Poor performance and gas mileage checklist

[Billabob]

I acquired my Comanche about a year ago now and its been a great first car but I can tell it's lacking in power despite not having much to compare it to. The truck hasn't been treated very well by it's previous owners so I've been attempting to fix the many issues it has while also learning how to wrench. The problem I'm running into is that I have no idea which problems are the biggest culprits or if there is even much room for improvement. I guess what I'm asking is if this was your car, how would you prioritize these repairs? My biggest issue currently is the lack of torque in the low end (any incline will bring me to a stop very quickly if I'm under 3k rpm) and very poor gas mileage in the city (6-8mpg). The truck is a manual 4.0L with fresh oil, plugs, Bosch injectors, and about 170psi per cylinder. Here is what I've found so far:

1) 31" tires on 3.07 gearing

2) 2.5" exhaust with a very free flowing muffler

3) Exhaust header bolt is snapped at the head, causing a very minor leak

4) Low vacuum (12ish psi)

5) No EGR

6) Slow warmups, I usually idle for 5-6 minutes and it barely hits 120 degrees by then

7) Slight backfire when cold, especially when I let off the gas pedal

 

I have a feeling all of these things contribute slightly and I will fix all of them when money allows. I should also mention that I get about 22mpg on the highway which leads me to believe the gearing is the biggest culprit here. I would appreciate any opinions.

[Ωhm]

34 minutes ago, Billabob said:

4) Low vacuum (12ish psi)

 How did you measure that?

[Billabob]

14 minutes ago, Ωhm said:

 How did you measure that?

Vacuum gauge attached to an unused nipple on the intake manifold

[Ωhm]

That's poor intake manifold vacuum. Tells the Manifold Absolute Pressure (MAP) add more fuel. Are you running RICH at idle? RICH all the time?

[watchamakalit]

31's and 3.07. Thats your problem.

[87MJTIM]

Yr? 

 

If RENIX, the do Cruiser's Mod's first.

 

Look in to getting Nick-in-Time-Design "Renix Engine Monitor"  (REM).

 

Mine had a somewhat similar MPG issue.  Turned out I was stuck in Open Loop - bad O2 sensor.

[gogmorgo]

In addition to spark plugs, the spark plug wires and distributor cap and rotor are also wear items that should be replaced if you don't know how old they are.

As mentioned, Cruiser's tips are definitely something worth working through. www.Cruiser54.com

As far as slow warmup is concerned, the thermostat would be the first thing I'd look at, because they're cheap and easy to change out. It's also apparently very common for people in warmer climates than mine to put in a lower temperature thermostat than factory recommended in a misguided attempt to make the engine run cooler, which doesn't really work out how they think. The thermostat only controls the lower end of the operating range, so a lower temp tstat will only slow down the warmup, and won't ever prevent overheating. In my climate, it also means you'll have an inadequate heater. Running at low temperature in a fuel injected vehicle will cause the computer to stay in open loop, essentially a warmup mode that is functionally similar to running a carburetor with a closed choke, in that it will run very rich, which will be mostly noticeable in terms of fuel consumption. I don't think it would cause the entirety of your poor MPG issue, but it will definitely be part of it. It can also carbon foul your spark plugs and cost you some performance or at least smooth running if it rarely warms up entirely, so it's worth at least checking and/or cleaning those again, even though they're new.

The easiest and likely cheapest way up from your 31's on 3.07 gears is almost definitely going to be to find a Dana 35 out of another MJ with 3.55 gears, paired up front with a non-CAD Dana 30 from a '90's XJ if yours in 4x4. It's the setup in my daily driver and it does alright, it's basically the equivalent ratio to 3.07's on 3.55's. Although if big tires are in the future, now would be the time to future proof.

I wouldn't be too concerned about your exhaust. It's possible the broken manifold bolt and subsequent exhaust leak also burnt your intake manifold gasket, but that would very likely cause a high idle. As far as the system and diameter go, it's not such a huge increase. It will shift your torque curve up in RPM somewhat compared to stock, but it shouldn't be that big a deal unless you like pulling hard at idle speeds.

[Tex06]

On 1/15/2022 at 10:00 PM, watchamakalit said:

31's and 3.07. Thats your problem.

Naw, that's what I've got and she'll pull right up hills around North Georgia, even loaded. 

[omega_rugal]

its seems that your engine is running ice cold all the time, maybe your thermostat is stuck open

[88towmanche]

On 1/18/2022 at 3:13 PM, Tex06 said:

Naw, that's what I've got and she'll pull right up hills around North Georgia, even loaded. 

