Down shifting kills my engine...

[kstjohn]

I have a '91 Explorer (manual 5 speed transmission)that has had an intermittent problem with the engine stalling when down shifting. It's consistent if I'm descending down a mountain trail. I remember when I first bought it from the dealer...I pulled out of the parking lot then downshifted to stop at a red light. The RPM's were descending (struggling for life) as the engine finally died. It always starts just fine or if I depress the accelerator and let go, the rpm's climb to idle and all is well. The Ford mechanic told me it was the EGR, well guess what? There is no EGR on a '91 (as he found out and was embarressed). Because it's intermittent (unless I'm descending down a mountain trail)), Ford doesn't have any solutions. After getting angry with these so called professionals, I attempted to replace sensor after sensor hoping it was electrical(you should see my stock pile of spare parts!). Well after everything was replaced, the problem was still there (not to mention the stupid pinging that I've also had since day one but that's another story).
Any suggestions would greatly be appreciated (and problem let me live a few years longer).

Thanks in advance

Ken

 

[TPLYNCH]

Your stalling is probably the Air Idle Control or the Throttle Position sensor. If you've replaced those already, double check your vacuum lines. Make sure they aren't cracked, loose, missing etc. As for the pinging, have you cleaned your Mass Air Sensor? That's the usual culprit. Could also be carbon build up, especially on an older Explorer. If you haven't already done so, check out this site. It has loads of info and procedures to check what I've mentioned. Good luck! Dead Link Removed

 

[SteveVB]

Along with checking the MAF as above, try cleaning the IAC-BPA (Idle air control bypass something something), it should be opening when you decelerate to keep the idle up, it may be blocked, inoperative, or just dirty. Check to see if the solenoid is working, and clean the valve itself. Also may want to clean the intake passages for the valve and the TB while you there.

 

[kstjohn]

I have replaced the high speed air bypass valve in the past with the same results. I replaced all sensors initially. I noticed there was a TBA for the throttle body but it was just cleaning of the residue buildup of carbon.
Because this problem is consistent when the truck is descending down hills, I thought maybe there was something wrong with the fuel delivery system. I changed the filter and had the fuel system checked for proper pressure. All looks good there.
I should have returned this vehicle to the dealership after purchase in '91. Unfortunately, I was living in Saudi Arabia and didn't have the time to mess with it. The good news is that I only have 48K miles on it since it was in storage for 7 years. I just wish I could solve this problem.
I cleaned the MAF sensor with brake cleaner but the engine still pinged. I put 91 octane gas and an octane booster (3 points) and the pinging is now gone. I shouldn't have to do this to get rid of the pinging though???
I did try a larger MAF intake and this solved the pinging problem but then the engine idled kind of rough (even after disconnecting the battery and letting the system relearn). Someone had told me that if you want the engine to breath more air in then you have to increase the output of the air. So with that in-mind, I removed this and went back to the stock MAF intake.
Anymore suggestions?


Thanks again for the responses

Ken