Replaced 4.0l, now worse mileage, lower power. Ideas? - JeepForum.com
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post #1 of 22 Old 08-05-2019, 05:56 PM Thread Starter
StellaTheZJ
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Replaced 4.0l, now worse mileage, lower power. Ideas?

I spun a rod bearing in the original 4.0l in my ZJ, so at 540,000km I figured I'd simply replace the whole engine. I got a low miles 4.0l from a 1992 YJ, polished the crank, put in new rod and crank bearings, new seals and oil pump, and put it in. I reused almost everything from the outside of my original 4.0l. Now, however, the mileage in the ZJ is not as good as it used to be, and although it starts and runs smoothly, it doesn't make as much power as the old one, as evidenced by having trouble holding mountain highway speeds that it used to maintain. What could be different in the YJ engine?

Here's what I know about the Jeep after the engine swap:
1. Same O2 sensors, and the front one is making crosscounts from .2 to .8v on the scanner.
2. Same cat and exhaust, so I don't think it somehow got plugged while the new engine went in??
3. Compression reads 153 on #1, then all the rest are 168 to 175. No, not perfect, but within factory specs of a max 30psi variation??
3. Same sensors, injectors, throttle body and air cleaner up top.
4. This new engine burns no oil, has no blow by going into the throttle body, and doesn't smoke at all after a long compression brake. The old one was pretty clean too, except for major smoke after a long compression brake!
5. I am running the same wheels and tires, and I changed the plugs again at 1200km just in case. No improvement with the new plugs.

Is it just that the 1996 cam is so much better than the 1992 cam? Or maybe the old, high-mileage 4.0l had such loose tolerances it spun easier?

Any suggestions on what to check next, before I look for someone who wants to trade a really nice 1992 4.0l for a 1996-1998 4.0l??

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post #2 of 22 Old 08-06-2019, 03:57 AM
Timo_90xj
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Here are the specs for the stock cams and head flow numbers:

http://jeep4.0performance.4mg.com/tech_specs.html

http://www.angelfire.com/my/fan/Jeep4.0Camshafts.htm

1998 Grand Cherokee 5.9 LX daily driver, 1.75" BB, 32" KM2s, HPD30 Eaton e-locker/D44a stock LSD, 4.56 gears, custom- fabbed tube bumpers and tube fenders,...


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1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***

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1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***
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post #3 of 22 Old 08-06-2019, 08:36 AM
LordHobbit
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More than likely the 4.0 from the YJ is not a HO engine. All 4.0 in ZJs were HO (high output). I doubt you'll ever get the power from that engine you did from the original.

If you put that much work into the YJ engine, why didn't you just do that to the original engine?

93 red ZJ 196k 4.0. Spent more money than I should have ...
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post #4 of 22 Old 08-06-2019, 08:45 AM
jtec
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Some thoughts-
I suspect you did check and there are NO codes, if scanner is capable- check fuel trims, MAP, TPS, and ECT.

If scanner is unable -
Put a vacuum gauge on it.
That .2v-.8v sounds 'hunting' from lean to rich...
Be sure ECT is reaching operating range.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #5 of 22 Old 08-06-2019, 09:42 AM
CatSplat
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Quote:
Originally Posted by LordHobbit View Post
More than likely the 4.0 from the YJ is not a HO engine. All 4.0 in ZJs were HO (high output). I doubt you'll ever get the power from that engine you did from the original.

If you put that much work into the YJ engine, why didn't you just do that to the original engine?
A '92 YJ should have had an HO engine, unless somebody swapped in a pre-'91 Renix motor at some point.

The '96 cam should have a wider power band but it's not night and day compared to the old one.

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post #6 of 22 Old 08-06-2019, 10:07 AM
Timo_90xj
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Yeap, all 4.0 engines '91 and after are HO. '91-'95 have the best cylinder head, '96+ have the wider power band cams, and the latest style "horseshoe" intake manifold is supposedly the best flowing. Difference however is pretty minimal to the log style.

Combining those with some intake- and exhaust port work you get the best flowing 4.0 without major modifications. That being said, '91-'95 HO, '96-'99 HO and '00-> HO should all feel much the same. You might notice the difference, but on dyno they differences are pretty damn small so it's basically your butt dyno telling your brain there's a difference because you know it.

If after an engine swap there is a notable difference in engine performance - slower acceleration, reduced ability to maintain speed, worse MPG, etc.. - there has to be something wrong with the new engine or how the new and old parts were put together.

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1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***

***Under construction***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***
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post #7 of 22 Old 08-06-2019, 11:37 AM Thread Starter
StellaTheZJ
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LordHobbit:
The YJ was definitely NOT Renix, so as far as I understand that makes it HO. It had the intake manifold on it when I got it, and it has a 1991 build code cast into it, because it was sold in a 1992 YJ.

As to why I didn't put that work into the 1996 engine, the work I did on the low-miles YJ engine would have been just the start for the high-miles ZJ engine! At a minimum, the wrecked ZJ engine needed valve guides (I'd already replaced the seals, to no apparent effect), a con rod and a crank (and that requires pulling the head too), and at 540,000km, likely cam bearings etc. As far as I knew, the YJ engine needed no work, although it was missing the valve cover, the timing cover, the oil pump, and the pan. I bought a new timing chain and oil pump for the YJ engine, then got the rest of those parts (including the main cap girdle) from the 1996 engine.

jtec:
No, there are no codes. I borrowed a SnapOn MODIS to check.

