dmoe69

Doublechaz, Thanks for the input, you have the rich/lean indications right. Watching the last video of the REM data, it appears to me that the engine is running rich (by indication, low ST around 35ish) at and slightly above idle and the ECM is trying to correct. At the point the engine hits the “rev limit” the O2 sensor goes steady and high around 4.7 volts +, which would indicate a lean condition, even though the ST doesn’t completely concur, maybe climbing to around 165. The first time he rev’d it up, it didn’t go into open loop until a few seconds after it settled back down to idle. If you watch the ST, it is trending rich, right out the bottom (0), but the lowest seen is 13, probably because the REM doesn’t update fast enough. The second time he rev’d it, it goes into open loop while it is “rev limited”. It appears because the O2 sensor pegs high for too long.
In other words, indications are telling the ECM it is running rich at one moment, and lean at another. The question that needs to be resolved is whether it is a physical/mechanical problem(s) or bad sensor(s) input before he continues to throw money and parts into it.

doublechaz

Yes, the ST is "correct" at one point and "wrong" at idle, but ST is a calculated value of what the ECU is *doing* in response to what it *believes* is going on with the engine. The ECU can be wrong if a sensor is broken or reading a mechanical fault that its software isn't programmed to detect and deal with. My go to example is when an exhaust leak adds oxygen to the exhaust, the ECU registers this as running very lean and adds fuel even though the engine will barely run it's so rich. Newer ECUs will notice the difference between "air in", "fuel commanded" and O2 and report a code for "implausible X". A safer way to proceed is to check and verify the *inputs* to the ECU and only then check if the *outputs* of the ECU are in line with that engine state. The O2 sensor should not peg to a single number. Is there a bad ground that is saturating at the higher rpm so the O2 sensor builds a backup of charge? Is there a short that happens when revved? Is the REM not able to read the O2 when the ECU goes to open loop mode? This generation of ECU is very basic in what it does so a decent multi meter is critical to working on them. There is a wealth of information at cruiser54.com for how to do this. Plus all the head scratching going on in this thread.

dmoe69

I would like to recommend disconnecting the O2 sensor, start/warm the engine, check the “rev limit” with the ECM in open loop and a base fuel curve of 128. Unless the engine is actually running lean it should rev without undue correction from the ECM. Unfortunately, other than narrowing down the search to rich or lean, it doesn’t otherwise rule out much else.

The symptoms are not classic O2 sensor failure. I'm very interested in forum input and suggestions. I would recommend testing the O2 wiring. I'm also interested in forum input on the non/short projected spark plugs.

Thank you Ecomike at NAXJA forum for the following (test parameters in last sentence):

"For the Renix years, 87-90, the O2 sensor has 3 wires, 2 black and 1 orange. The orange wire (largest gauge of the 3) is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall, and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immediately after start up. Loss of that power will hurt gas mileage even with a good O2 sensor.

One of the black wires is a common ground for the heater power and O2 signal to the ECU, so a poor ground will give a voltage feedback from the heater power input, to the ECU causing poor mileage even with a good O2 sensor.

The third wire, also black is a voltage feed wire, 5 volts, from the ECU to the O2 sensor. The O2 sensor is an O2 concentration sensitive variable resistor. At optimal O2 concentration the 5 volt input feed to the O2 sensor drops to 2.45 volts due to losses across the O2 sensor to ground. That same wire if disconnected from the O2 sensor will read 5 volts constant to ground.

At idle that voltage should read 1-4 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 2 and 3 volts max, and is optimally running between 2.3 and 2.6 volts at 2000 rpm (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the ground and the 12-14 volts to the heater and the 5 volt feed from the ECU with power on and engine off."

dmoe69

You have a few other threads for this problem with a lot of good input. I've been reading through them. Someone suggested you get a noid and check each fuel injector connector to ensure they were all firing. That is something I was going to suggest. No indication you ever did that after given that advice. In one of your other threads on Nov 30, 2020 you state “Got my bolts for my CPS, and got that installed up, ran differently so it didn't rest against the engine... and now my engine won't rev past 2k rpm .” Is that when this started?

I suggest you get up to the CPS with an inspection mirror and see where the pick-up is in relation to the windows on the flexplate. I have an aftermarket flexplate installed and the CPS pick-up was over the very front edge of the windows. I had to add two washers (just large enough for the shoulder bolts to go through) between the CPS and the engine.

