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93 Explorer, rough idle & low power, clears up when warm

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93 Explorer, rough idle & low power, clears up when warm

 
  #16  
Old 02-18-2019, 11:23 AM
little dutch
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Originally Posted by cooksmtn View Post
Have you checked fuel pressure? Didn't you say something about a lean code? The fuel pressure isn't monitored electronically, just controlled by the vacuum actuated regulator. May not be getting enough fuel at cold temps. I had that problem with my '90 F250 460. Started and ran fine at moderate or warmer temps. Would start fine cold on one tank, but not the other, which is what led me to check fuel pressure.
ETA: on the f250, my coolant temp sensor threw a code, so I replaced it, didn't help the start issue, so I dug deeper.
The code was "176 (M) Oxygen sensor not switching - system is or was lean Left or Front HO2S - Fuel control"

I considered fuel pressure, but dismissed it because I thought I would see a problem under heavy acceleration if it were low. So I have not checked the pressure. In hind-site, I suppose I was seeing problems under acceleration, just restricted to cold temperature mornings...

I think there is a single vacuum line from the vacuum T running to the fuel pressure regulator. It should be easy enough to trouble shoot that whole piece of the puzzle.

Thanks for the reply, I will look into it.
It's not supposed to get above freezing here until Friday, so I likely won't have an update until the weekend.
 
  #17  
Old 02-22-2019, 04:10 PM
little dutch
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Update. Still don't know what the issue is.

Spark is good (I replaced plugs and wires already)
I have not done the gaskets yet.
We've tested the MAF sensor, and then replaced it (temporarily) with a new Motorcraft sensor to be sure. Same problem, so the old one went back on.

I decided to spend the $100 on a diagnostic. Here's what that showed:
The fuel pressure tested fine. The fuel trim numbers tested negative, but only mildly. Fuel filter looked good (It's pretty new too).
The engine vacuum starts at 12", then drops to 6" as RPM increases. It doesn't recover like it should. They did not find a vacuum leak.
I verified the Engines back pressure was fine, and that the cats are not the problem (Had a muffler shop check that for me.). So, whatever is causing the vacuum issue isn't a restricted exhaust.

It's back in for further diagnostics.
 
  #18  
Old 03-19-2019, 02:30 PM
little dutch
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I think I'm finally getting to the end of this issue. The the intake manifold is definitely leaking.

First, the battery was bad. The cold cranking amps were toast. When I replaced the battery, the problem improved noticeably. The rough idle was still there, but the computer was attempting to correct for it and power surging automatically. I have a theory as to why the bad battery was compounding the issue, but it's secondary and not really relevant.

Not knowing what else to do with it, I took it back in to the mechanic to work on the problems identified earlier. Mostly fixing leaks. He started taking things off and the intake manifold leak was immediately visible.
 

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