I recently picked up a 1990 4.0 5speed. It's my third Cherokee, but my first Renix. It was exclusively a trail rig for several years so I gave it the basics to get it road worthy (decent front bumper, new front end components, functional signals, etc) The only problem left is a terrible loss of power, bucking and backfiring at any serious load.

It starts and idles well (bad check valve so a little cranking, but is rock steady after that). If I give it throttle in the first 10-15 seconds after it has started it pulls nicely. Over the next 15 seconds or so it's ability to accelerate diminishes to the point that at anything above ~20% throttle it begins to buck and backfire impressively. Below that it runs smooth and will cruise in any gear as long as you keep the load minimal. While revving in neutral it is smooth so it is a load dependent problem. This did not happen when I test drove the truck so it developed the problem between buying the truck and my first drive as owner.

Things I have tried:
-New MAP
-Tested Intake Air Sensor, within spec
-Tested coolant temp sensor, within spec
-Tested TPS voltage is smooth from 0.50-4.37 (should be 5.0, but close enough for now....)
-Replaced The destroyed O2 sensor
-Checked for obvious vacuum leaks, though it's entirely possible one or more are hiding....

Other relevant things:
-Newish fuel pump and tank
-Oldish gas, perhaps 6 months, but there are several fresh gallons in it and Techron cleaner
-Fuel pump resistor eliminated
-Pulled a plug. Blackish, but not caked on
-No gauge cluster due to a shorting/melting/burning wire of some sort (previously like that)
-Aftermarket intake tube and Mopar exhaust

At some point I will get to all the usual tune up stuff (coil, plugs, wires, fuel filter), but I would like to solve this problem before just throwing money and parts at the truck.

 

We, (or at least I), have had trouble nailing this one down. Will it go like crazy if you really hit the gas, like past 80% throttle?

Going through Cruiser's tips found right on top in my signature is always a good idea. I just caught my dipstick stud with 17 ohm's resistance!

When mine was backfiring out the intake it was one of the wires to the lower sensors grounding. Unfortunately I don't know which one. You might put an ohmeter between ground and various wires, like the 02, the coolant and Knock sensors, and jiggle the whole harness, all the way back to the firewall,see if any particular move make the readings change.

Even it it DOES go when you floor it, it's free to borrow a fuel pressure gauge. Also I've had ongoing problems with the "new" fuel separating and fouling my carbed engines. Short of draining it I might at least fill it up. (the Jeep has a tray inside the tank for the pickup, and return. It COULD recycle heavier stuff. (like water laden ethanol))

 

Several good ideas, thanks.

First, I am going to get the usual tune up parts since I would be doing them anyway, though I doubt that will cure any of this.

I'll check grounds. This Jeep has been modified, driven hard, and has mud in every orifice so who knows what has been soaked, corroded, or loosened.

It does not go when floored. I think typically TPS voltage of 4.5 or higher will put it in open loop/full throttle and I am below that on my TPS. The more throttle the worse it stutters. I had been leaning toward air, metering, or spark issues due to all the backfiring, but I will look in to fuel pressure. I have a tester around somewhere. If only my garage wasn't such a mess.....



Oh, I have looked at many of those tips (very useful!), but I will read through again as I work through the little stuff for ideas.

 

OK....That Renix will ignore a bunch of sensors and just use a few like the map and CPS at over 80% throttle. Tune up, fuel and fuel pressure...can't hurt!

 

It looks like I have found the problem. Fuel injector cleaning seems to be in my future.

 

Get some Volvo 746 injectors.

 

Do those injectors work on a 98 XJ? Because, I would much rather swap to them..

 

No, but there are other upgrades for the 98.

 

Injectors, O-rings and pintle caps ordered. Hopefully it will be running smoothly by Monday. Thanks for the advice.

It's already somewhat better after blowing the lines out from the fuel rail back. What possessed someone to bypass the fuel filter I can't comprehend....

 

Some folks can screw up an anvil with a rubber mallet......

Or bend a digging bar in a sand pile.

 

If you go with the 746's, feel free to post up what you think here> https://www.cherokeeforum.com/f2/199...r-swap-163105/

 

Once you get everything back together, check the fuel pressure. Your symptoms are classic failing fuel pump. I know you mentioned a "newish" pump and tank but it also sounds like you just got the rig and I'm assuming the PO told you it had been replaced. Maybe the "newish" means it came from the junkyard.

 

i agree with this guy.

 

Me, too.

 

A shorted wire in a harness will cut right through the insulation of the others like a hot knife through butter. As you go through it, keep en eye out for anything that looks like it got hot. Might be only the 02 heater wire that caries much current. Just one cutting through can connect anything to anything. Just something to keep a eye out for.