Hey everyone, after a few months of minimal problems (there it is, jinks my self) I had a couple hours driving to do round trip in a day. About 6 hours into my drive (on the way back luckily) I realized I wasn't able to maintain speed going uphill. As in I'd be doing 70 on a flat, start going up hill and my speed would start to drip at a fairly steady pace as long as I was on the incline. As soon as it got flat again I could pick up speed.

Wasn't running rough, same tank of gas all day, no weigh change and NO check engine light. I just lost a bunch of power. It still shifted fine, braked fine, temp., battery volt, oil pressure all fine.

I knew something was wrong but I didn't have time to stop and find out. An hour or so later still going up and down the hills of South Georgia, I noticed that even at highway speeds (~70ish mph) with plenty of air flow over, around and through the radiator my temp. gauge started to climb. Not super quick, but it wasn't a happy 180 like normal.

Roughly 1 1/2 hours later, with the temp rising a little above 210 (probably 220-230), back in the city, stop and go lights, I got to where I was going and the engine was HOT. I gave it about an hour to cool down and drove the 10 minutes home.

Over the next few days I noticed very similar things happening, just less dramatic as I was driving only a few miles.

One start, it would start and idle just fine, 600ish RPM like normal, little stumbly but nothing new. But I still had a lack of power, and even while moving and pushing plenty of air through the radiator it would still get a little hot.

YES, both fans worked as they should. The coolant temp. sensor is only a few months old. Mechanical fan clutch is brand new, works great. Electric fans cuts on and off as it should.

At this point I had a check engine light. The codes read 12: Positive battery cable removed within last 50 key cycles, 51: A lean air/fuel ratio detected and 52: a rich air/fuel ratio detected.

Reset the ECU, replaced upstream O2 sensor, codes 51 and 52 fixed. Still had a lack of power and the temp could still creep up while driving.

At the time I decided to not worry about the over heating, holiday season things were busy and I just didn't feel like it. Plus I had just replaced the water pump, thermostat, serpentine belt and flushed the engine and radiator during the summer after my water pump seized and belt exploded. What a joyous road-side fix that was...

To attempt a fix at lack of power I replaced the distributor cap, rotor, wires, plugs and ignition coil. Now this engine has a little (LOT) of blow-by so the plugs were a little (VERY) on the ashy side of things. This helped a little, knocked most of the stutter at idle out, gave me a little power back. I figured with the proper functioning ignition system and new )2 sensor the ECU would need to relearn its fuel map, so I gave it a few weeks. Drove it to work, the store, ya know the usual places around town.

Fast forward a month, put about 600-800 miles on it, and things only got worse. When this all started I had enough power to still safely and reliably get around town, now I could barely hit 40 mph even on long flat straights, and sure as heck couldn't maintain it on any sort of incline.

I did a compression check, it sucks. Fuel pressure perfectly in spec., fuel pump is only about a year old. Spark check was good, crank position sensor is only a couple months on, vacuum leaks everywhere (nothing new, been like that for years). The usual exhaust leak at the head. It has an after market intake so I cleaned and oiled the filter. Has a throttle body spacer, all the cables are tight, throttle body is tight. I couldn't find anything externally wrong.

Fast forward a few months, I haven't had time to work on it. I can barely make it the few miles to work safely. Massive power loss.

This morning with the torrential down pour that happening on the east coast, I was forced to drive it. It's been sitting about 2 weeks, half tank of gas, nothings been done to it.

Got in, turned the key, fuel pump primed, tried to start and it wouldn't start, it turned out just fine but wouldn't even stutter. Yet ANOTHER new problem. Gave the starter a few seconds to cool down, tried again. Fuel pump primed, cranked but wouldn't start.

Now I'm a little irritated.

A couple seconds later I tried again. This time while I was cranking I gave it a little gas, maybe 1/4 throttle and it stated right up, but as soon as I let of the peddle it died fast and furiously.
A couple more times of this, pump primed, crank, throttle, start, close the throttle, dies, I was over it. Decided to just call a co-worker.

With the extensive 10 minutes of reading I did on my phone, seems like the general consensus for the not starting is the throttle position sensor. I haven't changed it, so it's probably original '94.

But with all of that, I turn it over to the experts. I'm working on rebuilding another 4.0 HO to swap with this one because of the leaks and blow by. But that's at least a couple weeks off.

Any brilliant guesses as to what this might be, in the mean time?

My thoughts are throttle position sensor for no start, either torque converter or distributor pick-up for lack of power, and no clue at all for over heat, though that didn't happen much during the winter.

Any help is greatly appreciated, thanks every one!!

 

I would drop the tank. I had the same issue with hills and the tube from the fuel pump was loose inside of the tank. It wouldn't be as much of an issue with a full tank because some of the fuel pumped would fall back thru the non-tight seal with resistance of the full tank. The only way to see this is with the fuel pump assembly removed. I'm not sure whether 1994 is like 1992 in which you can remove without dropping the tank.
Poor performance from a lean fuel-to-air ratio wouldn't turn the water pump as well and would circulate the coolant as well. The pump priming is not a sure check of the pump. That is mostly an electrical check. The fuel rail has a pressure testing port. You can "rent" a free gauge at a parts store to test this.
Even that isn't a real world test, though, if it is the same issue as mine. The hill and inertia won't be present like when you are driving upwards on an hill. That causes the gasoline to shift lower and rearward in the tank.

