99 Sport w/PO171 & PO174 codes.

[Mike65]

My wife's 99 Sport at times when we start the truck it will idle low & stall but it will restart fine but it had no CEL or codes. Two days ago I drove it to work & the CEL came on, & when I checked it I got the PO171 & PO174 codes & the truck has 114K miles on it. I did a search & found where people speak of the pcv elbow cracking but they are all for 2001 & up vehicles, does the 99 SOHC have it?.

 

[96eb96]

Either a vacuum leak or dirty MAF sensor.

 

[donalds]

Intake gaskets

 

[Lee's Automotive]

My wife's 99 Sport at times when we start the truck it will idle low & stall but it will restart fine but it had no CEL or codes. Two days ago I drove it to work & the CEL came on, & when I checked it I got the PO171 & PO174 codes & the truck has 114K miles on it. I did a search & found where people speak of the pcv elbow cracking but they are all for 2001 & up vehicles, does the 99 SOHC have it?.

Change the upper and lower intake gaskets and replace the EGR O ring while your in there also check your fuel pressure and if its low start with changing your fuel filter on a 99 with the 4.0 SOHC it should be between 60-65LB of pressure from the fuel rail

 

[drdoom]

Intake gaskets

My thought as well.

 

[Mike65]

I replaced the fuel filter in the fall & the fuel pump is only 1 year old..

 

[DintDobbs]

@Mike65 Mine threw those same two codes when it got cold two years ago, idle was rough and occasionally felt like it was going to stall out after coming to a standstill. Rev up at all, and it ran fine. Fuel mileage was horrible. I concluded that, to compensate for a false lean reading, the engine was running rich and choking itself out by injecting too much fuel. Replaced the oxygen sensor right behind the cat, and it fixed both the rough idle and the bad mileage. Didn't see the code again until it got cold this winter, and now every couple of days the CEL comes on, I erase the code, and it never causes the rough idle problem.

Definitely check the things the others listed, but if all is A-OK, I'd bet it's the OS. I have a single pipe with one OS, yours may differ, but that one piece is like $25. Not a big waste of money even if it doesn't solve the problem.