1998 C70 2.0 T5 (B5204T3) – WOT Misfire

[Gregory91PL]

Hi everyone, the patient is a 1998 B5204T3 in a C70 body.

​After purchasing, the car didn't want to go at all. It turned out that the wastegate rod had fallen off; it was put back on and incorrectly overtightened by a mechanic. Initially, this didn't cause too many issues because, due to a weak ignition system, it didn't drive super fast anyway. It felt sluggish, but once it built revs, it seemed to manage, and it felt like those ~225 HP might actually be somewhere in there.

​Parts replaced:
​New OEM Volvo spark plugs
​New Bosch ignition leads
​New Bosch ignition coil
​New OEM Volvo distributor cap
​New OEM Volvo distributor rotor
​New Pierburg TCV
​New vacuum lines
​New timing belt kit
​New Bosch MAF sensor
​New fuel filter
​
Checked / Verified:
​Fuel pressure tested with a gauge – holds 3 bar plus boost
​Intake system is completely leak-free
​No DTCs / fault codes stored on the ECU
​
The car came back to life, but instead of pushing 0.95 bar, it started hitting 1.1 bar, accompanied by misfiring/hesitation starting right around 3000 RPM.
I connected the dots regarding the wastegate preload adjustment and figured it was overboosting and triggering a safety cut. I checked the crack pressure and adjusted the rod movement to 0.15 bar (found this spec in a Volvo service sheet).
​
On a test drive with a boost gauge, it now holds around 0.95 bar, but under 100% WOT (Wide Open Throttle) it still misfires/cuts out. At 80% throttle input, it pulls smooth and clean.
​
I have a Launch CRP 919, so I took it out for more log runs and amateur data analysis.
​Air/fuel mixture stays rich all the way to redline, no timing pull/retard, fuel trims look very healthy, and coolant temp is completely normal.
​
Then I finally noticed the MAF readings.

​The Volvo starts misfiring the exact moment the airflow hits 408 kg/h. This happens between 3000–3300 RPM in 3rd and 4th gear. I found information in a Bosch tech bulletin stating this MAF unit has a measurement range up to 480 kg/h.

​I bought a "New" NOS (New Old Stock) Bosch MAF, and the behavior is 100% identical.

​So here is my question that hopefully someone here knows the answer to:
Is 408 kg/h some sort of hardcoded airflow limit imposed by the Motronic software/ECU?

​Best regards!

[RickHaleParker]

Check your plumbing.

Unmetered air escaping through loose inter-cooler piping or torn hoses creates a lean condition under boost.
Would also explain a lower then expected flow.

Don't just look at it. Grab it and give it a shakedown. Somewhere it is opening up under pressure (~1.0 bar).

[Gregory91PL]

I checked the intake with a smoke machine and it was tight, but obviously that wouldn't show a leak under 1 bar boost pressure. I was thinking about doing a proper pressure test, but I wasn't really sure how to set it up.
I'll definitely give it a good shakedown by hand and see if anything feels loose.

[RickHaleParker]

I'll definitely give it a good shakedown by hand and see if anything feels loose.

Might be a crack in plastic somewhere that opens to a hole under pressure.
You could make it a weekend project to tear it down, clean and inspect.

[abscate]

A cracked intercooler is a real pain to find but would fit symptoms. Maybe getting off the car and filling with water ?

[Gregory91PL]

I think the water test doesn't solve the problem of a potential leak under boost pressure.

[RickHaleParker]

I think the water test doesn't solve the problem of a potential leak under boost pressure.

A good cleaning and a visual inspection is your best chance. Keep a screwdriver on hand to probe any cracks you find.

[matthew1]

Welcome to MVS, Gregory. The intake manifold hose is a fairly wide diameter and with age can deform/collapse under boost. Difficult to check besides feel. If you have a "GoPro" camera or any small device capable of video you could mount it nearby, pointed at this hose, then click record and take a run.

[Gregory91PL]

I'm starting to consider the do88 Big Pack intercooler set with the RIP kit, but I've got a few doubts. First off, I'm still not 100% sure if the intake system is even causing this whole mess. On top of that, looking at it from here in Poland, the price tag is honestly pretty steep. And lastly, I really value keeping my C70 in fully stock, original condition.

That being said, you actually make a really good point about the hose collapsing under boost. What was bugging me is that a regular boost leak would normally cause fluctuating MAF readings. But a soft inlet hose getting sucked flat under vacuum makes total sense—it would act as a physical bottleneck and restrict airflow to that exact same capacity every single time.

At this point, a collapsing hose seems like the only mechanical scenario that actually explains why the MAF freezes at 408 kg/h so consistently, unless it's a hardcoded limit in the Motronic software itself.

[abscate]

I’m not sure this is right

fuel mixture stays rich all the way to redline, no timing pull/retard, fuel trims look very healthy, and coolant temp is completely normal.

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