep said:

I tried the distributor indexing because the timing gets off the higher the RPMs are. It basically wouldn't run in any other index.

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Indexing the dizzy doesn't change the timing, it changes where the rotor contacts the cap. You have to cut the locating tab off the base of the dizzy and use an old cap with a hole in it to locate the rotor in relation to the cap post. I'll try to find a link, but it was a TSB from jeep.

Indexing the dizzy will change the injector timing on a RENIX, other than that there is little effect no matter where you put the dizzy as related to ignition timing. With that said, on my 90 a tooth either way made little difference but 2 teeth either way and it would not run.

88 Wagonman said:

Indexing the dizzy doesn't change the timing, it changes where the rotor contacts the cap. You have to cut the locating tab off the base of the dizzy and use an old cap with a hole in it to locate the rotor in relation to the cap post. I'll try to find a link, but it was a TSB from jeep.

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Even then, it's suspect. Considering I was able to cut the tab off of a dizzy, start the engine, then turn the wretched thing 180* in either direction without a hitch in its idling...

I still haven't figured that out yet, but it did confirm that the dizzy doesn't have anything to do with timing (and that the CMP is largely ignored once the engine is running.)

The last point was tested by unplugging the CMP and trying to start. It coughed a couple of times when it was "guessing" which cylinders were coming up, then idled smooth as glass once it caught. Revved and drove fine as well.

There's a lot about RENIX that I'd really like to know...

5-90 said:

There's a lot about RENIX that I'd really like to know...

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Me too. :tears:

Exxon Valdez Jeep said:

Me too. :tears:

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Me three. :rattle: