1998 C70 2.0 T5 (B5204T3) – WOT Misfire

[Gregory91PL]

I checked the rubber hoses today, and they flex under even very light pressure. At least one of them near the throttle body is cracked, so I went ahead and ordered a full set of silicone hoses.

Once they arrive, I'll tear it down, inspect all the hard plastic pipes, and put everything back together with the new silicone hoses and fresh clamps.

I’m not sure this is right

Isn’t the timing pulled back hard on boost? It sure is on ME7

I happen to have a screenshot of a chart from one of the runs on hand. I know it's a bit hard to see, but you can still read the parameters by following the lines on the graph.

[Gregory91PL]

I've replaced the rubber hoses with silicone ones, and the plastic pipes are completely intact with no cracks. The issue is still there, though the only difference is that the throttle feels a bit more responsive.

Could it be the fuel pump in the tank? Maybe it just can't keep up under full load?

Or... Overboost.

[hausmeister]

Probably not ignition? Because that feels like the car is shaking apart under WOT.

A boost leak usually just feels like missing power... not so much misfiring (in my experience). You can get something similar to this to really test the induction system: https://www.amazon.de/Turbolader-Ladelu ... B0GKFHW6YM
If it really misfires maybe do a compression test to be sure?

Sorry, no idea about the MAF. Might be fuel cut or unmetered air or leak if it feels like a soft wall. Can't read fancy graphs on my old car to compare But if it's overboosting or something I'd still think the ecu would do something instead of misfiring.

Edit:
you might also replace the O2 sensor as they can go bad over time. Injector cleaning might help in some cases...

[Gregory91PL]

I got a bit ahead of myself with the "misfire" thing. It's more of a slight hesitation or stuttering. It happens exactly when the MAF reads 408 kg/h.

I just managed to find some info on a forum showing that 408 kg/h is actually the maximum value that Motronic and the MAF can read.

Now I need to figure out why I'm hitting that max value so early, at around 3200–3300 RPM, when the engine revs all the way past 6000.

Either something is causing excessive airflow, or something is messing up the sensor reading.

New Bosch MAF.
And new silicone hoses.

[hausmeister]

Well, ok. Sounds more like overboost/protection to me. I'm not sure on the c70, but the 850 stock ecu is very tame and will feed the boost gradually, you won't have full boost at 3k rpm I think. Is it tuned? maybe wastegate or boost solenoid setup incorrectly?
Or someone put a big turbo on it and it doesn't have the tune for it.

[Gregory91PL]

The car is completely stock, with a stock turbo.

According to this table, it already hits full boost at 3000 RPM, which matches what I see on the boost gauge connected to the manifold.

The TCV is a brand new Pierburg unit, and I actually checked if it was connected correctly for the fifth time today.

The wastegate crack pressure (the moment the rod starts moving) is set to 0.15 bar. That’s how it should be.

Two questions come to mind:

Could the wastegate not be moving as smoothly as it should?
Could a potential vacuum/air leak before the turbo cause the MAF reading to be too high?

[hausmeister]

Alright, then I'm out of ideas. You can test the wastegate, just disconnect the arm and see if it moves freely.
Air leak b4 the turbo is after the MAF, so imo it would read low and be lean.

Edit: 195 kpa is 1.95bar. So I assume thats including atmosphere so it should have around 0,95 bar. Nice that this already has it at 3k rpm. the maximum MAF reading should be a lot more than that I'd think, though.

[Gregory91PL]

Quoting Volvospeed.com user Midnight Caller:

"From the VOLVO bluebook it seems that MAF measurement should be up to 408kg/h which seems consistent with the Bosch HFM2. Also, if we multiply 255 with the 1.6 scaling factor in the M4.4 RAM scaling we also get 408."

This aligns perfectly with the readings from both of my mass air flow sensors.

[matthew1]

This is fascinating. This phenomenon is what I may have been hitting in 2012 in my 850 T-5, but I had a manual boost controller, flirting with 17-18 psi. I had a "628" R ECU, but maybe that also was bound to this 408 kg/h ceiling.

What's strange in your situation Gregory, is that you're hitting it stock.

[Gregory91PL]

My only idea left is some issue with the MAF wiring harness that is corrupting the readings. Possibly a bad ground.