Random "rev limiter" sensation - update - found the problem

[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.

[Chilezen]

UPDATE

As mentioned in the first post,

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.

I decided the most logical route to diagnose any issue would be to address the codes first, foregoing any inspection on the fuel system. One important detail relevant to the O2s is that they will control whether the car enters closed loop. Turns out, if the heater circuits don't register, then the ECU won't consider the O2s to be up to operating temperature, and it will run in open loop indefinitely. That was my first clue; my console-mounted scan gauge can display data like this at all times. It confirmed that the car never reaches closed loop. So, I suspect, this "rev limiting sensation" is related to the ECU not entering its normal operating conditions, so it limits engine power when the load is too high. I assume.

Therefore I investigated the O2 sensors. First I checked for resistance in the heating elements. Came up as OL both times; no continuity. That confirms they are bad. Then I checked the resistance of the sensors. Those also didn't give me results, but maybe that's because they're cold? I don't know.
Then I checked for 12v coming from into the heater circuit, which I did by just testing from the loom side of the plug. I went from power to ground in the plug. I got about 5v. This threw me off; I expected 12 because in the wiring diagram, I didn't see any reason for the input voltage to be that low.
So I asked my tech buddy and he came over to help. He explained to me that because the ground goes to the ECU, and not battery ground, that I would be reading 5v due to normal internal resistance. That made sense. We checked the rest of the curcuits, for any voltage drops or shorts, and found none.
I was worried that if I replaced the sensors, but if there was still a problem with the circuits/harness, that it would fry the new sensors. But since we concluded the harness is fine, that it will be okay to replace the sensors.

However, I can't reasonably understand what caused both heating elements to die at the same time. But I can admit that I had been working on my exhaust recently, where I did unscrew the sensors to remove the exhaust. I wiped off the carbon buildup, and I applied a tiny bit of copper anti seize to the threads, which was advised in the Bentley manual. How did I destroy the heating elements doing this? I don't know. Did I cause this, or did they fail coincidentally together some other way? I don't know.

So now I'm going to buy a new front and rear O2 sensor.
FCP Euro only offers Genuine BMW, while Pelican offers a few less expensive options. Of that list, what brand(s) would you go with? Walker, Pierburg, NTK, or BMW? These are the results for the '97 Z3 engine I have.
Thanks!

[TobyB]

For narrowband sensors, NTK would be fine.
If the heater circuit's not metering low resistance, then yes, the heaters are shot. They do fail.

You won't get a resistance reading off the sensor itself- you'd be looking for a voltage in operation.

Since the car is in closed loop under full throttle, I will be curious to see if this is the problem,
as there's anecdotal evidence that o2 data is used as a modifier on the closed- loop tables...

t

[moatilliatta]

Do you have the revised crank sensor and 12v harness?

Sometimes 5K limiters happen if it drops wheel speed or if you don't have wheel speed coded out.

[Chilezen]

For narrowband sensors, NTK would be fine.
You won't get a resistance reading off the sensor itself- you'd be looking for a voltage in operation.
Since the car is in closed loop under full throttle, I will be curious to see if this is the problem,

Cool, NTK is available for the pre-cat. But for the post-cat, would I choose Walker or Pierburg?
Good to know.
The car didn't enter closed loop even under full throttle.

Do you have the revised crank sensor and 12v harness?

The harness has the updated 12v loom.

Thanks guys!

[TobyB]

Oh. Ugh. Me, I'd grimace (remember him?) and buy Pierburg, because I KNOW Walker doesn't make oxygen sensors...

In theory, the post- cat's only used to monitor the cat itself, so it shouldn't matter too much.

t

[Chilezen]

Oh. Ugh. Me, I'd grimace (remember him?) and buy Pierburg, because I KNOW Walker doesn't make oxygen sensors...

Lol. I had to go with Walker. Pierburg wasn't available. But at least I got it with Pelican's lifetime warranty, so when it fails, I'll throw in another one. And another. I'm curious who Walker buys them from though. And yeah, since it's just monitoring the cat, I cared less about the rear.

Will update in a week or so when stuff is installed!

[Chilezen]

Alright. New O2s installed.

Wiped the codes. Drove around. I can see on my scanner that the car enters closed loop. Yay!! Codes haven't returned. Yay!!



But the random rev limiter problem still exists! So that problem was completely unrelated to this one :(

So here's what I know:

• During a hard acceleration/full throttle, I'm approaching redline, but then engine power hits a ceiling, and doesn't continue accelerating. It will hold that RPM, while full throttle, below redline (typically between 4.5k to 6k). It will stay there as long as I stay on full throttle.
• If I let off the throttle a little, the car will slow down a little, but, if I get close to full throttle, the car will slowly accelerate past that rpm it was previously stuck at. In other words, if the engine load is less, it will make it to redline.
• If I stomp on the throttle from idle, it will hit redline, and stay there until I let off.

So,
there is sufficient fuel flow to sustain redline. Both under no load and under light/medium load. But, there is an issue under full load.

So what parameters dictate a high engine load?
What inputs will cause the ECU to determine it needs to limit power?
Is the ECU detecting knock? I don't hear any pinging, and since I can rev higher under other conditions, I think I would have heard pinging then.

[jbontke]

If you had knock, the ODB2 would flash the check engine light (do you have a light connected?). What DME version do you have? Was it flashed or modified in any way?

[Chilezen]

If you had knock, the ODB2 would flash the check engine light (do you have a light connected?). What DME version do you have? Was it flashed or modified in any way?

Yes and no; Yes I have the light, it is operational, except I have it disconnected because I have a code all the time, for an emissions sensor (fuel tank pressure sensor) that I need to install. I have a scanner permanently installed in the car to check codes on the fly.
In this instance, I could reconnect the light, to see if it flashes.
I have the stock ECU, MS41.1 I think, and it was flashed only to remove the EWS. I do plan to have the car pass California BAR one day.

Edit: update: no, the engine light did not flash during the issue.

[Chilezen]

Final update: It's fixed!

And no, it wasn't anything that has been mentioned yet.

Over the course of 3 years, 38,000 miles on this new/rebuilt engine, I've had 2 spark plugs fail. That seems rather premature, don't you think? So I started to wonder if another was going to fail soon.
These plugs were installed by the shop that built the engine: NGK BKR6E

I did 3 things tonight: cleaned the air filter, sprayed the MAF clean, and I changed most of the spark plugs. Most, because I kept the newest one in there. I installed NGK BKR6EK.
I took photos of the plugs in case anyone's interested, to see their condition.

After that, took the car for a test drive. It was nearly a WORLD of difference! The car pulled harder, with no stumbling! It was amazing!

So new plugs and a little maf spray did the trick!

Why? I can't figure this one out. I could try to reason that the maf was dirty and giving inaccurate airflow readings. I could maybe assume the plugs were too weak to keep up with the load. I really don't know. But there it is. Go figure.