So here's the scenario...

At first, before the LIM gasket went out, the engine was losing power under load, but only in 3rd and 4th gear. Once the speed was low enough, it downshifted to 2nd, and got back up to speed quickly, then went to 3rd, 4th, then TCC lockup.

After the LIM gasket replacement, it's still doing the same thing, but not all the time. I work nights, so last night, I decided to unplug the battery while I was at work to clear any codes stored in memory (engine light was not on prior to this). I reconnected the battery, fired it right up, no CEL, and it was running good.

Drove the truck across the street to get a coffee for my ride home, then drove down the road to get to the highway onramp. Drove up the onramp without issues, merged at speed into traffic, still no issues.

About 1km down, there was an accident, so of course everyone was slowing down. I slowed with traffic to about 60kph, then once in the clear, accelerated back up to speed... This is where it starts...

When I hit about 90kph, the CEL came on, and the engine just started bogging down, not misfiring from what I could tell, just not producing power. The trans shifted to 3rd, still bogged down albeit not as bad. At this point I was going 60kph on the highway, then the trans kicked down to 2nd, the engine roared to life and it got back up to 100kph fairly quickly. It then shifted to 3rd, 4th, then TCC lockup and cruised up to 120kph without issue.

I set the cruise at 120, and went along my merry way for a good while, then on a slight downhill section, it started bogging again. It got down to 80kph (I left cruise engaged to see what would happen). It again downshifted to 2nd, and went through the gears until it was cruising again at 120kph. It was fine again for a while. Exited off the highway to an 80kph road (everyone travels it at 100kph), set the cruise to 90-95kph, and it went up a fairly steep but short hill without a hitch. I came to another hill that wasn't as steep, but longer and it started to bog down close to the top. Repeated the same shifting.

On my drive home, there is a very steep hill, that is a good 3km long. Most cars have to downshift just to keep speed going up. The truck started at 95kph at the bottom, and actually carried 4th gear in lockup all the way up until the crest of the hill when it started to bog again. At this point I let off the gas, as I had to slow down for a 60kph zone anyways.

It continued this random quirk for the rest of my trip home, sometimes doing fine under load, sometimes not.

Notable observations :

When the trans shifted to 2nd, it carried well, but the temp rose to a notch below the halfway point, but no further, and quickly went back down to regular temp once it shifted into 3rd. No real concern there.

Tested fuel pressure, 62psi key on ign off, holds 57 psi for longer than 10mins (I gave it about 15mins and relieved the pressure and disconnected the gauge).

TPS has a smooth transition through the whole range (0.1-4.5V)

I can't see it being the cat clogged up, other second gear wouldn't be able to do squat, same goes for fuel.

The dist cap and rotor did not look too bad when I reinstalled it.

There are no vacuum leaks that I can tell, the engine idles nice, smooth, and low.

Before someone asks, no I didn't pull the codes. Apparently I let a friend borrow my scanner, and he's gone MIA. I ordered a new one, but I'll have to wait a day or two for it. I ordered a bluetooth reader, and I'm going to use my laptop to get some good logs of whats going on.

Regardless, anyone seen anything like this before? I'm at a loss...

 

Disconnecting the battery loses your radio presets & clock, it sets a code in the TCCM if equipped with 4WD, and if equipped with automatic climate control, it will recalibrate when you reconnect the battery. It does not reset or clear any of the 3 memory files, and nothing is relearned. If the check engine light is on when you disconnect the battery, it will turn off the light ONLY if there are no hard faults present. Basically a waste of time

Your service port leakdown results are good, there is no excessive leakdown in the fuel delivery system. Your regulated pressure reading of 62psi is good. However, if 62psi is all the pump can build, that's a problem. Fuel volume will be insufficient during a sustained load, (fuel starvation). Make sure the fuel filter is not restricted, and check fuel pressure at the filter outlet. All pressure and flow from the pump must end at the tester. Pressure must be 73psi to 108psi. It must remain above 55psi for at least 10 minutes after the pump shuts off, (we already know leakdown is good though).

 

So interesting thing this morning going home from work... I decided to unplug the MAF sensor for ****z and giggles to see if anything would come of it. I figured why not, the CEL is already on LOL

Started it up, and I noticed right away that it was idling lower then it was before. The truck was sitting all night, so it was dead cold. Did my normal routine of getting my coffee for my trip home, and set off.

Would you believe that it drove perfectly the whole way home?! Not a single hiccup, misfire, weird shifting, nothing. Just smooth driving... The cruise control actually had to slow the truck down on downhill road, which I may be crazy was absolutely delightful.

So now the question is, with the MAF unplugged and the truck running good, what have I narrowed it down to? I could venture a guess and say the MAF itself is bad, it didn't look dirty, but I will try cleaning it anyways. I read somewhere here that with it unplugged the PCM relies on the TPS and MAP sensors for fuel/spark and ignores the O2 sensors? Is this correct?

If so, I may have to replace O2 sensors and such...

 

When you disconnect the MAF, the PCM will switch to open loop. It will ignore oxygen sensor data, and apply a richer fuel mixture. If disconnecting the MAF improves the performance, it indicates that it's running lean in closed loop, (not enough fuel, or too much air). Best to check streaming data & fuel trims on a scanner to figure out what's going on. When oxygen sensors get old and lazy, or completely die, fuel mixture goes rich, so that's probably not the problem. Several things can cause it to run lean though. Low fuel pressure and/or volume are two of those things Make sure the fuel filter is not restricted, and check fuel pressure at the filter outlet. Wouldn't hurt to check volume too, should pump 1 quart in 30 seconds.

 

When I was under the truck checking the 4x4 components today, I noticed I had a fair size exhaust leak just above the Y pipe merge. Do you think that could be playing a part? I'm still waiting on my scanner... That storm has got everything delayed...

EDIT : Forgot to add that I checked the fuel pressure at the filter (replaced it while I was there). 98 psi when pump is live, I left it on the line with the pump off for quite some time (I actually forgot about when I went inside to warm up and got caught up playing tea party with my baby girl lol) I went back out and it was at 59 psi, so it seems that the pump checks out fine. I also cleaned the MAF, no change. I'm starting to think that the exhaust leak may be letting air at or before the O2 sensor and wreaking havoc on the readings...

 

Fuel pump is fine, no problem there.

On your 1997: If there are any exhaust leaks upstream, (forward) of B1S2 oxygen sensor, it will affect performance, no doubt about it. Any leaks downstream of B1S2 will not affect engine performance. B1S3 monitors the catalytic converter, that's all it does, it does not affect engine performance.

 

There is definitely a leak upstream of b1s2, its coming from above the y pipe merge. Guess I have to replace that pipe before I go any further... Great, more ordering and waiting lol