Random "rev limiter" sensation

[Chilezen]

'87 chassis with an M52 OBD2.
I have this problem during a hard acceleration, and it feels like fuel starvation. But it's not every time. I'll rev to redline at 6800 or so in 2nd gear, no problem. But in 3rd, it like stalls around 5k. Foot is on the floor but the RPMs stay constant.
What's weird to me is, I'll clutch in as this is happening, and the RPM's will go up! Obviously I'm not accelerating anymore, so maybe fuel can return to the lines, but once I release the clutch, the problem still exists.
I have nearly a full tank of gas. Everything seems normal about the engine, however, I do have 2 codes (that have been present for a little while now, well before I noticed this new problem): P0135 and P0141. Those are for a O2 sensor heating element circuit, and I can't imagine what that would have to do with this acceleration issue, but it was worth mentioning.
This problem just started (that I'm aware of) today, so I haven't done any real testing with other conditions beyond what I've mentioned.
I'm aware the duel fuel pump setup can be wonky, and starvation is a known issue on early cars. But I've accelerated hard before and never run into this. I've never replaced the in-tank pump, but the inline pump was replaced in 2018.
Maybe the issue isn't fuel-related, but I haven't any ideas as to what else could cause this.

Any thoughts? Thank you!

[Panici]

My bet is fuel pressure or loss of spark.

Can you throw a mechanical fuel pressure gauge on the supply line or the fuel rail?
If you see pressure dropping when you have the issue, you can check the fuel filter, FPR, pumps, and then the hoses/lines.

I would assume a crank/cam sensor issue or a coil induced misfire would throw a code on a stock OBD2 ECU?


On my car, I had the in-tank pump split the in-tank rubber hose between the pump and the top fitting (but that was because I did a conversion to an aftermarket single fuel pump and that little hose isn't rated for high pressure).
This could have also failed due to age, more so if you have ethanol in your gas.

[TobyB]

+1 for checking your fuel pressure. That would track the 2nd- 3rd gear behavior, in my experience...

t
load related

[reelizmpro]

This may or may not be your problem but I would not trust the original transfer pump. A new one would support S52 HP but they are expensive nowadays and aftermarket transfer pumps are hit and miss. That's why many, including myself converted to a single high pressure intank pump and bypassed the external main pump. Anyway, it only takes a few minutes to pull the transfer pump out and inspect it. Look for anything obvious like loose connection or corrosion build up. I once had a leak at the hose on top of the transfer pump caused by a loose clamp and the hose was cracking. I also experienced a leaky hose between the fuel filter and main pump under the car so that's all worth a look as these cars age.

[Chilezen]

I've done some "testing" and do believe it is a pump issue. I will stall between 5-6k in those gears... oddly not in 1st though. Surprisingly, it happens in 4th gear too, north of 6k. It's not consistent in terms of missing every time I do the test, but I will confirm it only happens during hard acceleration aka high g forces. I can make it to redline if I approach it slowly.
Fuel tank level is irrelevant to its condition.
Next step is to inspect the in-tank pump.

Can you throw a mechanical fuel pressure gauge on the supply line or the fuel rail?

I would assume a crank/cam sensor issue or a coil induced misfire would throw a code on a stock OBD2 ECU?

This could have also failed due to age, more so if you have ethanol in your gas.

As this problem exists during high G forces, I'd have to screw on the gauge to the schrader valve at the front of the rail and run the hose all the way into the cabin to watch it as a drive. It's possible I guess, a good idea, but not my first step.
Yeah, no codes related to a misfire have appeared.
Age is likely the culprit, in addition to California gas.

I've looked up the single pump conversion (and saw your input there Barry) but for some reason, I'm still not convinced... I'd rather take a mortgage out on the house to buy the OEM pump. We'll see. I wouldn't be surprised if it's dirty and worn by now.

On a separate note, I've read a tip that, one way to avoid starvation in early cars is to run an extended length of fuel line between the pickup and high pressure pumps... essentially storing fuel in the line like a reservoir but without the tank itself.