Stock 93 Explorer runs as if it has a vacuum leak but

[ddenton749]

I can't find a vacuum leak anywhere.

It all started suddenly. The engine started running very rough. It would almost stall at traffic lights and I had to drive two-footed to keep it running to nurse it home. It is running extremely rich to the point where it blows black soot out of the tailpipe when I start it.

It will idle ok for several minutes then it will start to gag and sputter, then if it doesn't stall it will clear up and run ok for a few minutes. The exhaust smells rich. The CEL will come on and then go out again. It runs better when the engine is cold but then when it warms up it idles rough.

When it idles rough I can open the throttle a bit and the engine clears up and the power comes back. Then it will start running rough again.

I had trouble accessing the computer due to poor connections. When I did get it connected there where no codes stored. Weird since the CEL did come on.

I took the upper intake apart to look at the gaskets above and below the fuel rail - both appeared good.

416,000 miles on the original engine.
Compression test - good 180 psig in all cylinders.
Vacuum - 14-15 inHg.
Spark plugs black and dirty but all are the same.
I checked the vacuum components: hoses, brake booster, cruise control, interior vent system, cold air valve, trans modulator, fuel tank vent, etc., - all good.
Tuned up a couple of years ago but I didn't drive it at all last year.

Injectors are very old. I replaced them many years ago with junk yard injectors. I applied 25 psig of air pressure to them and they don't leak.
Coil pack is original.
MAF sensor is original.
Air tube between MAF and throttle body new.
IACV original.
TPS original
Fuel pressure regulator is original.

I have been having trouble with electrical connections lately. Most recently a dirty fuel pump fuse prevented it from starting. The cruise control will drop out sometimes when I hit a bump. Those kind of things.

Any and all suggestions and ideas are welcome.

 

[RangerX]

One place to look for a vacuum leak is on the air intake tube, right at the accordion pleats. Take the tube off and flip it over and check the bottom. Those pleats are known to disintegrate.

 

[15Limited]

Have you done a smoke test to see if/where you have a leak? Have you tested the O2 sensors?

 

[Roadrunner777]

Well, two quick things that you may have ruled out during the work you have done already, but for what it's worth:

1) A bad fuel regulator does this. There is a vacuum port on the regulator and if the diaphragm in the regulator breaks, you end up sucking gas through the vacuum line into the vacuum tee. You can disconnect the vacuum line to test... it won't run great but it would prove or eliminate the regulator as a cause.

2) A failed transmission modulator will do this too, but not as severely. You end up sucking air and trans fluid into the vacuum tee. You can look at the line from the modulator and see if it is wet with trans fluid.

So, two things you may have already ruled out, but if not, the fuel regulator fits your symptoms exactly.

 

[ddenton749]

One place to look for a vacuum leak is on the air intake tube, right at the accordion pleats. Take the tube off and flip it over and check the bottom. Those pleats are known to disintegrate.

Yes. This was actually the very first thing I replaced.

 

[ddenton749]

Have you done a smoke test to see if/where you have a leak? Have you tested the O2 sensors?

I used good old ether to look for a leak. I found none.

 

[ddenton749]

Well, two quick things that you may have ruled out during the work you have done already, but for what it's worth:

1) A bad fuel regulator does this. There is a vacuum port on the regulator and if the diaphragm in the regulator breaks, you end up sucking gas through the vacuum line into the vacuum tee. You can disconnect the vacuum line to test... it won't run great but it would prove or eliminate the regulator as a cause.

2) A failed transmission modulator will do this too, but not as severely. You end up sucking air and trans fluid into the vacuum tee. You can look at the line from the modulator and see if it is wet with trans fluid.

So, two things you may have already ruled out, but if not, the fuel regulator fits your symptoms exactly.

1.) I checked the fuel pressure regulator with a vacuum tester. It held vacuum.

2.) I checked the transmission modulator in the same way. It held vacuum.

 

[ddenton749]

Have you done a smoke test to see if/where you have a leak? Have you tested the O2 sensors?

How do I test an O2 sensor?

 

[ddenton749]

I took the upper intake and the fuel rail off to look at the gaskets. They appeared to be intact.

I used my air hose and 25 psig air to check the injectors. They all held pressure.
Is a bad injector(s) a possibility still?

 

[15Limited]

How do I test an O2 sensor?

Several videos on YouTube can show you how to bench test the sensor(s)...if the sensors are original, probably time to replace anyway.

 

[ddenton749]

Several videos on YouTube can show you how to bench test the sensor(s)...if the sensors are original, probably time to replace anyway.

The O2 Sensors are only a few years old.

 

[Turdle]

To me, rich burn combined with vacuum loss = bad fuel pressure regulator passing fuel through the vacuum line.

 

[ddenton749]

To me, rich burn combined with vacuum loss = bad fuel pressure regulator passing fuel through the vacuum line.

I put a Mighty-Vac vacuum tester on the fuel pressure regulator. It holds vacuum.

 

[AGodlyCanuck]

Have you tried disconnecting the MAF to see if it runs better or the same?

 

[ddenton749]

Have you tried disconnecting the MAF to see if it runs better or the same?

I did not try that. I have the upper intake off at the moment. If new injectors don't help I will try that.

 

[ddenton749]

I did not try that. I have the upper intake off at the moment. If new injectors don't help I will try that.

So I got it all back together with new injectors, plugs, and wires and it still runs like crap.

I DOES run better with the MAF sensor disconnected. (Not perfectly but it clears up and the idle calms down.)

Does that imply a bad MAF?

 

[ddenton749]

**Update**

I replaced the upper intake and fuel rail gaskets. I replaced the injectors, plugs and wires. It still runs the same.

At the suggestion of AGodlycanuck I unplugged the MAF and it runs better disconnected.

Any ideas?

 

[RustyMacintosh]

MAF disconnected I believe it will run richer

 

[ddenton749]

MAF disconnected I believe it will run richer

With the MAF connected it runs so rich the exhaust burns my eyes and my clothes smell like gas. With the MAF disconnected it runs leaner and better.

 

[RustyMacintosh]

you sure your 02's are working and are not aftermarket crapola from tinbucktooland?