running rough and erratic

[speed_demon]

I just bought a 91 Explorer 4.0 "as is" advertised as being a good runner but slowly lost power and eventually would stall with the application of throttle. I figured if the fuel pump cycles its a plugged filter. The pump cycled but sqealed a little. I then thought install the filter and see how it goes. Sure enough the old one was plugged bad and after cycling a few times the fuel pump sounded better. It runs and idles but stumbles and misses. At lease I know the IAC is working. As it warms up the miss gets worse and I see a black spot in the grass where the exhaust exits. I pull a vacuum line to induce a leak and it runs better but still rough which confirms it's rich.
I figure its probably a temp sensor, a dirty mass airflow sensor, or possibly a bad or fouled o2 sensor. I'm also guessing the rough idle is due to fouled plugs as a result of the rich condition.

Which of the two coolant temp sensors is in charge of the fuel mixture?
How do I test? Can I use a multimeter to check resistance cold and hot? What should it be reading?

How do I test the MAF sensor? ohm specs needed.

What else should I check?

That isn't all though. At idle the voltage bounces around when the engine stumbles. The battery seems strong so I figure maybe a bad chassis ground but can't be sure with all the other issue happening at the same time.

Thank you so much for any help. I'm kinda screwed if I can't get this truck running well. I have to pass emissions and I'm a little nervous right now.

 

[MAKG]

Neither is "in charge" of the fuel mixture, but the two wire one affects it. I doubt it will make it anywhere near rich enough to foul the GROUND.

Run the self tests. It won't tell you exactly what the problem is, but it might help narrow it.

Does "slowly lost power" refer to a single drive cycle, or over multiple warm-ups? If the former, run a vacuum test to look for restricted exhaust. Note that if you find it, it's almost always a secondary problem. But it does mean you'll need a cat before getting it sniffed.

I agree that poor grounding is a possibility. Look for voltage drops between the negative battery post and all the critical parts -- engine block, computer ground (pins 20, 40, 60), oxygen sensor case, and so on.

Also, FIRST, pull the vacuum line off the fuel pressure regulator right after running. If fuel drips out, replace the regulator (this will make it NASTY rich -- basically pumping fuel directly into the engine at several PSI).

Probe the oxygen sensor to see what it is reading. Then probe the computer to make sure it's the same.

Where are you? If this is a California inspection, the seller is responsible for making it pass smog. Go after him and if he refuses, take it to a shop and sue him in small claims for the cost (he WILL lose).

The Mitchell books give a table of all the computer inputs under several different driving conditions. This may be quite valuable (try the library).

 

[speed_demon]

Neither is "in charge" of the fuel mixture, but the two wire one affects it. I doubt it will make it anywhere near rich enough to foul the GROUND.

Run the self tests. It won't tell you exactly what the problem is, but it might help narrow it.

Does "slowly lost power" refer to a single drive cycle, or over multiple warm-ups? If the former, run a vacuum test to look for restricted exhaust. Note that if you find it, it's almost always a secondary problem. But it does mean you'll need a cat before getting it sniffed.

I agree that poor grounding is a possibility. Look for voltage drops between the negative battery post and all the critical parts -- engine block, computer ground (pins 20, 40, 60), oxygen sensor case, and so on.

Also, FIRST, pull the vacuum line off the fuel pressure regulator right after running. If fuel drips out, replace the regulator (this will make it NASTY rich -- basically pumping fuel directly into the engine at several PSI).

Probe the oxygen sensor to see what it is reading. Then probe the computer to make sure it's the same.

Where are you? If this is a California inspection, the seller is responsible for making it pass smog. Go after him and if he refuses, take it to a shop and sue him in small claims for the cost (he WILL lose).

The Mitchell books give a table of all the computer inputs under several different driving conditions. This may be quite valuable (try the library).

I live in Anchorage, AK. This guy registered it out of town when it failed emissions. He says it was because of a rusty tail pipe that has been replaced but he failed to produce the IM report. Funny he lives in anchorage and before said the title and reg are clear and ready to go. Yeah right! I'm a little worried the cat is fouled now but worst case I'll sell it in wasilla where there are no emission laws.
As far as the temp sensor goes I know if doesn't reach around 190 degrees it will never go into closed loop mode and will act as its still ice cold. I had this exact same problem on a different vehicle. Once warm it doesn't do much but if you can't get out of open loop it's like leaving the choke on. Thanks for narrowing down the sensor. My dad used to be a ford mechanic/service manager so I'm sure if I pay him a visit he will have a sender that will work. He likes to pack rat that kind of stuff.

"The Mitchell books give a table of all the computer inputs under several different driving conditions. This may be quite valuable (try the library)."

^^^ That's a life saver!

As far as the rest I'll print it off for tomorrow.

Thanks!

 

[speed_demon]

I haven't had much time to work on it but I did clean the MAF and that did help. It seems to run smooth but still somewhat rich. The plugs are dark and if you pull a vacuum line it idles up.
The temp sensor tests out and the fuel pressure reg doesn't have a ruptured diaphram. Does anyone know the ohm specs on the tps? I plan to test it and the o2 sensor tomorrow. Other than that the only thing I can think of is the MAP or bad connections to the ECU. Any ideas are greatly appriciated.

 

[smokey]

for the tps it should just read a smooth sweep of the needle on the meter as your move it through out the range. the voltage spec is .5 volt IIRC don't have a manual near me.

 

[MAKG]

It's supposed to be 1V or so at idle, but honestly, it is not a carburetor idle stop and it isn't going to cause noticeable problems unless it's WAY out.

Have you run the self tests? Guessing is a bad idea.

 

[speed_demon]

It's supposed to be 1V or so at idle, but honestly, it is not a carburetor idle stop and it isn't going to cause noticeable problems unless it's WAY out.

Have you run the self tests? Guessing is a bad idea.

I don't think the tps is out of adjustment, I think it might be toasted. As far as the self test goes I don't have a manual so I don't know what pins to jump or what codes are what.

 

[speed_demon]

Shouldn't I get a trouble light if I have a code? So far no light but then again it could be burnt out or the previous owner could have removed it to hide problems.