'88 4.0L Renix - Super Rough Idle & Stuck in Open Loop (MT2400 Video)

[boxyjeep]

I've been systematically getting my '88 4.0L Comanche into better running condition and now am encountering an issue with super rough idle.  I've verified the TPS voltage is good and just replaced the IAC.  The video will show how the engine tries to go into closed loop mode, the short term fuel trim spikes up, then it flips back into open loop, all the while stumbling with idle.  Before I replace the O2 sensor, does anyone have ideas that could cause this?  The coolant and air intake temps appear good, as well as the MAP voltage.  The O2 sensor reads a constant 4.98v and never fluctuates (although I've read that is expected because an analog voltmeter is the only way to read it).

 

 

[Ωhm]

21 minutes ago, Chargum85 said:

The O2 sensor reads a constant 4.98v and never fluctuates (although I've read that is expected because an analog voltmeter is the only way to read it).

Your scanner is reading digital data and is correct.

 

Engine is running very lean (O2=4.98vdc) . Note INJ PW climbing to 8-9 mS prior to going into OPEN loop. Check fuel pressure.

[boxyjeep]

Your scanner is reading digital data and is correct.
 
Engine is running very lean (O2=4.98vdc) . Note INJ PW climbing to 8-9 mS prior to going into OPEN loop. Check fuel pressure.

Thanks. Will check that. When I got the truck I noticed the fuel resistor ballast was bypassed and I plugged it back in. Will try bypassing it to see if that has any changes. Will also check fuel pressure. Prior owner also recently changed the fuel pump and filter.


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[cruiser54]

Your O2 sensor is bad or is not getting heated by the red wire going to it. Or, the O2 sensor ground is no good. 

I'd like to see what the MAP sensor does when the throttle is opened.

 

[boxyjeep]

11 hours ago, cruiser54 said:

Your O2 sensor is bad or is not getting heated by the red wire going to it. Or, the O2 sensor ground is no good. 

I'd like to see what the MAP sensor does when the throttle is opened.

 

I believe the wiring to the O2 sensor is good (I recently replaced the plastic connector).  I'll get a video of it running at 2500 RPM's.  I also got a few comments when I posted this on Reddit.  Mostly focused on either an exhaust leak or bad O2 sensor.  Going to also try plugging some vacuum lines as a process of elimination to see if I can isolate a leak.

[cruiser54]

When it stays at one reading, the O2 sensor is not working. Check for battery voltage at the red wire with it running. Check the ground to the block at the same connector. Black is ground. 

 

Have you ever visited my website and completed tips 1 through 5? 

[dvitha]

Check the wiring on the O2 relay. The Relay may be good, but with bad connection because of the dried electrical grease as well as degrading wire near crimp points on relay plug to be a cause. Your o2 sensor is not heating.

 

[boxyjeep]

Thought you guys might want an update on this, since it seems like rough idle on Renix common problem.  After fiddling with a variety of potential causes over the past year and using a REM to verify all sensors were good (I did replace the O2 sensor as recommended above), I finally ended up replacing the cylinder head.  This solved the issue and it was clear upon removal that the old head had problems.  The combustion chambers were rusty and I doubt the valves were sealing very well.  Gasket didn't look too bad, but it was also leaking oil out of the passenger side of the block.  I couldn't see any obvious cracks or warping, but I'm sure it wasn't true.

 

Parts Replaced:

1. IAC
2. O2 Sensor
3. Coolant sensor
4. Knock Sensor
5. Complete exhaust: manifold, cat, muffler
6. Intake/exhaust gasket
7. Injectors, Plugs & Wires, Distributor
8. Timing chain (checked for tightness, not replaced)

 

All of these parts incrementally improved how it ran, but never really fixed it.  The PS pump would also constantly whine because the engine didn't have enough low RPM power to spin it fast enough. 

 

Moral of the story is to pull the cylinder head on any older trucks that aren't running well.  Even if it is good, you'll also be able to check the cylinder walls, head gasket, etc, and verify everything looks good (and I'd suggest upgrading to a HO 7120 or 0630 head at the same time...).

[cruiser54]

How about a cylinder leakdown test right off the bat?

 

FWIW, swapping to the later head is not worth it. 

[boxyjeep]

Just now, cruiser54 said:

How about a cylinder leakdown test right off the bat?

 

FWIW, swapping to the later head is not worth it. 

