Renix stumbling around 2300RPM - SOLVED IT!

[Smash6]

My son and I are working on his first car. Details in this thread lined below. To call this rig a bucket-of-bolts is a compliment. It was rescued from a field and the PO did a lot of work to get it running. She goes well except for this stumbling/backfiring issue that I need to get resolved. The block is from a 2000 with all Renix stuff. 

 

Under load, the comanche stumbles and backfires around 2300-2700 (approx) RPMs under mid-ish throttle. I can press through it and it cleans up just fine. The motor pulls hard otherwise. If I manage the throttle when accelerating, I can pull to redline without a hiccup. But under load with the throttle in the wrong position and she's going to spit, sputter, buck, and backfire. It's also running rich, by smell. 

 

Things done to date:

1. Cruiser Tip 1 - ground refresh

2. Cruiser Tip 2 - C101 connector cleaning

3. New NTK O2 sensor

4. New Standard Motor Products TPS

5. Vacuum lines all tight and sealed

6. Plugs, wires, distributor cap/rotor, and coil are all relatively new and in great working order

 

I've noticed some mild improvements (maybe?) and I keep hoping to find the problem and I just can't.

 

I purchased a Renix Reader and here are my readings:

Good:

Ignition at Idle - 14*

Knock sensor - 0 at idle

Inj pulse width - 5.5ms at idle

Injector Duty Cycle - 4.2% at idle (I need to check this at WOT, but haven't been able to yet)

Water Temp checks good

MAP - 12Hg

Relative Manifold vacuum - 17.7VAC

O2 goes into closed loop and indicates sweeps that I think I should be happy with

Bad/questionable:

Short Term Fuel Trim - ~141"The ECU needs more fuel then base to run properly"

Long Term Fuel Trim - 151 "The ECU needs more fuel on average then base to run properly"

AFR - 14.9-16

REM swaps between "LEAN EX" and "RICH EX"

 

I MAY have a small exhaust manifold leak. I'm not positive it's from the manifold and if it is, it's faint. 

 

Thank you for anyone that has an idea for me to try. I'd be in your debt if you can help me solve this one!

 

 

 

[pizzaman09]

Are all of your header/manifold bolts in place and are they tight?

[Ωhm]

Sounds like crossfire in the distributor cap.

 

Cruiser Tip 13 - Renix Distributor indexing

 

 

Fuel Trims values are well within the ECU's range of authority. Yes, ECU did add fuel but still in Closed Loop (CL).

[Smash6]

The rear most exhaust manifold bolt was not completely tight. It is now and it made no change. 

 

Ohm, thank you for the response. I'd really appreciate you taking the time to look at the below video to see if you still think it's a crossfire issue. With your volume on, you can clearly hear the problem. After chasing sensors, grounds, connections, and vacuum I think you're onto it being an ignition issue. What's baffling me is how well the Jeep runs outside of that specific RPM range under that specific throttle setting. If I holeshot the Jeep and keep the pedal mostly down, it runs great and never stumbles. I'm capable of driving it in a way that mostly avoids the misfire. THANK YOU!

 

 

[Smash6]

Building on this sage some more:

 

I pulled the cap and rotor. Before I had only done a visual inspection and saw they were new. Now that I've pulled the rotor, I have concerns about how much play there is.

 

The distributor shaft has about .15" rotational play, .03" vertical play, and no lateral play. This feels like a lot in my hand, despite how it measures. Been a while since I've had a vehicle with a distributor and I don't remember ever feeling play like that. Thoughts?

[Eagle_SX4]

I had my cam shaft eat my distributor gear. It could be that but I have not found any other instances of that happening so it is a real long shot. 

 

This is what my distributor gear looked like.

[Smash6]

So we removed the distributor. I didn’t like how much play there was. The gear shows some wear and there was oil in the distributor housing. It appears to be the original motorcraft. So we’ll be replacing it. 

[Eagle_SX4]

Look down in the distributor hole and check the gear on the cam shaft. If it is worn out it will just eat your new distributor gear even worse than the old one.

[Smash6]

Thanks! Peaked in and it isn't too bad. I think the long block is about 13 years newer than that distributor. It isn't perfect...but at this point, if that cam is hosed, the motor is coming out for a rebuild. F-it. 

[Eagle_SX4]

The date code on the engine is just above the distributor hole towards the front slightly.

First number is the year, second 2 are the month and the last 2 are the day.

[Smash6]

OHM, YOUR A GENIUS! If we ever cross paths, beers on me.

 

You completely pointed me in the right direction. The distributor was bad so we took the opportunity to index it per Cruiser54's instructions. The Jeep fired right up and drives so well! We've been chasing this problem for AGES and I was at a complete loss. 

 

My son and I truly thank you for your wisdom. Thanks to the others who also posted. What an awesome cummunity. Now we're off to the next project - follow our bucket of bolts thread for updates!

[Ωhm]

[Tsalagi]

Love the community and teamwork!