Pending DTC P0171 and P0174 But explorer runs great.

[Explore10FL]

So I have recurring p0171 and p0174. But Explorer runs well. And freeze frame doest look bad. Any explanation for this?

 

[C420sailor]

Drive the car while looking at the live data. See what the trims look like at various conditions.

Check the usual suspects. If it’s a SOHC, intake manifold gaskets.

 

[Lee's Automotive]

So I have recurring p0171 and p0174. But Explorer runs well. And freeze frame doest look bad. Any explanation for this?

View attachment 427528 View attachment 427529

Upper and lower intake gaskets need to be replaced Use Fel-Pro MS90890

 

[Explore10FL]

Drive the car while looking at the live data. See what the trims look like at various conditions.

Check the usual suspects. If it’s a SOHC, intake manifold gaskets.

Unfortunately my scanner does not display trims in live data, which I also heard is a common issue on '97s.

 

[Explore10FL]

Yes I am planning on it here in the next week. I just thought it was strange that freeze frame data looked good. Yet I still had the fault. Thank you.

Upper and lower intake gaskets need to be replaced Use Fel-Pro MS90890

 

[4pointslow]

Both banks are adding 11.7% fuel according to the fuel trims (add STFT and LTFT together).
Fuel trims should be 0 +or- 5%, if I saw 6 or 7, I wouldn't worry about it but once it starts getting above that I start to look for a reason why.
Your RPM in freeze frame shows 2400, it would be helpful to see what the fuel trims were at Idle too.
If they are higher at idle (IE 20%) and not as bad at 2,000 rpm then it is a vacuum leak. If the fuel trims went back to around 0% at idle (and 11.7% at 2,000 rpms) then it would indicate a fuel starvation problem (like a failing fuel pump). The idea is if it is running lean at idle it is most likely a vacuum leak (too much air) but if it is running lean at higher rpm it is not getting enough gas (fuel filter or failing pump).

If the Centech doesn't allow you to look at live data, then I would suggest picking up an Autel AL319. They can be purchased at Walmart or Amazon etc. These can be bought for 20-40 dollars. The tool is inexpensive, and it can pull codes, erase codes, view live data, and even check the readiness monitors to see if your vehicle will pass inspection, if you have to do that in your state.

My old blue 1998 kept coming up with P0171 and P0174, but when I looked at fuel trims (engine warmed up), they were 0% + or - 5% at idle.
Finally, I looked at the fuel trims right at start up when the engine was cold, I found them to be in the 20% range at idle.
The O-rings can expand and contract with engine temperature, so they can swell when warm and the problem may seem to go away.

The fuel trims will temporarily fix the lean condition up to a point, so the vehicle will seem to run fine.

At least if it is the upper and lower intake O-rings, they don't cost too much for the parts.

 

[Explore10FL]

Both banks are adding 11.7% fuel according to the fuel trims (add STFT and LTFT together).
Fuel trims should be 0 +or- 5%, if I saw 6 or 7, I wouldn't worry about it but once it starts getting above that I start to look for a reason why.
Your RPM in freeze frame shows 2400, it would be helpful to see what the fuel trims were at Idle too.
If they are higher at idle (IE 20%) and not as bad at 2,000 rpm then it is a vacuum leak. If the fuel trims went back to around 0% at idle (and 11.7% at 2,000 rpms) then it would indicate a fuel starvation problem (like a failing fuel pump). The idea is if it is running lean at idle it is most likely a vacuum leak (too much air) but if it is running lean at higher rpm it is not getting enough gas (fuel filter or failing pump).

If the Centech doesn't allow you to look at live data, then I would suggest picking up an Autel AL319. They can be purchased at Walmart or Amazon etc. These can be bought for 20-40 dollars. The tool is inexpensive, and it can pull codes, erase codes, view live data, and even check the readiness monitors to see if your vehicle will pass inspection, if you have to do that in your state.

My old blue 1998 kept coming up with P0171 and P0174, but when I looked at fuel trims (engine warmed up), they were 0% + or - 5% at idle.
Finally, I looked at the fuel trims right at start up when the engine was cold, I found them to be in the 20% range at idle.
The O-rings can expand and contract with engine temperature, so they can swell when warm and the problem may seem to go away.

The fuel trims will temporarily fix the lean condition up to a point, so the vehicle will seem to run fine.

At least if it is the upper and lower intake O-rings, they don't cost too much for the parts.

Great information. I was wanting know about a scanner that is known to work on this truck that shows fuel trim. I will Check fuel pressure now as well, also I was led to believe a +/- 10 was acceptable for fuel trim when added together. All of this is very helpful, thanks for your time.

 

[C420sailor]

As he said. An engine creates most vacuum worn the throttle plate closed…not under load. So if you’re at 10% cruising and you have a vac leak, I bet it’s pretty massive at idle.

 

[4pointslow]

As he said. An engine creates most vacuum worn the throttle plate closed…not under load. So if you’re at 10% cruising and you have a vac leak, I bet it’s pretty massive at idle.

Nothing you stated is untrue here.
I would like to take this concept a bit deeper if I could.

Engine vacuum is stronger with the throttle plate almost closed, when it is sitting in its resting place, compared to being wide open where a lot of us like to drive including me. LOL.
Engine vacuum will go even higher(24-25 inches Hg) on deceleration from a higher rpm, more pumping action from the engine as it is turning faster and the throttle plate is closed. When the rpm gets lower as in idle speed (600-800 rpms), the vacuum drops back to aproximately 20 inches Hg.