 

Yeah, I didn't have a leakdown test kit at the time, but should have started there.  Will always be doing a compression and leakdown test on any new engines I get.

 

From my initial seat of the pants testing, the HO head was well worth it.  The truck feels completely woken up.  I also plan to swap in a HO horseshoe intake and TB at some point soon, which should bring some additional improvements.

[JustEmptyEveryPocket]

7 minutes ago, Chargum85 said:

From my initial seat of the pants testing, the HO head was well worth it

From the above post, you said the truck wasn't running well to start with. I would guess that any refreshed head with better seals + new gasket would have given the same seat of the pants xp. I doubt it was just the fact its an HO head. 

 

This seems like a common misconception across the forum. HO parts (or similar) don't give substantial gains (or any gains really) besides the fact you are replacing old worn parts with newer less worn parts. Not saying its bad to do, just keep in mind you are comparing 30+ years of abuse to new. *shrug* 

[boxyjeep]

17 minutes ago, JustEmptyEveryPocket said:

From the above post, you said the truck wasn't running well to start with. I would guess that any refreshed head with better seals + new gasket would have given the same seat of the pants xp. I doubt it was just the fact its an HO head. 

 

This seems like a common misconception across the forum. HO parts (or similar) don't give substantial gains (or any gains really) besides the fact you are replacing old worn parts with newer less worn parts. Not saying its bad to do, just keep in mind you are comparing 30+ years of abuse to new. *shrug* 

 

Yep, totally understand that (and I know "seat-of-the-pants" evaluations are BS to begin with).  I'm planning to run a HO intake once I'm out of CA and don't have to deal with smog.  I went through and rewired the whole Renix electrical system and can now fully appreciate it's simplicity ;-).

 

With the HO head and intake/TB, I would expect some gains.  But in all honesty, it's a 30+ year old truck that won't see much time on the road.  If/when I need more power, I'll solve that with an LS swap or gears.

[cruiser54]

1 hour ago, Chargum85 said:

 

Yep, totally understand that (and I know "seat-of-the-pants" evaluations are BS to begin with).  I'm planning to run a HO intake once I'm out of CA and don't have to deal with smog.  I went through and rewired the whole Renix electrical system and can now fully appreciate it's simplicity ;-).

 

With the HO head and intake/TB, I would expect some gains.  But in all honesty, it's a 30+ year old truck that won't see much time on the road.  If/when I need more power, I'll solve that with an LS swap or gears.

HO myth buster

Renix in 90 made 182 HP. HO in 91 made 190 HP. That's 8 HP difference. 

HO only made more HP than Renix at higher RPMs and not a bit more torque. HO had a 58 mm throttle body versus a 52 mm throttle body on a Renix. That’s 20% more air available through the HO throttle body. The HO also had a better design header. See where I'm going with this? 

It’s only a 4% horsepower increase…..

The whole 8HP was not mostly from the head, but from the bigger TB and better exhaust manifold.

Put a 60mm TB from www.strokedjeep.com on your present manifold using the Renix head, eliminate the "crush" in your headpipe with proper re-routing, and go for it. 

HO stands for Highly Overrated.
__________________
 

[boxyjeep]

5 minutes ago, cruiser54 said:

HO myth buster

Renix in 90 made 182 HP. HO in 91 made 190 HP. That's 8 HP difference. 

HO only made more HP than Renix at higher RPMs and not a bit more torque. HO had a 58 mm throttle body versus a 52 mm throttle body on a Renix. That’s 20% more air available through the HO throttle body. The HO also had a better design header. See where I'm going with this? 

It’s only a 4% horsepower increase…..

The whole 8HP was not mostly from the head, but from the bigger TB and better exhaust manifold.

Put a 60mm TB from www.strokedjeep.com on your present manifold using the Renix head, eliminate the "crush" in your headpipe with proper re-routing, and go for it. 

HO stands for Highly Overrated.
__________________
 

 

Yep, I know.  I've read this post before.  I am running a '90 ECU as well.  Will probably stick with the current setup for awhile and re-gear if power becomes a concern.

[Ωhm]

3 hours ago, Chargum85 said:

After fiddling with a variety of potential causes over the past year and using a REM to verify all sensors were good (I did replace the O2 sensor as recommended above), I finally ended up replacing the cylinder head.  This solved the issue and it was clear upon removal that the old head had problems.

That's one thing a REM doesn't do, is tell you that you have a mechanical problem. Glad you figured it out.