4.0 flat spot in the power band

shelby27604

shelby27604

Well-Known Member
Joined
Jun 6, 2013
Location
Efland NC
Finally got the new to me 1997 4.0l / AX15 XJ out on the road, did all the basic maintenance stuff prior to the test drive (filters, fluids, etc). In the 1st & 2nd you don't notice it, but in 3rd gear there is an exhaust note change and no power gain from 3k - 4k, above that it kicks in like a rocket.

The jeep will be getting a new fuel filter / regulator and a fresh set of injectors after I get a full tank of gas through it, I would think this is fuel related, except for the fact that there is plenty of power above 4k.
 
Could be ignition related too. If it isn't advancing the timing properly, that'd do it. Just something to consider.

I'd start with what you said though. It definitely won't hurt.
 
I had an XJ that would idle fine but bog out under load. The filter sock on the fuel pick up was super clogged. New pickup/sender assembly fixed it. Of course, this was after we changed everything we could underhood :eek:
 
GotWood said:
Check wiring loom at firewall above fuel rail.
Click to expand...

Did a quick check this morning, I didn't see any issues, am I looking for points where the insulation is rubbed off of wires and grounding out wires to the valve cover?

Jody Treadway said:
Unplug the downstream O2 sensor and road test
BTDT
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Tried that, no discernible difference noted, I put 50 miles on it like that....I did trip the check engine light, but no improvement in the flat spot.

I will have more time to check the MAP and TPS tomorrow, I couldn't dig up my multi-meter and scan tool in time this evening before headed out for dinner.
 
shelby27604 said:
Did a quick check this morning, I didn't see any issues, am I looking for points where the insulation is rubbed off of wires and grounding out wires to the valve cover?



Tried that, no discernible difference noted, I put 50 miles on it like that....I did trip the check engine light, but no improvement in the flat spot.

I will have more time to check the MAP and TPS tomorrow, I couldn't dig up my multi-meter and scan tool in time this evening before headed out for dinner.
Click to expand...
Sometimes the wiring harness ends up laying on the back of the fuel rail and rubbing a hole in the insulation. I usually fix those with a zip tie.
 
I did find a spot where the harness would rub, but not damage to the wires, zip tied up and out of the way.

I tested the MAP at using a volt meter and (0 in Hg @ 4.7v, 5 @ 3.9v, 10 @ 3.0v and 20 in Hg @ 1.1v) all measurements were within 10% of values listed (which I know is less than ideal, but I am dealing with a hand pump vac. gauge and running a multimeter, I can't say I was exactly at 5 in Hg when testing and less than 10% seemed like it wouldn't be a major contributor to this issue)

I checked the TPS, fully closed it measured 0.89v (0.95v allowed max) and at WOT 3.96v (4.49v required). The scanner confirmed these numbers, showing 16% throttle (when closed) and 78% at WOT. I tossed in a new TPS from NAPA and now read 20% with a closed throttle and 80% at WOT.

I see 12 in Hg from the MAP at idle, the idle itself is steady at 740 when the engine is at temp.

A test drive shows the flat spot is not considerably better if I keep my foot at 75% throttle, but the motor bogs if I go full throttle.

The TPS only goes on one way, but I did have to clock it / pre-load it to get it to engage the flag on the throttle. The TPS doesn't look adjustable, do these numbers point a guru toward a good logical next step?
 
Continuing down the checklist, I checked the fuel pressure, I got 20 psi key on, engine off, and 26 psi engine running. The spec. I saw was 49psi +/- 5 psi. I have a filter/regulator I was going to throw on, but at this point I guess it would be a safer bet to buy a complete filter/regulator/pump setup and toss that in. Rock Auto has an Delphi unit for $125, any concerns with this? Any better recommendations? I like to buy a good fuel pump whenever possible.
 
shelby27604 said:
Continuing down the checklist, I checked the fuel pressure, I got 20 psi key on, engine off, and 26 psi engine running. The spec. I saw was 49psi +/- 5 psi. I have a filter/regulator I was going to throw on, but at this point I guess it would be a safer bet to buy a complete filter/regulator/pump setup and toss that in. Rock Auto has an Delphi unit for $125, any concerns with this? Any better recommendations? I like to buy a good fuel pump whenever possible.
Click to expand...

I almost bet this solves your problem. I almost bet this solves your problem. I had an XJ acting the same way and once a new fuel pump was installed the problem went away. I was trying to remember what the hell I had done to fix it to tell you but you just reminded me.
 
Fuel pump came in yesterday, would have been a quick job except the last mechanic that was in there cobbled together a fuel line out of 3/8" low pressure rubber fuel hose and a couple of regular hose clamps.....how this didn't leak is beyond me! As the part is no longer available, I had to make something, and that took 3 trips to the parts store.....it is an odd ball, 3/8" metal fitting on one end, 5/16" fitting on the other.

Incase someone need them, doorman 730-4927 (3/8 fitting), 730-4926 (5/16 fitting), (4) 3/8" fuel injection hose clamps, and 9" of 5/16" FUEL INJECTION rated fuel line.....if you need to make this part.....double check.

Flat spot is gone, and fuel gauge is actually accurate, previously 3-gallons of gas registered as 1/4 full....now 3 gallons is on E with the low fuel light on (my wife was VERY happy she can now safely run the jeep to just shy of zero gallons).
 

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