Now fuel trims are tested in park at idle, and approximately 2,000 rpms (found in pinpoint tests for P0171). Think about how far open the throttle plate will be if you slowly accelerate the engine to 2,000 rpm's in park, not too far open. So the vacuum could actually go up at 2,000 rpm's compared to idle because the plate is barely open and the engine is pumping harder. So why does a vacuum leak effect idle more than 2,000 rpm's?

It has to do with the quantity of air and fuel being used at idle vs 2,000 rpm's.
At idle there is just enough A/F mixture to keep the engine running, a small vacuum leak can now double the amount of air going in.
Around 2,000 rpm's the engine is using much more air and fuel so that same size vacuum leak is not throwing off the A/F mixture as much.
So for a visual....
Idle might look like this with a 14.7 A/F ratio

Quantity
AAAAAAAAAAAAAA.7 to F

2,000 rpm's might look like this.....

Quantity
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F

Now envision a vacuum leak to look like this...
AAA

3 A's will effect the quantity of A/F at idle way more than it does the quantity at 2,000 rpms.

If this doesn't make sense I apologize, when I was teaching I just use to hold my hands up showing an amount small for idle and large for 2,000 rpms. LOL

 

[Fix4Dirt]

agree with what everyone else said! one thing that happened to mine was the iac tube (idk if thats what its called), but it comes off the intake tube and has that y joint at the iac (in my 98 at least, an 01 didnt have the y, it was an elbow), but that joint cracked and would occasioanlly cause a small high idle and created a vac leak occasionally (it became more frequent as time went on)

 

[4pointslow]

agree with what everyone else said! one thing that happened to mine was the iac tube (idk if thats what its called), but it comes off the intake tube and has that y joint at the iac (in my 98 at least, an 01 didnt have the y, it was an elbow), but that joint cracked and would occasioanlly cause a small high idle and created a vac leak occasionally (it became more frequent as time went on)

Were you able to find a replacement part for that Y?

 

[Fix4Dirt]

i just used the iac tube off a later sohc which was an elbow... after the Y it just went to a dead tube that didnt connect to anything, so i figueed it didnt need it.. and the 01 thst i took parts off of didnt have the Y, just the elbow

 

[Explore10FL]

Nothing you stated is untrue here.
I would like to take this concept a bit deeper if I could.

Engine vacuum is stronger with the throttle plate almost closed, when it is sitting in its resting place, compared to being wide open where a lot of us like to drive including me. LOL.
Engine vacuum will go even higher(24-25 inches Hg) on deceleration from a higher rpm, more pumping action from the engine as it is turning faster and the throttle plate is closed. When the rpm gets lower as in idle speed (600-800 rpms), the vacuum drops back to aproximately 20 inches Hg.

Now fuel trims are tested in park at idle, and approximately 2,000 rpms (found in pinpoint tests for P0171). Think about how far open the throttle plate will be if you slowly accelerate the engine to 2,000 rpm's in park, not too far open. So the vacuum could actually go up at 2,000 rpm's compared to idle because the plate is barely open and the engine is pumping harder. So why does a vacuum leak effect idle more than 2,000 rpm's?

It has to do with the quantity of air and fuel being used at idle vs 2,000 rpm's.
At idle there is just enough A/F mixture to keep the engine running, a small vacuum leak can now double the amount of air going in.
Around 2,000 rpm's the engine is using much more air and fuel so that same size vacuum leak is not throwing off the A/F mixture as much.
So for a visual....
Idle might look like this with a 14.7 A/F ratio

Quantity
AAAAAAAAAAAAAA.7 to F

2,000 rpm's might look like this.....

Quantity
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F
AAAAAAAAAAAAAA.7 to F

Now envision a vacuum leak to look like this...
AAA

3 A's will effect the quantity of A/F at idle way more than it does the quantity at 2,000 rpms.

If this doesn't make sense I apologize, when I was teaching I just use to hold my hands up showing an amount small for idle and large for 2,000 rpms. LOL

I does make sense. I am trying to learn all of this a/f mixture, vacuum, fuel trim, etc.. so I will use this as a kind of cheat sheet in my learning process or if I am explaining it to someone. Thank you.

 

[509Explorer]

So I have recurring p0171 and p0174. But Explorer runs well. And freeze frame doest look bad. Any explanation for this?

View attachment 427528 View attachment 427529

Do a quick check for vacuum leaks, there can be a tricky one to the side and up against the intake manifold. Otherwise maybe try a quick spray clean of the MAF with a cleaner made for it. That solved the code for me about a year ago.

 

[J_C]

If all else fails, do a smoke test to try to find vac leaks.

 

[Explore10FL]

Do a quick check for vacuum leaks, there can be a tricky one to the side and up against the intake manifold. Otherwise maybe try a quick spray clean of the MAF with a cleaner made for it. That solved the code for me about a year ago.

Thanks for your reply. I have narrowed it down to a intake manifold gasket leak which I am planning on replacing next week.

 

[C420sailor]

Replace your front tensioner while you’re in there

 

[410Fortune]

Replace your front tensioner while you’re in there

Gooooooooooood advice!!

 

[Explore10FL]

If all else fails, do a smoke test to try to find vac leaks.

Had one done and the lower intake gasket showed smoke exiting. Got the gaskets, now finding time will be the difficult part. Thanks.

Replace your front tensioner while you’re in there

Yes, I plan on it and was also gonna clean injectors and replace o-rings. Also valve cover gaskets, while I have good access. Replacing tensioner won't break the cassettes and guides with the added pressure?

 

[J_C]

^ No, "usually" a good (Motorcraft) will keep the chain from rattling around as much until oil pressure builds up, so reduce wear on the cassette guides. I wouldn't use any aftermarket brand, though possibly someone knows of a good aftermarket...