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1990 Buick Century -- 15 year mechanic is stumped

Hello, all. I?m a first time poster who really needs some help. My wife and I own a 1990 Buick Century (1990 BUICK CENTURY LIMITED with 3.3L 204cid V6 MFI (N) engine). It has 111,000 miles on it. I absolutely love this car, though I?m not exactly sure why. I take all my vehicles to Tran?s Auto Repair in Renton, Washington due to stellar service and reasonable prices. Recently the Buick had a major 100,000 mile tune-up. The car was running superb until a few days ago.

It completely stalled as we were driving it. As we were driving! I was able to start it but only if I kept the gas on the floor board. We smelled a lot of gas, of course, but as soon as I let up ever so slightly on the gas, it died immediately.

Long, the guy who works on my vehicles at Tran?s Auto Repair, is ?stumped.? He is a veteran mechanic who has over 15 years of experience. It is NOT the mass airflow sensor. That checks out. The Buick gets a spark, gets gas, but no oxygen. The spark plugs are black and there is a strong odor of gas, but the Buick will not stay running. However, if the FAN is not working the car runs fine! Very strange, but it gets better? the fan is constantly on. Long has come to a conclusion that it has something to do with the Buick?s computer. He has taken upon himself as a personal goal to figure this out but I thought that I would try and help with suggestions.

Any help? Thanks everyone.

Other than the throttle position sensor being broken, there is the good chance that your engine computer is dead. Your car is old enough for this to happen. If you are lucky. you may have the cheap computer for under $150. I don’t know when GM advanced beyond solid state electronics. My 85 Cadillac had transistors in the computer, and a PROM chip but that didn’t make it “cutting edge technology”.

I am sure you want long to think you came up with all the good ideas about what is wrong with your car all by yourself,you would not like it if he knew you cheated and asked CT.

So your mission is to start “googling” everything you can about your cars computer,ignition system,and fuel system. Remember you have to convince Long you know what you are talking about.

Post back in the morning and I will prep you for your “impress Long sortie”

I don’t think that someone has schooled himself in electronic engine controls (It shows). Here are some instructions in that area:
The engine coolant temperature sender (sensor) sends its temperature indication to the engine computer. The engine computer uses this information to enrich fuel flow during cold start and idle, and, turn on the engine cooling fan when the engine reaches a certain temperature, or anytime the a/c is turned on to cool the cabin or to defrost the windows.
The engine coolant temperature sender may be erratic, and causing the engine to run too rich, and turn the engine cooling fan(s) on.
Let Long measure the voltages and ohms of the sensors…rather than throwing parts at the problem.

If the fan is constantly on, how do you know the car runs fine when the fan is not working? This seems obvious, but is the check engine light on and if so has he pulled the codes?

The CT sensor might be a possibility, but usually it won’t throw the mixture off enough to make the thing not run and it would be accompanied by a very high idle. Also, I’m fairly sure that the fan is controlled by a separate temperature switch in the radiator on this car, not by the computer.

It sounds like it’s running very rich and I’d say if the MAF and/or MAP sensor has been ruled out, the fuel pressure regulator is the only other thing I can think of that will cause it to run super-rich. It might be worth putting a fuel pressure tester on it.

obviusly the “tune up” disturbed something, loosened a hose, or ? Ask what they do during this service and your answer is there if they back track

Thank you very much for the responses, everyone. I forget to mention that the check engine light does come on but there are no codes that indicate anything is wrong. Again, very weird. Thanks for the links and the advice. I especially like the fact that my mechanic should “measure the voltages and ohms of the sensors…rather than throwing parts at the problem.” Agreed. I’ll continue to do some Google searches and learn as much as I can. Thank you CT! What a great place to learn…

You still did not clarify your statements, “…if the FAN is not working, the car runs fine!” You also state, “…the fan is constantly on.” These are contradictory statements. Please clarify. The state of the nation depends on it, or something.

Yes, sorry about that.

So, if the fan motor is literally plugged in then it is tremendously hard to turn the vehicle over. Once on, it runs “very rich,” as someone pointed out above – lots of gas is required (pedal to the metal) and only stays on for a minute or two. During this time the fan is on, even with a cold start, and the fan stays on. However, when the fan motor is unplugged and not receiving any electricity, the car starts fine and seems to get rid of the extra fuel that had previously built up. There is a strong gas smell and blue smoke comes out the tailpipe.

Looking at the fans for the car it appears to be an electrically powered fan. First stab at the cat a short in the fan and the fan needs to be replaced.

What does “First stab at the cat a short in the fan and the fan needs to be replaced”?

Did somebody drink their 100 proof mouth-wash, or what? I, too, those words, not understanding, am.

I’m not real convinced the fan has anything to do with this problem.
Since it’s obviously running rich (sooted plugs, gas odor, etc) then you might consider the following.
Faulty fuel pressure regulator.
Weak and dying fuel pump. Weak pump means low fuel pressure and that in turn can mean lousy fuel atomization out of the injectors.

If one factors the fan into this problem then what about inspecting the battery positive cable junction terminal?
Maybe a poor junction terminal connection or even a faulty battery cable could be behind this since both the fuel pump and fan are high current draw items.
Maybe the poor connection is heating up due to high resistance, lowering the voltage being provided to the engine computer, etc. and the ECM is simply shutting down due to low voltage.

Just some wild guesses for you perusal and hope it helps.

You say: the engine runs smoothly until the wiring to the radiator fan is connected. Then, the radiator fan runs, and the engine begins to run roughly, and gasoline is smelled,
I think that ok4450 is onto something. The cause of the problem may be (mostly) electrical. The voltage and current may be falling too low for things to operate properly.
Scroll down to Fig. 61 and Fig. 63 for the wiring diagrams for the engine management electronic controls, and the radiator fan circuitry.
There may be poor wiring connections which causes voltage to drop. The fan may be the straw (current draw) which broke the camel’s back. The radiator fan always running, may be a separate problem.
When the radiator fan is running, it may be reducing the current and voltage available to the fuel pump (and, of course, to other things). The resultant drop in fuel pressure would affect the fuel injector spray atomization and force. The large droplets of fuel won’t burn completely, and pass on through the exhaust unburned (raw).
Have the battery and alternator checked. Some auto parts stores will do this, curbside, for free.
Check the wiring and connections, including ground points. Check current to the fuel pump. Connect the radiator fan wiring, and watch the fuel pump voltage and current.

No codes? You will get around to changing that computer soon enough. If there are any other problems like fast idle with no power, digital readouts that freeze and cruise control that doesn’t work sometimes, bucking and stalling when turning, you will be certain. I wish it was cheaper.

About the engine cooling fan constantly running: The radiator cooling fan is turned on (in normal operation) by the A/C switch, and defrost switch, and by the engine computer (ECM) when it gets a “I’m hot” signal from the engine coolant temperature sensor (ECT).
If the wire were disconnected (say, accidentally) from the ECT sensor, would the ECM turn on the radiator fan? Maybe. Actually, check the wire to the ECT sensor. Is it connected?
A stuck radiator fan relay will keep the fan running. The relays for the fan, the fuel pump, and the A/C Compressor Control are all the same type (MR44). Swap the fan relay with one of those and see if the radiator fan stops running.
Give feedback, even if it’s negative. We needs it.