1996 850 NA 11mpg, MVSOLVED , MAF O2 sensors

[mhdorson]

I have a 1996 850 GLT wagon, non-turbo. It has never been great for fuel economy, but it recently took a turn for the worse. It started dying randomly and blowing black smoke out the tail pipe at the same time the fuel economy dropped from about 17mpg to about 11mpg averaged over two tanks of fuel.

A remanufactured Bosch MAF made it stop dying regularly, but it still gets about 11 mpg on the highway.

It has zero codes and I know the CEL works because it will trip if I pull the MAF connector. I took it to tihe dealer who reported high fuel trim and no codes. This made me suspect that the two new O2 sensors I installed might be seeing air in the exhaust so I disconnected both of them.

Removing the O2 sensor connectors did not change the performance. It also did not set a code or turn on the CEL. Is this normal? I would expect a CEL with both O2 sensors disconnected.

I have checked for vacuum leaks (none), replaced the plugs rotor and distributor cap, both O2 sensors, the engine coolant temperature sensor and the MAF. Fuel rail pressure is about 43 psi at idle and seems to hold steady when revving in the driveway. The plugs are black, the exhaust is sooty and smelly.

I like this car but it will have to go if I can't figure out why the ECU wants to put so much fuel in the engine. Anyone ever seen a bad ECU play these sorts of tricks?

--Matt

[matthew1]

Bad ECU's are rare, despite there being one or two on this board lately.

I'd suspect the O2 sensors. If you can remove them from the system and it still functions the same... that says something. Something like "we're not functioning". Well, the upstream sensor at least.

What brand are the sensors you put in?

[mhdorson]

I used Bosch sensors. What really throws me is that the ECU does not seem to care enough to set the CEL with them both disconnected.

[abscate]

Try disconnecting the MAF and reporting mileage. If it is still low, I would guess dirty fuel injectors or FPR is gone

[matthew1]

ECU is simple to test. Get another and pop it in, no wrenches to turn, although you might need a screwdriver to pop two tabs.

[Ben850]

Your "re-manufactured" MAF is likely the fault.

I had luck with an original scrapped Bosch MAF sensor on my '96 wagon while trying to diagnose an issue.

Remanufactured really has nothing to do with Bosch any more. (except maybe the plastic casing.)

I even used scrapped Bosch O2 sensors because it was for diagnostics at that point.

All the sensors are still working great together so I left them in. They have made all the adjustments to my 3" intake alterations and the R 628 ECU.

I guess original Bosch (even if not new) is what these cars want.

[mhdorson]

I may still try a real Bosch MAF, but I got some new data from my new live data reader in the meantime.
At idle contitions I see the following
RPM 840 SparkADV 5°
MAF 3.99 (g/s) TP% 9
LongFT% 25 ShortFT% 0
ETC 95°C O2 sensor 0.255 Volts

If I disconnect both O2 sensors, the O2 reading sets at 0.48 Volts and does not move. With the O2 sensors connected, if I open a major air leak after the MAF I can force the O2 reading to zero, but it never goes above 0.3 Volts when connected to the ECU. Forcing this air leak makes it run smoothly, stop smoking and generally act correctly at idle. It would be hard to go down the road and manipulate this leak at the same time.

This tells me that either the connection to the O2 sensor is bad and it can never reach 0.9 volts to let the computer know it is rich, or that there is an air leak in the exhaust before the O2 sensor that prevents the sensor from seeing a rich condition.
Anyone here know what voltage on the O2 sensor tells the ECU it has a rich condition?
Anyone know what pins on the ECU I should be looking for to check resistance between my O2 sensor and the computer?
Anyone have tips on removing and plugging the secondary air injection port on an old rusty manifold? The compression fitting is loose in the manifold hole and the compression nut is seized on the injection tube.

--Matt

P.S. I tried the old Bosch MAF that has been out for a week. It reads 7.5 g/s at 840 rpm idle and makes the car smoke like a chimney. There are real differences in these sensors.

[mhdorson]

I'd suspect the O2 sensors. If you can remove them from the system and it still functions the same... that says something. Something like "we're not functioning". Well, the upstream sensor at least.

^^^^^
|||||Good call. I swapped the front (2 week old) O2 sensor for a new Bosch sensor and now I see the O2 voltage swing up and down between about 0.05 and 0.9 volts at idle and steady state operation.

It appears that I had a bad MAF and then changed out that and the O2 sensors at the same time. The new MAF is good but the new O2 sensors not so much. This will teach me to swap one thing at a time in the future.

I also pulled the 2nd air supply line and added a copper washer to seal the leak on the exhaust manifold. This might not have helped the sensor, but it did improve the exhaust note to my ear.

Thanks for the help.

--Matt

[abscate]

Nice, thanks for the final report