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08 Mitsubishi Eclipse GT, 122K on the ticker, 3.8 V6, with six speed manual and drive by wire. No check engine light and only one code for network fault U0073 which the Modis only defined as BUS off.
It’s got a very sporadic condition where the idle will fluctuate between 600-250RPM and then stall out. I got to enjoy a short streak when it would stall out, & I often couldn’t restart the engine without applying a little throttle and then holding it to maintain the idle. That streak ended by the time I got a scan tool into the car to collect PID data. It will otherwise maintain its target idle of 680 RPM perfectly.
My immediate thought is throttle control motor in the throttle body, but I want to prove it first. I did find one of the signal wires with a shitty butt connector repair right off the harness connector end that I’m suspicious of being a problem, but the PID data I could collect is saying the throttle position signals are fine. If you graph the data, the signals are all identical between the TPS and the APP signals.
Once I lost my symptoms, I did a little playing with the throttle motor just to get aquainted with its behavior. It will choke itself fully when cranking and when you shut the car off, it will cycle the the throttle wide open, fully closed, and then open up a crack. I also scoped the power and ground circuit of the motor. I should have scoped the TP signals too, but that butt connector is riding tight on the harness connector. I just really didn’t want to break that wire. At any rate, I have three screen shots for you of the scope patterns. They’re all of the motor circuit only and all KOEO. The first is idle position, the next is wide open throttle, and the last will be part throttle.
What struck me as odd, is the dirty signal on the WOT compared to the part throttle. They both also look like PWM signals, but the frequency looks about the same between WOT and part throttle too. Shouldn’t they have different frequency with different throttle positions? I’m pretty sure I should have had battery voltage on both traces with throttle closed too. I might have not had good contact with the back probe. At any rate, is there anything in the pattern trace that condemns the throttle motor?
Fopeano, I’m sure you know I’m looking at you for advice on this one, but I’ll take any useful information from anybody that’s got it to share.
Thanks in advance.
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I’ll look into this more at work, but my first instinct is not a throttle problem. I’ve never actually seen one of these engine run badly because of a engine management failure or defect. It’s always something somebody has done to it. As such, I’ve never had a reason to scope the throttle and I don’t know offhand if the graphs show a problem.
Something I’ve seen that causes a similar issue is when the purge valve hoses are not connected properly, but that usually causes a P0171. It will be idle fine until the purge valve is activated and then it gets rough and sometimes will stall.
Post up the wire color and stripe you scoped and of the one with the butt connector on it.
A really neat thing about that engine, which may help to diagnose it, is that it can run and drive around at parking lot speed with the throttle disconnected. That crack open it defaults to is enough air and when you start it, it runs on just the front bank of cylinders with no fuel being delivered to the rear bank. Then when you press the gas pedal, it switches on the rear bank to produce more power. If the problem happens while in that state, you know it’s not the throttle body. It does get a little mad when that happens though and you have to cycle the key and clear the codes created multiple times before they stay gone and it’s happy again.
Like I said, I’ll investigate this a little more at work. I’m in the middle of putting that Evo engine with the cracked piston back together today, so they’ll leave me alone.
Time was a little short, so I didn’t get to look more at the car. I should be getting the car back next week and I might be able to give it some proper attention then. In the meantime, the picture tells you what you want to know. I can’t remember which channels correspond to which wires, but I’m pretty sure I matched the green trace to the solid green wire on pin 2 of the connector.
Unless that butt connector is a nice heat-shrink type that’s totally sealed, there could easily be a high resistance issue there (especially if it’s open and old). Suspect that anyway. If there is any corrosion in there or a poor connection, it will drive the PCM nuts. That wire affects both the main and backup throttle position sensor signals. If the PCM commands the throttle plate to a specified position but sees it somewhere else, it will try to correct accordingly and cause problems like this one.
I’m not 100% sure it’s that, but I have a feeling that playing with that butt connection while it’s running might cause the problem. I’d expect it to make a throttle code with that kind of thing, but it might not be doing it for long enough to set one.
On the paper, it says you scoped pins 1 and 2, but which pin was shown in the pictures?
Aside from all of this, you should really try driving it around the shop or lot with the throttle unplugged. It’s a PITA to clear the codes like I said, but it’s just plain cool. Mitsu does a lot of smart and creative things like that with their engine programs, and that is the easiest way to see and feel how smart they are.
Both. That mostly flat line along the bottom was the other channel. It should be yellow, but on my screen, the line turns up looking kinda white. I figured it was TAC motor ground or I didn’t make a good connection with the back probe. I’ve made that mistake a few times but you can see a little twitch that matches the voltage drops on the green trace so I figured I should have made a good connection.
Before you get in too deep I’m just going to suggest you eliminate the simple. Check for vacuum leaks and hoses that might not be connected properly as suggested for starters. It would be a shame to go through all that you’ve gone through only to find it’s the result of a simple vacuum leak. More info here
http://www.ericthecarguy.com/faq/solving-automotive-performance-issues
Good luck and keep us posted
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