96 XLT 4.0 surging and stalling

[Ohiocrewcab]

I've got a problem that I can't seem to figure out. It's a 96 XLT with a 4.0 4x4 and auto trans.

I've replaced all three(3) O2 sensors, replaced the plugs with factory Motorcraft platinums, new plug wires. The issue is the fuel curve goes into open loop fault and stalls and surges. As soon as I touch the throttle it smooths out till I let off or Ive driven it a while. Also the coolant temp gauge fluctuates while driving. I hooked up the scanner and the only codes I get are P0171 and P0174 banks 1 & 2 lean. I can watch the live data display and see all the things going on and when it went into OL-FLT I hit the freeze frame and then took pictures of the screen. Can anyone look at these and decipher what possibly could be my issue?

 

[XLTrunner]

I'm not the one to tell you what all that data is telling you. But, the symptoms are classic vacuum leak. I suspect it's related to the upper intake gaskets leaking due to age and when the engine warms up, they expand enough to prevent further leakage.

 

[Ohiocrewcab]

That's the one spot I haven't checked yet but it does it at times weather the engine is hot or cold. It's something I'll check. And it's a quick fix from what I've seen.

 

[gatzdon]

Is 181 degrees hot enough to go to closed loop?

A corroded ECT sensor reading colder than the real temp would prevent closed loop operation.

Everything you are describing sounds like a warm engine operating in open loop.

 

[wcjeep]

The upper gaskets are not difficult to replace. They just take a little time. Be sure to verify the bolt torque specs. My Chilton or Haynes was incorrect. Snapped one of the bolts.

 

[4D X]

do you notice a hum or buzzing type sound at all just before it dies?

 

[Ohiocrewcab]

A friend said to replace the ETC sensor so I was gonna go ahead and replace the sensor, thermostat and intake gaskets while I'm there.

There wasn't any buzzing or humming sounds just before it dies.

 

[koda2000]

From your description and CEL codes, it definitely sounds like you have vacuum leaks. Intake gaskets are a very likely source of vacuum leaks on the job 1 SOHC engine (job 2's only have one set of intake manifold O-rings and they're much thicker), but a large vacuum leak anywhere will cause the same symptoms. Have you checked your PCV elbow? They tend to split when they get old and cause a huge vacuum leak. Even a bad power brake booster will cause a large vacuum leak, but there are other symptoms associated with a leaking P/B booster.

As far as your coolant temp, IDK if 181 degrees is enough to get into closed loop mode (possibly not), but as your temp gauge is jumping around I'd replace the thermostat with a new 195 degree t-stat.

BTW, The SOHC intake gaskets leaks tend to go away once the engine warms up and all the parts expand, but not always. They may not leak at all in the summer time. The gaskets/O-rings shrink and get brittle with age.

 

[Ohiocrewcab]

It's not a SOHC 4.0 but still gonna do gaskets temp sensor and Tstat

 

[Ohiocrewcab]

Well I replaced the Plenum gaskets and re torqued the studs to the intake and then torqued everything else to spec and also replaced the Tstat and ECT sensor and all is good now. I did spray so carb cleaner on the intake before and got an rpm change and did it after and rpm stayed steady. Thanks for the help guys.