1987 Cherokee Renix 4.0 - rough Idle/misfire

Hey everybody, so I have read what feels like hundreds of posts about this issue and it seems like many of them just disappear or decide to sell the truck because they are so tired of dealing with it. Hopefully I will be different and I can help the next guy in diagnosing his issue. I have been working on this for a few days now, but this Jeep off a guy that said it had been sitting for about a year and developed a misfire. I have been going through cruisers tips to see if any of those would make a difference but to no avail. I recorded a video that can be seen here showing the symptoms and talking about some of the things that I have done. Hopefully the video helps you see what I'm dealing with and also maybe you'll see something that looks wrong.

https://youtu.be/gQxmtOmTrkQ

Here is what I have done so far:
Checked ground on TPS = no resistance
Set TPS to 17% of 4.81 = .83
Refreshed grounds
Cleaned and swapped MAP and IAC with working one from my other Renix 4.0 - can swap other parts too, my 89 is a good running Renix.
Cleaned throttle body
Changed plugs to new champion copper gapped at .035
New wires
New distributor cap
New distributor rotor
Checked compression = 150ish on all cylinders
Checked spark with each wire and each plug in each cylinder
Tightened intake bolts on top
Sprayed all vacuum hoses with starting fluid to check for leaks
New O2 Sensor
Cleaned coolant temp connection
Checked EGR function
Ran seafoam through tb
Made sure gas was good and added more

I know there are few things I still need to do like tighten the bolts on the bottom of the manifold, double check that all hoses for vacuum are clear and a couple other things but if you can think of anything that you feel would be attributing to this issue I would greatly appreciate the suggestion. I will work on this thing every night until it is ready to go!

Brian

Unplug injector connectors one at a time and see if anything changes.
Also, consider that the vehicle is almost 30 years old, and the injectors may need to be replaced.

Oh and the muffler was cracked and vibrating and the cat was toast, all the catalyst was cracked up and rattling. I pulled both off to ensure I didn't have a blockage.

Hmmm, I didn't think of that. I'll give it a shot tomorrow and report back at night. Thank you for the input!

Seems like a sensor issue, it's pretty methodical with it dropping out. I'd pull the computer and dielectric the connector and try that. But def could be an injector sticking as well.

Also the tube on the filter box is a warm air diverter. Below a certain temp there is a bimetallic strip in the box that diverts vacuum to the thermactor valve on the opposite side of the hose. It then draws air that is pulled over the header so that it's warm when it hits the engine. Just FYI. Mine broke and I just run without it. But I do live in FL and don't really need it.

X2 on what walker said. Also with it running wiggle every sensor plug and listen for changes. I don't know if you done it yet but Google "cruiser54 mostly renix tips" and do all of them.

I honestly don't know what that is.

First impression is something with the injector timing. Maybe unhook the sync sensor and see what happens. And just for giggles unhook the battery and reconnect it and run it with the sync sensor unhooked. You have a fifty percent chance of it being in sync this way. Some people will argue with this, but seriously it will only take half an hour, why not?

Does it do this hot and cold. On a cold motor many of the sensors are bypassed. If it only does this on a hot motor it is likely to be a sensor issue.

I keep a little kit of two inch pieces of vacuum line with screws in one end to plug off all of the vacuum lines to the intake (except the regulator vacuum line). Reason is many of the vacuum lines, if leaking, are off center on the manifold, you can get lean and rich cylinders at the same time.

Getting spark doesn't mean getting good spark. Open up an old plug to -.060 hold it on a good ground and look at the spark, yellow is weak, blue/white that you can actually hear go snap is good. Tip of the day, don't press your crotch against the fender while checking spark.

Check your voltage into the ignition module, the larger yellow wire. Do an ohm test at the ignition module ground. There will typically be some voltage loss, between battery voltage and what actually makes it to the ignition module, sometimes too much. The ignition module and coil are fairly high amperage and need enough power. A couple of volts low isn't likely to affect much, too low sure will.

I've had a hard miss at idle for various reasons, One of the hardest to figure out was the coil to ignition module contacts were old and weak and making poor contact. Be careful with this connection, it is supposed to be kind of springy, when it gets very old it can get soft. It can break off or fall apart on you if you fiddle with it much.