Timo 90XJ:
I love Dino's site! So much good info on the 4.0l there. Interesting to note that his chart suggests that the '92 YJ is supposed to have 10hp less than the '92 Cherokee. Surely that must be from exhaust or something, because the intake, injection, and cam are identical, I think? But yes, the cam did change for 1996 to make more torque at lower RPM. I feel like this '92 engine is making less power at all revs because it doesn't hold speed on highway hills like it used to. I've owned this ZJ for 11 years now, so I feel pretty confident that the "butt dyno" tells me it's noticeably slower now! And the overhead display is definitely reporting worse mileage than before.

Is there something in the tranny that could be dragging somehow, something other than the engine that would cause this?? I'm grasping at straws here, but given how well the engine starts and runs, and given how well the compression test turned out, maybe I should be looking at something else on the Z, even though it still has the same transmission, transfer case, rims, tires, and gear ratio (but with new gears and a True Trac in the back).
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post #8 of 22 Old 08-06-2019, 02:36 PM
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I suspected there were NO codes or you would have posted them - but had to ask.
The Snap on would allow you to view live data - getting into closed loop, ECT, MAP, TPS, Fuel trims, and check the o2 sensor voltages see if they tighten up from the wide range .2 to .8.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #9 of 22 Old 08-06-2019, 11:37 PM Thread Starter
StellaTheZJ
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I never saw the O2 voltages go any "tighter." I know that the number of crosscounts matters (I think it's something like 6 times per second or more). But as long as it's crossing .4v or so, is the range important? If so, is there an ideal range??
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post #10 of 22 Old 08-07-2019, 12:29 AM
RedRiverT
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It's starting to add up to something on / in the engine not being right. The overhead display mileage is calculated from the sensor readings. If the O2 sensors are varying too widely and like jtec said it is "hunting" for the correct mixture, something could be causing the conditions to change, maybe a vacuum leak, or that 153 compression cylinder. The head or a valve could be bad.
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post #11 of 22 Old 08-07-2019, 08:33 AM
jtec
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be more specific with problem - Am I correct poor MPG (mile per gallon) ?
what was it and what is it now.

You are not replying to suggestions- simple ECT question ? open loop is it entering closed loop - and other data.
That snappy scanner will help take a look at the system inputs outputs mentioned above, quick easy and sitting in driver seat with a coffee in hand..

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #12 of 22 Old 08-07-2019, 12:09 PM Thread Starter
StellaTheZJ
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Sorry jtec. It goes into closed loop very rapidly, and then reports switching from rich to lean with the crosscounts of the front O2 sensor. Timing sync is rock steady. Ignition timing varies with engine RPM. TPS varies with throttle position. I didn't memorise the short or long term fuel trim data, and I don't have the MODIS with me this morning to re-check it. Is there anything else I should be looking at in it?

And yes, MPG is "Miles Per Gallon." I live in a flat valley, and I have been driving the ZJ here for 11 years now, so I have a few roads where I have observed the MPG readout time and again, in all kinds of conditions. The ZJ used to report about 24mpg at 100 kilometers per hour (kph) on cruise control on a highway near me, now it is reporting more like 17 to 19mpg on that same highway. Going about 75kph on a different road near me, it would report around 28mpg on cruise control, but on that same road it now reports about 21 to 23mpg.

The overhead readout might be nowhere near right, but it should still be reading the same numbers now as it did before swapping the engine. The MPG is calculated with some simple math as "speed sensor pulses
in a certain time / (injector pulse width x number of injector pulses during that time)." Since I haven't changed any sensors, the inputs for that equation should still be the same. "Injector pulse width" is directly related to the load on the engine, and the harder it is working, the longer the pulse width will be. I expected to see about the same numbers with the new engine, but since I don't I am trying to figure out what has actually changed to affect the injector pulse width at any given speed. Also, the ZJ will not go as fast as it would up the mountain highways I take to go camping and exploring. I "feel" like it is slower off the line too, but that is subjective - the change in MPG and the lower top speed up steep hills is objective. For some reason I can't yet figure out, this engine is less efficient at turning fuel into power.
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post #13 of 22 Old 08-07-2019, 12:46 PM
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IIRC the 96 was derated in hp due to emissions. That might be only in the ZJ.


I see where you did the compression test. Have you tried doing a cylinder leakdown test?

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The 4.0 is dumb old tractor motor, itís too stupid to know itís supposed to die.
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post #14 of 22 Old 08-07-2019, 02:49 PM
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look at ECT, people love to use wrong thermostat.
The fuel trims will help. paint picture.
The compression leak down might be wise OR maybe a vacuum gauge to ck for valve issue.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #15 of 22 Old 08-08-2019, 01:05 AM
RedRiverT
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Quote:
Originally Posted by StellaTheZJ View Post
"Injector pulse width" is directly related to the load on the engine, and the harder it is working, the longer the pulse width will be. I expected to see about the same numbers with the new engine, but since I don't I am trying to figure out what has actually changed to affect the injector pulse width at any given speed.
The PCM will change the injector pulse width in response to other inputs too, won't it? So maybe if an 02 sensor is giving goofy readings, the PCM has jacked with the pulse width.

It just struck me: Experts, are these symptoms of a cracked header? Maybe it was made worse during the engine swap, or OP, did you swap headers too?
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