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Author Topic: Going Down The Rough Idle Rabbit Hole  (Read 32504 times)

Offline weitrhino

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Going Down The Rough Idle Rabbit Hole
« on: 11:32:05 PM / 28-Oct-15 »
Just drove my '85 on a thousand mile trip and she ran like a top. Zero complaints other than I wish she had a damn cup holder! Seriously, I couldn't be happier with how she ran. So I've been home just over a week and using the car to run around town on various errands and everything continued to run well until coming home from the store I noticed she wanted to die at the stoplight. I could goose the throttle and keep her going so I made it home OK and decided to up the idle a bit because before the 1K trip I turned it down a tad. The car recently went from an auto to a 5-speed and I'd never readjusted the idle down to the 750 range. It still uses the auto ECU to no apparent ill effect. Anyway I could only keep her running with my hand on the throttle while adjusting the idle back up. Now it idles rough and won't hold steady. Once above, say, 1500 RPM the engine runs smooth.

I intend to inspect the idle air adjust (IAA) and the air regulator and look over all the vacuum hoses again because the ECU didn't show any trouble codes. Perhaps the engine needs to run awhile longer before a code is picked up but I see these components can have trouble without throwing a code. One thing I don't want to do is throw parts at it. I'm not a diagnostician so without a code to follow or an obvious broken part to replace I'm left flailing around for an answer. Is it possible the IAS (idle adjuster) could have an issue? It's really the only thing that's been fiddled with since right before the trouble began. I replaced all the vacuum hoses for good measure in July 2014 after I got the car.

Intelligent pointers and personal experience welcomed.

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Offline rednucleus

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #1 on: 01:06:04 PM / 29-Oct-15 »
Could this be due to a cylinder misfire? Like bad plug, wire or injector. Can try unhooking one cylinder wire at a time to see if you hear a change. Vacuum leaks worth checking too. I remember needing to change the regulator years ago - will have to check my old notes as to why!

Offline Cajun1guy

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #2 on: 06:12:01 PM / 29-Oct-15 »
Stuck fuel injector? I used to have occasional issues where I had to keep the throttle open as it wanted to die (CA18ET). Never could pinpoint it, I always wondered if a nozzle stuck open as it was trying to flood. Try a dose of Seafoam.

Offline weitrhino

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #3 on: 08:51:08 PM / 29-Oct-15 »
Could this be due to a cylinder misfire? Like bad plug, wire or injector. Can try unhooking one cylinder wire at a time to see if you hear a change. Vacuum leaks worth checking too. I remember needing to change the regulator years ago - will have to check my old notes as to why!


Certainly could be a misfire although I'd expect the ECU to throw a code but there's been none so far. I tried a second ECU today to rule it out and observed he same behavior. I also took out the air regulator and peered inside and it was open in a cold state. The FSM says to make certain it moves freely but there's no discussion on how to do that. There is a nut on the outside that's glued into place so I didn't muck with it. The air regulator has continuity and voltage reaching it properly. I may have to look at the plugs and wires next. The plugs only have 2000 miles on them but the wires came with the car. Probably wouldn't hurt just to throw a new set on there. I pulled and reseated them all with no change.


Stuck fuel injector? I used to have occasional issues where I had to keep the throttle open as it wanted to die (CA18ET). Never could pinpoint it, I always wondered if a nozzle stuck open as it was trying to flood. Try a dose of Seafoam.


After I rule out a spark issue I'll consider all aspects of fuel delivery.

« Last Edit: 08:55:37 PM / 29-Oct-15 by weitrhino »

Offline rednucleus

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #4 on: 09:55:53 PM / 29-Oct-15 »
The glued nut can be used to adjust the cold fast idle setting - but I would not touch that if your idle was previously ok - something else is the problem.

Offline BOSSMAN

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #5 on: 02:06:23 AM / 30-Oct-15 »
Did you happen to fill the tank up in the pass day or two.  My father had something similar, car idled rough a few days after he filled his car up.  After filling a new tank from another gas station it run a little better, but the fix was he added a bottom of fuel system cleaner and now his car ran better then when he got the used car a few years ago.
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Offline weitrhino

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #6 on: 08:16:50 AM / 30-Oct-15 »
Did you happen to fill the tank up in the pass day or two.  My father had something similar, car idled rough a few days after he filled his car up.  After filling a new tank from another gas station it run a little better, but the fix was he added a bottom of fuel system cleaner and now his car ran better then when he got the used car a few years ago.


In fact I had very recently filled the tank but I'm a little hazy on the exact sequence of events. Last night I ordered a full set of 10 plug wires from Rockauto, shipped for a puny $12 on a closeout special. Pouring rain today so nothing further will happen.


The glued nut can be used to adjust the cold fast idle setting - but I would not touch that if your idle was previously ok - something else is the problem.


That's exactly what I figured so I left it alone and simply reassembled everything.
« Last Edit: 08:19:01 AM / 30-Oct-15 by weitrhino »

Offline rednucleus

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #7 on: 02:08:04 PM / 30-Oct-15 »
How about cap and rotor and plugs - when were they last changed - do those as well when you do wires if no recent change. I did my wires in the last year and found a coil wire was intermittently arcing to the coil tower and resistance in several wires was way too high. Huge diff after the change. Wire brands seem to make a difference too. What did you buy?

Offline weitrhino

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #8 on: 06:14:08 PM / 30-Oct-15 »
http://www.rockauto.com/catalog/moreinfo.php?pk=644061&cc=1208618&jnid=437&jpid=3

I consider these wires a cheap diagnostic tool because I'm not certain the set in use has a problem. Rockauto has 3 sets remaining on close-out.