 

Sounds a little like a fuel pump that is getting weak and not able to keep up the supply when the demand gets heavy. Causing a lot of lean running, even if the computer commands more fuel (eventually throwing a code for lean mixture) it won't get any more gas than the fuel pump can deliver. This will be especially evident on inclines, which is probably the highest demand for fuel under normal operating conditions. Hard acceleration, and startup also being relatively high demand moments.

You can measure fuel pressure, but it can be hard to catch the fuel pump in the act of poor supply during high-load, because it is not easy to measure this in the driveway. You might be able to get create with a fuel pressure test gauge and have it pop out from under the hood so you can see the gauge while driving, but most do not have a hose that long (just 12-18" typically).

 

Hey guys,

I just got home and tried it again. It fired right up, but idled low (like 300-500rpm). I got out while it was running and gave the IAC valve a couple good wacks. The idle steadied off and held about 800rpm. It throttled up just fine, didn't die when I let off.

So I'm betting the IAC valve got stuck from 2ish weeks of sitting. And it looks like its original to the engine so I'm just gonna replace it. BUT I still have no power.

While it certainly could be, I'm hesitant to think its fuel pump, though it is certainly possible.

HERE ME OUT, I read it on on the internet

I had the fuel pump go out a little while back, man that SUCKS. It felt like I hit a giant pothole one day and then it just died on the side of the road. Easy change though! Didn't have to drop the tank, just jacked up the body, let the suspension hang and pulled it out. The old pump had actually started to melt (probably because of the incredibly well clogged fuel filter) so I was lucky my jeep didn't catch fire.

When I did that I also picked up a cheapo fuel pressure gauge off Amazon. I spliced in a long enough hose to poke it out the back of my hood and zip-tie it to my drivers side wiper blade. Drove around like that for 4 days before I decided I trusted the pump.

So when I started having the lack of power issue I threw it back on. Fuel pressure was perfect.

36-38 psi at idle with vacuum attached to the pressure regulator.
30-34 psi at speed with vacuum line attached.
40 psi at idle with vacuum line removed, no vacuum to pressure regulator.
36-40 psi at speed with vacuum line removed.

So in my experience that would rule out the fuel pump. Still possible though, I was wrong once but we don't talk about that anymore....

So where do I go from here! Obviously a new IAC valve, but what's my magic fix for no power?

Thanks so much guys!!!

 

What were the compressions that sucked? We're those wet or dry?

 

Sounds like you did the right test. I don't know the fuel pressure spec for a 94, but it sounds like you do.

Did you lose RPM when losing power on an incline? Or did the RPMs stay high? If you are not sure, you should test this. Because if your torque convertor is not locking up, or your transmission is slipping under load/heat, it will feel like a loss of power and will generally generate a lot more heat - which would be seen in the engine coolant temperature as well because it shares the radiator.

 

perform a "Torque Convertor Stall Test" outlined in the FSM, will indicate if TC is stuffed or not
although if the engine is lacking full power, not sure it will be conclusive

how bad is the compression?
how much oil does the motor consume?

in my experience, a worn out engine starts to deliver very low power if it blowsby, and the plugs become compromised
the entire combustion area becomes overwhelmed with oil & carbon

 

To perform a full test start at "FUEL SYSTEM FUEL PRESSURE" at http://jeep-manual.ru/index.php?page=320 and go from there.Record the values that you measure across the tests so that you can post them here.
If your problem for lean isn't too little gasoline, then it should be too much air. I'd check for cracks, gaps, and holes in the air intake side of things visually and perhaps with a spray bottle of plain water listening for rougher idle.

 

Hey all,

It's been a couple weeks since I did a compression check, but values at the time were

Cylinder #1: 108
Cylinder #2: 120
Cylinder #3: 118
Cylinder #4: 113
Cylinder #5: 124
Cylinder #6: 112

And that was dry. I didn't have any oil handy obviously I couldn't check them wet. That was also on a cold engine, probably mid-40's and approx. 1,000Ft. above sea level.

When going up an incline I loose engine RPM relative to speed loss. With additional pedal it also doesn't increase engine speed much if at all until the transmission shifts. It doesn't feel like the transmission is slipping or dropping out of gear or anything like that. The transmission oil looks fine, it was changed about 30K miles ago when I changed the leaky transmission cooler lines. Not weird smells to it. And the ATF filter was changed at the same time.

How exactly do I perform a stall test? Can't say I've done anything like that.

During the winter I burn/leak about 1/4 quart of oil a month. I've got SAE10W30, with 20psi of oil pressure at idle and 45psi of oil pressure at speed.
During the summer it'll go through a full quart or more a month depending on how much I'm driving. In the summer I use the same SAE10W30 and have roughly the same pressures.

I really don't think it's fuel delivery to the rail. With the values stated above, it seems to the rail even while driving that I'm keeping enough fuel pressure. They were roughly the same (I don't remember exactly what they were) back when I replaced the pump before I had the lack of power. And I've been through several tanks of gas since then, so I don't think I had water or something like that in the fuel that it started to suck up on my way back.

Thanks so much guys!