Intermittent stalling in gear - thinking it is tranny related

[Apu]

Hi guys

Just wanted to say this is a great forum, I just discovered it recently and it has been a great wealth of knowledge.

Anyways, here is my problem. I have been battling a low idle condition for the last few weeks on my 2000 XLS SOHC 5 speed auto and now I am starting to think that I may have something going on in the tranny. When the truck is warm, it idles nicely at around 650 - 700 rpm in park. Sometimes, but not all the time, when I put it in gear with the brake on, the rpms dive to almost stalling and then bounce back to 500 and stay there. Also, when I am creeping in stop and go traffic without my foot on the accelerator, when I try to stop it feels like a standard with the clutch out. It dips to 300rpm, then "breaks free" and idles normally at 500 rpm again. The tranny works flawlessly in regular driving other then I really have to mash it to get it to kick down into the lowest allowable gear. I have replaced the IAC, cleaned the throttle body and had the transmission oil changed, but still is the same.

I was thinking either a TPS or TCC solenoid may be the culprit, any thoughts?

 

[glfredrick]

Without putting a scanner on the truck and checking codes and info, it might be difficult to say.

I'd start by checking your IAC (idle air control) which is supposed to compensate for engine load at idle. It may be sticking. The fix for the IAC is generally very simple -- just find it on the side of the manifold, take out the two small screws that hold it in place, and then clean the inside with throttle body cleaner spray. Re-install it and test it.

Another problem that you might have (again, with no codes it is impossible to know for sure) is that your lock-up torque converter switch isn't working correctly, leaving the TQ locked up at idle. That will definitely stall a vehicle, but I'd think that would be a somewhat rare problem.

A final problem may be a vacuum leak somewhere. As you put the truck in gear, you introduce load, which draws the RPM down. A vacuum leak at that point would produce the results that you are seeing (as the IAC compensates for the low idle). There are a lot of places to get a vacuum leak, including the intake manifold. I generally check for vacuum leaks in two ways --one with a vacuum gauge the other with a can of starting fluid. Of course, with the gauge, you simply check to see what the manifold vacuum is, and if it is low, find the leak. That's what the starting fluid is for. SMALL bursts of fluid sprayed in the vicinity of vacuum connections will indicate leaks by raised RPM as the fluid is drawn into the intake stream and burned. Any place you spray that revs up the engine will indicate a leak.

NOTE WELL -- We're talking about micro-bursts of spray here! If you just go to hosing down the engine compartment you WILL burn down your truck. Do this at your own risk! This stuff is FLAMMABLE TO THE MAX!

 

[Apu]

I replaced the IAC with a new one from Ford, still the same. When I cleaned the throttle body, I noticed the butterfly plate was almost completely closed to the point that when I sprayed the cleaner in there, it wouldn't leak through to the other side. I opened the throttle stop screw a few turns last night, and it seems to have helped. My idle is at the same rpm, but it seems not to drop as much when it is loaded up. I believe the extra airflow is helping the IAC out a bit.

I took it out in a parking lot and let it roll without touching the gas in drive. On flat ground it would pick up speed and it would almost feel like the converter would "lock in" once it reached a certain speed. Once I applied the brakes, it would load the engine for a second and then break free and idle normally. Is this the way that it is supposed to work?

 

[Apu]

Update: Set the idle stop a little higher and then set the TPS to .98 v. No more issues. Vibration is all but gone and no more stalling.

 

[glfredrick]

Glad you found the problem. Good example of how to systematically work through issues instead of just replacing parts and hoping it gets fixed.