My son's 1997 F150 has an issue where it wants to die when you come to a stop while in Drive. (4.6, trans U)
It used to not be too bad, but yesterday it became almost undriveable. if you shift into drive with the brake firmly pressed, the idle will surge up and down about 500 rpm and occasionally drop to 0 and kill it. its worse if you've been driving and come to a stop; it almost always dies.

The problem only appears to exist in drive. neutral, park, and reverse all seem fine.

The previous codes were p0171, 0174, and 0401. I'm assuming 171 and 174 are O2 sensor related. 401 is EGR. I had him pull the vacuum hose off the egr valve and it still does the surging, only in drive.

Then i started to wonder if its possible that the torque converter clutch is staying engaged and its stalling due to that?

I searched a bit, but didnt see anything jump out at me with this issue. Thanks.

 

Incorrect. The O2 sensors are doing just fine and telling you there is a vacuum leak affecting both cylinder banks.

My hunch, with the issue rearing its head with use of the brake, is that there is a leaky hose going from the intake to the master cylinder. I’d check that out.

 

Thanks for the advice. I’ll look into that here shortly.

Curious though why it woundnt be acting up in any transmission position other than drive?

 

Well, I went in and sprayed a bunch of starting fluid around the vacuum hoses with the engine idling and never had an rpm change. I even wiggled a bunch of hoses and found nothing loose.

I thought ‘what if it’s the brake booster itself that has a leak’ so I unplugged and capped the vacuum hose going to it, but that didn’t help either

Not sure what else to try.

 

Starting fluid will not stick around long enough to diagnose an intake manifold leak
Use either flammable brakleen or better yet the non flammable type
You are looking for a change in RPM is all indicating a leak
Sometimes it is just dirty injectors or a weak fuel pump that causes a lean condition
That is why the PCED H manual section has 200 pages of diagnosis
I'd start with either new injectors or pull yours out for testing if you find no vacuum leak

 

Before starting a search for vacuum leaks, always check the MAF sensor first. Fords are well known for MAF issues and a bad MAF can affect both banks and produce the codes 171, 174.

If you do decide that it is not the MAF and want to chase a vacuum leak, then do a smoke test.

 

is there MAF troubleshooting guide? I have a multimeter.

 

The MAF may just need cleaning.

Some tests can be conducted with a voltmeter, but to fully test you will need to also check the barometric readings . I'm not sure you can do that with just a voltmeter.

More videos on using a voltmeter are available - just search: f150 1997 4.6 testing a MAF

You can do a crude test by just unplugging the MAF sensor and see if the engine problem is worse or better. The computer will default to a prescribed air-fuel mixture ratio during the test. If the engine runs better then the likely problem is the MAF. If you replace the MAF always use Motorcraft.

 

Well, seeing the thing about the MAF sensor reminded me of my ford probe I drove 20 years ago. It would run like garbage if the MAF got dirty (thanks K&N). I would clean it off with a brake cleaner spray. Decided to try that. I also replaced the hose between the PCV and the intake. Now it dies immediately when I shift into drive where before it would try to keep running. Still works in reverse.

 

After seeing this, I went out and unplugged fhe MAF. No change.

 

That test doesn't completely rule it out. I would do the voltage tests as well. if those pan out then it's likely not the MAF.

Next logical step is to continue pursuing resolving those codes. It may be time to do a smoke test.

 

Didn’t do a smoke test yet. The whole reverse thing is still bothering me. I just had it running. Still doing the “dies in drive” but reverse still works. I stomped the brakes, put it in reverse and gassed it until it started moving against brakes and it still didn’t die. If it were a vacuum leak, wouldn’t that very large load on the motor have killed it?

IF my torque converter clutch had failed where it stayed locked, wouldn’t it die in reverse also? Is there anything mechanical in the 4R70W that tells the clutch to engage only when it’s in drive, or is it strictly electronic / solenoid driven?

 

Remove and clean the IAC.

 

Already have. No change. Swapped with a different one, no change.

Not trying to sound smart, but why would it work in reverse under load and not drive?

 

Okay, I didn't focus on that part. I was focused on the codes. So, ignoring the codes, it would appear you have a transmission problem. Did you try moving the vehicle by shifting into 1st or 2nd ?