“ Having the tech at the truck with the phone was a great help. He recorded events for playback at the PCM, and read data readings one at a time from the scanner. ”

Ever had a day when you seriously considered trading in your toolbox for a new, chrome-plated spatula ... thinking that a career in fast food cannot be as bad as this?

I got a call from a technician in just this frame of mind who was down for the count on a 1995 Ford F-150 with a 5.0L engine and an E4OD transmission. His customer complained of a stall on acceleration from a stop.

All the usual tests had been done. There were no codes in the computer ... fuel pressure increased slightly during the brief throttle movement before the stall ... the ignition appeared to be fine until the stall ... and adding external fuel did not make a difference. The tech used a lab scope to monitor the pip signal, the vehicle power circuit, and the computer and ignition grounds. Everything seemed perfect. The transmission had even been rebuilt on this truck since it was an E4OD, and because on three separate occasions it had stalled simply by moving the shifter into drive.

Having the tech at the truck with the phone was a great help. He recorded events for playback at the PCM and read data readings one at a time from the scanner. The one thing he hadn't monitored in his data stream was mph - but then why would he? This vehicle didn't even start to roll before the stall. When I asked him to watch mph and drop it into gear, he shouted so loud into the phone that my ears are still ringing - it hit 100 mph! To eliminate the stall, I had him unplug the rear differential speed sensor (DSS).

This vehicle is equipped with a programmable speedometer/odometer module (PSOM), an electronic unit made to look like a conventional speedometer. This unit receives an AC signal from the DSS and drives the speedometer needle to indicate speed. It also converts this signal into a digital signal to pass on to the PCM and cruise control servo (if equipped). Since the DSS signal feeds the ABS module and the PSOM ... there was no ABS problem or light ... and the speedometer read correctly, its signal would be OK. The fault would be in the PSOM's ability to read and generate the appropriate signal to pass on.

In this case, the almost nonexistent AC signal from the DSS caused by the slight twist of the exciter ring in the differential on gear engagement was causing the PSOM to send the PCM an input of 50+ mph. When this value reaches the 100 mph mark, PCM strategy limits vehicle overspeed by locking the convertor and shutting off half of the injectors. Since this vehicle hardly started moving when the PCM saw that kind of speed, the stall was immediate.

Replacing the PSOM fixed the problem. The last I heard, the tech had canceled his plans to serve “Happy Meals.”

Jeff Boskowitz is an IDENTIFIX Ford team leader. He is ASE master and L1 certified, with 29 years of diagnostic repair experience.

On '93 Subaru Imprezza: Throttle Body Cooling Off Causes High Intermittent Idle

By Todd Erickson

“ By using a temperature probe on the compensator, the technician was able to see that it was cooling down when cruising on the highway. Under certain conditions the throttle body cooled off enough to cause the idle compensator to start pushing the throttle back open. ”

Symptom: High intermittent idle when coming off the highway. Talking to the owner of the car did not reveal a pattern. Checking for codes revealed no problems seen by the power train control module.

The customer did not want the car back until the shop found the cause of the high idle. After several days of driving, a pattern emerged. The engine would idle at normal speeds all day long in the shop. Driving through town did not cause any higher-than-normal idle speeds. And when the technician took it out on the highway, nothing unusual happened for several miles. Then, pulling up to a stop light after about 5 miles of steady cruise, the idle showed 1100 rpm. By the time the technician got the car off on the shoulder of the road, it was at normal idle again.

The technician test drove, test drove and test drove until he could get it to repeat the high idle. Finally, he saw more patterns. If he drove more than 5 miles at a steady speed over 50 mph and if the outside temp was below 40 degrees Fahrenheit, the engine would idle higher than normal when coming to a stop. Within minutes of slowing down or stopping, the idle speed returned to normal.

Here's what was happening: The cold idle compensator for this engine package is a wax pellet-type plunger that physically opens the throttle. By using a temperature probe on the compensator, the technician was able to see that it was cooling down when cruising on the highway. Under certain conditions the throttle body cooled off enough to cause the idle compensator to start pushing the throttle back open.

The PCM did not see this as a abnormal condition, so no codes. Flushing the compensator pushed out a piece of plastic wrap that was restricting the coolant flow. Road testing on the next cold day proved the compensator was staying up to operating temperature again.

Todd Erickson is an IDENTIFIX Asian specialist. Erickson is an Accredited Automotive Manager (AAM) with 16 years of diagnostic experience. He is a certified Subaru senior master technician, as well as ASE master certified, L1 and alternate fuels.

This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time. For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. www.identifix.com. © 2008 IDENTIFIX. All Rights Reserved. © 2000 IDENTIFIX. All Rights Reserved.

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