1998 Volvo V70 GLT, High Manifold Pressure

[Zodiark1593]

Today, I got a check engine light, being far from home, I took it to the nearest place I could find that would read the trouble code (which ended up being the Midas next to my work). The code tells me the sensor in the intake manifold is reading too high a pressure. This actually relates to the recent feeling that my car has more power than it normally does (I can actually hear the turbo spin up about at about 1900 rpm, when normally, it only kicks up at about 2300-2500 rpm), which began right after I took it in for an oil change Saturday.

Anyhow, I'm getting a diagnostics done tomorrow, but for now, any thoughts. My research tells me a malfunctioned wastegate for the Turbo can result in increased boost pressure if it is stuck closed. What sticks out though is I only noticed the increased power right after the oil change Saturday, so I'm wondering if the wrong oil can also impact how the Turbo spins up?

I'm rather nervous though. I've actually had dreams where I get the check engine light, and something bad would soon happen, and I drive like a granny 99% of the time, so I'm on the paranoid side :'(

 

[tortillasoup]

Get the PCV system inspected/worked on etc. Common issue of eurotrash cars, ask me how I know... (Hint: I own one)

 

[Zenmervolt]

Get the PCV system inspected/worked on etc. Common issue of eurotrash cars, ask me how I know... (Hint: I own one)

If you think that a clogged/malfunctioning PCV system can cause high manifold pressure, then it's clear that you're not familiar with what a PCV system is or how it works.

The PCV system provides ventilation for high crankcase pressure. The crankcase and the manifold are two entirely separate things. When functioning normally, the PCV system vents excess crankcase pressure into the engine air intake; in the case of turbocharged cars, this happens before the turbo. It is impossible for a PCV malfunction to cause excessive pressure in the manifold, which is after the turbo.

While Volvo's turbo 5-cylinder engines are indeed known for collecting some gunk in the PCV system (though primarily it's the high pressure 2.3 liter turbo in the T5 models and not the low pressure 2.4 liter engine used in the OP's GLT), it's not possible for that to be causing the issue the OP described.

To the OP:

Unless the shop was fiddling with everything in the entire engine bay, it's not possible for a routine oil change to damage the wastegate. Nor will the wrong oil cause the issues you're seeing. Of note, the low-pressure turbo engines like yours are set up so that the turbo is supposed to spool at about 1,800 RPM. The specs for the LPT (low-pressure turbo) engine give peak torque of 199 ft-lbs from 1,800 RPM all the way through 4,998 RPM. It's only the high-pressure T5 models that don't fully spool until about 2,500 RPM. So you may have already had issues with the wastegate being stuck slightly open before it recently stuck closed.

ZV

 

[tortillasoup]

I misread OP's topic but nonetheless he should have the PCV system cleaned/repaired/etc if it hasn't been done already in the vehicle's service history.

 

[Zodiark1593]

Just got word from the mechanic. He said it was the Manifold Air Pressure sensor that was acting up. I'll probably pick up a pressure guage anyway, just to keep an eye out for trouble.

This does pose an additional question though. Does type of oil and frequency alter turbocharger performance (oil changes seem to make the turbo work better for awhile)? As per what the manual specifies, I use conventional 5w-30 and change every 5k miles. Should I use synthetic, or change the frequency of changes?

To quote above, the PCV system has been serviced at about 145k miles. I'm at 185k now, though I put on about 25k a year.

 

[phucheneh]

I've seen turbo 850's (98-99 S/V70 are basically the same) with 200k+ miles still pull negative pressure on the crankcase. You measure it at the dipstick tube with a manometer (a very sensitive vacuum gauge, basically, but Volvo calls it a manometer). Their PCV systems work very well...when they're working. The first stage of failure is a restricted oil trap box and/or associated external hoses. Second stage is blocked passages inside the engine (this sucks; it means pulling the oil pan and HOPING that you can use complex specialty tools...such as coat hangars...to clear out any carbon and/or sludge that is blocking the way).

However, as ZV plainly stated, your issue is not with the PCV system. You could have a perfectly-functioning PCV system, or you could have a totally non-functional one with copious amounts of cylinder blowby...and it would not make much difference with what's going on in the intake.

