Fun with RENIX. Thoughts?

Well, now that I've read every RENIX thread I could find that seemed to be relevant to the issues I have with my truck, I figured I'd make one for myself. Somehow I almost feel like it's some sort of rite of passage for new Jeep owners.

Anyway, after going on a sensor testing, replacing, and testing again binge, I have improved the overall drivability of my XJ, and have narrowed the symptoms of my RENIX problems down to a (thankfully) much shorter list. They are:

*Random high idle. The truck idles perfectly at about 700rpm when it's in it's cold start cycle. Most of the time, the truck will idle at 900-1100 when warm in drive with my foot on the brake, and at 1200-1600 in park/neutral. Sometimes it idles at 1000-1100 in park/neutral when warm. I haven't been able to pin down any pattern to this at all. The idle speeds are stable, there's no hunting at all. The only other thing I noticed that may be related is that the egnine speed jumps to 2500-3000rpm immediately after starting the truck, before it settles down to whatever idle speed it desires at the time.

*HORRENDOUS fuel economy. I'm talking 10-11mpg consistently. The best I've seen is 14mpg, and that was an all-highway trip. The truck has a 2" BB and 30" BFG ATs. Otherwise the drivetrain is stock. I have a heavy foot, but it's not heavy enough that I get what seems to be about half the MPG that most people seem to be getting from XJs similar to mine.

*Backfiring/loss of power. I could help this a bit if the truck didn't insist on shifting into 4th gear and overdrive as soon as possible, but I suppose that's the woe of owning a vehicle with an automatic(my first, and likely my last). Anyway, if I accelerate away from a stop(say from a traffic light), then let off of the gas completely(to avoid running into the person in front of me who can't operate the skinny pedal properly), when I get back on the gas again, the truck backfires, hesitates, and then accelerates normally. This is worse when it's cold, but also happens when it's warm. It also happens at highway speeds when I do the same thing(throttle, release, throttle again).

Aside for these issues, the truck runs very well. It has plenty of power, responds well to throttle inputs at all speeds(short the backfiring thing), and upshifts/downshifts as well as I can reasonably expect from a 19-year-old truck.


Any thoughts you guys have on this would be greatly appreciated. I love driving this thing, but I need to rectify these issues so I can trust it 100%, as it becomes my daily driver in a few months when the lease on my other car ends.:worship:

Tighten your intake manifold bolts. Replace the O2 sensor. Check all the sensor grounds for resistance. Shake the injector harness around a little up near the firewall while you do the ground testing.
The hot idle mode on your Renix uses every sensor, is sensitive to vacuum leaks, sticky IAC and sensor grounds. Unlike some motors a small vacuum leak may not raise your idle much if any, but can make it run rich under most circumstances.
The popping back through the TB can be a bad O2 sensor, if it runs lean it will do that. The O2 sensor can run lean under some conditions and way rich under other conditions. The O2 sensor can also mess with your idle.
Check the injector harness where it runs down the front of the motor and make certain it isn't cooking on the exhaust manifold.
I've had a Renix in the driveway since 87, temperamental on occasion, but generally tough and reasonably dependable.

Make sure your grounds from motor too fire wall are clean an tight..

scottmcneal said:

Make sure your grounds from motor too fire wall are clean an tight..

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Clean the stud on the rear of the head. Also, use a wire wheel to clean a spot about the size of a postage stamp around the screw hole in the firewall down to bare metal. Clean the lug as well, and apply corrosion inhibitor to both before you reassemble. Clean or replace the screw as well. That ground strap is called "the RENIX killer" for a reason...

Mine did the same things but the back firing. Tightened the intake bolts ,mainly the rear ones, and it now idles fine.

Tighten your intake manifold bolts.

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Done. I got maybe a 1/4 turn on each of them.

Replace the O2 sensor.

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Done.

Check all the sensor grounds for resistance.

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All grounds have been checked and cleaned. Additionally, every connector in the engine bay has been disconnected, cleaned, and had dielectric grease applied.

Shake the injector harness around a little up near the firewall while you do the ground testing.

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Did that about a month ago when I was checking my work after replacing all 6 injectors.

The hot idle mode on your Renix uses every sensor, is sensitive to vacuum leaks, sticky IAC and sensor grounds. Unlike some motors a small vacuum leak may not raise your idle much if any, but can make it run rich under most circumstances.

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The only place I can think of that might have any type of vacuum leak would be the valve cover. And the only reason I say that is because it tends to leak at the back after the truck has been driven hard. But the problems I have are occuring all the time, not just when the truck is driven hard. And I have never seen a minor, intermittent valve cover leak cause backfiring through the intake and a 50% drop in fuel economy. How sensitive are these things to leaky valve covers?

