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Help me diagnose my van's terrible idle when warm.

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FrankTheCat

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Help me diagnose my van's terrible idle when warm.

PostTue Mar 05, 2013 10:12 am

So I'm currently still stuck with my hand-me-down 2000 Plymouth Voyager with the 2.4L EDZ, due mostly to the lack of funds necessary to replace it with something more hoon-y (hopefully I'll rectify that this summer.)

For the past year or so I've been trying to diagnose (as cheaply as possible) idle quality issues the car has when warm (after the thermostat opens.) The idle goes a bit lopey and has a bit of a misfire feel, but no codes have been triggered, not even misfire (P0301) codes. Any throttle past idle, and the engine's as smooth as a Chrysler-designed I4 can get.

I've checked the fuel pressure, spark, and compression (cyl. 1 is 115PSI, cyl. 2 is 120PSI, cyl. 3 is 116PSI, cyl. 4 is 121PSI,) so all that's left is the air now.

But I noticed something after running a bluetooth ELM327 OBDII reader and OBDScope on my phone (wanted to get semi-accurate MPG estimates; it was slightly depressing); the idle quality drops as soon as the idle management switches from open loop to closed loop. What does that narrow my problem down to?

I should mention that I had a malfunctioning PCV for a while (~3,000 miles) that first spit oil into the intake, and then blocked up completely. Idle quality issue existed before this. Didn't notice until I checked my oil and it was all sludged up after only 1,500 miles on Castrol GTX. I replaced the PCV, cleaned the crap out of the throttle body with isopropyl acohol, and switched to Pennzoil Platinum to clean out the sludge. Helped the clattery valves when cold, but the idle quality persisted.

-edit-
Looked through the service manual, and the only difference between closed and open loop, is that closed loop looks at O2 sensor data. Guess I'll have to look at the sensor voltages and see if it needs replacement.

-edit deux-
Upstream O2 sensor reads normally (between .010v and .95v,) downstream reads a little sluggishly, but that has no effect on the fuel trim, and I passed emissions testing last month so I'm not too worried about that. The only other thing I could think of would be the timing?

-edit tres-
Regapped the spark plugs. They were at .042", when the sticker on the firewall called for .050". Made a definite difference (no more 'chugging' at idle, and the vibrations are much less noticeable,) but the idle is still a bit rough. I think I can hear the hiss of a vacuum leak around the throttle body, possibly related to the use of RTV instead of a proper gasket by some past ham-fist of a mechanic.
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jeep_jeff

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Re: Help me diagnose my van's terrible idle when warm.

PostWed Mar 06, 2013 1:09 pm

IIRC, the other thing that happens once it warms up is the Idle Air Control valve changes how much it's open. If it's stuck or dirty, it will cause idle problems. It will be attached to your throttle body. Try cleaning it (both the IAC and the whole throttle body, for that matter) first.

EDIT: And, of course, this will let you redo that RTV gasket...
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Stu_Rock

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Re: Help me diagnose my van's terrible idle when warm.

PostWed Mar 06, 2013 3:12 pm

Wait, so the van goes from idling fine to idling poorly the exact moment it enters closed loop control, regardless of the temperature? That means a sensor input to the control algorithm is wonky. If this is the case, change your upstream O2 sensor(s).
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FrankTheCat

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Re: Help me diagnose my van's terrible idle when warm.

PostWed Mar 06, 2013 4:15 pm

jeep_jeff wrote:IIRC, the other thing that happens once it warms up is the Idle Air Control valve changes how much it's open. If it's stuck or dirty, it will cause idle problems. It will be attached to your throttle body. Try cleaning it (both the IAC and the whole throttle body, for that matter) first.

EDIT: And, of course, this will let you redo that RTV gasket...

IAC passages are as clean as they can get. Need to pull the solenoid itself and test if it actually moves (enough; if it were dead altogether I wouldn't have an idle,) though. I also need to get the remainder of the oil residue out of the intake plenum from the PCV disaster.

I'll do both when it gets a little warmer. Also want to replace the upper intake plenum gasket, and the EGR gasket (which is crumbling into dust.)

Stu_Rock wrote:Wait, so the van goes from idling fine to idling poorly the exact moment it enters closed loop control, regardless of the temperature? That means a sensor input to the control algorithm is wonky. If this is the case, change your upstream O2 sensor(s).

Yeah, if you ignore the horribly clattery valves from the sludge-induced low oil pressure to the head when the engine's cold, the idle is actually smooth. As soon as the ECU switches to closed loop control, the idle quality decreases noticeably. If you put your ear to the tailpipe, every 3-5 seconds it'll 'chuff' like there's been a misfire.

Confuses me that there haven't ever been any codes triggered for misfires though. Every other car I've worked on that's running like this is throwing codes out from everything.

I should also mention that the gas mileage is pretty awful (~16mpg) even for a 2 ton minivan shaped vaguely like a rounded brick.

But yeah, before I spend $60 on a new upstream sensor, I'll have to lug my 20lb, 1970's vintage scope and check if the wave form isn't high or low.
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Stu_Rock

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Re: Help me diagnose my van's terrible idle when warm.

PostWed Mar 06, 2013 4:41 pm

If the vehicle does not have any trouble codes, the sensor is still giving alternating high and low voltages. As the sensor ages, its response time to changes in O2 concentration increases, so it could be inaccurately reporting. You could look at the "cross counts" on the scan tool for more diagnostic information. A new part is less than $35 for this van, though.
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FrankTheCat

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Re: Help me diagnose my van's terrible idle when warm.

PostThu Mar 07, 2013 12:22 pm

Well after driving 150 miles back and forth to school on the regapped plugs, things have gotten a lot better. I presume the ECU 'relearned' its fuel/air maps?

Clattery valves are slowly getting better too. During my last oil change I switched from 10w30 Castrol GTX to 5w30 Pennzoil Platinum with a much heavier duty oil filter, in order to flush the residual sludge out of the head. My sister borrowed the car for ~6 months when she didn't have one out at college, and despite driving almost 8,000 miles, she never changed the oil (which was a non-synthetic!) The 4.5ish quarts that drained out looked like crude, it was so black. Never going to let her live that down.

I still plan on putting a scope on the O2 sensor to check if it's reading low or high (I don't have the money to just outright replace it; run a tight budget,) and replace the gaskets on the intake.

Thanks for the help, though!

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