I'm not sure if those cars are capable of setting a code for too much boost. I'm pretty sure the later Volvos (S80, S60, S40, ect) can, but still can't say 100% for sure. If they can, I don't think I've ever seen it.

It sounds like you have P0108 stored, which is 'manifold absolute pressure circuit high.' Not 'air pressure,' but 'absolute pressure,' because it is not measured in comparison to the atmosphereic ~14.7psi. Pressure is transmitted via a voltage that corresponds to a standard pressure unit like kPa, with no negative numbers. The MAP sensor of naturally aspirated engines would max out at about 100kPa, or atmospheric pressure. Boosted engines will see a higher reading.

A circuit/signal high DTC is not actually saying 'too much pressure.' It's saying that it is seeing reference voltage (5v), which should correspond to an impossible stupid-high pressure that it should not ever see. So the ECM/PCM had turned on the CEL to let you know that it sees a fault, and is disregarding that sensor input. It is now depending on MAF, IAT, and other input sensors to determine how to fuel your engine.

If you had a 1990 TBI Chevy truck, it would be running like complete dogshit. Luckily, the engine controls on your car are a little more robust, and the PCM is probably still doing a pretty good job of managing the EFI based on the valid inputs still present.

 

[Zodiark1593]

It sounds like you have P0108 stored, which is 'manifold absolute pressure circuit high.' Not 'air pressure,' but 'absolute pressure,' because it is not measured in comparison to the atmosphereic ~14.7psi. Pressure is transmitted via a voltage that corresponds to a standard pressure unit like kPa, with no negative numbers. The MAP sensor of naturally aspirated engines would max out at about 100kPa, or atmospheric pressure. Boosted engines will see a higher reading.

A circuit/signal high DTC is not actually saying 'too much pressure.' It's saying that it is seeing reference voltage (5v), which should correspond to an impossible stupid-high pressure that it should not ever see. So the ECM/PCM had turned on the CEL to let you know that it sees a fault, and is disregarding that sensor input. It is now depending on MAF, IAT, and other input sensors to determine how to fuel your engine.

If you had a 1990 TBI Chevy truck, it would be running like complete dogshit. Luckily, the engine controls on your car are a little more robust, and the PCM is probably still doing a pretty good job of managing the EFI based on the valid inputs still present.

That's the code I got. And yes, the other sensors are doing a pretty fine job of keeping the engine going. I have the sensor ordered, about $100. Pricy, but would otherwise run me $150+ in most other shops I went to.

 

[Zenmervolt]

This was really tripping neurons in the back of my mind and thanks to phucheneh's post, now I know why.

The 1998 S70/V70 line doesn't actually have a MAP sensor. There is a barometric sensor outside the engine just sort of hanging out under the hood and that's what will trip the CEL as a P0108 code. The sensor is used for altitude correction (the higher you are, the leaner the fuel map can be when you're off boost), but without it the engine is just going to default to assuming sea-level operation and leave the fueling maps alone. Some parts lists just call it the "altitude sensor."

I recently had the same code thrown on my own '98 S70. I unplugged the sensor, cleaned up the connectors with contact cleaner, plugged it back in, and it's been working perfectly for the last 1,000 miles with no recurrence of the CEL. I can't guarantee that will work for you, but it's worth trying. Of note, you can find an off-brand version of the sensor for about $80 from FCP Euro, but I generally prefer to stick with OE Bosch sensors myself.

You measure it at the dipstick tube with a manometer (a very sensitive vacuum gauge, basically, but Volvo calls it a manometer).

"Manometer" is the term for the old-style pressure measurement devices that used a column of liquid. Even though a lot of newer gauges don't use that method anymore, the term is still pretty common in European and Asian manuals from what I've seen. I've mostly seen it in descriptions of the process for synchronizing carburetors on vehicles that have one carb per cylinder where the instructions are, basically, hook a manometer to each cylinder's intake and then adjust the carb linkage until the vacuum is the same for all cylinders. (For a simple process it can be a royal pain.)

ZV

 

[tortillasoup]

Dumb they would put the barometer in the engine bay when they could have incorporated that directly onto the circuit board of the ECU like most other manufacturers do. Even on the 80s Honda Fuel injection systems did that. I wonder what other vehicles have such a stupid setup for a Barometer.