The popping back through the TB can be a bad O2 sensor, if it runs lean it will do that. The O2 sensor can run lean under some conditions and way rich under other conditions. The O2 sensor can also mess with your idle.

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The 02 Sensor is brand new less than 2 months ago. I also double checked the wiring for continuity and resistance pre- and post-replacement.

Check the injector harness where it runs down the front of the motor and make certain it isn't cooking on the exhaust manifold.

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I'll look at that, but would a burned up injector harness not cause a permanent fueling issue? I would think that if the injectors weren't getting a signal due to damaged wiring, they wouldn't work at all.

5-90 said:

Clean the stud on the rear of the head. Also, use a wire wheel to clean a spot about the size of a postage stamp around the screw hole in the firewall down to bare metal. Clean the lug as well, and apply corrosion inhibitor to both before you reassemble. Clean or replace the screw as well. That ground strap is called "the RENIX killer" for a reason...

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Did that, too. I added 4-gauge ground cables in 3 places: Battery to firewall, firewall to the block(behind the dipstick), and firewall to the stud on the back of the head. All of the grounding points were cleaned to bare metal, and as with all other connections in the engine bay, dielectric grease was applied.

I appreciate the input, guys. Keep it coming! :wierd:

check that the throttle linkage itself is not your issue.
the MJ had a random high idle, I chased it for a month in sensor and electrical issues, couldn't find anything.

Then one day it did it and I got out, looked at the throttle linkage and saw that it wasn't closing properly. Some grease later on the linkage has cured that problem.

Did that, too. I added 4-gauge ground cables in 3 places: Battery to firewall, firewall to the block(behind the dipstick), and firewall to the stud on the back of the head. All of the grounding points were cleaned to bare metal, and as with all other connections in the engine bay, dielectric grease was applied




You should have the main neg battery cable attached to the engine block also behind the dipstick. New sensors on a renix do not always work even if they are new for some reason.

:looney:

The injector harness where it goes down the front, has the O2 sensor, knock sensor and engine temp sensor wires in it. I've seen half a dozen XJ (mostly early models) with that harness cooked and the insulation falling off of the wires. Which can't be good.
Maybe back to basics and check your compression. Then put a vacuum gage on there and see if the needle swings wildly, the reading is less important than if the needle stays relatively stable.
I've never gotten better than around 15 MPG, mixed city and highway.
You can plug off every vacuum line except the MAP and still drive it. I plug all of mine off periodically just to see if it acts differently, so I know to go look for a vacuum leak.
Check your TB hold down bolts, the gasket shrinks.
There is a splice in the harness about four inches after the bend on the firewall, where various sensors come together to a common ground. My TPS ground wire was loose there, tested fine, but was just barley sitting in a very loose crimp.
Pull your MAP vacuum line plug up one end and suck, stick your tongue in the hole on one end and your finger over the other, see if it holds vacuum, this line gets cracks.
At least two people I know off have had bent pins at the MAP, though this should run you rich and not lean.
Double check your spark plug cables, especially your coil to cap cable. In the dark you can often see spark jumps. Even new coil to cap wires act up and can induce a miss, they seem to be resistance sensitive, stock is best in this location. The spark plug cables seem less sensitive.
The distributor cap tends to cross fire and wear fast. I change mine yearly.

Still sounds like an O2 sensor issue, if it isn't the sensor it may be in the wiring, you may have to test both the ground side the input side wiring and the heater circuit function along with the heater relay.

Low fuel pressure can also cause bucking, anything that causes the motor to run lean is a possibility. Cross fires, when the spark jumps to another spark plug cable or in the cap, initial timing (loose timing chain and worn distributor gears), and bad valves can also cause pre ignition or cross fires.

Have you replaced the fuel filter and/or the pump? If not, you may want to check the pressure at the rail and at least replace the filter.

Additional -
Add a ground directly from the battery to the chassis (unless I missed it, you haven't done that yet.) Treat similar to the others.

Check the stud nuts on the exhaust collector flange (there are two - use a 9/16" socket.) They also work loose over time, and the resulting exhaust leak can screw with HEGO readings and get your fuel trim to go all stupid.

Use the "carburettor cleaner trick" (q.v.) to check for vacuum leaks at the base of the throttle body and the base of the EGR valve - both are more common than you'd think. The TB gasket would require no special treatment - the EGR gasket is helped by a light coat of RTV copper before installation. Likewise the manifold gasket, should you replace it.