Two Sensors Die at Once? 98 v70 T5

[abscate]

You might just be hypersensitive to everything " not quite right" with your car..so let's get our sleeves rolled up

Ignition
When were plugs cap rotor wires last changed? Let's pull the plugs and have a look at them.

You need to go over all your vacuum hoses and look for a split hole, gash, funky chicken. That could be the whole problem here. Trouble spots of known history are the idle air control boot and the vacuum elbow Under the manifold at front left of the motor ( front of motor is to passenger side of car)

Induction path, the big tunes from the turbo to the intercooler and then the throttle body are another part that can go wrong. There are three (?) rubber parts that can split here that will give you trouble

The fuel pressure regulator, FPR, Hausa vacuum hose on it as well as a diaphragm inside that bears and dumps fuel into the manifold, causing sputtering

[WagonLagan]

Hey abscate! Your guidance has helped me often when finding solutions in other people's posts, thanks!

I can assure you it's not a "not quite right" situation. With 260k on this car, I'm used to it not being perfect. The shuddering I describe is the RPMs repeatedly dipping down to 500- when coming to a stop or when I am stopped. The engine shakes and sounds like it's going to stall. Before I replaced the TPS, it was so bad it was forcing the car into Limp mode. Something is significantly off. I wasn't too awful worried until my brake assist gave out and I saw white smoke, which I haven't seen before from my car (excluding water vapor in winter obviously).

I'll follow your troubleshooting guide as soon as I can, hopefully Sunday, and report back what I find.

Any thoughts on why these things would all happen around the same time? Seems like WAY too much coincidence to me when the car was running quite steady for a long time.

[mrbrian200]

Loss of brake booster vacuum assist is probably key to the whole thing.
According to product description of this particular booster/master cylinder on FCP "Fluid leakage from the master cylinder can enter the booster and cause damage to the diaphragm". Might the smoke you saw in your rearview not be coolant, but possibly brake fluid?

[Clemens]

The brake booster is vacum operated, no electronics involved here as I see it. So if the motor dies or almost stalls, there could be too little vacum for the booster, i guess.

[WagonLagan]

Update. Haven't gotten time to try abscate's troubleshooting yet.

Thank you guys for trying to work through this weird collage of problems.

Car dropped back into Limp mode again . I really hate driving it like this but I have pretty much no choice. Hopefully I'm not damaging anything.

I got a video of the RPM dipping/shuddering I'm talking about, so that you all can see the severity. Unfortunately you can't hear the engine shuddering in the audio, but rest assured, it sounds and feels awful. This usually happens when coming to a stop and/or sitting at a light. It seems to level out after coming up to speed.

During these videos I was not touching the gas.
https://vid.me/9sXQI
https://vid.me/IBybZ

[WagonLagan]

Loss of brake booster vacuum assist is probably key to the whole thing.
According to product description of this particular booster/master cylinder on FCP "Fluid leakage from the master cylinder can enter the booster and cause damage to the diaphragm". Might the smoke you saw in your rearview not be coolant, but possibly brake fluid?

I don't know. I'm unfamiliar with what color brake fluid burns off as. The brakes only faltered that one time, and I haven't seen any more white smoke since.

The brake booster is vacum operated, no electronics involved here as I see it. So if the motor dies or almost stalls, there could be too little vacum for the booster, i guess.

I hadn't considered that. So it's likely the brake boost loss is a red herring. Especially considering it only happened that one time.

[sleddriver]

I'd pull the idle air control motor, thoroughly clean it out with throttle body cleaner, then clean the throttle body itself. Next check all vacuum connections right at the intake manifold. There's about 4 or 5. Pull the vacuum tree and inspect the gasket. Be sure the tree isn't cracked nor are the "branch" caps rock hard. They'll leak.

Smoke is the easiest way to find vacuum leaks. I built a simple smoker from a new paint can, fireworks punks, an old aquarium pump & some tubing.

With your foot off the gas and on the brake at idle, the Motronic system is in closed loop mode and the idle control motor is in charge of regulating idle speed. Any leak there will cause havoc.

[mrbrian200]

DOT4 is glycol ether/borate ester. DOT5 is silicone based. Either one I would probably expect to make white smoke. If you've ever flushed your brake system with DOT5 be aware silicone is a known nemesis of both MAF (if your car uses one) and 02 sensors. I wouldn't expect DOT4, technically a solvent, to be particularly good for them either.

However, if you determine the loss of brake boost was because the engine wasn't running or barely chugging then this line probably isn't what's wrong.

You know, massive amounts of fuel can manifest as white smoke if there's no combustion/not getting hot enough to carbonize. Gasoline is technically a very light oil. If you pour gas into the carb on a cold lawnmower engine and try to start it with the spark plug disconnected you'll see this.

[mrbrian200]

I reread this whole thread and decided this sounds more like a wiring or ECU problem. DTCs for 2 sensors, intermittent. So I looked up the schematic.

Fenix engine management correct? ...Fenix uses a MAP, Motronic uses a MAF from what I see. You are getting a DTC for MAP/baro and TPS.

1998 V70 with Fenix management, TPS and MAP share a reference voltage on a violet wire. Pin 15 at ecu, splits to TPS and MAP, nothing else. I don't know what voltage you should see on that circuit. Probably either +5v or +12v. Volvo diagrams never say.

Both sensors ground back to ECU, but that ground circuit is shared with three more sensors, you'd likely be getting DTCs for those too. But you never know. Ground circuit is Brown/Black stripe wire from various sensors under the hood. They're all tied together at a junction buried somewhere the harness and end up back at the ECU pin 18.

My experience with engine management systems from this era when 2 or more sensors are whack at the same time the whole thing can go just completely haywire (such as so much gas it comes out as white cloud instead of black) An electrical/connection/wiring issue could certainly explain the intermittent nature as well.

Sorry about the little brake fluid conspiracy theory. Without the car in front of you can be easy to visualize some yellow brick road that isn't there.

[abscate]

You really still haven't ruled out a simple vacuum leak, and poor running throwing spurious codes. The vacuum loss and smoke are red herring symptoms from poor running.

If you scan MVS, poor running is dominated by vacuum leaks over component failures about 10:1

Ok, I made that up, but it's a big ratio.