 

[Zenmervolt]

I wonder what other vehicles have such a stupid setup for a Barometer.

Essentially every Bosch or Bosch-derivative system ever built (without a MAP sensor).

Which, despite your comment, covers "most" cars. Honda's PGMFI is unique to Honda and not used by other manufacturers. Their system of having the altitude sensor soldered into the ECU is not common among most manufacturers.

Ford, Chevrolet, Chrysler, Nissan, Mazda, and Subaru all use, or have used, separate barometric pressure sensors. And that's just off the top of my head and intentionally omitting European brands because I know you'll just claim "eurotrash" and avoid the actual topic if I bring any European manufacturers into this. Toyota does this as well, though instead of a dedicated altitude sensor they simply use a MAP sensor and adjust fueling based on the MAP readings (which makes having an altitude sensor redundant).

What is more common in modern cars is to simply use a MAP sensor to directly measure intake manifold pressure. This eliminates the need for a separate barometric pressure sensor as fueling adjustments can then be done based on actual conditions in the manifold rather than the, frankly, crude method of applying a fixed lean-out based on going above a pre-set altitude (typically between 4,000 and 5,000 feet).

The '98 S70/V70 occupies an odd transition year and the '99 and later models do use a MAP sensor in the actual intake tract. The '98 models (and the 850 models before them) appear to use a substantially similar sensor, but connected outside of the intake tract.

ZV

 

[tortillasoup]

Essentially every Bosch or Bosch-derivative system ever built (without a MAP sensor).

Which, despite your comment, covers "most" cars. Honda's PGMFI is unique to Honda and not used by other manufacturers. Their system of having the altitude sensor soldered into the ECU is not common among most manufacturers.

Ford, Chevrolet, Chrysler, Nissan, Mazda, and Subaru all use, or have used, separate barometric pressure sensors. And that's just off the top of my head and intentionally omitting European brands because I know you'll just claim "eurotrash" and avoid the actual topic if I bring any European manufacturers into this. Toyota does this as well, though instead of a dedicated altitude sensor they simply use a MAP sensor and adjust fueling based on the MAP readings (which makes having an altitude sensor redundant).

What is more common in modern cars is to simply use a MAP sensor to directly measure intake manifold pressure. This eliminates the need for a separate barometric pressure sensor as fueling adjustments can then be done based on actual conditions in the manifold rather than the, frankly, crude method of applying a fixed lean-out based on going above a pre-set altitude (typically between 4,000 and 5,000 feet).

The '98 S70/V70 occupies an odd transition year and the '99 and later models do use a MAP sensor in the actual intake tract. The '98 models (and the 850 models before them) appear to use a substantially similar sensor, but connected outside of the intake tract.

ZV

It's fine to use the MAP sensor for the purposes of measuring barometric pressure but to have a dedicated sensor with harness, connectors etc. just for that purpose is really dumb and I can see it isn't limited to eurotrash vehicles but some other models as well which I'd also consider trash. Toyota has done the same as well with both using the MAP sensor and NOT using the map sensor with the barometer on the ECU circuit board. I believe it depends on the model and whether it has a MAF sensor but no MAP sensor.

 

[LTC8K6]

It's so dumb that it's been working fine since 1998 and for 185K miles, and still might only just be dirty...

 

[Zenmervolt]

It's fine to use the MAP sensor for the purposes of measuring barometric pressure but to have a dedicated sensor with harness, connectors etc. just for that purpose is really dumb and I can see it isn't limited to eurotrash vehicles but some other models as well which I'd also consider trash. Toyota has done the same as well with both using the MAP sensor and NOT using the map sensor with the barometer on the ECU circuit board. I believe it depends on the model and whether it has a MAF sensor but no MAP sensor.

Why am I not surprised that when it's been shown that the method is vastly more common than you claim you dismiss the cars using it as "trash?"

When the car does not use a MAP sensor, it is much easier and cheaper to use an additional, external, sensor for altitude compensation than it is to solder a special sensor to the ECU (which also cannot be replaced if it fails). The Bosch units are typically rather modular designs and it makes a lot of practical sense to do things that way.

ZV