The car came into my possession 7/14 and although I don't know the exact age of the cap and rotor the cap appears to be newer than the rest of the engine. The PO had already replaced the radiator, brake master cylinder, water pump, alternator and by the looks of it I'd guess at least the cap as well. One step at a time.
« Last Edit: 06:14:54 PM / 30-Oct-15 by weitrhino »

Offline onephatser

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #9 on: 06:13:33 AM / 02-Nov-15 »
I just wanted to say that you can use your freezer and oven to check the operation of the air regulator.  Just observe the difference between hot and cold because it's not going to be a way obvious difference.  And you're right in not touching the glued screw.  Don't touch it!!!  The idle valve may be bad, but I doubt it.  Last time I checked OEM surplus had some of those.  If the engine is misfiring there will NOT be a code.  Our OBD systems are far too crude for those codes and these cars don't even have a code for misfires assigned.  You can always check your idle switch for the correct voltage (TPS).  IIRC closed is 0 V and wide open is 4.0V give or take a few tenths.

At one time I had a similar problem with mine, but mine was due to having a high lift cam put in.  Eventually I corrected it by giving it 32 degrees of timing and it never ran better.  Try checking your timing and although I'm not saying to advance to 32deg, try bumping it up a lil bit and see what that does.
« Last Edit: 06:16:23 AM / 02-Nov-15 by onephatser »

Offline weitrhino

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #10 on: 04:38:25 PM / 03-Nov-15 »
I can now rule out the wires as an issue. The new set is in but there is no observable difference at all. The engine still stumbles and although I had previously reported things smoothed out over 1500 RPM I really don't think that's the case. The fact that the engine is running higher sort of hides the stumbling but it's still present. I'm feeling a bit handicapped since moving into a rental house in TX as I no longer have a workbench and my tools are sometimes difficult to locate among all the stuff piled in the garage. It's not the best situation for getting things done.

Therefore I remain at square one today.

Offline rednucleus

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #11 on: 04:41:20 PM / 03-Nov-15 »
I would do my cap, rotor & plugs next.

Offline weitrhino

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #12 on: 05:52:29 PM / 07-Nov-15 »
Cap and rotor check out. I had the cap off earlier and all the contacts are clean with no corrosive build-up or wear. I kinda figured these were new judging by the relative lack of dirt on them compared to the rest of the engine bay. I think I can add them to the other items the PO replaced.  I'll pull the plugs and make sure they're gapped correctly maybe tomorrow but this was a very sudden onset issue and the plugs only have about 2000 miles on them.

Does anybody know if the ignition coils are dedicated intake / exhaust or dedicated by cylinder? Or neither?

Offline rednucleus

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #13 on: 06:15:04 PM / 07-Nov-15 »
One for intake, one for exhaust. Pretty sure they are identical parts.

Offline weitrhino

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #14 on: 07:52:28 PM / 07-Nov-15 »
Page EL 37 in the FSM shows a basic diagram with one coil for intake and the other for exhaust. Thanks.

I think I'll pull the exhaust coil off completely and see how the engine idles, then repeat the process with the intake coil disconnected. Seemingly, I'd have to have issues with both plugs for a particular cylinder in order for the plugs to be the cause of the stumbling rather than just one because the second would still ignite the fuel.  But in this way I might be able to determine if there's an issue on one side of the ignition or another. If it continues to stumble either way then it may be time to look into the injectors. Agreed?

Offline rednucleus

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #15 on: 09:21:58 PM / 07-Nov-15 »
Is rough idle your only problem - does the motor run well otherwise?

Offline weitrhino

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #16 on: 08:26:54 AM / 08-Nov-15 »
I'd have to characterize it as a misfire / stumble. It continues as RPMs build but because engine power is increasing the stumbling is less relevant compared to idle speed.

Offline rednucleus

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #17 on: 01:22:08 PM / 08-Nov-15 »
I have had two rotors that visibly looked ok but were shorting out intermittently thru the distributor shaft.  Cheap easy part to change/try.

Offline weitrhino

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #18 on: 05:23:34 PM / 11-Nov-15 »
Finally had a chance to have another look at this issue and did what I said I'd try above. With a completely cold engine, first I unplugged the exhaust side ignition coil and fired up the car. It started pretty quickly as usual but fumbled around at a low idle speed for a few moments before zipping up to 2000 rpm where it ran smoothly. Interesting, says I. So I shut it off, reconnected the exhaust coil and disconnected the intake coil then restarted. She fired straight away and zipped right up to 2000 rpm where once again it ran smoothly so I took her around the block to warm the engine expecting the idle to drop back down. With the engine warmed the idle remained at 2000. I shut it off and reconnected the intake coil, started it up warm again at 2000 rpm.

I had brought the idle speed up when it first began stumbling so now with a warmed engine I brought it back down to about 800 rpm and took her out on the highway. Zoom Zoom. Mazda's got nothing on Nissan. She ran absolutely perfectly.

The only thing I noticed was a quick whiff of a burned oil smell just after the idle jumped to 2000 rpm the first time. Since then there's been no such smell. So that's got me wondering if some hunk of greasy crud was lodged somewhere that finally got blasted into gaseous oblivion. I've still got a new cap and rotor on the way that I'll just hold in reserve.

Offline sideways_s12

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Re: Going Down The Rough Idle Rabbit Hole
« Reply #19 on: 05:55:13 PM / 11-Nov-15 »
Nice! definitely sounds like something was clogged. Glad to know to got it worked out...