Idle misses on the Renix can drive you nuts. I spent many hours trying to fix mine (87 and 88). I finally got my 88 to idle really good, exactly how I did it is anybodies guess. My guess is it was a combination of things and I accidentally fixed it while cleaning and inspecting.

If all else fails, the coil to cap wire is more important than the cap to plug wires. I've had to try a few before I was satisfied, I eventually went back to a very old OEM coil to cap wire, it seemed to have a resistance the ignition module liked.

I'm really proud of my 88 idle now, a little low around 500 RPM but it chugs like a metronome, never a miss. I took me forever to do it.

Something else you may want to do is to add some injector cleaner or HEAT to your fuel tank. Too much condensation in the fuel can also cause this. An occasional dose of injector cleaner through the gas tank seems to help sometimes anyway.

Too much fuel is as bad as too little, your exhaust didn't look bad, but the fuel ratio screw up may be subtle. Check to see if there is any fuel in the fuel regulator vacuum line, use your nose.

Lkwelch said:

Seems like a sensor issue, it's pretty methodical with it dropping out. I'd pull the computer and dielectric the connector and try that. But def could be an injector sticking as well.

Click to expand...

I will mess with the injectors tonight if I get a chance. I am trying to be very logical about all of this and I see a lot of people that just throw parts at these things and never figure it out... there has to be a reason right? if an injector is bad, maybe it is just leaking fuel into one of the cylinders and it isn't enough to fire it everytime but after a build up of fuel it fires and then goes back to missing... I don't know. Or maybe there is a vacuum buildup that occurs and once it hits a point it allows it to run good, but then loses the pressure and goes back to running poorly. I will take your information and apply it. Wouldn't hurt to pull the ECU and swap it with my working 89... thanks for your thoughts on this.

mtbxj87 said:

X2 on what walker said. Also with it running wiggle every sensor plug and listen for changes. I don't know if you done it yet but Google "cruiser54 mostly renix tips" and do all of them.

Click to expand...

Ive have been through Cruiser54's list a few times and did most of it, not all as there are some things in there that didn't seem to have any bearing on my issue, more of just a "good to do". One thing about living in San Diego is the corrosion issue isn't like it was when i lived in IL... no winter, no salt, I don't live next to the ocean so humidity is very low. All connections that I HAVE pulled open are clean and looking good. I will get some DieElectric grease though just to start getting some of that in the plugs. thanks for the thought.

8Mud,

Thanks a ton for the detailed response, lots of great information there! So to answer your questions and add to my initial information:

Basic answer, Yes, it runs much better when cold. I notice this misfire heavily when it is warm. Yesterday it was around 55 degrees and when I tightened the manifold bolts and started it up I thought I had done it! Drove it around and it felt better, not perfect, but much better and it didn't have that clanky misfire sound coming out of the exhaust so it sounded like all cylinders were firing. Once I let it run for a while and got it up to maybe 150 or so on its way up, it started with a slight misfire and then the hotter it got the more the misfire was apparent.

Now as I understand it... a bad sensor somewhere could be causing a very rich situation, but when the engine is cold, the rich situation could be more easily disguised as normal running, is that correct? BUT if you are telling me that certain sensors are bypassed when the engine is warming up, then maybe I should start with those sensors as once it warms, and those sensors get thrown into the mix, it runs poorly. Do you know by chance which sensors are bypassed when cold? aside from the EGR Valve which looks new and moves with an increase of RPM when the engine is up to temp? any chance the EGR could still be bad?

I can take the coil out of my 89 and throw it in there tonight, would that be a good test? I will still test the ground there as that will be constant no matter what the coil in there is.

I will try a plug with a .060 gap today as well and will let you know the result.

I did hold my ear up to the injectors using a screw driver and all injectors were clicking in the same pattern... although clicking doesn't meant they aren't clogged...

I guess the main question is... since it runs much better when cold than it does hot, should I hold off on some of these tests and focus in on some more important tests based on logic? If a bad injector were present, or a bad connection somewhere, or a bad CPS, or a bad TPS or IAC or plug, wire, dist cap, rotor or distributor for that matter... would engine temp have any effect?