I don't know...I have the same set up and it is weak-sauce with 31's & 3.07.  I also have an '89 XJ with twice as many miles that feels much stronger with what I presume is a different gear ratio (what did the Renix XJ's come with 4.0 5spd?)

 

That rig should easily haul around my fishin boat and is "ok" on the road but weak pulling up hills and ramps.

 

Have a parts MJ with 4.10 axles that is going to get swapped over (factory MJ D44!!) - once I finally get to that, expect we'll see if gearing is source of suckyness or not.

 

 

[88towmanche]

On 1/15/2022 at 7:00 PM, watchamakalit said:

31's and 3.07. Thats your problem.

Have to agree - not liking that set up on my rig, at least not for pullin a boat.

[Tex06]

18 hours ago, 88towmanche said:

I don't know...I have the same set up and it is weak-sauce with 31's & 3.07.  I also have an '89 XJ with twice as many miles that feels much stronger with what I presume is a different gear ratio (what did the Renix XJ's come with 4.0 5spd?)

 

That rig should easily haul around my fishin boat and is "ok" on the road but weak pulling up hills and ramps.

 

Have a parts MJ with 4.10 axles that is going to get swapped over (factory MJ D44!!) - once I finally get to that, expect we'll see if gearing is source of suckyness or not.

 

 

I've got a topper on mine (~150lbs) plus a bed that's usually got 200-300lbs of assorted tools, materials, and rugby stuff in it, plus a 140 lbs of dog a lot of times. I know that I can hold 60mph taking the I-75 to I-285 exit here in Atlanta without dropping out of 5th....I'm not gonna accelerate up the hills but it'll hold speed.  If I'm pulling a trailer of any sorts, yep gonna have to downshift. 

 

I do wish I got anywhere near 22mpg with mine. I was consistently getting 17-18mpg then in December I fixed a vacuum leak and replaced the oil fil cap (last one leaked and blew oil all over the motor over time) and now I'm at 15.7 mpg. No other changes and I took a 2mpg hit. 

OP, we really need to know what year your truck is to make further suggestions. The differences between the Renix 4.0 (pre-'91) and the HO motor ('91+) mean there's different "most likely" culprits. 

[Billabob]

Thank you for all the responses! It is a 1987 Renix Comanche with a good portion of the Cruiser54 tips already done. The distributor cap has been replaced and adjusted properly according to cruiser's guide. The cooling system has been completely re-done about 3 months ago and has had no effect on warm-up times and fuel mileage. It looks like the coolant hasn't been replaced in over a decade so the whole system got torn out and replaced with a Mopar open system from a 98 XJ, Mopar 210 thermostat, and Hesco pump. I can feel the truck being happier and less noisy now with a slight power increase however it still gets poor fuel mileage. I should also mention that it has never had an electric fan or clutch fan shroud, by all means it should be running hot if anything. My buddy has a REM reader that I borrow from time to time but last I checked (about 4 months ago), there were no problems at all. I have confirmed with the REM that it goes into closed loop as it should and theres a slight rpm drop when it does. The truck does start quite noisy/shakey when the temps are low but it evens out quickly and I make sure to not drive off until it hits closed loop and idles smoothly. Lastly, I have my eyes set on a Ford 8.8 with 4.10 gearing which will be my winter project but it probably won't get swapped in till spring since we do get a bit of snow here and I'd like to retain 4wd. 

 

This is still a huge mystery to me but I'll do a bit more probing this weekend and report back with some fresh REM readings.

[87MJTIM]

What are your LT and ST fuel trims?

[WahooSteeler]

4 hours ago, Billabob said:

Mopar open system from a 98 XJ, Mopar 210 thermostat

210? 195 tstat is correct for your motor......

[Billabob]

My mistake, it is a 195 degree thermostat.

 

Here is a full REM readout fully warm after driving for about an hour.

VAC: 18.8

fWT: 169

fIA: 111

Idle: 600-640

VLT: 14.1

vO2: ~1.2 but jumps to ~4.7 every several seconds

Fuel mixture: 14.6

vHT: 14.3v

Closed Loop

EGR off

128 IAC

TPS: 14

IGN: 15

KNO: 0

mS Injector width: 4.4

2.4% duty cycle

ST Fuel Trim: 99-120

LT Fuel Trim: 150

GPH: 0.44

MAP: 11.6-12

 

So it seems that my vacuum is fine after all and the only real outlier here is the water temp.. 169 degrees is way too low. Judging by how many people struggle to cool their jeeps below factory settings, I'm going to go out on a limb and say that the sensor or sensor ground is probably faulty. Does anything else look out of place? All measurements were taken while parked, would it help to get any other measurements while actually driving the car? 

