Pulsing idle speed

(Rob Reilly) #1

2000 S-Type 3.0 L automatic with 228,000 miles.
When the engine is cold and I’m stopped, trans in Drive or Reverse with the foot on the brake, the idle speed pulses at intervals of about 1 second. Its behaving as if I was tapping the gas pedal. Once in awhile it will stall. Seems to be ok in Park and Neutral. It clears up when the engine is warm. It is not generating any OBD codes.
Any ideas where I should start looking?

(Eric Capron) #2

Hi Rob,

Did you get anywhere with this problem? I think I’d start by dismantling and cleaning the throttle body.


(Harv XK 140) #3

Yes Rob … let me know if Eric’s suggestion helps as my 2001 seems to do that same thing off and on.

(Rob Reilly) #4

It is now sensing codes P1131 and P1151.

I found these:

Error Code P1131 refers to insufficient switching of oxygen sensor bank 1 or the sensor 1 is lean. This trouble code indicates that the system is correcting the ‘rich’ for an overly ‘lean’ condition in engine’s air fuel ratio.
P1151 is similar.
The code means that the Powertrain Control Module (PCM) has detected a lean system, which means that there is either not enough fuel or and excessive amount of air in the system. Start by checking for vacuum leaks.

Read more: https://www.autocodes.com/p1151_ford.html

So I’m looking for vacuum leaks in the intake manifold.

(Harv XK 140) #5

If you find a vacuum leak please let us know where is was so we can all go checking in the same spot

(Rob Reilly) #6

Well, the problem went away with the warm weather, but now that cold weather has returned so did the problem, with the same codes P1131 and P1151 and a new one, O2 sensor no activity.
P1131 Heated oxygen sensors (H02S) 1 (upstream), bank 1 – slow response

So I replaced all four O2 sensors. I bought Bosch same as last time, #15716 front upstream and #15717 rear downstream, $26.79 each from Rock Auto.
That cured the problem immediately, and its running so much smoother.
I got about 96,000 miles out of the last set so I guess I got my money’s worth.

Tools you need to do this job:
22 mm open end wrench
a brother with a garage lift and an oxygen/acetylene torch

Glad I didn’t have to do this on my ramps.

Squeeze the tab on the clip and the plug pulls out. If the old sensor won’t come loose with hammering on the wrench handle, heat up the threaded boss which is welded into the catalytic convertor until it is red/orange, while trying not to heat up the sensor, then hammer on the wrench handle. When you get it to move a bit, stop and go on to the next, letting it cool down, and it will come out easy. Worked for me anyway.
A propane or Mapp gas torch might work if you don’t have oxy/acetylene.
The new ones came with anti-seize already on the threads. The fronts have shorter cables than the rears so you won’t get them mixed up.

(Eric Capron) #7


Do you think it was the heaters that weren’t efficient but the summer ambient temps. were enough to overcome the problem?


(Rob Reilly) #8

I don’t know enough about them to guess whether temperature or ozone would have an effect. We had a lot of rain this summer.
Wikipedia has a lot about them. Here is one paragraph.
“The sensor does not actually measure oxygen concentration, but rather the difference between the amount of oxygen in the exhaust gas and the amount of oxygen in air. Rich mixture causes an oxygen demand. This demand causes a voltage to build up, due to transportation of oxygen ions through the sensor layer. Lean mixture causes low voltage, since there is an oxygen excess.”