I’ve been working on my own vehicles since I first started turning a wrench. Not because I wanted to — it was a simple matter of economics. I couldn’t afford to pay someone else to do it for me. For the last 20-odd years, I’ve had to work in the dirt driveway of my current place of residence but was blessed a year or so ago with the chance to build a real garage next to my house, complete with two-post lift (a gift from a very good friend)!
I refer to my shop as the “Motor Age Garage” in honor of the column that has been a staple of this magazine for as long as I can remember. Writing for that column was my first role as a new contributor to the magazine, and it’s where we start our newbies to this day. By the way, we’ve got some great young talent to introduce you to!
In addition to making life easier maintaining my own cars, it also provides me with the means to produce the videos you see on our YouTube channel and has opened up a variety of opportunities to share topics that I couldn’t before. On the other side of the coin, it opens me up to a lot of requests for help from my youngest son and his friends. I’ve done everything from oil changes to evaporator core replacements (Ford F-150, full dash removal) and soon, I’ll have his Chevy truck in for a valve “tick” that I suspect may be a wiped camshaft. Oh, the joy…
I’ll admit, there are times when I don’t feel like working on a car but I then find myself enjoying the work, getting greasy and beat up again. And I’m keenly aware that, in order for me to do my job here, I have to stay “in the trenches” as much as I can. After all, I can’t tell you guys to continue your education and skill development if I don’t!
Anyway, I recently had a project in the shop that I thought might be of interest to you. It starts with my very favorite customer – my wife.
The shuddering Scion
My better half owns a 2014 Scion tC that is her pride and joy. She’s gone full tilt with it; installing a TRD (Toyota Racing Development) exhaust, leather interior, custom lighting and more. When she cleans it up, she’ll spend a solid 3-4 hours making it look as good as it did when it first rolled off of the showroom floor. And she is just as picky when it comes to maintaining it mechanically.
The car has just over 37,000 miles on it and recently displayed a new behavior that my beloved finds truly annoying. She works at the local hospital and leaves the house when it’s still dark. One recent (and also our first “cold”) morning, she headed off to work as she normally does but on the way there, experienced a severe “shudder” when she came to a full stop. The vibration was enough to cause a loud “tapping” noise that could be heard in the cabin and was strongly felt in the steering wheel. Whatever was causing the condition, it wasn’t something the ECM was concerned with – the MIL light remained off.
When she returned home later that day, I met her in the garage to see what the car was doing. Of course, I couldn’t duplicate the problem and after reviewing the scan tool data, could find no clues as to what the problem might be. So I told my spouse I would check it the next morning and she could take my vehicle to work instead.
The following day I went out to the garage and started the car up. Following the same route she takes to work, I tried to duplicate the problem but again, without success. The only difference between my drive to work and hers was the temperature. It doesn’t stay cold long in central Florida!
Waiting for the chill
A few days later, the weather forecast called for another cold night. Here was my chance! Or so I hoped.
I have an exterior/interior thermometer I use for A/C work and I first recorded the air temperature in the shop. It was reading a chilly 41°F, which matched up with the ECT (Engine Coolant Temperature) and IAT (Intake Air Temperature) readings on my scan tool. I started the car and it dutifully went into its cold start mode, keeping the idle high as the engine warmed up. I decided to monitor the ECT, IAT and engine RPM along with STFT (Short Term Fuel Trim) and LTFT (Long Term Fuel Trim) as I retraced my wife’s commute.
A few minutes into the drive, the ECT had reached normal operating temperature and the idle speed had settled down. As soon as the car came to its next full stop, I could feel the vibration and hear the tapping noise my wife was complaining of. The engine rpm appeared to be lower than it should be, hovering around 620 rpm.
Specification in Neutral is 680-780 for this car so I shifted into Park to check. Yep, that was OK and at the higher idle, the vibration and noise were gone and the engine was running smoothly. Stepping on the brake and putting the car back into Drive immediately brought the rpm down to 620, and adding the headlights dropped it even further to 580-590. That had to be too low! A quick look at the trims, though, showed perfect fuel control with both numbers staying under +/- 4%.
Next, I applied the parking brake and released the pressure on the brake pedal. The rpm increased to 670-680 and the engine condition was gone. That seemed like a more normal loaded idle speed to me and I was surprised that applying the brake pedal would result in that much of a change. After all, the transmission was already loaded. What possible difference could the brake application make? And why only under “cold” weather conditions?
I think I got it!
I admit, I was at a momentary block in my thinking. I turned to some talented techs that hang out on Facebook and got some ideas from them, and I also talked to the shop lead at my local Toyota dealer. He wasn’t too optimistic about my finding a solution, though, sharing that this was a common complaint that, to date, they have been unable to solve!
One great idea that came out of the discussions was to consider the impact the brake booster may have on the engine. The vacuum booster could be leaking and doing so only when the brake pedal was in a certain position, the shop foreman shared. He told me that he had found several similar issues when troubleshooting P0171 (System Lean) problems for his customers.
It made sense, so I thought I’d try it. I disconnected the vacuum feed to the booster and closed off the line at both ends and went back out for another test drive. And I’d like to report that the complaint had been resolved.
I’d like to, but I can’t…
The problem was still there; the vibration, the accompanying noise, and the unusual drop in rpm with the brake applied. I did identify one additional factor though. The rpm would not drop any lower than 580. I mean, I could get the rpm to drop lower by adding load but the ECM would recover and bring it back to that minimum. Yet I’m convinced that the ECM’s “minimum” was too low for the car. What else could it be?
I should add, before your emails start, that I did clean the throttle body (it was a little dirty, but not badly coked) and performed an induction cleaning (setting a P0304 in the process – a possible clue?). I also performed the idle relearn procedure using the Toyota method and the aftermarket method many of my FB friends suggested. Problem is still not resolved. And, of course, I checked for any related TSBs (Technical Service Bulletins) but found nothing that helped.
But, faithful readers, I’m not done yet. My next step is to research every system that uses the brake pedal position sensor as an input. I’m also going to do some more in-depth engine inspections to see if the problem may be related to any weakness in an individual cylinder or in the VVT (Variable Valve Timing) system. I’m not ready to give up quite yet. I — and I can’t believe I’m saying this — only hope the cold weather remains around long enough for me to verify any fix I make! And I’ll be sure to let you know what I find out in a future Tech Corner column.