Put a jack at the back of the transmission. Undue the 5 bolts holding the cross member. Let the transmission down a few inches. There is a shifter linkage rod between the transmission hump and the transmission. This limits how far you can lower the back of the transmission. A few inches is all you need and you can reach the CPS from the bottom. If you think something was different in the way the engine ran at that point, get up there and see if something is wonky.

Earlier in this thread you seem to have changed out Bosch -710 fuel injectors for -746. I have -746 and they work well. In your other thread it seems you swapped in Bosch -943 at some point.

The spark plugs you seem to have installed are NGK gasket seated non-projected. The recommended 4.0L NGK spark plugs are all gasket seated projected plugs. I started out with non-projected plugs, they did not run well at all, but ebay said they fit a 1988 Jeep Cherokee.

Lastly, mine began to run really well when I had brand new fresh gasoline in the tank. In my case that also required another new filter and even new injectors. It was obvious when I tilted the filter and let it drain, it was not full of gasoline.

I have a REM and a good running engine. If you would like some comparison readings, just ask. Otherwise, seeing all the things you have done to your Waggy, you seem very capable and knowledgeable. I'm just adding to the voices telling you to check to see that your fan belt is buckled.

aroundincircles

Just as a follow up, I took it to a shop, and They see that it is perfect up to 2000 RPM, and then the ignition falls apart. They have the original CPS I took out (I didn't toss it). and that made no difference. they also swapped in my old distributor, ignition coil, etc. and new/old parts, all runs the same. so after a week, they too are clueless.

aroundincircles

Just heard from the shop again, Flex plate is out of round slightly, so at low RPMs everything is fine, but as things speed up, the cps just cannot read it correctly. so it stops sending spark. At least that's the only thing they can find. So having them replace the flex plate.

doublechaz

Sounds plausible.

Seems like from all I've heard the CPS is on the ragged edge of not working at the best of times. All the after market parts that are even 5% away from perfect don't work. Any wiring issue at all and it doesn't work. Slight metal dust contamination on the teeth and it doesn't work. Slight sensor alignment problem....

All the cars that I've worked on that never have CPS problems also use shielded sensor wires. Maybe someone who isn't up to their eyeballs in time debt can run a shielded CPS harness and see if it runs better. Wouldn't that be a hoot.

cruiser54

What's the CPS reading when this happens? It's testable.CRUISER'S MOSTLY RENIX TIPS

RENIX CPS TESTING AND ADJUSTING

OCTOBER 30, 2015 SALAD 127 COMMENTS



Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS itself as shown in Figure 2 as Connector A, crank the engine over. It won’t start with the CPS disconnected.

You should get a reading of .5 AC volts.

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from NAPA or the dealer.

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

A little trick for increasing the output of your CPS is to drill out the upper mounting hole to 3/8″ from the stock 5/16″, or slot it so the CPS bracket rests on the bell housing when pushed down. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

Another little tip to save tons of aggravation is to stick a bit of electrical tape to your 11mm socket and then shove the bolt in after it. This reduces the chances of dropping that special bolt into the bell housing.

lawsoncl

I'm not sure I believe a weak cps or increased flexplate gap on the renix causing stumbling at higher rpm because it puts out more voltage at higher rpms.

I assume they would have scoped the signal from the cps and the signal to the ignition module?

aroundincircles

The flex plate is damaged enough they could see it with their eye (inspection plate removed). so obviously it's a problem I didn't even think to check it. When I had it, I checked the CPS manually and it read fine at idle.

doublechaz

The bent flexplate would be likely to bend more at higher rpm the same way a bell is one shape when silent and flops around more as it rings louder.

aroundincircles

Flex plate was the problem. Runs great now, now it’s time to wrap up the rest of it.

cruiser54

That's one for the record books!! Glad it is fixed.

aroundincircles

The mechanic couldn’t figure it out until they had the inspection plate off and it was up on the lift, and they had one guy revving it up, checking on something else and noticed it out of the corner of his eye. And had to confirm it on their brake lathe.

Red1992XJ

I am so happy that you were able to finally get it fixed. You put a heck of a lot of work into the truck, and now you can really enjoy it.