'98 Volvo C70 2.5T atmospheric pressure sensor and high consumption

SvenGWK · Post by **SvenGWK** » 02 Mar 2018, 15:43

Hello Volvo friends!

I have a '98 Volvo C70 2.5T.
4 years ago, the engine control light came on in the middle of summer right after the start.
The car drove quite normally.
Then, using the OBD2 interface and the "Torque" app, I found out that P0107
is reported (Manifold absolute pressure sensor Low voltage).
I had exchanged the sensor, but the error came back on the next hot day.
Someone told me it might be connected to the air-conditioning pressure switch. Since my A/C didn't work anyway, I bought (scrap yard) a 3-pin high pressure switch and installed it. After filling the air conditioning system, it worked. Fine. But my barometric sensor error persisted (and still does).

In the meantime, the crankcase ventilation and almost all rubber hoses have been replaced. The vehicle was fitted with a new EGR valve, a new MAF sensor, a new timing belt, new spark plugs, new ignition cables, a new ignition distributor and new air and fuel filters.
It didn't do any good.

I don't find a garage that can handle the issue properly. They always want to replace components that have already been replaced.

In the city the consumption is about 18l/100km or 16mpg. CEL is permanently on, if i delete the error it will come back soon.
By the way, the consumption on the freeway (with speed around 120km/h) is about half as much as in the city . The CEL lamp stays off then, but it lights up as soon as I get back in town and have to wait at traffic lights.
The Long-Term-Fuel-Trim then goes up to 25%. After reaching this value CEL lights up. A Bosch tester reported that the fuel control system had reached the upper limit.

I have read out with VOL-FCR that my ECU sees a value of 1.6V / 36kpa when the MAP sensor is connected. That would roughly equal the air pressure at an altitude of 8000m, but I am almost at sea level. If I remove the sensor, the default value is set to 100kpa. Fits. Problem: The lamp then lights up because the control unit is missing the sensor.
Unfortunately, it seems the sensor is not defective at all. From the control unit the 5V arrive at pin 3 of the sensor, ground at the middle pin 2 is also OK. I have tested the sensor on my laboratory power supply, at normal pressure the output voltage at pin 1 is approx. 4.7V. That should be okay. If I use a vacuum pump to create negative pressure, the voltage decreases. But according to VOL-FCR, the ECU only sees 1.6V. Either the wire resistance between the ECU and the sensor is either damn high or something pulls down the voltage. Alternative 3: Control unit broken?

Anybody have any idea how I could solve the problem?

[Excuse me, English is not my mother tongue. I hope that my problem description is written in an understandable way.]

Post by **wizechatmgr** » 02 Mar 2018, 15:46

Sounds like a leak in the intake manifold somewhere.

Have you tried hooking up a "real" gauge to verify what the sensor is indicating as fact?

SvenGWK · Post by **SvenGWK** » 02 Mar 2018, 15:59

I measured the sensor with the multimeter, but only on my desk and not in the car. However, since the reference voltage of 5V (exactly 5.17V) is present at the connector, the sensor will probably also bring out the approx. 5V. The control unit sees a significantly lower voltage.
To avoid confusion: It is the ambient pressure barometer (Bosch part no. 1275342) near the wiping water tank. My car does not have a "real" MAP sensor in the intake tract.

Post by **precopster** » 02 Mar 2018, 16:01

When you "exchanged" the sensor was it a new genuine Bosch one? I had this code on my now sold 1998 V70R and found that the sensor was missing. On the V70R the sensor was measuring atmospheric pressure not turbo pressure which I believe is the same as yours.

SvenGWK · Post by **SvenGWK** » 02 Mar 2018, 16:05

No, I put in a Facet sensor. Part number fits. I ordered another used one from Bosch today, as a last resort. But I don't think it will do any good, because the Facet sensor does exactly what it should do. At least on the lab bench.
And yes, that's the sensor. Turbo pressure is not measured in Motronic 4.4.

Post by **oragex** » 02 Mar 2018, 16:41

How about a front O2 sensor? It's not expensive and may explain city mpg (only use the recommended brands, either the one that makes them for Volvo, or NTK/NGK if they have a replacement).

I remember main streets in Berlin are quite long before a red light, but if your are driving on smaller streets with plenty of stops, a 15-16l/100km is not that shocking for the turbo engine. My non turbo gets a decent 13-14 on short streets with plenty of stops/lights. By the way, is it manual?

Post by **sleddriver** » 02 Mar 2018, 19:50

As we both own '98s and they're getting old and you have already replaced the sensor, I'd check the wiring harness for degraded vinyl wire insulation which could cause a short. I've seen this the ECT, crank & cam sensors wiring. It also happens to the headlight low-beam harness inside the light assembly.

Insulation just breaks down over time, thus leading to shorts, green, oxidized copper, etc.

Post by **jreed** » 03 Mar 2018, 05:31

I have a '97 855 GLT at 202k miles with the same 2.5T engine. Two years ago and five years ago I got the same error code for the MAP sensor. I think the location of the connector right behind the right hand headlight may expose it to a lot of rain and salt spray.
I cleaned out the connector on the harness and the pins in the sensor. There was some green corrosion visible on the pins. The second time I used a contact cleaner on the pins and applied silicone grease around the connector housing to try to improve the water resistance of the connector housing. So far, the code has not returned.

SvenGWK · Post by **SvenGWK** » 03 Mar 2018, 08:31

@sleddriver
.... that would be the worst case scenario, to be honest.
The price of having the wiring harness replaced by a workshop is an economical total loss. I'd have to find someone to do some patchwork. I myself do not have the knowledge or the possibility to do so.

@jreed
Corrosion on the connectors is not visible, everything looks quite good according to age.

I did a test drive today without the sensor. After approx. 25km of city traffic, the on-board computer calculated a plausible 12.5l/100km. The car drives completely different. No more "sawing" at the traffic lights, clean gas acceptance. Only the idling speed was slightly higher after the engine was warm (1100-1200rpm). However, I didn't reset the ECU and the trim tables might still have too high values in them. I guess it would all normalize after a few more kilometers.
This barometer signal is obviously to blame for the whole misery.
My problem is that I can't get out of the ECU a.) because I don't have a removal tool and b.) if I could remove it, I wouldn't know at which pins this sensor line ends (a possibility would be to "ring through" all pins with the multimeter...).
The pin assignment of the connector seems to be as follows: (from left) 3 - URef=5V, 2 ground, 1 signal. I measured the lines during operation today. When the sensor is connected, the 5V at pin 3 will drop to 3.7V (?). Curiously, pin 1 has a voltage of 5V. Is it possible that this voltage is switched on by the control unit when a missing sensor is detected? I hadn't deleted the error. The tester also tells me that 5V/100kpa are detected if there is no sensor. However, I would not have expected 5V to be physically connected to the line.
Other possibility: My analysis of line meaning is not correct. This would indicate that a wrong sensor has been installed there, where the signal and URef are interchanged with the original sensor.
Does anyone have the correct pinout?

Post by **precopster** » 03 Mar 2018, 14:54

That's why you need a genuine sensor.

It would pay to remove the engine control module and clean the pins as corrosion here can play havoc with signals/voltages from sensors. Just pull back on the steel strap and she comes right out; you don't need a removal tool unless this is a 1999 model.