85/86 XJ fuel gauge question

OK My 1985 2.8 XJ with 20 gallon tank has a new weird problem.

Replaced the fuel tank not too long ago, right before that the fuel gauge died. Turned out the wiring running out the body to the plug was bad. Since I had to replace it with a new plug and wiring harness I got a plug compatible with a 1986 Fuel sending unit, the 86 fuel sending unit is the only new replacement available for the early years (1984-1986).

Its up and running and now that I am driving it a little more the gauges is acting a little weird, under load, driving at 45mph the fuel gauge drops to the first quarter mark below the full mark.
Stopped at idle the gauge slow climbs to just a little below the full mark. Revving the engine doe not have any affect, only on the road does it start to fluctuate.

I think the fluctuation is on par with the fuel usage / what is actually in the tank. I'm at a loss as to which reading is correct.

The Instrument cluster is out of around a 1989 model. (sending unit is a for 1986 w/20 gallon tank).

I don't know what I need to test, should I try to swap out the cluster, track down another electrical gremlin not sure what at this point

Any ideas what could be causing this?

Every XJ I have owned, the fuel gauge has gone down under accel and fluctuated. Some more than others. It is usually something to do with the float movement.

Johnnie Walker said:

Every XJ I have owned, the fuel gauge has gone down under accel and fluctuated. Some more than others. It is usually something to do with the float movement.

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You may be right it may all be down to the float boobing around. The norm I was used to is out the window I guess.

Welcome to the new norm. My 85, 87 and 89 all do that.

This is actually a relief to know. The old Jeep is such a hodgepodge of parts with the new tank, new plug/wiring, new sending unit, I was worried something else was wrong.
If this is the new normal I can live with it.

Green Mesa XJ said:

This is actually a relief to know. The old Jeep is such a hodgepodge of parts with the new tank, new plug/wiring, new sending unit, I was worried something else was wrong.
If this is the new normal I can live with it.

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Hodge podge, LOL, you should see my FrankenJEEP, 1985, it has a 1982 Nissan diesel engine and Nissan 5 speed manual Tranny in it, with duct tape and house wiring under the hood.

But the bumper sticker says "Don't laugh, it's paid for" and it gets 34 MPG on diesel!!!

But the AC is almost all new, and works great!!!

Ecomike said:

Hodge podge, LOL, you should see my FrankenJEEP, 1985, it has a 1982 Nissan diesel engine and Nissan 5 speed manual Tranny in it, with duct tape and house wiring under the hood.

But the bumper sticker says "Don't laugh, it's paid for" and it gets 34 MPG on diesel!!!

But the AC is almost all new, and works great!!!

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So it's still a 2.8

I've got a mix of pieces from Mustangs, VWs, but mostly other jeeps, and nothing as drastic a Datsun diesel.

My 85 doesn't have AC (played with the Idea of adding it but decided not to), where i belongs it doesn't need it. High altitude package, and it never did well on some of the roads going up the mountain.

No it is a 2.2 L, 61 HP. Not much good in mountains, and tough to climb hills from a dead start, but great in flat lands like Houston! I heard the 2.8 had pretty good power.

Ecomike said:

No it is a 2.2 L, 61 HP. Not much good in mountains, and tough to climb hills from a dead start, but great in flat lands like Houston! I heard the 2.8 had pretty good power.

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Ah the 2.2 (z22 motor). Had a 50/50 chance with the 2.8 Had boss that one but a 2.8 diesel 5 speed in a 280z. I don't know if he ever finished the project.
Must have been fun fabricating the mounts for it, although it should of had plenty of room.


I don't have many complaints about the GM 2.8 aside from it is pretty gutless at times. I hate taking it on the interstate but when it is at speed it does rather well, gets better gas millage than our 4.0.

here in Florida it does well in most circumstances, out where I bought it it was nerve racking on the mountain passes sometimes, people flying up behind not realizing this is a old carburated jeep struggling to breath. On the trails it was fine.

So it is the GM V-6 you have, not the 4 banger?

Ecomike said:

So it is the GM V-6 you have, not the 4 banger?

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Sorry didn't see this post before today.

Yep its got the V6, the prince of carburated 6 cylinder motors. Ugh, I had a ton of ideas of what to do with it when the motor gave out, but who ever rebuilt it before I got it did a good job.

Green Mesa XJ said:

Ah the 2.2 (z22 motor). Had a 50/50 chance with the 2.8 Had boss that one but a 2.8 diesel 5 speed in a 280z. I don't know if he ever finished the project.
Must have been fun fabricating the mounts for it, although it should of had plenty of room.

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Actually mine is an SD-22 Nissan diesel engine and 5 speed manual, transplant into what was an 85 Jeep 4 banger. The mounts are 1/2" think steel plate, welded into an angle shape, covered or topped off with the tread from an old radial tire (about 1/2" thick fiber reinforced, wire reinforced rubber) between the frame and the brackets. Extremely crude but they work.

Ecomike said:

Actually mine is an SD-22 Nissan diesel engine and 5 speed manual, transplant into what was an 85 Jeep 4 banger. The mounts are 1/2" think steel plate, welded into an angle shape, covered or topped off with the tread from an old radial tire (about 1/2" thick fiber reinforced, wire reinforced rubber) between the frame and the brackets. Extremely crude but they work.

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SD-22.... What was that in a old 720? Yeah that right the gas motor was the z22, not the body style of the truck. I actually seen a few of the old 720 around town recently. They were not very tough and often rusted out. And the gas Z22 was a crap motor.

Not a bad way to fab the mount. Was there any similarity to the OEM mounts or did you start fresh bending it to fit the need?

Green Mesa XJ said:

SD-22.... What was that in a old 720? Yeah that right the gas motor was the z22, not the body style of the truck. I actually seen a few of the old 720 around town recently. They were not very tough and often rusted out. And the gas Z22 was a crap motor.

Not a bad way to fab the mount. Was there any similarity to the OEM mounts or did you start fresh bending it to fit the need?

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I bought it as an incomplete project, so all I did was fix a sheered MM bolt in the engine block (that WAS fun, LOL) and came up with a better rubber isolator than the junk the PO used. I never saw the OEM motor or mounts.

The SD-22 diesel is still used in forklifts last time I checked. 500,000 plus potential miles motor. I am pretty sure mine came out of 1981 Nissan 720 PU truck. Probably 2 WD.