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Old 01-31-2011, 08:31 PM   #1
AndrewD
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Default 15 mpg highway, black soot from exhaust when cold, what to do?

Hey guys,

First off, I want to say how helpful this board is. I have learned a lot, being new to the brick world.
I recently bought a 1990 745ti and have been trying to diagnose a problem I have been having. I live in maine, and it is winter, so temperatures have been low. After a cold start, waiting to warm up, I have noticed a lot of black soot coming out of the exhaust that develops on the ground. Only when cold though. After reading up on this, I read that some people with similar problems have found it to be the 02 sensor. So, today I took out the sensor and it appeared to be white with a decent amount of deposits on it. Not knowing how old it is, I ordered a new one and should be in tomorrow, so we'll see what that does. I cleaned it with some rubbing alcohol and put it back in to see if it changed anything. Unfortunately, same result.
This also may be related-- I am getting very poor gas mileage... 15 mpg highway.

I have replaced the following since I bought the car:

-Both fuel pumps - solved the issue of bad starting
-Fuel filter
-Fuel pump relay
-Plugs
-Plug wires
-Fuel Pressure Regulator
-All vac lines
-CBV
-Cap and rotor will be replaced this weekend

Does anybody have any other ideas as what this might be? It has a 15g on it right now and has some up and down play in the propeller shaft. Will be replacing in the summer.

Here is a picture of the black soot that I am talking about. This is about after a minute or so of running:


It doesn't appear to be oil. It is watery, and pitch black.

Any help is appreciated!
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Old 01-31-2011, 08:32 PM   #2
DNAsEqUeNcE
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sounds like a bad AMM/ possibly o2 sensor
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Old 01-31-2011, 08:36 PM   #3
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Thanks for the quick response! I forgot to ask about the AMM. I cleaned it a few weeks ago. Is there any good way to test these? I read that one way is to unplug while idling. When I did this, there was a change in rpm's and then began to idle roughly. Plugged it back in and the idle went back to normal. Since I replaced the fuel pumps, it always starts right up.
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Old 01-31-2011, 08:48 PM   #4
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Did you check the ECT (Engine Temp Sensor)?
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Old 01-31-2011, 08:50 PM   #5
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It is more likely to be the FPR or coolant temp sensor than the AMM if the car is running decently and the O2 sensor doesn't cure it.
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Old 01-31-2011, 08:54 PM   #6
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yah, a non functioning ECT sensor will cause the car to run super bork rich or something, i dont remember what I heard.

I had problems with really rich running with a defective AMM as well, might want to try another if you have one kicking about.
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Old 01-31-2011, 09:12 PM   #7
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I haven't checked the ECT. The weird thing is, the plugs look really good. If anything they're on the lean side (white to golden brown). Unfortunately I don't have another AMM kicking around and there aren't any JY's around that have volvo's.

Once its warmed up, it doesn't run stellar. With the mbc in, set at 8-9 psi, it will cut out under full boost at WOT.
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Old 01-31-2011, 10:12 PM   #8
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ECT part number is 8788200 used on some 92-95 Saab's and some 87-95 Volvo's
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Old 01-31-2011, 10:25 PM   #9
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seafoam/ water torture it. cant do anything but help.

ECT/AMM. about all you have left. the test for the amm is if you unplug it and it will continue to run for a few minutes, its probably shot. after you do that you need to clear the codes if you have OBD.
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Old 01-31-2011, 11:32 PM   #10
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check catlitic cornverter
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Old 02-01-2011, 12:20 AM   #11
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check catlitic cornverter

It has a brand new custom 3" turboback with new cat.
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Old 02-01-2011, 01:00 AM   #12
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The catalytic converter cannot make it run pig rich.

Check coolant temp sensor first. You have an ohmmeter? Check it's resistance...... I forget what the spec is but low resistance when cold that will go up smoothly as it warms up.
It is worth the effort checking it at the brain so that you get the full effect of wiring. If the circuit goes 'open' it will be really rich. Edit: I may have that bass-ackwards. Check the manual.

Also worth checking....... Pull the vacuum hose off of the fuel pressure regulator and see if fuel drips out of it when cold. I've seen them drip when cold before failing altogether. If you can so much as smell gasoline in the vacuum line to the pressure reg it is suspect or worse. Pressure regs are a common failure.
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Old 02-01-2011, 10:23 AM   #13
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I'll check the coolant temp sensor. Thanks.

I have already replaced the fuel pressure regulator, but I'll pull the vac hose off anyways to see if its faulty.


Going to pick up the o2 sensor in a bit so lets hope thats it!
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Old 02-01-2011, 12:03 PM   #14
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1990 , if it is the original LH2.4 ecu, the ECU has gone bad, check it if it is a red label ending with 56x then you've found the likely culprit. A common failure mode is running rich, and poor MPG's. Good luck!
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Old 02-01-2011, 02:28 PM   #15
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Just replaced the o2 sensor with no luck...

