LT4 my 96 idles rough

[96 BAM]

My '96 idles rough once up to temp. Replaced the fuel pump, fuel filter, EGR valve, installed new plugs and plug wires, new fuel injection pressure regulator and still idles rough. Think the o2 sensors may be next, your thoughts will certainly be appreciated.

 

[Hib Halverson]

Have you checked for vacuum leaks?
Have you run a fuel pressure test?
Is the check engine light on or has it been on for a period then gone off?

How many miles on the engine?

 

[96 BAM]

Cannot locate any vacuum leaks and the fuel pressure is good. The check engine light comes on peridically and the motor has 137K.

 

[tmkassin]

It might be time for new injectors. Even if the ohms test checks out good they can have bad O rings. I had this problem on my 92 and replaced them with Bosch injectors. What a difference!

 

[96 BAM]

Thanks for the info on the injectors. I might try new o2 sensors first since it was a California car until I bought it and brought it to Denver. I'm thinking the extreme change in altitude might be an issue with the old sensors. Let me know if you still think I should consider fuel injectors first.

 

[Hib Halverson]

Cannot locate any vacuum leaks and the fuel pressure is good. The check engine light comes on peridically and the motor has 137K.

First thing to do is find out why the MIL is coming on. History codes will have been stored in the ECM. Do you have a scan tester and the Factory Service Manual?

It is possible that, at 137,000 miles and if the O2Ses are original, that there is a problem with them, however, the change in altitude from CA to CO would NOT cause their failure. It is more likely just a case of them having exceeded their life expectancy which is typically, about 75,000 miles. Best replacement sensors come from Denso, next best are Delphi or Bosch. I'd replace the two front sensors.

When you say the fuel pressure is "good", is that based on the fuel pressure tests in the Factory Service Manual? If the engine passed the fuel pressure tests as discussed in the FSM, then you do not need new injectors.

With 137,000 miles, valve sealing might be an issue. Have you run a compression test?

 

[96 BAM]

My buddy, also a mechanic, has tested the fuel pressure but has not run a compression test. He also has a scan tester that is not pulling up any codes. I will ask him to run further diagnostics and report any abnormalities. Appreciate your comments and hope to get this solved soon as I am very anxious to get back behind the wheel since my Explorer is not a very fun ride.


If anything else comes to mind that might be causing the rough idle (only after coming up to temp) please let me know. Didn't have any issues with the motor not running perfectly until the fuel pump was replaced last May.

 

[jcdouglass]

skipping

There is a big distributor behind the water pump.. If you get water up in it, like when detailing under the hood, you get trouble you describe. Do not ever, ever shoot water down by the water pump.
btw, ask your mechanic to run eng diags for free. It takes the guess work out.