Sorry if these sound like dumb questions, but going through repair manuals and taking apart the intake manifold on my truck has us stumped right now.

I have a '98 4L v6 ranger 4wd. My check engine light came on about a week ago and I ran a diagnostic on it for codes P0171 System Too Lean (Bank 1) and P0174 System Too Lean (Bank 2), and the recommended solution was to replace the intake manifold gaskets. No problem, I go ahead and order the parts from Mahle for a gasket set (MIS19316), which has lower and upper gaskets, valve cover gaskets, all that good stuff. I'm taking apart the intake manifold, pain in the rear by the way, and I see that the upper gaskets that go along the fuel rail aren't there, and the lower gasket is all one piece versus the parts that came in the set which are 2 pieces. So I've got a couple questions:
1. Do the 98 rangers come factory without the upper gaskets that go along the fuel rail? And if so, then why do they make an aftermarket part for it?
2. Is it fine to use the gaskets I got for the lower, or did I get the wrong replacement parts?

 

Welcome to the forum

The Lean on Bank 1 AND 2 wouldn't be caused by lower intake air leak, unless BOTH sides started to leak at the same time, long odds on that
On the 4.0l the lower intake bolts are held on using low torque, I think 14ft/lbs, good idea to use "lock-tite" on their threads, they can back out causing an air leak, but generally just on one bank

No, there should be a gasket on upper intake, its possible it got left off at factory or previous owner

The two piece lower intake gaskets are fine to use, just use some RTV(silicone) to hold them in place, the one piece is often easier to work with but either will seal air and coolant passages the same.
Like any torque pattern start in the center and chris-cross to the ends

Causes for lean codes on both banks of V6 at the same time are:
Dirty MAF sensor<<<< 4.0l is know for this one
Upper intake vacuum leak, check PCV valve
MAF sensor to intake air tube leak
Partially clogged exhaust, also has lack of power at highway speeds
Partially clogged fuel filter, also has lack of power at highway speeds

Failing fuel pump, low fuel pressure, should be 55psi in 1998, if pressure is under 30psi you will start to get Lean codes on both banks


For future reference
Easy test for a vacuum leak is to warm up the engine, let it idle
Unplug the 2 wire connector on the IAC(idle air control) Valve, it will close and idle should drop to 500 or engine will stall, either is GOOD, it means no vacuum leaks
If idle doesn't drop then there IS a leak

Vacuum gauge can tell you if exhaust is getting clogged up

 

thank you! Manifold is already disassembled, and we’re cleaning and replacing the parts anyway, but that’s good to know about the 2 piece lower gasket replacement. Is it possible the leak was coming from the upper intake due to the gaskets missing? Or is it more likely another problem? Guess I’ll find out once it’s all assembled again and we run it and see what else is wrong.

 

Depends on how long you have driven this Ranger, if lack of gasket was causing the air leak then you would have known that pretty quickly after buying it

 

only had it for a couple months but I’ve put thousands of miles on it, so I was wondering if it was even that in the first place, and why it would be causing an issue now of all times after driving it a lot.
At this point, Is it possible to just bolt the lower intake manifold back on top of the original gasket and reassemble everything as was, or is the sealant screwed and I should just replace it anyway?
Picked up some MAF sensor cleaner as well, so I’m gonns clean that out and put the upper gaskets on and hope for the best

 

If you have new gaskets use them, period
Once the pressure/compression is removed from a gasket it never quite aligns back up were it was before, so best to go with a gasket that has never been compressed

 

Just finished up putting everything back together today. Replaced and cleaned up all the gaskets and manifold parts, as well as sprayed out the MAF sensor. Started up the truck and runs great, check engine light is gone. Thanks for your help!

 

Good work

Thanks for the update