However, I might have found a lead on something. I pulled off the connector of the AMM and the idle raised and cleaned up. When I plugged it back in, the idle dropped, almost stalling, and then regained itself into a somewhat decent idle. Did I just find the problem? The next question is... if this is the problem, where can I find one? No volvo JY's around here and I don't want to spend $250 on a new one unless its necessary. Also, do you think this is the correct AMM? I thought it was weird that there is 6 prongs coming out of the unit, but only 5 terminals on the connector. Is this right?

Here's a pic..





It appears the ECU is a replacement... Its green and doesn't have the 56x at the end.



So what do you guys think about the AMM?
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Old 02-01-2011, 02:32 PM   #16
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Also, what are the signs of a failing ECT? It warms up quickly and stays at a correct temp..
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Old 02-01-2011, 02:47 PM   #17
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Quote:
Originally Posted by AndrewD View Post
Also, what are the signs of a failing ECT? It warms up quickly and stays at a correct temp..
that is regulated by the thermostat, not the ECT

The ect is for engine control/fuel delivery. I do not remember if there is a separate sensor for the dash gauge or not
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Old 02-01-2011, 04:36 PM   #18
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If the ECU doesn't receive a reading within a certain from the ECT it will adjust to something like -40 dgrees when the engine is cold and 68 degrees when the engine is warmed up. If the ECU thinks the engine is at 68 degrees instead of 186 degrees it will result in rich running. I think this is the way that I read it.
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Old 02-04-2011, 07:55 PM   #19
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Okay so a little update:

I replaced the 02 sensor, cap, rotor, and took out the injectors and inspected them. Didn't appear to be too bad as far as dirt and grime. I sprayed them with carb cleaner. After all of that, still same symptoms. Bad mpg's, black carbon? buildup out off the exhaust, and sputters when its in boost. I put the mbc back in after all of that and it made the problems more noticeable. When you get on the throttle it acts as if its cutting out, jerks the whole care. Doesn't sound like a misfire though. BTW, I replaced each of these items separately. I noticed after I cleaned the injectors, there was a decrease in performance. I also noticed that I could spin the injectors between the intake manifold and fuel rail so I'm wondering if this could be leading to the issue. I put new o-rings on before reinstalling. The injectors appear to be out of an old ford. They have mixed part numbers. Here's a pic:




When the car is sputtering, from the outside it sounds like it is hitting a rev limiter or something. But it happens as low as 2000 rpm. Its hard to tell whether its a lack of fuel, or too much.

What do you guys think the next step should be? Find some different injectors?

BTW, the car has a big intercooler, full 3", 15g, IPD cam, and those bigger injectors as far as engine performance mods go. All of which were done 2 owners ago. I'm not sure if anything has been done as far as tuning goes. Kind of a crap shoot. When I bought the car, it had a TB sticker on it and was told that the guy who did all of the work on it was frequent on here. His name was dave, not sure of a last name. Lived around barrington, nh. It'd be cool if I could get in touch with him so I find out what was really done to this thing and get it running properly.

Thanks for the input guys!
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Old 02-04-2011, 07:56 PM   #20
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Also, if I do replace the injectors with something else, what would you recommend? Brown tops? Or is there anything else that could be easily found at a JY?
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Old 02-04-2011, 08:00 PM   #21
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what about the ECT?
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Old 02-04-2011, 08:14 PM   #22
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Coolant temp sensor/ect sensor. CHANGE THE DAMN THING.

I've had it fail on two Volvos. Both times, it resulted in sooty blackness from the exhaust, crappy mileage, and a reluctance to start on cold mornings.

Just make sure you're replacing the SENSOR and not the SENDER.
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Old 02-04-2011, 08:36 PM   #23
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Yup the ECT is on order. Should be here tomorrow...

Now that should be a pretty simple job, right? Its right below the intake manifold, correct?
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Old 02-04-2011, 10:37 PM   #24
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Had very similar issues on my '89 240 a few weeks back.

Try this: Find the ECT and first start by checking the connection. Remove the connector and spray it out real well with a high-pressure air hose, as well as cleaning the connector end of the sensor. Reconnect and see if that cures it. If not, I definitely concur with the other contributors, which is to say "replace the ECT before doing anything else".

Good luck to you, btw - where in Maine are you? My wife is from South Thomaston (Midcoast). Sure love it up theyah.
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Old 02-04-2011, 10:50 PM   #25
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The ECT is the rear most temperature sender in the head, located below intake runner 3. Start by removing the expansion tank cap, this will release the pressure in the cooling system. Then put the expansion tank cap back on, this will create a vacuum in the system when you remove the ECT sensor. By doing this you will not lose more than an ounce or two of coolant when you replace the sensor.

Edit: Test the new sensor before you install it.
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