[87MJTIM]

Your LTFT is WAY too high.  The computer is enriching the fuel trim.  LTFT should be ~ 128.

 

 

[Ωhm]

With the ECU in CLOSED LOOP (CL), your REM numbers are kinda okay. Things I don’t like are IDLE, INJ_PW, and TPS. I’d look for ≈750rpms, 5.5mS, and 17. Can only suggest a few @cruiser54 tips.

 

http://cruiser54.com/?p=60

 

http://cruiser54.com/?p=94

 

Do this one last: http://cruiser54.com/?p=54 

[87MJTIM]

The LTFT above 128, the computer is detecting a lean condition (too much air), it will enrich the fuel.

 

 

Check for vacuum leaks and tighten the intake manifold bolts/nuts.  Check the fuel pressure, and possible clogged injectors.

[Billabob]

I’ve slowly been chipping away at this issue to no avail. My long term fuel trim now sits at 165 and this issue is currently at the top of my to-do list due to the rising gas prices. Since my last post I’ve fixed/found a few more things: 

1) Did a couple of C54’s tips including the c101 delete and the temp sensor ground update

2) Cleaned and Oxgard’d all sensor connectors
3) Adjusted TPS to 17 as instructed

4) Checked initial fuel pressure and checked again 30 minutes later, good. 
5) Found a minor leak in my MAP hose. 
 

The only real remaining outlier I’m seeing on my remreader is high fuel trim and low coolant temp. It seems to always live at 169° and pop open at around 175° according to the REM. It is still fairly cold here but I have a hard time believing its cooling too well, especially since it has no fan shroud and no e-fan. I’ll check the actual resistance measurements at the sensors this weekend when I re-do my MAP hose. In the meantime I’ve purchased a good quality infrared temp sensor to verify temps another way. What reading should I be getting at the head and at what emissivity? Currently getting a reading of 144° at 0.80 EMS with a bare aluminum head. 
 

Lastly, I found a wire without housing that was soldered into a group of ground wires slightly upstream of the temp sensor wires. It looks too clean to be a burned wire but I’ve also never seen a bare wire on any of my vehicles. Is this intentional or something to be concerned about?

[gogmorgo]

Reading off the thermostat housing when it's up to temp should give you more or less the opening temperature of the thermostat. I don't remember if it was mentioned or not, but if the temperature sensor isn't reading accurately it could think it's colder than it actually is, triggering all the same same issues as if it was actually cold. I don't really mess with the emissivity setting on my own IR thermometer, I just set it to whatever setting was appropriate for cast iron when I first got it and left it, but the manufacturer should have a table similar to this one: https://ennologic.com/wp-content/uploads/2018/07/Ultimate-Emissivity-Table.pdf. Also, your head should be cast iron, not aluminum, no? Tstat housing is cast aluminum.

 

As to bare wires, sometimes the insulation simply falls off. It gets old, age cracked, whatever. The second biggest concern with exposed wires is corrosion, the first being shorting, but if they're all ground wires connected to ground, does it matter if it shorts? Still worth addressing though. Corroded wiring will have higher resistance, and therefore could screw with some sensor readings.

[cruiser54]

Tip 5 been done? 

[Ωhm]

22 hours ago, Billabob said:

Lastly, I found a wire without housing that was soldered into a group of ground wires slightly upstream of the temp sensor wires. It looks too clean to be a burned wire but I’ve also never seen a bare wire on any of my vehicles. Is this intentional or something to be concerned about?

 

Note the metal foil with bare stranded wire (shield). This makes up the shielded circuit used for the Knock Sensor (KS). This shielded must find a path to ground. Protects against Electromotive Force (EMF) Don't want to RETARD your timing unnecessarily.

 

Shielded circuit is shown using dashed lines.
 

[pizzaman09]

If the engine is pulling more timing than it should be, that can easily wreak fuel economy and power.  

Are there any ignition timing checks that can be performed?  My memory recalls that the Renix ECU is always striving for max timing just short of knock.

[cruiser54]

Put a timing light on it and see. I think 12 is normal at idle.