Stall, Code 66 when going downhill

[sd_iconoclast]

I have a '91 Explorer EB 4x4 with 4.0 and 5 spd.
I recently redid the heads and the whole top end of the engine.
Now, when I go downhill, particularly when I step on the brakes, the car stalls and a get a check engine light that turns out to be code 66 (Mass Air Flow out of range low). The MAF clearly has been changed once before, and it looks good and clean.
Somebody suggested to me that code 66 usually means a vacuum leak.
That would make sense to me given that it happens when going downhill.

Has anyone seen this?

 

[sd_iconoclast]

I decided to follow up on the vacuum leak theory by testing every hose off the vacuum christmas tree. In particular, I suspected the brake booster might be leaking as that would be operating when I go downhill.
I tested by attaching my vacuum hand pump and checked if each line could hold vaccuum.
1] Brake booster - Held vacuum until I pressed the brake pedal. Then all vacuum went away. Vacuum was still gone when I released pedal.
2]Cruise control diapragm - slowly leaked vacuum. It took about 20 seconds for 5 inches of vacuum to go away.
3]Air cleaner - Held no vacuum. The other end went to the heater door that allows hot air (from exh manifold) into air cleaner. This line (when disconnected from air cleaner) held vacuum.
4]One way valve connected to vacuum revervoir-held vacuum
5]Tube goes around back on intake manifold, but is broken - Where does this go? I looked at the diagram under the hood but it was no help.
Since it looks like 4 out of 5 are potential problems, please let me know which ines are problems.

 

[sirhk100]

Did you ever figure out the cause of the code 66?

 

[sd_iconoclast]

Did you ever figure out the cause of the code 66?

I did. Sorry for not posting it.
It turns out that the vacuum hose to my fuel pressure regulator had fallen off the regulator so I had a good size vacuum leak.
When I had bought the car a few months earlier, I had found a bunch of the hard plastic vacuum lines had cracked or broken so I replaced them with standard rubber vacuum lines. I did not notice that the vacuum line to the fuel pressure regulator was smaller on the regulator end. So my new hose was large enough to slip off on that end.
I replaced it with a smaller diameter hose and have not had the problem since.
I hope this helps you.

 

[sirhk100]

Well it sounds like this code 66 deal tends to typically be vacuum related even thought the code itself isn't a vacuum code. Interesting... I'm reading similar stuff elsewhere. I can drive or about 45-60 seconds and then it just flat out falls on it's face. I then just have to turn it off, let it sit about a minute, refire it and it runs great again for like another 45-60 seconds. Made for an interesting 6 or so mile limp home this afternoon. LOL

Thanks for the info, gives me more fuel for the idea fire...

Khris

 

[sd_iconoclast]

The break for me came when I realized that virtually all vacuum leaks must go through that vacuum tree on the intake manifold.
Start plugging each branch of that tree one by one and see if your problem goes away.

 

[sirhk100]

The catch for me is it's a little intermittent so getting it to do it is a little challenge. How rough/bad was yours running? When mine does it, it'll idle somewhere between barely noticeably rough to where you think it's going to stall. Hit the throttle at all and it tries to die, pops and sputters and even backfires thru the intake.

 

[sd_iconoclast]

Mine was intermittent at first, but became easily reproducible over time.
I never had any backfiring. To me, that suggests a problem with ignition timing. A vacuum problem can only lean out the mixture. It cannot make it too rich or change the timing, which would produce a backfire (through the intake or otherwise).
I think you can confirm this by just blocking off everything on that vacuum tree and go for a slow, controlled test drive. You won't have a brake booster or fuel pressure regulation but you should be able to see if the problem still exists

 

[sirhk100]

Messed around with a bunch of stuff last night and found that my fuel pressure gauge was faulty. Tested and tested and tested and something in my gut said don't trust the gauge cause I'd seen it at some goofy situations. Got my hands on another one and went for a drive. Ended up that when it falls on it's face, the pressure is down below 10psi so I'm guessing bad pump on it's way out or a plugged filter. The filter physically looks new underneath but I picked up a new pump, screen, and filter last night anyways. So tonight I'll be installing that stuff and we'll see how it works from there! Just running the pump without the engine on I'm sitting at 40psi, with the engine running it's down at like 27-32psi and when it falls on it's face it's down in the 10 and under range.

 

[sd_iconoclast]

A few months after, The same problem came back. But this time it was more persistent and I had no vacuum leaks. It turned out that the problem was the fuel pressure regulator.