Blowing Fuel/Running Like Hot Garbage

Plagueis

Registered
Hello All,

I have a 1994 Mark VIII that I have been driving for the last 10 years. Recently, it started burbling at idle, and then a bit later it started stalling. When it is at a high RPM it seems to run a bit smoother, but it will put black marks on the pavement behind the exhaust, as there is so much unspent fuel. I ran codes on it, and got 159, 121, & 96.

With those codes in mind, I put in a new TPS, MAF, Cam Sensor, Fuel Filter, Fuel Relay, Pressure Regulator, Plugs, Wires, and Coils. If anything, the car is running even worse, as it will only make it a single mile down the road. Oil doesn't have anything bad in it, neither does the coolant. When I changed the plugs, they were wet, much like a carbureted motorcycle when it is flooded. I would have initially thought it was a stuck fuel injector, but all the plugs were wet, not just one. My current theory is that maybe the ECU has gone bad, as I can't understand why it would be behaving this way. Winter is Coming™, and I am running out of time before I won't be able to wrench on it. This is the first time I have been stumped by anything the car has needed. Any thoughts on my mystery? I would appreciate any suggestions.

Thank you.
-Plagueis
 

tixer

Lincoln Evangelist
Staff member
Paid Member
This is speculative, but since I don't see 'em on the list.. How many miles are on your o2 sensors? You'd be surprised what problems replacing old sensors can fix. I feel like I did mine at the ~120k mark, but should have done them sooner.
 

driller

El Presidente
Staff member
I ran codes on it, and got 159, 121, & 96.

With those codes in mind, I put in a new TPS, MAF, Cam Sensor, Fuel Filter, Fuel Relay, Pressure Regulator, Plugs, Wires, and Coils.

Not to be critical, but I hate when people bring me their car to look at with a similar description... too much done at once. This is equivalent to a drag racer installing 6 different modifications only to find the car now runs slower than before when it should have more power. He has no clue what is at fault or even worse, what combination of mods is culpable. :unsure:

With any blatant drivability problem, the first 'go to' is obviously reading the codes. With multiple codes, I like to clear the codes and monitor which code comes back first. The reasoning is usually traced to a domino effect. That is say a fuel supply problem will manifest an air/fuel problem which leads to... you can see where this can get misleading. With multiple codes, each can either be the symptom or the side-effect.

Your post diagnosis of wet plugs is the old school carburetor "way too rich" equivalent. Given the car is EFI there likely is no reason to suspect the fuel injectors since all the plugs are wet. So, it is reasonable to presume the ECU is firing the injectors full time either due to a bad input (sensor or associated wiring) or the ECU itself is at fault. Obviously, fuel supply is not an issue, so we can rule out the fuel filter, fuel pump relay and fuel pressure regulator (though I probably would look at the fuel pressure and verify the vacuum source to the pressure regulator).

A TPS issue is unlikely to cause a full rich condition, the same with the MAF sensor. Since both were replaced, either neither was bad or both replacements were bad.

I believe tixer's advice on the O2 sensors is a good call. I have replaced them before on my cars 'just because'. You may also want to inspect the wiring for the fuel injectors but your instinct of a failing ECU could also be correct. The only other diagnostic advice I could offer would be to check the injectors with a noid light but I think it's safe to assume that would only confirm what we already suspect.

Best of luck and be sure to update us later with your findings. 😉
 

billcu

Head Moderator
Staff member
Good advice above. I've also heard the fuel pressure regulator can go bad and leak fuel into the system. It's easy to check remove the vacuum line from it and see if there is fuel in it.
 
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