One thing that may be important now that I think about it is that i have to jump start this jeep everytime I start it as the battery only holds around 8-9 Volts. I have a 12 Volt jumper pack and I have left it on to see if that added voltage helps and it makes no difference... when the pack is not connected, the battery level on dash is around 8-10V... not sure if that makes a difference. I do know the alternator works and the battery is shot though, that is for sure.

I will try uplugging the battery for a little while just for the heck of it. I am not familiar with a Sync Sensor though. I will research it and unplug it as well.

Thanks again! I can take more vids or whatever you need, just ask. You guys are great, thanks.

I disconnected thebattery for a while while I ate and no change. I disconnected the sync Sensor and started it and no change. Continuing on!

Ignition coil yellow wire was 11.84 volts and battery is 12.44.

Battery volts being low can definitely wreak havoc on sensors and electronics. I'd get that battery up to snuff first.

If your getting faulty info from the O2 sensor it can mess with your fuel. EcoMike says unplug the O2 sensor and see what happens, the ECU should default to a standard fuel curve instead of sensor regulated fuel, I've never tried it.

Many of the outputs from the ECU are standard (programmed) until the temp. gets above 140F or so. Then the ECU uses more sensor inputs to trim fuel, set timing etc.

The most common sensor failures are the O2 and the CPS. The Ohms in the CPS can rise as it heats up and can cause a poor signal to the ECU or intermittent weak pulses. 175 ohms on a hot CPS and it can be crap shoot whether the motor will start at all.

Two things to check is the CPS wire and make sure it isn't cooking on the exhaust manifold. And the O2 sensor wires where they go up the front of the block, again they can cook on the manifold. The front harness for the O2 sensor, the knock sensor and engine (to ECU) temperature sensor can be hard (almost impossible) to see where it tends to contact the exhaust manifold. Cruiser has a post showing the best way to route this harness to avoid issues.

The MAP sensor vacuum line sometimes contacts on the bottom and may rub through making a tiny crack or hole. You can often feel it with a finger tip run on the under side of the vacuum line. Be careful of this line, it is plastic, it gets old and very brittle.

I wish you luck, been there done that, it can take a long time to find the issue or issues. But whenever you get done you will know your XJ pretty darned well.

One of the hardest things to find is harness issues. The first place to start is by shaking the harness around a little and see what happens. The injector wires can fatigue in the harness where the bend is by the firewall. The copper inside the wire can fatigue and break under the insulation. I've found the copper core of the wire broken except for a few stands.

If oil has leaked onto any of your connectors, spray them out good with a quality contact pray. Very low voltage going through those sensor connectors, it doesn't take much to screw up the signal.

So I have finally identified that it is cyl 6. After some testing with pulling plugs and also injector wires I had definitely noticed that cyl 6 is causing the issue. I know we are getting spark there and I know it has 150 compression. I did not know about the Injector but I swapped one out of my 1989 Cherokee and it made no difference. I checked to see if I could read voltage on cyl 5 and 6, cyl 5 pulsed up to 1.5v and 6 pulsed up to 1.25 v. Not sure what that means if anything. A leaky manifold wouldn't cause a single cyl to fail would it? A alive issue would cause compression issues right? This is strange! Tight valve maybe? Or is that voltage an issue maybe? Or a rat's nest in the cyl 6 intake maybe? Haha. Maybe I will pull the intake..

I had a very strange issue with the #6 on my 89. No start/rough idle. Ended up being the ECM. I had heard through here that that plug is known for breaking. So check the resistance from the ECM to the injector and wiggle the harness to make sure.

go to parts store and rent the noid light.
they probably call it an injector tester or something.

anyway, that will show if the injector is getting a signal from the ECM to pop.
I'd probably pull the valve cover and make sure that it's not something dumb like a rocker arm that's loose.

Also, follow the harness from the injectors back to the firewall, looking for something chafed.

Run just like an 88 I have in my driveway that needs a new motor because of blowby caused by a worn motor. That filter that was added looks like it was added to keep oil form getting into the air box which is what happens when you have blowby. Take the oil cap off with it running and see if you feel air buffing out the valve cover.